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Mon Apr 07, 2008 1:38 pm

I know this is a dumb question, but I have been checking everywhere and can't seem to find the answer. What is the length of the stock vw connecting rods on the 1600 DP engines? I know that when you are adjusting your compression, changing the length of connecting rod you use is one factor so I just wanted to know what the starting length is for stock rods. (I am trying to plan out my next engine upgrade and want to know- and fully understand- all the variables that go into making for a better performance engine). Thanks for your time :D

Mon Apr 07, 2008 2:33 pm

http://www.stahlheaders.com/Lit_Rod%20Length.htm

Happy reading.

Tue Apr 08, 2008 2:37 pm

That is good information, but what I was trying to find out is how long the stock connecting rods are for a stock 1600 DP engine. I see several part providers sell longer than stock rods and some sell shorter than stock rods all of which they give the length of, but I can't seem to find any information on how long the stock rods are. I would like to know that so I have a base length to determine what change if any in length of connecting rod would work best with different engine configurations. So does anyone know what the stock length for a 1600 DP connecting rod is (my engine is from a 73 super)?

Tue Apr 08, 2008 4:07 pm

Even the factory manuals don't give that. They give all the diameters, but not the lenght.

Tue Apr 08, 2008 6:25 pm

Stock rod length is 5.394".

Thu Apr 17, 2008 2:19 am

Thanks for the info, believe it or not that helps alot :D

Thu Apr 17, 2008 10:30 am

when you do find a site that talks about rod length,, the stocks ones will be referred to as 5.4

AND if you looking and they don't list rod lengths don't buy from those people.

If you ever see an old (70's) CB performance catalog get it,,, ( claudes Buggies) in the backs pages is about the best explanation of weber's and Hi po engines I've ever seen.

The rod length will determine where you want your power band,, not so much of the compression ratio

Fri Apr 18, 2008 12:24 am

bcbug wrote: Stock rod length is 5.394". It is actually precisely 137mm.

Thu Oct 02, 2014 2:44 pm

Is that length the stock length for ALL vw engines?

Thu Oct 02, 2014 3:38 pm

It should be for 1300/1500/1600 (stock 69mm stroke) rods. 1200 and type 4 were different.

Tue Mar 03, 2015 5:43 am

A few more good arguments about rod ratio which help in determining a choice: if you search TheSamba forum for Rod Length you will get a result of "Search found 12652 matches" and searching Rod Ratio returns "Search found 14100 matches." I decided to add a few post to relocate info elsewhere to a spot to help those looking to increase power and reduced friction wear at the same time without hurting longevity as much as in their control by their choices:

I located this at a header manufacture's website at Stahlheaders.com which is posted below my comments w/address for you to go check it out. I hope it helps you in choosing ratios. Just normalize the info from water-cooled engine to AC-VW, however the base info is what is missing in determining rod ratio. i.e piston rock, side wall pressure, journal force and etc.. in relation to longevity of a engine, hence friction & wear. Most I read quote from a "Top power" stance in their arguments/debates over and over, but, some of us want torque without creating too much wear/friction in the design hurting the reliability/longevity. Increase the life of our work and $$$ laid out along side power is missing in too many commits on too many forums. Planning every mod to increase reliability alongside with power has a crowd as well.

However missing in this article is in relation to Air-cooled engines how longer rods have slower piston speeds at the top of the travel which allows for more heat transfer to the heads which needs to be addressed in your builds cooling system.

I made a quick chart of ratios:

There are many discussions in using a rod size arguing using 5.??? versus 5.??? rod since they're listed in inch measurements and seeing a 5.4 is shorter than a 5.7 rod mentally; however what is not apparent in just looking at a rods length differences is the final ratio (ratio not inches) differences when calculated compared by stroke. An 82mm stroke with a 5.394 stock rod at 1.6727 ratio compared to a 5.5" at 1.7037 rod is an increase of .0328 rod ratio.

That is your rod ratio difference you are worried about and as stated below and on many sites for well founded engine builders world wide "" very little differences in instantaneous peak piston speed-both in magnitude and position, even when you go up in con rod length by 10 mm." Since this would be very small fraction of a degree of rotation and the slightly longer rod would produce less friction meaning less hp loss to friction and translate to in a small amount less engine stress over all.

Search most builders and parts distributors and they have their simple formula to tell you what rod to use with what stroke. This experience has taught then at certain higher strokes you have no choice but to use a longer rod since the stresses with break the rotational mass at some point along it's travel and why put just a long enough of a short rod to get it to pass the test of swinging and "HOLDING UP." I come from the old school of factoring in a margin of error in your favor.

Now ask them how to increase engine life and reliability for a daily driver for your teenager and all of a sudden your rod length goes up and your strokes come down, even your carbs get smaller.. LOL or ROFLMAO.

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Article at Stahlheaders.com:
http://www.stahlheaders.com/Lit_Rod%20Length.htm

Rod Length Relationships

You are invited to participate in this attempt to understand a part of internal combustion engines. I invite any/all criticisms, suggestions, thoughts, analogies, etc.-- written preferred, phone calls accepted from those too feeble or who have arthritis. Contributors are invited to request special computer printouts for specific combinations of interest to them.

