This may not be the best forum to post this question, but it really is DIY prom based from my way of thinking, and all the real guru's reside in here as well. Move this if you have to, no hard feelings on my part.

Here's my question: What effect does the size of the intake plenum have on the characteristics and manners of a motor?

Does a smaller plenum increase the air velocity and hence increase or increase the lower end torque? If so, what would the impact be on the upper end RPMs? Would a smaller plenum actually decrease throttle response? Or maybe restrict air flow and hence power on the top end?

Conversely, would a larger plenum volume increase throttle response since more air is "available" to draw from before the air flowing thru the now more open throttle blades as we accelerate?

Just doing some thinking, but it seems if we strive for good velocity in heads, wouldn't improving the velocity of the air thru the runners be kind of a "head start" on this? If so, it seems a smaller plenum volume might be beneficial.

I'm also not sure how effective runner length would be affected by changing the plenum volume, any ideas?

Fire away folks - shoot down my idea, correct me where I'm wrong. This is an important question/matter to me right now. I can see how it could change performance, both good and bad. Guess i've been thinking on it too long

Seriously though, what should be the benefits and/or detractions from changing the plenum volume? And for the sake of discussion, please think in terms of a major change in volume, say something like increasing or decreasing it by 50% ....

Thanks in advance for any thinking and replies anyone does on this. It is not a trivial or stupid question (at least not to me).

Have a Great Day!

 

Alot.
More then you might ever imagine, that said, it also depends on alot of other factors.

I've spent a fair amount of time on the GN's intake, and every increase in volume has resulted in more HP. While T/C, things are a bit different with it then with a N/A design. I've also cut back the runners lengths as much as possible to increase it's volume, again made more peak HP.

It has a huge effect on the way AE is going to be used. The car got real fussy about what it liked, and didn't like. More the a few percent off on VE, and a few degrees timing, and the car literary wouldn't come off idle.

Converter and cam have to be matched for what you want to do (well so does the other components, but they're real biggies when want to get the car to drive right).

 

Another thing to be concerned about with plenum volume is idle control. I've had a real hassle trying to get the idle right on a 4 cylinder SD ('808) application with a large plenum. With too big a plenum, you lose the relationship between IAC steps and air into the engine (hunting idle because the IAC control tends to overshoot the correction required).

John

 

Thanks for the replies, gents.

I know I'm sounding a bit nebulous at the moment about this, but it's because I'm trying to get a feel for the plenum volume effect as part of an experiment I'm about to run. Grumpy - I'll send you a PM on the details - but I don't want to prejudice anyone's thoughts before they reply...