what are the effects of plenum volume?
I am in the process of cutting up my stock upper plenum. I cut out the runners, and basicly, its just a big open box. should I try to reduce the open volume of the plenum? does it even matter?
I have seen people who have added "boxes" on top of their plenum, saying it adds volume, thereby helping "throttle response" - how/why does that work?

Too much area can make throttle response seem vague. Curious, why did you cut the top off? The plemium is not where the problem, it's narrow neck and tight turn to middle intake and to lesser extent the lack of velocity stacks that make the stock intake a very nice-looking but otherwise P.O.S. design.

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didn't cut the top, I cut the bottom. like the "improving the stock intake" thread.
I was just wondering what affect having the insides basicly gutted will do?

Actually, looking at your pic in that thread, it looks like you will dramatically increase your plenum volume but practically eliminating your runners completely in the upper. the runners now effetively start just about at the bend/joint to the middle, as I am guessing that you will seal those gaps between cyl 1 and 3/5 and 2/4 and 6 making the top a large plenum.
I would guess that the net effect of that is loss of lowend torque (shorter runners) but some improvement at higher rpm.
the larger pelnum will make for a spongy throttle too.

how does the 2.8 camero divided plenum compare volumewise ?

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Do a search about "Intake resonance". Basically long runners are good for high RPM's. Short runners are good for low end torque. The intake is balanced just like your cam is to a certain torque and HP range. Also exhaust flow factors in and sort of mimics the intake flow. If you change one thing, to take full advantage of it you must change all others. Resonance is a basic "engine physics" law for performance. Unfortunately, things like the intake, cam, exhaust, valves and ports are "hard" items and can't be variable. Thus the engine has it's peaks "ACCORDING" to them.

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If you don't also open that restrictive TB-to-plenium neck up a bunch and do something about the runner bends, runner lenght won't mater much with respect to high RPM, because you still wont get useful higher RPMs. The engine is going to go extremely rich above 4.5k and fall flat on it's face. This is not a guess, I've seen it on the flow-bench and on the dyno.

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the throttle body & plenum neck is bored to 57mm
the plenums have been gasket matched
the heads have been ported & gasket matched
the cam is Crane 272
WCF long tube headers

the upper plenum is the last peice of the puzzle - I am opening the neck & removing the runners - making them start at the bend.

I am wondering if I should leave the area open or fill it in, to reduce plenum volume.

if you are talking about the gaps between the runners, I would say yes fill them in. if you don;t those 2 single cylinders will have longer effective runners than the doubled ones, as they will still be the same length as stock.
While I;ve seen people do stuff like that to have a mix of cylinders running a differrent power band, I would not reccomend it as then there is a fuel mix discrepency between the cylinders at different rpms.

To reduce plenum volume would be an interesting excersize with that layout, will be tricky to do without compromising air flow or creating a runner effect to a cylinder. possibly a pointed elipse shapeput in dead center would do the trick without doing too much collateral damage. if you don;t get what I am saying, Icould draw somethign up on that cut open pic of yours....

yes, the 2 gaps are getting filled. I thought about leaving them, and going with the "dual plane" concept. I dont think the fuel/air mix is gonna be very affected by this.

for reducing the volume, I was gonna just put a "V" like thing thru the center, and just make sure their is plenty of space, so it doesnt become "runner" like, and fill it with "Great Stuff" foam

ahh I think we are on the same page in out thoughts on this. I see where you are going. (I hope) and I think you may want to modify the v concept to an elipse, tho rounded on the dead end, not continueing the v flair to it;s natural termination.

I don;t know now that I have an extra middle and top plenum that I am going to go stock looking. I was actively trying to figure a dual plenum, short and longer intake path, but have since decided I cannnot implement this without taking the car off the road, which is not possible as it is my daily for work. I wiwll be modifying the middle and upper to smoooth the bend out, allow valve cover removal, and have shorter runners, aiming at the top rpm the motor was laid out for.

quote Originally posted by frankenfiero1: Do a search about "Intake resonance". Basically long runners are good for high RPM's. Short runners are good for low end torque.

frankenfiero1, isn't it the other way around, as tjm4fun suggests in the previous post?

Namely, longer runners favor low RPM, and shorter runners favor high RPM, right?

quote Originally posted by project34: frankenfiero1, isn't it the other way around, as tjm4fun suggests in the previous post? Namely, longer runners favor low RPM, and shorter runners favor high RPM, right?

That's what I say also.

so, I wont have any real issues with the extra plenum volume I will now have?
or will my light throttle "cruiseing" be better, due to the larger volume to "buffer" small vacuum changes?
or will heavy throttle changes be subdued from the delay of the larger area "building" vacuum?

I am thinking it will make little difference, and may improve general drivability

Honestly, I don;t know what the exact effect will be, I suspect at light throttle you won't see any change, at constant you really shouldn;t see any change. I would think from a light throttle to wot stomp, you will see some delay, whether it is a problem, I don;t know, might be like a lite turbo lag.
with the shorter runners tho and opening up the neck and tb, you should see a nice power shot in the upper rpm range. I think cutting out the runners to that nasty down bend will help a little, as now that will be the start of the runner. anything you can do on the inside edge of that joint on the to side to smooth the bend will be a definate plus. I am thinking that if you angle towards the middle on the inner side it will help, and may act like a velocity stack. I won;t know how much material there is ther til I cut mine open. but I am also planning on reworking the middle plenum also to get that sharp bend out of the picture.

that nasty bend - yes - the metal is pretty dang thick around there, and I was able to take out ALOT. almost an 1/8" - it almost paper thin on the top side of that turn now. got a full "bell" shape all around the port, which is gasket matched to the peice below. the radius I kept as wide as possible, and was sanded smooth back when I gasket matched the plenums.

good, I figured you would address that, sounds like you did all you can do there, that bell shape should act like a velocity stack and help the flow.

Plenum volume will directly affect the torque curve. Too much velocity (too small of a plenum) will give a good throttle response, but not so good for performance. A larger plenum will shift the torque curve up in RPM, and give you a higher peak (numerically).

http://www.grapeaperacing.c...inductionsystems.pdf

See page 8 for a better explanation

quote Originally posted by ryan.hess: Plenum volume will directly affect the torque curve. Too much velocity (too small of a plenum) will give a good throttle response, but not so good for performance. A larger plenum will shift the torque curve up in RPM, and give you a higher peak (numerically). http://www.grapeaperacing.c...inductionsystems.pdf See page 8 for a better explanation

good stuff - wish I saw that before - but after reading it - I feel even better about the work I am doing.

be careful of that reference, that page (8) is taking about smaller spit plenum arrangements, ie that would be an excellent reference for those building the enterprise dual log setups. would explain why they seem to work so well....

even when they discuss volume, they specifically say 2 plenums, 3 cylinders each for 6 cylinder. confusing since they do not really mention sizing for single plenum 6's, tho I quess you could use the numbers for the 8 cylinder as an indication.

quote Originally posted by tjm4fun: they do not really mention sizing for single plenum 6's

Because there are too many pulses for helmholtz resonator to work for 6 cylinders on a common plenum.

Generally speaking though, you want a minimum 75-80% of the engine's displacement for a single common plenum.

Sorry, that dyslexia is kicking in again. Yes it is vice versa of what I said earlier. This is a public service annoncement: "Friends don't let friends drink and type"....

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