Plenum volume?

General engine tech -- Drag Racing to Circle Track

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travis
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Plenum volume?

Post by travis »

Noob question here...

When is more (or less) plenum volume beneficial? Is most of the benefit of a bigger plenum from making it easier for air to turn into the inlet runner? Does a bigger plenum work better for a bigger displacement engine, or for higher rpms? Does it simply act as a bigger "tank" for each runner to draw from?

I was reading something the other day, can't remember where now, but it said something along the lines of it (plenum volume) affected the pulses seen by the carb and boosters...that a smaller plenum is going to be more responsive at lower rpms because the boosters got a stronger signal, where a bigger plenum kind of delays the signal seen by the boosters until airflow demand is higher. Does this sound right?

I've got questions but I want to make sure I understand the basics correctly first.
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Re: Plenum volume?

Post by Tom68 »

No expert but I think it's room to turn and help distribution.

Possibly also helps at high rpms by giving the runners a clearly defined end point, for instance an old school 2 barrel dual plane runner goes all the way to the venturi, on a race single plane it is just runner length from the expansion chamber/plenum.

No plenum (long runners) suited the Side Valve rev range.
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Re: Plenum volume?

Post by mag2555 »

Besides being a storage area for runners to grab air and fuel from the size of the Plenum also dampens out reversion pulses when intake valves slam shut that then back the flow up making some runners richer then others and pressurizing the carb the wrong way.
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Re: Plenum volume?

Post by gunt »

, i have being asking this , i see a rule i=of 2.5time the engine cc , but this i only see when refered to boosted engines , but none of the inlets look 5lit on a 2lit engine , but [ still not i cannot get an answer to the second question ] maybe its at the intended boost pressue as , this would compress to smaller space , this is not just a drag race ting either
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Re: Plenum volume?

Post by af2 »

Funny thing is the fact I like the Eldelbrok X ram that has a huge plenum X2..
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Re: Plenum volume?

Post by HQM383 »

Maybe this from Darrin Morgan can give a little insight

Darrin Morgan on plenum volume

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.
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Re: Plenum volume?

Post by HQM383 »

More from Darin here too

Plenum volume is interrelated to so many other variables such as, Engine size and rpm. Engines indented use such as Drag Racing, oval track, truck tractor pulling, boat racing, off shore boat racing and does it have an automatic transmission or five speed. These are all questions that need to be asked before deciding on how large the carbs and plenum should be. Its also the questions I ask when designing the manifold itself. If your asking for a magic number or formulas for any given engine, it does not exist. Its one of the last tuning factors we explore on the dyno. Its also very easy to get the plenum to large when chasing power numbers on the dyno! You can get a plenum way to big if you just look at power numbers on the dyno. Then, when you take it to the track the engine wont recover on the shifts and wont accelerate very well. An AED competition eliminator engine is good example. They have a sheet metal single four barrel manifold. On the dyno you can just keep stacking vertical spacers under the carb and the engine will continue to make more power. I have seen 7 one inch spacers make a lot of power on the dyno but the engine would lay on the converter and not recover. The farther you get the carbs from the runners, the more the signal suffers. There is a magic spot on every manifold as far as carb height goes. The track has the final word on that.

Links to this and previous quote
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viewtopic.php?t=1142
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Re: Plenum volume?

Post by panic »

Grumpy, 50 years ago (paraphrased):
"More plenum volume will continue to add dyno power, but the car won't launch. Deduct 10% volume and try it out for best ET (rather than best power)."

Very relevant: how many cylinders? A 3 liter L4 vs. V8 will be very different.
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Re: Plenum volume?

Post by travis »

I guess I'm having a hard time wrapping my head around making more power with more plenum volume, but the engine won't accelerate even as the power goes up. Does it have something to do with trying to move a bigger column of air from a lower engine speed, such as against a convertor or when the rpms drop on the upshift? Like it effectively narrows the useable power range?
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Re: Plenum volume?

Post by hoffman900 »

travis wrote: Mon May 22, 2023 1:27 pm I guess I'm having a hard time wrapping my head around making more power with more plenum volume, but the engine won't accelerate even as the power goes up. Does it have something to do with trying to move a bigger column of air from a lower engine speed, such as against a convertor or when the rpms drop on the upshift? Like it effectively narrows the useable power range?
I suspect sweep rate and the transient effects have an effect.

Sweep it slow enough and the carb has time to respond to changes. Most engines don’t accelerated 300 or 600rpm/s in real life (can be much higher). Also think about what happens at shift recovery, the entire pressure waves in the engine reset in an instant - carbs respond to pressure waves.
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Re: Plenum volume?

Post by Tom68 »

Sounds like too much plenum is just that, similar to too much throttle opening at too low a rpm.
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Re: Plenum volume?

Post by Walter R. Malik »

A really big plenum can show atmosphere to the ports.
The right size plenum is actually a good pulse and signal tuning tool.
An even a smaller one can be a secondary sound wave tuning chamber at usually the cost of some airflow.

Determining what is best for the application takes a lot of testing and testing as no set rules will apply.
There are some formulas to get it closer to one of the three.
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Re: Plenum volume?

Post by SchmidtMotorWorks »

My experience at Edelbrock with a dry tunnel ram manifold with forward facing throttle body was that plenum shape made much more difference than plenum volume.
A plenum that looks like a big sausage performs much better than a "cool" looking shape with sharper corners.
Intrusions or extrusions ruined it too.

You might wonder why the sharper cornered plenum was tried.
I had a great performing manifold, marketing said "no one will buy that".
They had an artist design a "cool" looking plenum, they even went straight to foundry tooling with it (no prototype or testing).
It turned out to be a dog, they were stuck between a cool looking dog and a sausage they couldn't sell.
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Re: Plenum volume?

Post by In-Tech »

Hi Jon,
Tough question but can you provide or PM a drawing or pic of what "works" and what is not so good?
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Re: Plenum volume?

Post by HQM383 »

SchmidtMotorWorks wrote: Mon May 22, 2023 9:00 pm My experience at Edelbrock with a dry tunnel ram manifold with forward facing throttle body was that plenum shape made much more difference than plenum volume.
A plenum that looks like a big sausage performs much better than a "cool" looking shape with sharper corners.
Intrusions or extrusions ruined it too.

You might wonder why the sharper cornered plenum was tried.
I had a great performing manifold, marketing said "no one will buy that".
They had an artist design a "cool" looking plenum, they even went straight to foundry tooling with it (no prototype or testing).
It turned out to be a dog, they were stuck between a cool looking dog and a sausage they couldn't sell.
Having a hard time picturing what was meant by sausage in the context of manifold plenum. Do you have an image, even hand drawn?

(Edit. Posted the above before reading In-Tech post).
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
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