In general, most observations relate to engines used for some type of competition event and will in general produce peak power higher than 6000 RPM with good compression ring seal as defined by no more than 3/16 CFM blowby per cylinder.

Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.

I. LONG ROD

A. Intake Stroke -- will draw harder on cyl head from 90-o ATDC to BDC.

B. Compression Stroke -- Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDC--may change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPM--measure CO, CO2? Find out!!

C. Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDC--allows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.

D. Exhaust Stroke -- see above.

II. Short Rod

A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.

B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.

C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.

D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.

III. NOTES

A. Rod Length Changes -- Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%--ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.

B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.

C. Stroke -- Trend of OEM engine mfgs to go to longer stroke and/or less over square (bore numerically higher than stroke) may be a function of L/R. Being that at slower engine speeds the effect of a short rod on Intake causes few problems. Compression/Power Stroke should produce different emissions than a long rod. Short rod Exhaust Stroke may create more reversion--EGR on a street engine.

D. More exhaust lobe or a earlier exhaust opening may defeat a longer rod. I am saying that a shorter rod allows a earlier exhaust opening. A better exhaust port allows a earlier exhaust opening.

E. Definition of poor exhaust port. Becomes turbulent at lower velocity than a better port. Flow curve will flatten out at a lower lift than a good port. A good exhaust port will tolerate more exhaust lobe and the engine will like it. Presuming the engine has adequate throttle area (so as not to cause more than 1" Hg depression below inlet throttle at peak power); then the better the exhaust port is, the greater the differential between optimum intake lobe duration and exhaust lobe duration will be--ie. exh 10-o or more longer than intake Carbon buildup will be minimal if cyl is dry.

IV. DEFINITIONS

Short Rod -- Min Rod/Stroke Ratio -- 1.60 Max Rod/Stroke Ratio -- 1.80

Long Rod -- Min Rod/Stroke Ratio -- 1.81 Max Rod/Stroke Ratio -- 2.00

Any ratio's exceeding these boundaries are at this moment labeled "design screw-ups" and not worth considering until valid data supports it.

Contributors to Date: Bill Clemmons, Jere Stahl

Top of Page Back

Connecting Rod Length Influence on Power

by William B. Clemmens

A spark ignition (SI) engine and a steam engine are very similar in principle. Both rely on pressure above the piston to produce rotary power. Pressure above the piston times the area of the bore acts to create a force that acts through the connecting rod to rotate the crankshaft. If the crankshaft is looked at as a simple lever with which to gain mechanical advantage, the greatest advantage would occur when the force was applied at right angles to the crankshaft. If this analogy is carried to the connecting rod crankshaft interface, it would suggest that the most efficient mechanical use of the cylinder pressure would occur when the crank and the connecting rod are at right angles. Changing the connecting rod length relative to the stroke changes the time in crank angle degrees necessary to reach the right angle condition.

A short connecting rod achieves this right angle condition sooner than a long rod. Therefore from a "time" perspective, a short rod would always be the choice for maximum torque. The shorter rod achieves the right angle position sooner and it does so with the piston slightly farther up in the bore. This means that the cyl pressure (or force on the piston) in the cylinder is slightly higher in the short rod engine compared to the long rod engine (relative to time).

Table 1
ROD LENGTH RELATIONSHIPS*
(with Crank @ 90 deg ATDC)

Piston Position Crankpin/Rod Angle

Stroke Rod Length Rod Angle from TDC ATDC
3.5 5.70 17.88 2.025 72.12
3.5 5.85 17.40 2.018 72.59
3.5 6.00 16.96 2.011 73.04
3.5 6.20 16.39 2.002 73.60
Table 2
ROD LENGTH RELATIONSHIPS with CRANKPIN/ROD centerline @ 90o @ 7500 rpm

Stroke Rod Length Rod Angle Piston Distance Crank Angle Piston Accel
3.5 5.70 17.07 1.487 72.93 2728.35
3.5 5.85 16.65 1.494 73.35 2504.72
3.5 6.00 16.26 1.500 73.74 2324.26
3.5 6.20 15.76 1.508 74.24 2097.27
*data from Jere Stahl

Another concern in selecting the rod length is the effects of mechanical stress imposed by increasing engine speed. Typically, the concept of mean piston speed is used to express the level of mechanical stress. However, the word "mean" refers to the average speed of the piston in going from the top of the bore to the bottom of the bore and back to the top of the bore. This distance is a linear distance and is a function of the engine stroke and engine speed, not rod length. Therefore, the mean piston speed would be the same for each rod length listed in Table 1.

