TheSamba.com :: View topic

Fri May 01, 2015 9:52 am

ok so i got a really good deal recently on a set of dual webber 40's. they're stock jetted right now. the motor is stock minus a glass pack and electric fuel pump. it has a vacuum distributor. my question is will this be over the top? i tried to look it up and could only find people doing these on motors with a few more performance mods. the stock carb is shot so thats not an option. thanks for any help fellas

Cheers

Fri May 01, 2015 10:14 am

it's better than a progressive.

Fri May 01, 2015 10:32 am

The stock German carb could be well rebuilt for about $200 you know.

Fri May 01, 2015 12:40 pm

DarthDrew wrote:
could only find people doing these on motors with a few more performance mods.
Cheers

Because that's when more than a stock carb is needed. On a stocker, dual 40's may be overkill. Maybe a single would be better.

Sat May 02, 2015 7:41 am

Throw them on with a merged exhaust system and the power increase will be noticeable. Nothing wrong with them on a 1600.

Sat May 02, 2015 8:29 am

Bashr52 wrote: Throw them on with a merged exhaust system and the power increase will be noticeable. Nothing wrong with them on a 1600.

this ^^

but dont be butthurt when you realize you also should have freshened up the heads, put in a counterweighted crank and installed heavy duty vavle springs and even added a mild cam....all things you will discover after you kill your motor.

Sat May 02, 2015 9:16 am

http://www.thesamba.com/vw/forum/viewtopic.php?t=132830
cfm = Cubic Feet per Minute (flow rate)
Quote: Source: CB Performance 1991 catalog- no flow specs or venturi sizes
I assume these numbers are @28" H2O per venturi

Dell'Orto 48mm DRLA 388.5cfm
Weber 48mm IDA 372.1cfm
Dell'Orto 45mm DRLA 329.3cfm
Weber 44mm IDF 292.3cfm
Dell'Orto 40mm DRLA 266.4cfm
Weber 40mm IDF 212.8cfm
Dell'Orto 36mm 205.8cfm
Since the above numbers are per venturi, you would quadruple them for a pair of carbs with dual venturis. That comes out to 851.2 cfm.

I found the below calculator to determine the cfm flow of an engine and it includes a volumetric efficiency adjustment (engines are not 100% efficient, for each full intake stroke the cylinder does not "fill up" all the way).
http://www.csgnetwork.com/cfmcalc.html
Setting the calculator to CC (Displacement type = 2):
CC=1585
Safe Engine max = 5000rpm
VI = 80% (this is generous considering use of stock heads and exhaust, I would use 60% as a more realistic value for a stock engine)
The adjusted cfm needed to feed this stock 1600cc engine at 5000rpm = 112 cfm
Basically your dual carbs will be 4x larger than they need to be. Even a single 40 IDF will be overkill.
I have heard of people running these small IDFs, but they needed serious re-tuning (smaller jets and venturis).

Stock engine = stock carb.

Sat May 02, 2015 9:57 am

ashman40 wrote:
Since the above numbers are per venturi, you would quadruple them for a pair of carbs with dual venturis. That comes out to 851.2 cfm.

WRONG. That is if you feed the engine with all four venturis, and nobody does that, well, to be more accurate a few have and they learn quick.

This is what we call a four cycle engine. The intake stroke is 1/4 of the time. So a carb can feed 1 cylinder it just works 1/4 of the time. 2 cylinders it works half the time, 4 cylinders it flows all the time.

What were you thinkin????

Sat May 02, 2015 1:55 pm

Is your left Weber fitted with the small vacuum pipe (circled green below) so that you can use that vacuum distributer? If not, then you need to use a centrifugal-advance distrib.

And I've been running 40IDFs for years on a stock 1600 which has only a 1 3/8" header and mild ported stock-valve heads. Runs great. 28mm venturies, 110 main jet, 50 idle jet.

Sun May 03, 2015 1:21 am

I think it might be overkill if you cant get the air fuel ratio to work on your side. keep in mind the stock undercarbed engine floats fuel through a 34pict and a maze of intake to distributefue through a slit Iin the intake. haveing those on would be better off going with aset of baby webbers and like the dude said a merged header.

Sun May 03, 2015 4:56 pm

modok wrote: ashman40 wrote:
Since the above numbers are per venturi, you would quadruple them for a pair of carbs with dual venturis. That comes out to 851.2 cfm.

WRONG. That is if you feed the engine with all four venturis, and nobody does that, well, to be more accurate a few have and they learn quick.

This is what we call a four cycle engine. The intake stroke is 1/4 of the time. So a carb can feed 1 cylinder it just works 1/4 of the time. 2 cylinders it works half the time, 4 cylinders it flows all the time.

