Plenum volume?

[hoffman900]

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.

And shape matters too. More than just just volume.

[SchmidtMotorWorks]

I didn't take many pictures, but this was my dyno cart.
I usually printed at least 1 manifold a week.

The one labeled A has the base that had the runners in it, there were about 8 variations of the base.
Many more of the plenums, I didn't store stuff there, once it was not scheduled for anymore testing it went in the dumpster to prevent concerns about my budget.
One similar to B made the most power, it was different in that it was thicker in height and was fully rounded.

I don't have any images of the one marketing had cast, the idea of it disgusted me, it looked like a Transformer toy.
As far as I know, those of us there at that time experienced peak Edelbrock, that era is in the past.

dynocart.jpg

[SchmidtMotorWorks]

Funny thing is the fact I like the Eldelbrok X ram that has a huge plenum X2..

That manifold had so many problems in the beginning, it was almost abandoned.
It is a tribute to the guy developing it that he had enough persistence and ingenuity to make it work.
Look at the runners carefully, there are cross-overs.
The path to that design was long and expensive.

If you just make something that looks similar to that, expect frustration.

xram.jpg

[Tom68]

Funny thing is the fact I like the Eldelbrok X ram that has a huge plenum X2..

That manifold had so many problems in the beginning, it was almost abandoned.
It is a tribute to the guy developing it that he had enough persistence and ingenuity to make it work.
Look at the runners carefully, there are cross-overs.
The path to that design was long and expensive.

If you just make something that looks similar to that, expect frustration.


xram.jpg

Thanks for pointing out the cross-overs, without them it would need a flat plane crank to be any good.

[HQM383]

I didn't take many pictures, but this was my dyno cart.
I usually printed at least 1 manifold a week.

The one labeled A has the base that had the runners in it, there were about 8 variations of the base.
Many more of the plenums, I didn't store stuff there, once it was not scheduled for anymore testing it went in the dumpster to prevent concerns about my budget.
One similar to B made the most power, it was different in that it was thicker in height and was fully rounded.

I don't have any images of the one marketing had cast, the idea of it disgusted me, it looked like a Transformer toy.
As far as I know, those of us there at that time experienced peak Edelbrock, that era is in the past.

dynocart.jpg

Sausage comparison makes sense now in the context of fi. OP was talking carb signal so I had traditional single central carb manifold in mind. Does/did the problem of looking good v best in situ design exists with those carb manifolds too?

I think what goes on in the plenum of a running carb engine and the size and shapes required for desired operating range is a mystery for most of us. Eric W’s test of manifolds on a 406ci sbc and the old uncomplicated Vic Jr coming out on top demonstrates that. Yes, different engine combo may yield different results but the fact remains on that combo the Vic Jr bettered a swathe of manifolds with decades more technological development. If we truly knew and understood by now what was going on in a carbed manifold plenum and what a V8 engine wanted would we not be seeing the most up to date manifolds toward the top of the hp/tq output list on such a test? Is it now and always will be a crap shoot with carb manifolds plenum?

[barnym17]

Your point is very valid on that PARTICULAR engine it was the best now change cam,carb,exhaust who knows?

[Tom68]

Is it now and always will be a crap shoot with carb manifolds plenum?

Nah, just gotta sort the wheat from the chaff.

The beauty of getting the right manifold for your combo is, if you are rock solid set on the long motor combination you can at least just throw some best guess manifolds at it and pick what suits.

Nobody is going to build a serious race motor without port matching so the one thing we can learn from EWs tests is keep the taper as constant as possible, don't go flaring up your 195cc or whatever ports to a 1206 if you don't have offset pushrods and potential port pinch widths to suit.

If your manifold's too big for your heads just bolt it on, tapering the heads port entry could kill it.

[HQM383]

Your point is very valid on that PARTICULAR engine it was the best now change cam,carb,exhaust who knows?

That’s right and backs up my point of the seemingly inexact science or black art of wet manifolds. I did say “different engine combo may yield different results”. Change one factor of the test mule engine and a different tested manifold may come out on top just as you say. The mystique would live on. Eric’s testing didn’t prove the venerable Vic Jr is ‘the’ best manifold for a 400 ci sbc as is at times misinterpreted, but displayed that the test engines bore and stroke combination along with its valve timing, cylinder head port csa and shape, valve size, discharge coefficient, combustion chamber, piston, comp ratio and ignition timing preferred whatever the architecture of the Victor Jr plenum and runner was creating in the airstream and how it was influencing the air/fuel mix and carburetor signal. When all is said and done it’s about maximizing air fuel density in the cylinder with a homogeneous air fuel mix that is in a state to promote the most effective complete combustion. The Vic Jr appeared to assist combustion the best in that regard and there is not a whole lot intuitive about that when stacked up against some of the other manifolds tested. What I have learned from his manifold test results, or at least has been reinforced is that it MUST be more than porting for flow and/or sizing manifold for ci and rpm target to achieve optimum results.

[HQM383]

Nah, just gotta sort the wheat from the chaff.

Not so easy for the average guy with a small budget and plethora of choices. The 300-110 is considered wheat by some but ended up in the chaff pile in Eric’s test.