Empirical experience; however, indicates that the mechanical stress is less with the longer rod length. There are two reasons for these results. Probably the primary reason for these results is that the profile of the instantaneous velocity of the piston changes with rod length. The longer rod allows the piston to come to a stop at the top of the bore and accelerate away much more slowly than a short rod engine. This slower motion translates into a lower instantaneous velocity and hence lower stresses on the piston. Another strong effect on mechanical stress levels is the angle of the connecting rod with the bore centerline during the engine cycle. The smaller the centerline angle, the less the side loading on the cylinder wall. The longer rod will have less centerline angle for the same crank angle than the shorter rod and therefore has lower side loadings.

Classical textbooks by Obert ( ) and C.F. Taylor ( ) provide little guidance on the rod length selection for passenger or commercial vehicles other than to list the ratios of rod length to crank radiuses that have been used by various engine designs. Race engine builders using production blocks have done quite a bit of experimentation and have found many drivers are capable of telling the difference and making clear choices along with similar results from motorcycle flat track racers/builders.

Because of recent developments in computer modeling of the engine cycle by R.D. Rabbitt ( ), another factor may be critical in selecting a given connecting rod length. This new factor is the cylinder head flow capability versus connecting rod length over stroke ratio (l/r) versus engine speed. To understand this relationship, let us first review previous techniques used to model air flow during the engine cycle which as Rabbitt points out is founded on principles initiated in 1862 and refined in 1920. These theories are documented in Taylor's textbook ( ). To calculate air flow throughout the cycle these models use such parameters as mean or average inlet mach number for the port velocity and an average inlet valve discharge coefficient which compensate for valve lift and duration. In these models a control volume is used to define the boundaries of the combustion chamber. The air flow determined by the previous parameters crosses this boundary to provide air (and fuel) for the combustion process within the control volume.

However, this control volume has historically been drawn in a manner that defines the boundaries of the combustion chamber in the area of the inlet and exhaust valves as if the valves were removed from the cylinder head (ie. a straight line across the port). With the valves effectively removed, the previously mentioned average port flow and valve discharge coefficient (ie. valve restriction) are multiplied within current computer models to quantify the air flow (and fuel) delivered for each intake stroke. But, as Rabbitt points out, this approach totally ignores the effect of the air flow direction and the real effect of valve lift on the total air flow that can be ingested on each intake stroke.

Rabbitt reaches two important conclusions from his study. One, because of the direction of the air flow (angle and swirl) entering the combustion chamber, three dimentional vorticies are set up during the intake stroke. Two, that above a certain piston speed, density of the mixture at the piston face is a function of valve geometry and valve speed. Rabbitt further discusses the effect of the first conclusion as it relates to the mass of air that is allowed to flow through the port and by the valve. Vorticies can exhibit different characteristics and in general conform to two general types--large scale bulk vorticies that could be described as smooth in nature and small scale eddies that are highly turbulant.

If one can consider that the vacuum produced by the piston on its downward travel to be the energy that causes the air to flow through the port when energy losses throughout the intake tract (including losses at the valve) are at a minimum, the flow delivered to the chamber will be maximized. If the area between the piston face and the valve is also included in the consideration of flow losses, the effect of the type of vorticies created can be more easily understood. Large scale bulk vorticies comsume less energy than highly turbulent eddy vorticies. Thus, more of the initial energy from the piston's downward movement is available at the port-valve-combustion chamber interface with which to draw the intake charge into the chamber. Small scale eddies eat up energy which reduces the amount of the initial energy that reaches the port-valve-combustion chamber interface which in turn, reduces the port flow.

Rabbitt's second conclusion follows that at some higher piston speed, the vorticies within the combustion chamber (which are assumed to be large scale bulk type at low speeds) transition from the bulk type to the small scale eddy type. At this point the flow into the combustion chamber ceases to increase in proportion to increases in engine speed. It is theorized that this flow transition point can be observed on the engine power curve as the point at which the power curve begins to fall off with increasing engine speed.

As indicated earlier, piston speed is normally viewed as mean or average piston speed. Thus for a given engine, the mean piston speed increases as the rotational engine speed increases. However, in Rabbitt's model the piston speed of concern is the instantaneous piston speed during the intake stroke near TDC. For any given engine, changing the rod length to stroke (l/r) ratio changes the instantaneous piston speed near TDC. For the purposes of flow visualization, the type of vortex formed should not care whether a given instantaneous piston speed had been achieved by a given rotational speed or changing the (l/r) ratio and operating at a new rotational speed. As long as the instantaneous piston velocity is the same, the type of vorticies formed should be the same and the amount of air inducted into the cylinder should be the same.

If other factors influenced by rotational speed such as the time distance between slug of intake air flow and valve opening rates relative to the acceleration of the air slugs were ignored, one should be able to predict the location (RPM) of the peak power as a result of a change in the (l/r) ratio. Note, that even though power is a funtion of air flow and air flow should be roughly constant for the same instantaneous piston speed (neglecting the afore mentioned factors), the power may not be the same because of the lever arm effect between the crank radius and the connecting rod. (As we noted earlier, the shorter rod should have the advantage in the lever arm effect.)

