Intake Plenum Volume

Hi all, I conducted an experiment on my own cars plenum volume. Using 10 psi of boost.

First up it ran a stock plenum of roughly 800ml volume , Dyno'd it and got X amount of power.

Then made a custom plenum using the stock manifold with a modified plenum about double if not more than the stock size one.

Re dyno'd it without making any other adjustments. and it did not differ more than 0.5 hp anywhere across the entire rev range.

I realize that the ignition timing was not altered to suit the possible change in VE but thats just what I found anyways.

If anyone with experience could discuss plenum setups and why you would want to change it (bigger / smaller, what effects it has ect) that would be great as so many rush out to buy the biggest baddest plenum with out really knowing why, let alone back to back testing.

This test would probably work best with an engine that is not knock limited. To test the changes (if any) in MBT.

This is one case that size isn't key. The design of the plenum should take into consideration air speeds/ volumes/ pressures internally, the air should be evenly distributed across each of the runners so that each chamber gets the save volume/pressure and speed of air. This comes down to internal design and also tapers to increase air speeds.

It's not something I've personally experimented with, the lack of fabrication ability sees to that, however it is something that's always intrigued me when I see lots of home made manifolds/plenum's which have obviously been designed all about the show rather than the flow.

To experiment properly you would need a hole host of extremely accurate sensors to measure air speed, temperature etc going in and also EGT and lambda going out, if you want to really go into it you could also get chamber pressure sensors to see how different designs affect the chamber pressures and therefore power outputs

I always thought that for a general rule of thumb (not taking into account all the items CHris250 mentions which are also important) a plenum's size should match the airflow capacity of the engine at max torque (or is it max power). I don't remember where I heard this but it has stuck with me so I am very interested to see others opinions.

I've never been in a situation where I have been able to back to back test a plenum where the only change has been to the volume. Before I designed the plenum for my drag car back many years ago I did some research into the subject. As you could imagine a lot of it was conflicting data which isn't helpful. The general rule of thumb I formed was that a plenum volume of approximately double the engine capacity is a good place to be.

As Chris mentions though, plenum volume is probably one of the less critical aspects of the plenum design. Runner size and length are a key aspect of the engine's torque delivery. It's also important to focus on getting smooth airflow into the plenum and then the runners. I'm a huge fan of smoothly bell mouthed entry into the runners. I'm not a fluid dynamics engineer, but I've always felt that if the design looks right, it should work right. Not very technical I'll admit but I've got no better explanation :)

Here's a very well written article from Professor Blair and Associates that may assist in some understanding of the intake runners, but not soo much the plenum.

Bellmouth