I had this question come up a while back and the "SBC distributor in an LT1" thread got me thinking about it again. Basically, how does a larger or smaller plenum volume effect power output and the powerband? Would the decreased plenum volume of cutting the LT1 intake to fit a traditional SBC distributor hinder performance on a big-inch engine? Or how about the throttle body spacer I put on my 2.2L S10? Will the additional plenum volume improve top-end performance? Is there a scenario where you can have too much plenum volume? Assuming that you keep the same size and length runners on an intake manifold is there such a thing as "too big" in regards to the plenum area?

I guess my question only applies to fuel injected motors. I understand how plenum volume and runner size effect the vacuum signal on a carburated motor.

-Chris

 

Ideal plenum volume is 50-70% of engine displacment.

 

My understand of intake manifolds is as follows,

Total plenum volume is the plenum + the volume per the sum of all the intake runners. (This from a very reputable source)

Too much plenum volume is a bit ambiguous. Obviously a pro stock engine is gonna need a different plenum volume for what it needs to do..... most of the 2hp+ per ci drag race motors are using plenum volumes 2x that of engine displacement, that's ~1000 ci for a 500 ci motor.
Most of the builders I've talked to recommend starting out with something about .75x engine displacement for street engines (V8's) and more volume for higher rpm (racing/high-performance).

wrt the drawbacks to too much plenum volume.... my understanding is that "too much" hurts throttle responsiveness. I can agree with that as I tried using a custom sheetmetal intake previously on one of my race engines (6000-9000 rpm) on a larger but much milder street engine... didn't have much for throttle response. This was a carbed engine by the way and the bigger plenum just compounds the booster signal problems there.
For MPFI stuff, I think it still plays a part, although I'm not sure how big. The main thing there is restriction... if you have too small a plenum, its understandable that you'll have a higher velocity through the throttle and once it reaches a certain speed, you'll start to lose power. For that same reason, I can see throttle response suffering a bit too.
The other thing is buffering.... a bigger plenum, as I understand it, can be of some help in buffering pressure wave interaction. Seems reasonable to me because the more "space" the waves move in, the lower their intensity by the time they reach the port of the cylinder next in firing sequence.

So, plenum volume should be matched to the rpm range of the engine. Same as the length/size of a header collector.... theres some power there too for guys willing to experiment.

Just my 2 cents, hopefully one of the intake guys will respond with their real world results.

-Mindgame

 

Question....

Lets say you're using forced induction.

I would THINK that once the plenum comes up to a particular pressure that, all things being equal, it would tend to drop slower with a bigger volume because each valve opening event is going to take a smaller percentage of total volume out of the plenum.

On the other hand I also can't help thinking that a forced induction engine would behave exactly as an N/A engine because the engine wouldn't have an "preconceived notion" of what atmospheric pressure was supposed to be.

my head hurts....

 

First thing… you don’t have to cut the plenum on an LT1 intake to use a distributor with it, there is plenty of room back there, it’s layed out similarly to a miniram for a SBC. You will loose the egr (no real room left for it and the tubing feeding it).

Under boost at WOT pretty much the whole volume between the compressor and the intake valves acts as plenum volume. This is significant since if your inlet ducting has a lot of volume the boost will not rise as crisply if that makes sense, it could even be misdiagnosed as “turbo lag” on turbocharged engines. On 2 strokes, engines with smaller numbers of cylinders, and ‘odd fire’ engines it actually helps quite a bit when running turbos, since it acts as a bit of a buffer allowing the turbos to spin up more evenly and not feel the pulsation of the flow through the intake. On roots blown engine it makes for easier tuning since roots style blowers pump air in pulses that match the number of lobes their rotors have.