Plenum volume

[Steve.k]

How can you tell if you have enough /or not enough plenum volume. What are the tell tale signs. I was chatting with Another builder and he said when your peak tq stacks up for 3-400 rpm thats a sign. What are your guys idea on that.

[PRH]

I’m not exactly sure what you’re asking......

You’re saying the tq plateau’s near the peak for a few hundred rpm....... and that’s a “problem”?
How much does it vary, over how wide of a range?

I suppose it depends on what the intended use is........ but looking through some sheets I have here....... some of what I consider the more successful builds have the tq numbers within 10 numbers or so for 600-1000rpm.

[F-BIRD'88]

Unless you are racing engine dynos, let the track ET slip guide you.

[gunt]

not really fair to say , racing dyno's as a response to a question . and not everyone is on a drag strip

i for one hav't had the time to look in to this ,and am eager for answers , my rule for myself is 4k rev ranger from peek tq to peek bhp , and hopefully a flat tq curve , i have seen some engine where the cam companies mad smaller cams as the std cam was made for the soul purpose of a sales pitch and when any fitting larger it lost , in the heal of the hunt the intake was a full restriction to any thing bigger , and in particular this 4 cylinder even came with 8 injectors , so why did the oem get it so wrong , ? or was it right considering emissions .

as for the boosted stuff which i play with alot , i see it said 2.5 time the cc of the engine , when i look at the manifolds they don't look like 5 liters for a 2.0 engine , or is this at the pressurized at the level of anticipated pressure

[Steve.k]

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What im saying is when you have your peak tq numbers hold nearly the same for 4-600 rpm. Peak say at 5400 then at 6000 still nearly the same. Some say your plenum should at least match cubic inch of engine. So if you have 400 cubes you plenum should be same. ? Similar to this Dyno sheet. Some say it should peak then fall off. The plenum and or intake to small. Has anyone heard this. And yes drag motor.

[rebelrouser]

From what little I have seen, plenum volume testing, or just adding spacers on a dyno, the results varied wildly. I have seen engines that liked a large spacer, I have seen engines that liked none at all. Seems like most dyno operators have lots of them hanging on the wall as it is the most easy item to swap out. I built one intake kind of from scratch and it worked out pretty good, and when reading here and there while building it, I got the following advice. I settled on a plenum about the size of two cylinders, I got a good flat torque curve.
I drag race, and large plenum manifolds seem to suffer losses in 60 feet, but many times make it up on the big end, and just the opposite for a small plenum. Lot of times it is a wash. Just my experience.

"Plenum volume should be 1 1/2 times a runner volume. And that's the total runner not just the head."

"I found that typically intake manifolds have a volume that is 50 to 80% the displacement of engine."

"The problem with a larger plenum is that it hurts throttle response. Throttle response is very much affected by throttle pressure (or in other words how fast the engine can consume all the air in the plenum and create a significant amount of vacuum in the manifold to draw in fresh air). The smaller the plenum (or smaller the runners), the higher the gas velocity, the faster the pressure drop, the sooner the new air rushes in, the faster the throttle response."

"As the plenum volume gets smaller, it becomes easier for the engine to rapidly consume all of the air in the plenum and thus it would have to spend a lot of effort (after the initial draw of air) trying to suck air in all the way through the entire intake system to stay alive."

" It depends on your RPM range, Engine size and intended use of the engine. For instance, An engine utilizing an automatic transmission will have a much smaller plenum than an engine using a five speed. An engine with a five speed behind it can use another 20% more plenum volume. You must be careful here! The dynamics of the race track vary slightly from the dyno and in this regard, they vary a LOT! On the dyno you can tune an engine to produce max power by adding a considerable amount of plenum volume. In some cases you can almost double the volume and see a considerable power gain. On the track the engine will lag against the converter or worse yet, just not accelerate at all. Smaller engines are much more susceptible to this than larger engines. We know from extensive track tuning that our P/S engines will not take any more plenum volume but on the dyno, I could add more volume and see more power. In this case, equations take a back seat to experience and practicality. "

"David Vizard suggests the following workable volumes to target the 5-6,000 rpm band for different engine configurations:
• V8 (if a single plenum is unavoidable): 40-50% of total displacement "

[Steve.k]

This engine has displacement of 434 and intake is 377.

[cardo0]

How can you tell if you have enough /or not enough plenum volume. What are the tell tale signs. I was chatting with Another builder and he said when your peak tq stacks up for 3-400 rpm thats a sign. What are your guys idea on that.