What were you thinkin????
Let's do the math...
4-strokes = 2 crank revolutions (each stroke is 180 crank degrees)

Since the OP was taking about dual 40 IDFs, this means each venturi will feed just one cylinder each. So let's just look at just one cylinder.
1-cylinder volume = sq (8.55cm/2) * Pi * 6.9cm = 396cc or 0.0140cubic-ft (one cylinder)
This volume is drawn only every other revolution of the crank. So at 5000 crank rpms, the cylinder volume is swept only 2500 times per minute.
2500 * 0.0140 cubic-ft = 35 cfm@5000rpm

So taking into account the point that the cylinder only draws its volume every other revolution, the volume the cylinder draws (35cfm) at 5000rpms is still much smaller than the max flow of the single venturi of the 40IDF (212.8cfm).
35cfm is at 100% efficiency. If we use 80% VE the 35cfm becomes 28cfm@5000rpm.

BTW, 28cfm x 4 cylinders = 112cfm. Isn't that what I posted above? :?

Sun May 03, 2015 6:59 pm

Still wrong. the intake stroke is not 720 degrees. yes the cylinder eats 35cfm every two revolutions(720 deg), but it does so in a period of 120-180 degrees. What is the CFM in this period?

Try it again assuming the intake stroke is 120 crank degrees. With a stock cam this is about right. If you get about 5x higher then you did it right.

Sun May 03, 2015 8:44 pm

what do you mean a merged exhaust system? this??

http://www.jbugs.com/product/3730.html?Category_Code=vw-bug-super-beetle-baja-off-road-exhausts

also is a sycronizer tool thing really necessary to tune the carbs? or is by ear/feel good and make them all match? ive also heard about a balance tube but i dont understand how i would go about doing that.

yes ive got a vacuum dizzy and have it hooked up to the left carb.

money permiting ill get around to a cam and and some mild bits in the engine. but for now this is what i have. ill rebuild it when it overheats and dies.

Mon May 04, 2015 4:30 am

67jason wrote: Bashr52 wrote: Throw them on with a merged exhaust system and the power increase will be noticeable. Nothing wrong with them on a 1600.

this ^^

but dont be butthurt when you realize you also should have freshened up the heads, put in a counterweighted crank and installed heavy duty vavle springs and even added a mild cam....all things you will discover after you kill your motor.

Are you kidding? A new exhaust and carb system will not automatically kill the engine unless you drive it like an idiot and just rev it to the moon all the time. Then yes you can pound out the main bearings. You will however get used to the new power and want more, then you will be getting into the internals :lol:

Mon May 04, 2015 5:16 am

Two words for you rev limiter. petronix has a decent digital system. I would not exceed 5000 rpm or even around that for very long without the proper crank and coresponding valve train parts. in fact read up on the damage you can do in the how to hotrod book it will make your skin crawl. I used a stock crank with solid steel full circle centers and my engine will prob easily rev over the safe zone for my non counterweighted crank. keep in mind if your motor is already do for a tune up it might not be a good idea. dont get off track with merged headers and performance upgrades if it will hurt you in the long run. the best line of strategy is to persue a mild or custom performance build if you really want reliable hp.

Mon May 04, 2015 10:20 pm

modok wrote: Still wrong. the intake stroke is not 720 degrees. yes the cylinder eats 35cfm every two revolutions(720 deg), but it does so in a period of 120-180 degrees. What is the CFM in this period?

Try it again assuming the intake stroke is 120 crank degrees. With a stock cam this is about right. If you get about 5x higher then you did it right.
I see what you are getting at... that the vacuum of the cylinder against the individual venturi is only 120-deg of a 720-deg 4-stroke cycle (1/6th or 16.7% of the 4-stroke cycle). But this is a static view of a dynamic model and doesn't apply once you get into the higher rpms, which is what we are talking about (PO trying to determine if the 40IDFs are correctly sized for his stock engine).

Example1 (simple/static model): Engine running at a theoretical 2rpm (720-deg/min). This works out to one intake stroke per minute. The intake stroke only lasts 10-sec (120-deg). So the single carb venturi needs to flow 0.0140cubic-ft in 10-sec. If you extend that same flow rate to the whole minute it is 0.084cfm. This is the simple model you are describing. The air through the carb is strong during the intake stroke, but air through the carb stops flowing between intake strokes.

Example2 (dynamic model): Take the above model to 2000rpm. That works out to 1000 intake strokes per minute... 16.7 intake strokes per second. Now the carb needs to flow 0.0140cubic-ft every 0.06 sec. This is a very short interval and it cannot be looked at individually (static model above does not apply) because after the intake stroke completes the next intake stroke starts 0.3 seconds later. The air doesn't have time to stop moving. In the same 10-sec period when there was one cylinder volume at 2rpm, there is now 167 cylinder volumes (2.338cubic-ft) in the same 10-sec period! The non-intake periods between intake strokes becomes less relevant than at 2rpm.
Double the engine rpms to 4000rpm and the interval between intake strokes is now only 0.15 sec! The intake strokes at this speed create very fast pulses. In between the pulses the momentum of the air flow will continue to draw air into the carb venturi as a continuous vacuum. The intake strokes become a continuous string of pulses that appear to the carb as a continuous flow of air through the carb. Place your hand over the carb opening at 4000rpm. You feel the vacuum from the intake strokes drawing the air into the carb as a continuous flow of air.... even though 5/6th of the time the intake valve is basically closed. This illustrates that the carb sees continuous vacuum from the cylinder at higher rpms.

