TheSamba.com :: View topic - The ideal length for an intake

Thu Jan 06, 2011 8:41 am

I came across some interesting reading at a site called grapeaperacing, specifically the induction systems section

http://www.grapeaperacing.com/tech/inductionsystems.pdf

On page 7 there they have an equation to determine the ideal length of an intake manifold, which I tried out.

The goal is to optimize the runner length in order to keep up pressure and velocity of the mixture into the valve the full time it's open; to fill up the cylinder as much as can be during that time.

I've got a webcam 86 grind in my 72 bus; it's got a 270 degree duration. So by their equation

EVCD = 720 - (Duration - 20)

my Effective Valve Closed Duration is 470 degrees.

Their formula for ideal runner length is
L = ((EVCD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

where
L is length of intake

V is Pressure Wave Speed which they say will be 1250 - 1300 feet per second

RV is "reflected value" -which seems to be to say that to tune for the primary 'harmonic' the intake would need to be too long and so you have to tune for some harmonic of that, meaning you can put whatever number you want in there but it seems the lower the better - as they say nascar uses 3.

D is Runner Diameter - the intake hole in the type 4 1.7 head I'm looking at is 1.32 inches in diameter.

So it's weird to me that the pressure wave speed is plugged in as feet/sec while the runner diameter is in inches, but plugging in actual numbers

I'm thinking 3800 for the rpm to ideally optimize for as I've read that's the rpm that is optimal for cooling as vw set it up, and also the cam says it's optimal for 2500-5500 rpm range.

L = ((EVCD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D
L = ((470 × 0.25 × 1300 × 2) ÷ (3800 × 3)) - (½ * 1.32)
L= 305,500 ÷ 11,400 - .66
L= 26.138

So 26 inch intake manifolds? That's optimized for the 3rd harmonic (reflected value) as in nascar. I haven't actually measured my manifolds, which are the long ones from cb performance for dellorto drla 40s. I'm thinking they aren't more than a foot long, and the math tells me that with a 12 inch runner it's tuned, at that rpm, for like the 8th harmonic, which is pretty weak.

What do you think, 26 inch manifolds?

Thu Jan 06, 2011 9:10 am

check your math for the 1st&2nd wave. I have ploted this years ago using the flow bench& data. I never heard of a spefic formula but I dont doubt you can do it. the first is very strong but hard to get and it would be a very narrow rpm windo,the 2nd is a good one to shoot for. does the formula account for a IR system like your or a plenum like a v8? and if the runners are like a atock dp that changes things again. but yes that typ of supercharging does make some very good power if you can get it.

Thu Jan 06, 2011 9:43 am

mark tucker wrote: check your math for the 1st&2nd wave. I have ploted this years ago using the flow bench& data. I never heard of a spefic formula but I dont doubt you can do it. the first is very strong but hard to get and it would be a very narrow rpm windo,the 2nd is a good one to shoot for. does the formula account for a IR system like your or a plenum like a v8? and if the runners are like a atock dp that changes things again. but yes that typ of supercharging does make some very good power if you can get it. Exactly correct. I didn't see where the OP mentions runner length including the port length in the head......hope he figured that. Then there are the effects in V-8 Nascar stuff of the adjacent intake port and plenum that allow the tuned length to be shortened and tuned by the runner dividing wall and plenum volume. Then port taper comes into play where average diameter is the key and tuned length varies depending on the degree of port taper. Straight tube type design does not account for friction losses in the pipe length and need a friction factor calculated in. It isn't as easy as running a formula and welding up some tubes.

Thu Jan 06, 2011 10:51 am

Ah I missed on page 2 where they talk about how with individual runners and no plenum that "the basic carb sizing formula does not apply". - it does get into taper, ideal port size relative to valve size, and plenum volume in page 8 there. Good call, Rock Crusher, about including the port that is part of the head in the calculation of length.

