Critic my custom intake manifold (larger plenum-shorter runners)

I would agree that the plenum volume appears minimal. Keep in mind that the purpose of the plenum is to simulate individual intake stacks. In other words, the flow velocity, within the plenum, should be quite low. As I understand your modification, you have not changed the plenum cross sectional area perpendicular to the runner axes, so I'm not certain you have significantly changed flow velocities at the runner inlets.

Manufacturers minimize plenum volume to minimize part cost and provide more engine compartment room. So, like everything else, it becomes a compromise.

Longer runners also cost money, so, again, the desires of the engine developers might be compromised. Those looking at the dyno output want short runners to yield maximum power. Those calculating performance want longer runners to maximize acceleration performance. The bean counters don't want any runners at all, if it'll save a buck.

You say you want to tune for about 6000 rpm. I assume this is approximately in the middle of the rpm range you encounter as you go through the gears. This means the distance, from intake valve head to plenum, should be about 13 inches for maximum effect. The next "harmonic," or whatever you want to call it, hits at a length just under 10 inches, but it is not nearly as strong.

Power Dubs,

Well I did not read every line but... Did you take any flow bench numbers before you started??? Your idea looks good, plenum size etc. as long as you did not hurt the total flow or the ballance between the cylinders. On a recent test on a Porsche manifold we found the ballance between the cylinders was less than 1%, no small feat. The intake in question was left un touched.

PFM

PowerDubs,

I really can't comment too much on your intake mod, as a dyno has proven me wrong more times than I'd like to admit. I will say, that at a glance, your setup seems fairly logical and straight forward.

I will agree with PFM about checking the flow on a bench. It's surprising the things you can find.

A couple years ago we were having problems with a Honda F20B engine that had been modified for road racing purposes. The engine was making use of a custom intake manifold, replacing the stock PGM-FI unit. We soon ran into a problem of running lean on cylinder #1. Soon detonation and burned pistons became all too common. After checking injectors, fuel/spark curves, and the like, we concluded that the problem was in the manifold.

We put the manifold on a flowbench and to our surprise we had relatively even flow across the runners. We scratched our heads a bit, then decided to mount the throttle body, and tested again. Not much different. Sure we saw the normal changes in relation to throttle position, but nothing unordinary. Then we got the idea to mount the entire intake track (induction tubing, couplings and filter). Presto! We now saw the flow differential that was causing our lean-out condition. A slight modification to our intake plumbing, resolved most of the issue. The next manifold we constructed, went with a slightly larger and mildly reshaped plenum, and our problems went away completely.

Just another lesson that an engine is a symphony…

Good Luck,
Bryan Carter

I'll keep you posted once I dyno before and after....

Josh,
You might want to take a look at an airbox design article wriiten by Prof. Gordon Blair in the latest issue of Race Engine Technology. It certainly looks like more plenum volume is a good thing, although that particular engine was designed to run at very high speeds. In any case the article is any interesting read.
Dan

Here are the DYNO RESULTS.