This momentum of air into/through the carb is not 100% efficient (the cylinder drawing 100% of the cylinder volume 100% of the time) but the momentum of the air flow can exceed the simple model where the cylinder is creating vacuum only 16.7% of the time.
This is where the Volumetric Efficiency (VE) percentage comes into play. At 2rpm the VE would be below 16.7% as your static/simple model suggests, but as the rpms increase the VE increases and exceeds just the percentage of the time that the cylinder is actually creating vacuum. Unless you have a turbo or supercharger the VE will never reach 100% but it can get very close.

This pattern pulses working like a continuous vacuum is analogous to a house fan (the oscillating table top ones, not the ceiling type). At the high speed setting, the fan feels like a continuous flow of air. But it is actually pulses of air from each of the fan blades. But the speed of the pulses is so fast and the momentum of the air flow makes it appear like a continuous flow of air. You can hear this when you "talk" into the spinning fan blades. You can hear your voice vibrating off the spinning fan blades.

Tue May 05, 2015 6:56 pm

ashman40 wrote:

Example2 (dynamic model): Take the above model to 2000rpm. That works out to 1000 intake strokes per minute... 16.7 intake strokes per second. Now the carb needs to flow 0.0140cubic-ft every 0.06 sec. This is a very short interval and it cannot be looked at individually (static model above does not apply) because after the intake stroke completes the next intake stroke starts 0.3 seconds later. The air doesn't have time to stop moving.

You must be from a planet where the air is far heavier. venus? LOL

It is true that past a certain RPM the air "will not have time to stop moving", however you have no concept of where this is.
2000 rpm is WAY too low.
For IDFs on standard manifolds it would be about 20,000rpm, yes twenty-thousand rpm.

at 2000 rpm, the airflow will reverse and bounce about 15 times between strokes. There is plenty of time for the airflow to accelerate and decelerate.

As far as your "take your hand example"......
Take a look down the throat at WOT and you'll get fuel in your eyeball.
It's called standoff! and that's it bouncing between strokes!

100 years ago most experts didn't know this, but we won the war anyway, which proves..........something. I think it proves you don't have to know how something works to do it. go figure!

Tue May 05, 2015 7:09 pm

Very well modok, you got me stumped.

Please provide the PO the useful formula for determining the proper sized carb (based on cfm rating) so he will know how close (or far) his carbs' rated max flow is to the stock engine requirements.

Tue May 05, 2015 7:56 pm

If he isn't using a carburetor that is rated in CFM then why do we need to know?

My rule of thumb, for IR four cylinder: venturi size squared, divided by 8= engine hp potential. That's for pretty wild engines tho,
To account for different "states of tune", lets multiply by 2/3 for a mostly stock engine. (why 2/3?? note I mentioned "effective intake stroke" can be 120-180 degrees) so 28mm vents are ideal for 65hp. Pretty close I betcha!!

I or you could create I formula that has CFM in it, however it would be less accurate.
The reason they don't rate webers in CFM is because it is MORE complicated than just having a chart LOL. But I think we COULD do it, for kicks.
The realty is there is no magic correct number anyway.
You can make the venturi as large as will still function ok in the driving you do, and that's the right size.

Wed May 06, 2015 10:41 pm

Bashr52 wrote: 67jason wrote: Bashr52 wrote: Throw them on with a merged exhaust system and the power increase will be noticeable. Nothing wrong with them on a 1600.

this ^^

but dont be butthurt when you realize you also should have freshened up the heads, put in a counterweighted crank and installed heavy duty vavle springs and even added a mild cam....all things you will discover after you kill your motor.

Are you kidding? A new exhaust and carb system will not automatically kill the engine unless you drive it like an idiot and just rev it to the moon all the time. Then yes you can pound out the main bearings. You will however get used to the new power and want more, then you will be getting into the internals :lol:

did i say it will automatically kill the engine?

my point being that once the duals and exhaust are added to the motor it will breath easier, it will rev higher....many times this will lead to winding the engine up to 5k + to show off, have fun etc and this will shorten the life of the motor and can possibly lead to catastrophic failure especially concerning on older tired motors with loose tolerances.

if you want to throw on some bolt on power then go for it, but be prepared for the potential consequences. i feel it is better to to it right. pull the motor, freshen the heads and do some work to the block.