There's not a lot of info it seems on tuning a port for use with carbs. It talks about in the article the general confusion of the term tuned port, it being confused with having to be part of a fuel injection system when it really doesn't have to be. -Info on tuning a runner for a carb without a plenum seems scarce, maybe because of the tricky nature of it/ the large effects of small variations in such a setup. I guess that's where the flowbench would come in.

It does seem that the carb manifolds you typically see, none of them are longer than a foot or so, that they would be better longer. By the calculations previous, if they were to apply, the intake would be optimal at 40" long to take advantage of the primary pulse wave!

Thu Jan 06, 2011 11:09 am

Nice to see you're paying attention.

No matter what you do, it all comes down to the dyno and the track. No amount of calculation can cover every nuance of a system and in tuned length systems you can't discount the cam timing as a MAJOR contributor. See how deep it can get? No matter what design you come up with you have to play for optimum cam timing.....then jetting, ignition timing......then you make a small mod and have to do it all over again. This is why real race engine development takes so long and costs so much. Think they just put their heads together and came up with the ultimate package on paper, built it and ran 7's in the Red Baron car? No way, it took a long time and a bunch of heartache to get it there.

Most commonly available manifolds for the ACVW are a matter of convenience, economy of manufacture, engine fitment in the car, etc......not at all optimized and for most guys that's fine. That is where a good head guy comes into play and why you pay such good money for a righteous engine build. You're paying for some expensive and hard won knowledge..... :)

RC

Thu Jan 06, 2011 12:41 pm

I believe in most cases you should tune so the third harmonic lines up near the top of the rpm range, because the zone between the third and second harmonic would be pretty dismal. Only time folks tune for the second is for VERY high revers.
Since this is a bus maybe this is a rare case where you want a runner that long, but as you will see the average IDf setup is about 16" with 2 1/2" tall stacks.
Now I know you are thinking that cb sells 6" tall stacks, but unfortunately super tall stacks will have jetting side effects, better to lengthen the manifold.
If you only rev it to 5000rpm I bet 20" would be better than 16" nevermind the harmonics.
You'd want to build braces to steady the carbs so you don't break your ten inch tall manifolds!

Thu Jan 06, 2011 2:17 pm

Lets make it simple - the optimum you are looking will be a fixed velocity at a fixed RPM, which will be a fix speed in each gear.. This is a bus, figure the speed you will be mostly using it for , lets say 60 mph and what ever your bus does in RPMs at that speed. That is were you are going to gain the most from the modification you will be making. That said, this will occur at your top speeds and upper rpms. Again this is a bus you will be losing low end torque if unless you rev the heck out it in every gear. So great for cruising and for better fuel economy if you cruise at that targeted rpm, but not if you run though the gears.

I have a friend that has an old Alfa 2000 and he travels from NY to Chicago to visit family on a regular basis. Dual Webbers. For the trip he changes the velocity stacks for some tall monsters and changes his main jet sizes. He loses low end but gains top end and it is a bit more comfortable at cruising speeds. Also several MPG better.

Thu Jan 06, 2011 8:31 pm

This is just the sort of formula I was looking for, Thanks!

Fri Jan 07, 2011 6:08 pm

Nice thread and that link is very goo information. One note, it is mostly if not all...based on carburettor tuning.
If you are dealing with injection...and most expecially a plenum system....the information about the third harminic is virtually meaningless.
Plenum systems use the reversion harmonic to actually push air into diagonally adjacent runners whose intake valve is next in line to open. At virtually any rpm over 1200 to 1500 the overlap time of valve open versus time to move reversion air through the runner and back pressurize the plenum...means that unless your plenum is grossly undersized.....its virtually impossible to have reversion exiting the TB like you do on a carb. The harmionic on a plenum system is totally different. And.....cast manifolds that use "planar" sub plenums (single and dual plane plenums)....have virtually no relation harmonically to the tubular type manifolds we even use for carbs on VW's. Not saying the info is wrong by any means....just that much of it does not apply to our systems Ray