MSefi.com read only forum (Jan.15,2018)

if you were sourcing an intake manifold, or fabricating your own, how does plenum size affect engine performance, assuming that throttle body size and intake runner diameter/length were fine? Can a plenum be too large for a given application? Is there a rule of thumb/formula for determining its volume? Maybe the combined volume of each runner?

I just finished fabricating a new plenum for my engine. I spent a lot of time searching the web for information, and I would suggest that you do the same.

The info I got indicates that there is an optimum size for the plenum, throttle, and intake runner volume/cross section, that will tune the setup to increase volumetric efficiency at a certain very narrow rpm range.

The way I see this is that if you are setting something up to run most efficiently at a particular rpm range you could spend the time to make the calculations and you may see some results. But the efficiency at other speed ranges may suffer due to restriction of air flow.

My plenum is slightly larger than the stock one and the runners are about 20% shorter and 10% fatter. My throttle is huge (70mm) for my 2.4l engine bu I did this so that I could get the air in the plenum to slow down and increase in density so that the engine (at WOT) would be pulling air through the runners as though they were pulling from open air.

I'm sure that this has had some effect on low speed performance but it still runs pretty good down there, and it rips right past 6000rpm

I would recommend spending some time searching the net using plenum size, runner size and hemholtz resonator as search criteria. there is a lot of math involved and I think you would likely need to try several configurations to arrive at the optimum setup.