Ideal Attainable Runner Length
 

I know I've asked some people before; but I'd just like the input of more people.

We'll just say I'm building an All-motor rotary racecar...

Large 4port Streetport (think bridgeport, minus the bridge) built enough to make make and handle power to ~10k. 4 38mm TB's, 1.25" intake runner diameter.... how long does the over-all length of the manifold need to be (stacks to port-face)? I'm looking for a nice broad powerband (car will get a lot of street use) but I've also got manifold simplicity and under-hood room to think about.

Any input appreciated; and please give a little input, not just "18inches". ~CE

Well from what I have seen and read most theories on how to calculate the ideal runner length for particular application are based around fairly simplistic mathematical models of a piston engine induction process. An example is the book "The Scientific Design of Exhuast Manifolds" as recomended by RICE RACING in the past.

Now even if these models hold true for your rotary setup they are still only a guide so that you have a starting point for experiments. That is a starting point to make your first experimental manifold and then get it on the dyno and experiment with different versions of the manifold, with longer and shorter runners, to find out which one effects the power curve in the desired manner.

So I would be wary of anyone stating some magic runner length that's gonna suit all applications.

HEY nothing will work well unless it is variable length, it has to be adjustable in its length, thanks RON

you could try making the set-up as long as possible and still fit it under the hood, then because your using hose-clamps to secure the TBs you would just keep trimming off the manifold until the power peaked where you want it to. This is very subjective but it would work fine with some patience.
Grant

And that's the only true proven method of finding proper length for both exhaust and intake.

This is exactly how my new intake manifold is being designed, except room will not be a problem. (think trombone)

This is exactly how my new intake manifold is being designed, except room will not be a problem. (think trombone)

Yeah but you've only got 2 runners to deal with;)
But seriously I thought of trying an electrically variable length set-up at one time, but ultimately I decided that I would never be capable of feeling the difference unless it was long enough to get 1st level harmonics.
Grant

hmmm, been wondering about these things myself cuz i wana design my own manifold. first i think u have to deside whether u are gona use a vacuum chamber design, individual TB desing, or a stock type dynamic design. heres a graph of dyno power with dif. length stock type designs...

18" equal length

I'm gonna have to check this out to see if the Racing Beat 6 port is this length. I'll post my findings.