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Originally Posted by SaabTuner
From what I've seen, the intake manifold for the Saab Viggen hillclimb car is almost identical. Same cone/plenum/runner setup. I haven't had a close enough look to be sure, but it looks pretty dang similar. The exception is that the injectors are mounted on the plenum and shoot the streams of fuel straight down the runners. Pretty neat. 
Still not sure why you're worried about the length of the runners in relation to flow. The length of the runners serves primarily to pulse-tune the intake setup. There's a good formula on this borrowed from the book I mentioned previously:
Runner length (from valve to plenum) = (90 * 1,100)/RPM
The value of 1,100 will be loosely dependant on intake temperature. (IE: speed of sound) The RPM is the approximate RPM at which peak pulsetuning effect should result. There will also be a significant change in the effect when different camshafts are used; a longer duration cam will require a longer runner tube to create the same pulsetuning effect. (Though the "same effect" may not be desirable.)
For an engine running peak torque at saaay 6,000 RPM, you get a value of about ~16.5 inches, give or take a couple.
Sometimes people tune to the second pulsewave, which reduces the requisite length by quite a bit. (Not exactly half, IIRC, but somewhere in the vicinity.) In either case, it's worth thinking about.  |
The original design concept was taken from a 24 hour race car designed by HKS. It has changed a decent amount in me trying to get it to be perfect. Not an easy task at all
About the runner length yes, there are a few different ways to measure the pulsing. I am assuming the one you mentioned is a helmholtz equation broken down to be simple. Similair to this:
The problem I have found with this equation is it gets less and less accurate at higher RPMs. There are a few other equations that do pretty well. Using sound wave design/emulator programs seems to be the best and most efficient way so far (And it works for companies like Ferrari). The other advantage of using the sound wave programs is it can also tell you the volume of the plenum as well as the lengths/volume of the runners.
It just becomes a serious pain trying to first get a design done, then testing it to ensure the best possible flow and then trying to fit that design into a sound wave type program. All this takes an extreme amount of time but its something to do
The good news is I dont actually have to make anything to test the designs. I couldnt imagine if I just starting making things with no idea of how well they actually worked. Im sure after 5 designs that take a while to fabricate, bolt up and dyno I would get close but I will never know how well they actually do work compared to what is possible.