In reality, the analysis must be viewed by stroke (ie intake, compression, exhaust, power) the selection of exhaust valve opening time combined with the exhaust system backpressure and degree of turbulance the exhaust port experiences. If the exhaust port has good turbulance control then you may run a shorter rod which allows you to use more exhaust lobe which reduces pumping losses on the exhaust stroke.

Tue Mar 03, 2015 6:45 am

If you observe the chart you will notice the total overall rod ratio increase actually looks numerically smaller compared to the rod length Even though the rod length seems substantial as in a 5.4in to a 5.7in which is .300" or just under 5/16ths of an inch; however looking at the rod ratio the number increase is considerably smaller. After-all it is a ratio.

Seems huge until you look at the actual final rod ratio and do look at the ratio difference and not the rod length difference. Do the math and not get caught up in the semantic of rod 5.4" vs. rod 5.5" etc.... However consider heavily how long you want those pistons, rings and rods to last in your engine; error on the side of longer is time I can enjoy my ride and a micro second of safety when I was so heavy footed it didn't blow my engine :twisted:

Also notice once you decide to stroke your engine you will always end up having a "Overall Short Rod Ratio" or OSRR comparred to the original 1600cc engine no matter if stock rods(2:1) are used or longer ones. You will be surprised in how little difference the final ratio is compared to the rod length you are considering between. Yes less than 1.81 is considered short rod ratios and 1.81 and higher are long ratios, but, never forget the 1600cc was a fun beast in it's day with 1.99 ratio or lets just say 2:1.

For those trying to build a stronger type 1 engine and gain as many ponies as you safely can without sacrificing to much rotational mass stresses in doing so should consider "Overall Short Rod Ratio" or OSRR as the stand point of the VW design having long rods and now you are shortening them overall as you stroke up.

Awhile back a guy at a VW pro speed shop (VW machine shops as well with dyno for tuning) told me the 2:1 rod ratio VW used doesn't mean they knew what they were doing and never use that as a reference point in any build. I find the 2:1 rod ratio VW used was probably used to reduce rotation mass stresses, reduce the wear on rod bearing, piston rings and cylinders and allow the engine(piston speed) to stay together at high RPMs for the little machine. "YOU DECIDE!"

increase your torque and ps with a bigger stroke and consider your rod length, but, also the stresses or stress reduction you are willing to live with.

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Posted on by Marquis_Rex http://www.pistonheads.com/gassing/topic.asp?t=117450

Ok, let me tackle the rod-to-stroke length ratio one first. Alot has been written on this subject- by people like Smokey Yunick and Vizard etc etc.

I'm not one to boast and parade my credentials if I can help it, but I'm sick to death of people contradicting me online on technical engine design issues- something I do for a living- perhaps quoting vague articles by people like the above.
I've spent years devloping the performance and analysing the gas dynamics of lots of engines, V8s, V6s and even inline 4s- from a clean sheet perspective. From an OEM manufacturers point of view- we have alot more resources at our disposal then some back street bloke with a dyno-jet.
This is what I wrote on another forum.

Con-rod to stroke ratio-exploding the myth. It s not only the amateurs that come up with misguided opinions. A supposed expert in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity the piston being driven less into the bore (thrust). If you re a clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you re trying to modify an existing engine I would say you re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
When I see performance gains I split them up into the following areas

Increases in engine breathing or charge efficiency-commonly known as Volumetric efficiency, often due to less restrictions-improvements in outright flow,or tuning changes of some kind, whether they be simple ram induction tuning or perhaps resonance tuning

Then there are combustion improvements- perhaps from a faster burn, higher CR, better-more thermally efficient chamber. I measure alot of this using a useful parameter called BSAC or Brake Specific Air Consumption- basically a measure of how well an engine uses it's own breathing. Just because an engine has a high VE, it doesn't automatically mean the engine can utilise this breathing well!

Then there is friction and improvements associated with all the various components and ancilliaries

Now my illustrated numerical example shows that there's very little differences in instantaneous peak piston speed-both in magnitude and position, even when you go up in con rod length by 10 mm. From this I would doubt I'd see a benefit in cylinder charge efficiency. 1 dimentional "quasi 3 -d" Cycle simulation codes have corroborated this and I haven't seen benefits on our test beds/dynos.

I can't see how they would effect combustion if the compression height is kept the same.

Most likely the biggest of a longer rod engine is down to engine friction due to the reduced angularity of the con rod, and the favourable geometry-less thrust into the bore. Perhaps the benfit is more apparent at higher engine speeds
If there is a contributory effect from combustion I have found on several test bed runs that the difference is within dyno to dyno variation, however I don't discount it's effect totally, only that's it tends to be exagerrated by people, and that it certainly isn't a phenomenon that exhibits itself on airflow/Volumetric efficiency of an engine- due to the minimal effect on instantaneous piston velocity I've just outlined.

There is a limit however, and my collegues/contacts at BMW agree that going for a ratio smaller then 1.5 on an engine that has a typical working rev range is ill advised.