What im saying is when you have your peak tq numbers hold nearly the same for 4-600 rpm. Peak say at 5400 then at 6000 still nearly the same. Some say your plenum should at least match cubic inch of engine. So if you have 400 cubes you plenum should be same. ? Similar to this Dyno sheet. Some say it should peak then fall off. The plenum and or intake to small. Has anyone heard this. And yes drag motor.

That's pretty easy to answer. You don't have enough when the manifold becomes a restriction. The power curve drops at some rpm value. Don't know how you can "stack torque up" which sounds more like a fallacy/bogus statement. A flat torque curve is more of a function of the bore to stroke/over square or under square ratios.

What is important is how the plenum turns the air to fill the cylinders as equally as possible and reduce reversion between runners within the plenum. Its hard for air to make sharp turns so the runners closest to the throttle body don't get the same volume as the far runners. Fix is to enlarge the plenum near the throttle body and extend the plenum at the far end beyond the far runners. Runner entry shape very important also.

So lets look at a 350 ci chevy at only 2000 rpm. That's 350 cubic inches of air times 1000 per minute at 0.8 efficiency = 280,000 cubic inches of air per minute and divide by 60 = 4,700 cubic feet per second. Well a 55 gallon drum has only 7.4 cubic feet of air so the engine would empty that 55 gallon drum plenum in 1.6 milliseconds. I don't think you can have too much plenum volume to delay throttle response.

This engine has displacement of 434 and intake is 377.

While there may be some mathematical relationship for the plenum volume to engine displacement I have never seen any. All the information I have read about is from empirical testing (results based on one dyno test at a time). But from what I have read matching the plenum volume to engine displacement is more than enough for a V8. 377 divide by 434 = 87% could be well enough or very close.

Hope this helps.

[Adger Smith]

Years ago when we did 9-10,000 RPM comp engines we started with a cc per cu in. . add till it fell off on the dyno. then take out tell it fell of when it was in the car. yes I've seen too much. The car gets lazy off the shift or 60's fall off.

[tuffxf]

Plenum may be fine, may need more camshaft lobe area
Cheers

[dannobee]

So lets look at a 350 ci chevy at only 2000 rpm. That's 350 cubic inches of air times 1000 per minute at 0.8 efficiency = 280,000 cubic inches of air per minute and divide by 60 = 4,700 cubic feet per second. Well a 55 gallon drum has only 7.4 cubic feet of air so the engine would empty that 55 gallon drum plenum in 1.6 milliseconds. I don't think you can have too much plenum volume to delay throttle response.

There are 1728 cubic inches in a cubic foot. The small block in your example has an air flow of 162 cubic feet per minute, or 2.7 cu ft per second.


We tried a lot of tunnel rams back in the day. The dyno always seemed to want more volume, but at the track we were gluing wooden blocks into the plenum and the car went faster down the track for the same reasons outlined above. It bogged down on launch.

[Steve.k]

6A48DAA4-F6CA-4FE5-AB4C-66F6CF80C284.png

Plenum in question. 434 cubes now. Tuffxf i was just talking with some guys who were questioning size if this plenum. Still could be little shy on cam.Thanks for feed back guys. I know nothing about sizing plenums. Always bought why was available off shelf. The one in pic I bought from fella who built for a3 headed 393.

[Steve.k]

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Look at inside.

B81CD665-2D47-44C7-914F-FF54D49BDD4E.png

Fairly straight shot in

[PRH]

The tq curve on the sheet looks pretty good to me.

To put the plenum volume for that combo into perspective.......
Here is a 452” motor with a Trick Flow 4150 intake, 2” super sucker, and a Holley 80496 carb.
This intake is not huge in the plenum by any means.

I’d say your TR has probably double the plenum volume, and a few less cubes than this combo..... yet the shape of the TQ curve around the peak isn’t all that much different.

My guess is your heads probably have more area, and higher flow as well.

[PRH]

I’m not manifold guru by any means....... but...... it would seem to me as though more of a radius from the plenum into the runner might help some.
In the pic, it looks like the transition is pretty abrubt.

Be interesting to flow the head with the manifold bottom mounted to it.
As is, and then with a clay radius added around the top of the runner.

I realize there is a lot that goes into making a manifold “work”...... and that adding the radius to the runner entry might actually make less power....... even if it flowed better.