Tue Mar 03, 2015 7:58 pm

Remember it's the "HUGE" rod ratio changes where all the actual differences are felt, tested and are "TRUE" for the rule; however the small ratio changes everyone is fussing, quoting and stressing about is where IMHO some are worrying to much by apply the "Ratio Rules" where they really don't apply as much.

I dropped these links in since there is a good argument about rod length vs. ratio stating to not worry about the rod ratio as much & why on smaller strokes and why on larger strokes; hopefully to be seen as a resource.

You will always have the post from individuals who see one statement and it rules all their decisions & comprehension from that moment on and weeding out all the misinformation here and at other sites takes several full work hour days just to locate good info.

I suggest you look at longer rods when the ratios are close to reduce stress on your rotating mass. The argument how longer rods move the power band up the rpm range just "normalize" the comment by remembering the stock 1600 has nearly a 2:1 rod ratio and any build with a stroker crank will inevitable have more torque & shorter rod ratios anyways no matter the rpm range compared to a stock motor. Those who race just ignore me; I'm here to reduced stress and gaining the power gains from "Cubic Inches."

However the the camshaft lift, duration and valve overlap, the intake size and flow design, head setup, and the exhaust size all play a larger roll in placing your torque/power where you would like it. 99% of your thought process should be in those areas and get longer rods as you stroke up to increase engine life, reduce wear and increase upper RPM piston safety for those daily drivers who want to merge into the fast lane.

When you ask for advice on using a longer rod all the advicee can hear is the word "LONGER" and I hate to say this is like a deer to headlights or a fly to the flame the word "longer" rules their thoughts; even if your going from from a at 82mm stroke with rods of 1.7037:1 (5.5") rod ratio to a 1.7656:1 (5.7")ratio will not matter much overall but the word "longer" prevents them from noticing the slight overall differences. However the stresses will be reduced slightly if your willing to adjust for the slightly longer rod since you will still be in a OSRR in the range classified as a short rod ratio. read this below here because this guy does a good job in helping you see the ratio paradigm.

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This post at http://www.shoptalkforums.com/viewtopic.php?f=34&t=142738 Will answer a lot for you:

by: gearheadgreg
Sat Jun 15, 2013 10:29 am

The connecting rod is a lever. Longer rods make more torque if all else is equal.

We have the complexities of calculating "rod ratio" which is vital to engine performance. This is one of the engine building areas that separate the winners from the losers.

CONNECTING ROD RATIO of STROKER ENGINES

(rod length divided by stroke length equals ratio)

The lower the ratio number, the more power the engine can produce

5.394" (137mm) VW ROD
5.352" (136mm) PORSCHE ROD

69mm 2.7165" VW STROKE
74mm 2.9134" PORSCHE STROKE

1.98 VW ROD RATIO, STOCK
1.99 PORSCHE ROD RATIO, STOCK

Stroke (in mm) times rod length (in mm)

69mm X 137mm = 1.98
74mm X 137mm = 1.85
76mm X 137mm = 1.80
78mm X 137mm = 1.75
82mm X 137mm = 1.67

Rod length moves the power band higher or lower in the rev range. The length of the rod and the length of the crankshaft stroke give the rod ratio. Rod length (mm) divided by the crank stroke (mm) = Rod Ratio.

The lower the ratio, the lower the power band, the higher the ratio the higher up the range the power band will move.

As a comparison the stock 1600cc set-up is 137/69 = 1.98 (considered high). If you wish to hill climb (torque) you will require a lower ratio (such as 1.76) and to drag race (rpm), a high one.

Standard VW rods are type 311. This is cast into the bottom of the big end. 311's are good for most engines up to 100 BHP, with a 69mm stroke crank. A stroker crank will either require these rods to be machined, or buy a set of previously cleared rods. The advantage of buying a set of rods is that they will have been balanced end for end and as a set, which will help engine longevity.

Like crankshafts, con-rods are available with different journals. You have no need to change from the stock VW 55mm diameter.

Chevy 327 rods are 145mm long, but require machining as the big end is only 51mm instead of the 55mm of the VW. This does save on changing the crank just to match the rods. These used with a stock length crank will give a high revving engine with a high power band, giving a rod ratio of 2.10, ideal for a small bore drag engine.

The Porsche 912 rod is a little shorter than the VW, but the VW is stronger, not a bad length, and it fits.

A long rod will give a slower piston speed, and therefore a longer life for the rings and cylinder walls, but this also causes the cylinders to fill up slowly when the piston moves down during the inlet stroke.

Of course, once the rpm builds this is overcome, hence the higher power band. A shorter rod will fill the cylinder much faster and therefore give more power at lower engine speeds, but be limited to a lower rev range.

Stock rods are nominally 137mm center-to-center (5.394") and get slightly shorter each time they're rebuilt. So with a stock 69mm stroke the ratio is 1.985, and with a 76mm stroke it's 1.80.

Many factors come into play when considering the "ideal" rodlength-to-stroke ratio. The lower the number, the faster the piston moves near TDC (and since it has to cover the same total distance in the same time, that means it's moving slower in the lower part of the stroke. This affects the port velocity and cam selection; with short rods a given engine will need bigger intake ports to keep from "running out of breath" at high RPM.

A lower number also increases the rod angularity near TDC - that promotes better torque due to the added "leverage".

Most production engines will make more total horsepower with a higher rod ratio - but that's because they tend to come with too short of a rod to begin with in order to reduce the total height of the engine. A 350ci Chevy, for example, has a rod ratio of 1.64 and people go to a great deal of trouble to squeeze in longer rods just to get that up to 1.72 - they'd be delighted if they could get even close to what the ACVW has stock.

For drag-racing, most ACVW builders go with a ratio considerably lower than stock (in the 1.7 ballpark) - to keep it up near stock with a big stroke requires really long rods which makes the whole package come out a couple inches wider than stock. Only a very small percentage of engines have ever been built this way, so almost all development of heads, cams, etc. has historically been done with lower rod ratios....in other words, the tail has wagged the dog here - the popularity of low rod ratios has much more to do with what fits than with what's best.

FJC

==========================================
Rod Length for Performance Engines
by John Connolly http://www.type2.com/library/enginem/rodjohn.htm

So what is the relationship between power and rod length? Any other sage advice on building a 2110 that will not likely see 5000 or more RPM? For instance, heads and carbs?
With a rpm powerband that you need, use the shortest rod you can get. Stock or even Porsche length.

All else being equal, a longer rod moves the powerbanD UP higher in the rpm range, and a shorter rod is torquey at the low rpm band.

If you have some spare parts, assemble this (or imagine it if you can see in 3 space well): an assembled engine with a long rod on one journal, and a short one on the other. Deck height is the same (adjusted, of course). Now, as the cylinder fires, and the piston moves down say, .060", you will see the short rod engine moves the crank MORE crank degrees because of the short rod, and the long rod moves barely at all. Imagine a rod length of 0, and a rod length of infinity to better understand this.

As a result, the long rod pistons "dwells" at TDC longer, and allows the pressure to build up. Obviously at high rpm this is good for maintaining power since the short rod engine would have the piston halfway down its stroke in no time (no push)! Conversely, the short rod engine at low rpms and high loads will not ping and the engine has "leverage" to spin the crank, where the long rod engine "stalls".

Since the piston "timing" is different on the two respective engines they have different characteristics regarding camshaft selection and timing requirements. Also, the shorter engine will "pull and push" the charge thru the ports harder, running out of "breath" at a lower rpm while the long rod engine keeps pulling! VW managed to incorporate this characteristic on the T-4 engines which have a VERY short rod, which is ideal for the heavy bus. Obviously high rpms are a "no no" on this engine without a rod change (and length change). Some high rpm engines use insanely long rods to "slow" the piston down to keep parts together and power up at these rpms. The biggest drawback is engine size, since we can only move the piston pin up so far (the ring stack gets in the way) to keep things close to the same size.

Lastly, rod bearing life is shorter on short rod engines, and so is piston and ring life (due to the side loads associated with the "extreme" angles the short rod engine's see).

Hope this makes some sense. This is one aspect of engine knowledge that most people are clueless about.

On a 2110 you can NOT use the stock rebuilt T-1 rod without asking for rod failure. This rod is only safe to a 78mm stroke. If you go to a better rod, don't be afraid to crank it to 86mm.

Wed Mar 04, 2015 12:09 am

towd wrote: when you do find a site that talks about rod length,, the stocks ones will be referred to as 5.4

AND if you looking and they don't list rod lengths don't buy from those people.

If you ever see an old (70's) CB performance catalog get it,,, ( claudes Buggies) in the backs pages is about the best explanation of weber's and Hi po engines I've ever seen.

The rod length will determine where you want your power band,, not so much of the compression ratio

Why would rod length not affect the compression ratio? Seems like if you shorten the deck height you increase compression. If you increase the deck height you lower compression. But I could be wrong.

Wed Mar 04, 2015 5:30 am

burdpete wrote: towd wrote: when you do find a site that talks about rod length,, the stocks ones will be referred to as 5.4

AND if you looking and they don't list rod lengths don't buy from those people.

If you ever see an old (70's) CB performance catalog get it,,, ( claudes Buggies) in the backs pages is about the best explanation of weber's and Hi po engines I've ever seen.

The rod length will determine where you want your power band,, not so much of the compression ratio

Why would rod length not affect the compression ratio? Seems like if you shorten the deck height you increase compression. If you increase the deck height you lower compression. But I could be wrong.

You are correct, longer rods push the piston farther up in the cylinder which decreases deck and increases compression. That is a by-product of a longer rod however, you wouldnt build an engine with increased compression by just throwing in a longer set of rods (usually). Engines with longer rods usually have a larger torque range and lower overall revs. SHort rod engines can wind to the moon! :lol:

Sat Mar 14, 2015 1:08 pm

Though it's about water cooled engines all engine dynamics of rod length is the same result:

http://victorylibrary.com/mopar/rod-tech-c.htm

FastCounter by bCentral

Connecting Rod vs. Stroke Analysis
The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts. This ratio (normally represented by “n”) can be calculated as follows:
Ratio “n” = Rod Length ÷ Stroke
The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length. It’s not possible for the rod to be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and are not practical for most purposes (although used for racing).
The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of “n”) will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of “n”) will occur with a longer rod or a shorter stroke.
If the rod length is decreased, or the stroke is increased, the “n” ratio value becomes smaller. This has several effects. The most obvious is the mechanical effect. Motors with low values of “n” (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high “n” motors):
» physically shorter top-to-bottom & left-to-right (more oil pan, header, and air cleaner clearance)
» lower block weight (400 vs. 440, for example)
» higher level of vibration
» shorter pistons, measured from the pin center to the bottom of the skirt
» greater wear on piston skirts and cylinder walls
» slightly higher operating temperature & oil temperature due to friction
There are also differences in how the motor breathes:
» intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response
» on the negative side, a small or badly designed port will “run out of breath” sooner
» piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)
Spark advance is also affected:
» earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation
» the motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)
Effects of Long Rods
Pro:
» Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
» The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
» For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Con:
» They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
» Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.
Effects of Short Rods
Pro:
» Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
» The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
» Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Con:
» Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
» Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.
Rod Ratio vs. Intake Efficiency
An “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. There will be instances where the choice of stroke or rod has not been made, but the intake pieces (carburetor, manifold, and head) have been selected. Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “n” value can be used to compensate for less-than-perfect match of intake parts to motor size & speed. The reverse is also possible: the lower end is done, but there are still choices for the top end. Again, the “n” value can be used as a correction factor to better “match” the intake to the lower end.
The comments in the following table are not fixed rules, but general tendencies, and may be helpful in limiting the range of choices to those more likely to produce acceptable results. Rather than specify which variable will be changed in the lower end, “n” values will be used. Low “n” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. High “n” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.
Best Combinations of “n” Values & Intake Characteristics
“n” = 1.45 - 1.75 more compatible with: “n” = 1.75 - 2.1 more compatible with:
Large intake port volume vs. motor size
(”J” head on 273) Small intake port volume vs. motor size
(stock 452 head on 498” RB stroker)
Single-plane or 360° intake manifolds
(Edelbrock Victor, Torker & Torker II, TM7. Holley Strip Dominator. Offenhauser Equa-Flow, Port-O-Sonic. Weiand X-Celerator, Team G) Dual-plane 180° intake manifolds
(Edelbrock: LD340, CH4B, DP4B, Performer & Performer RPM, Streetmaster, SP2P. Holley Street Dominator. Weiand Stealth, Action Plus)
Large carburetor vs. engine size
(273 with 750cfm) Small carburetor vs. engine size
(440 with 600cfm)
Moderate engine speed
(pick-up, RV, towing) High engine speed
(peak power more important)
Tall axle ratio
(2.76, 2.93, 3.23, 3.55 and/or with tall tires) Short axle ratio
(3.91, 4.10, etc. and/or with 25 or 26” tires)
Planning a 383 Motor
This engine is generally overlooked in selecting a high-performance project. The motor has an excellent bore to stroke ratio: 1.26-1 (similar to 327” SBC, better than 340). The short stroke allows high RPM without destructive piston speed (7100 RPM = 4000 ft./min., the accepted “safe” limit for piston stress). The large bore permits big valves (2.14” intake, 1.81” exhaust).
A potential method of increasing peak power is to substitute the longer 440 6.768” (LY) rods for the original “B” 6.358” rods on the original crank. This has the following effects:
» Increases the rod ratio (“n”) from 1.884-1 to 2.005-1
» Reduces the piston compression distance to about 1.525” for a useful weight savings
» Slightly reduces piston acceleration
This should allow an advantage in peak power. For a start in piston selection, take a look at the KB224 for BBC: flat top, CD = 1.52” (just below zero deck), and .990” pin for more weight savings and moderate cost. There may also be “possibles” for the 400 (4.34” bore), but not discovered yet. Ideas?
Stroke vs. Rod Length in Common Automotive Engines
Motor Stroke Rod “n” Ratio
Mopar LA 273/318/340 3.31” 6.123” 1.85-1
Mopar LA 360 3.58 6.123 1.71
Mopar LA 340 with 3.79” stroker crank 3.79 6.123 1.62
Mopar LA 340 with 4.00” stroker crank 4.00 6.123 1.53
Mopar “B” 350/361/383/400 3.375 6.358 1.88
Mopar “B” 400 with 440 crank & std. rods (451”) 3.75 6.358 1.70
Mopar “B” 400 with 4.15” crank & std. rods (498”) 4.15 6.358 1.53
Mopar “B” 400 with 4.15” crank & BBC +.400” rods (498”) 4.15 6.535 1.57
Mopar “RB” 413/426W/440; “B” 383/400 with 440 crank & rods 3.75 6.768 1.80
Mopar “RB” 413/426W/440 with 4.15” crank (494”) 4.15 6.768 1.63
Mopar 426 hemi 3.75 6.86 1.83
Small Block Chevy 302 3.00 5.70 1.90
Small Block Chevy 327 3.25 5.70 1.75
Small Block Chevy 350 3.48 5.70 1.64
Small Block Chevy 350 with 6” rod 3.48 6.00 1.72
Small Block Chevy 400 with std. rod 3.75 5.45 1.45
Small Block Chevy 400 with Chevy 350 rod 3.75 5.70 1.52
Small Block Chevy 400 with 6” rod 3.75 6.00 1.60
Big Block Chevy 396/402/427 3.76 6.135 1.63
Big Block Chevy 454 4.00 6.135 1.53
Ford 289 (Windsor) 2.875 5.156 1.79
Ford 302 (5.0, Windsor) 3.00 5.090 1.70
Ford 351W 3.50 5.954 1.70
Ford 460 3.85 6.605 1.72
Angle Limitation
The angle of the rod at 90° ATDC is a good indication of how much stress the piston and cylinder wall will be subjected to with a specific rod/stroke selection (this is not the angle of maximum thrust, which occurs when the rod’ beam axis is at 90° to the crank throw or journal, typically between 70-76° ATDC; however, the math is easy to do). Angles beyond 17° (where the rod axis is 90° to the crank throw at 73° ATDC) promote excessive wear at the piston major thrust surface, and piston breakage could be the result. Before you purchase connecting rods that are shorter than previous or increase the stroke of the crank, calculate the new rod angle. High rod angles will require quality rods that have been checked for cracks and have quality (ARP, etc.) fasteners. Piston selection will be critical for the life expectation of the engine; maximum skirt length below the pin is desired.
Sine of Rod Angle = Stroke ÷ (Rod Length * 2)
(or)
Sine of Rod Angle = .5 ÷ R/S
To make your own calculations using the Microsoft Calculator (every Win95/98/00/ME has it):
Double-click the “Calculator” icon to open it
Click “View”, then “Scientific”
Input the result from the formula above
In the left margin of Calculator, look for the check-box that says “Inv” - check it
Make sure the box marked “Degrees” (not Radians) is checked
Click on “sin”
The rod angle in degrees will show in the window
Rod Angle “n” Ratio Examples Comments
13½° 2.142-1 High speed motor with small ports. Best breathing with small ports
14° 2.067-1
14½° 1.997-1 Long rods for good breathing with small ports
15° 1.932-1 Long rods to help breathing with small ports. Responds well to stroke increases (“n” value too large for intake port size)
15½° 1.871-1 Responds well to stroke increases (“n” value too large for intake port size)
16° 1.814-1 Mopar 383/400 Approximate “ideal” compromise between stress & breathing (1.81-1)
16½° 1.760-1 Chevy 327 Good choice for motors with good breathing
17° 1.710-1 Mopar 360
Ford 302, 351W, 460 ”Safe” limit for thrust angle. Approaching practical limit for street motors
17½° 1.663-1 Approaching practical limit for street motors
18° 1.618-1 Chevy BB 396/427 Approaching practical limit for street motors. Good power due to large intake port
18½° 1.576-1 Limited street use
19° 1.536-1 Chevy BB 454 Good power due to large intake port
19½° 1.498-1 Not practical for street use due to short pistons
20° 1.462-1 Chevy SB 400 Poor peak power. Longer rods are used in any serious application
See these other Victory Library booklets

Wed Mar 09, 2016 5:18 pm

Anybody knows, if I want to replace my 1200 ccm 40 HP stock rods to a same length of rod set, but with 22 mm pins, what do I have to look for??
Txs!!

Thu Mar 10, 2016 1:40 pm

akomjathy wrote: Anybody knows, if I want to replace my 1200 ccm 40 HP stock rods to a same length of rod set, but with 22 mm pins, what do I have to look for??
Txs!!

A machinist?

Fri Mar 11, 2016 7:34 pm

Quote: Anybody knows, if I want to replace my 1200 ccm 40 HP stock rods to a same length of rod set, but with 22 mm pins, what do I have to look for??
Txs!!

Custom made rods at about $200 a piece or I have heard of wrist pins that have two diameters, one for the piston and one for the connecting rod. A search on volvo pistons in a type IV might return some results for you.

Tue Feb 13, 2018 7:38 pm

Ok. Reviving the thread a little.
As my oil pump has destroyed the end of my camshaft, it’s time to split the case. Since I’m going in deep I’m going to install a new counterweighted crank as well. Thought about stroking it but I’m not certain if that would require tin and or engine bay mods to fit so I’m just going to keep it a 1915.

My question is:

I have a set of Bugpack 4005-09 I-Beam Race Rods still brand new in the box from 1994. They say on the box 5.352” length and VW journal.
So they’re shorter than stock 5.394”.
How much does that .042” screw me? Or should I use a mild stroker crank like a 76/78 with A pistons?
I’m using an Erson vw200 cam 441 lift 280 duration with CB Performance 044 super mags with 42x37 valves and Dahl dellorto 45’s

Just trying to make use of something I already have.