The beauty of getting the right manifold for your combo is, if you are rock solid set on the long motor combination you can at least just throw some best guess manifolds at it and pick what suits.

Is this crap shoot territory?

Nobody is going to build a serious race motor without port matching so the one thing we can learn from EWs tests is keep the taper as constant as possible, don't go flaring up your 195cc or whatever ports to a 1206 if you don't have offset pushrods and potential port pinch widths to suit.

If your manifold's too big for your heads just bolt it on, tapering the heads port entry could kill it.

Good points. What I noticed in his testing was matching of manifold to head port had minimal impact. However if going to the effort of building a serious engine port matching should be par for the course.

[ChopperScott]

I linked this in another thread. For those that haven't read it, this covers sbc intake manifold testing done over the years on different combos...

https://garage.grumpysperformance.com/i ... d-test.58/

[hoffman900]

I think there are two, maybe three aspects to manifold that make it hard to define best:

1) port alignment. From a strict flow sense, the engine never sees the cylinder head and manifold as separate entities, a duct is a duct. See Jon / Chad’s CFD thread. To compensate for the runners bends, they might need 4 separate ports to maximize every cylinder. Taper plays a roll in flow losses as well
1*) adding this on because for carburetors, booster alignment plays a role. Ultimately the location of the carb and boosters dictates the alignment of everything. It’s a big short coming of carburetors in general.
2) wave tuning. Length plays a big role, on V8’s with a single carburetor, each port has a different length that are going to tune differently (maybe this requires different cylinder timing or camshaft events to maximize). There is also charge robbing happening due to the firing orders used on these engines.
3) the plenum itself. Bigger always makes more power, even in on restricted fuel injection engines, but drivability. You have to define the use. Transients matter, shift recovery matters, etc. I suspect if everyone had eddy-current dynos where you could run simulated laps, their packages would look a little different and some of these rules for thumb would be thrown out the window. Secondly, besides volume, shape matters, and how the shape of the plenum is can cause standing waves or other interactions between cylinders (in a good and / or bad way). Dynos also don’t simulate g forces and the maldistribution that comes with that.

I remember Warp saying here he likes to go back to a baseline manifold every 2-3 manifolds when testing this stuff, and that’s after using million dollar dyno cells where they carefully control oil and water temps, atmosphere, etc. His statement was manifold tests are hard to do to get repeatable data.

I guess ultimately my point is there is no best manifold. It’s all about the combination. The entire package matters. A manifold that may tune well at 6500rpm might will likely not at 8000rpm. A manifold’s interaction with the head ports is going to he different due to bend radii, etc. I don’t see how you can develop a port without considering the manifold it’s being used with (again, the engine never sees them as separate entities), plenum is going to depend on line of sight, how big an engine is, shape, and a million other things.

If steady state and low sweep dyno power is all that mattered, then just make it bigger, it’s really that simple. And this holds true for carburetor manifolds (wet plenum) or stuff like this: https://www.sae.org/publications/techni ... 8-01-3007/

[steve cowan]

I have ported several 23 deg intake manifolds,when you bolt them on a reasonable cylinder head on the bench and flow test each runner there was always 2-3 runners 30 - 70 cfm down.
Meaning a head that flows 290 cfm with a radius plate can be down to 220 cfm with intake on..
If you up the depression it is obvious the induction tract will struggle with the demands.
If you spend sometime with a pair of dividers and measure at the runner openings you will see what I mean.this is the joy of as cast.the plenum usually needs to be opened up to get the area at the runner opening as you are raising and shaping the roof/ transition and then tapering down the runner which is fun to measure.

[David Redszus]

Let's take a look at the effect of plenum size.
Assume sonic velocity is 323m/s, temp is 80F, intake pipes are 450mm x 46mm.

Plenum L..Plenum dia.....Vol............rpm
250mm....75mm..........0.0011m^3.....5972
275.........75..............0.0012..........5694
300.........75..............0.0013..........5420
325.........75..............0.0014..........5238

250.........85..............0.0014..........5270
250.........95..............0.0018..........4750
250........105..............0.0022..........4266

Notice that increasing plenum volume will reduce the tuning rpm point.
But changes in plenum diameter are more sensitive than changes in plenum length.

Most important of all. The tuning target rpm of the plenum MUST match the
target tuning rpm of the inlet pipes. The plenum acts like a resonator for the
pipe pulses and they must be in phase with the pipe tuned length in order to
amplify the pipe pressure wave.
.

[In-Tech]

Let's take a look at the effect of plenum size.
Assume sonic velocity is 323m/s, temp is 80F, intake pipes are 450mm x 46mm.

Plenum L..Plenum dia.....Vol............rpm
250mm....75mm..........0.0011m^3.....5972
275.........75..............0.0012..........5694
300.........75..............0.0013..........5420
325.........75..............0.0014..........5238

250.........85..............0.0014..........5270
250.........95..............0.0018..........4750
250........105..............0.0022..........4266

Notice that increasing plenum volume will reduce the tuning rpm point.
But changes in plenum diameter are more sensitive than changes in plenum length.

Most important of all. The tuning target rpm of the plenum MUST match the
target tuning rpm of the inlet pipes. The plenum acts like a resonator for the
pipe pulses and they must be in phase with the pipe tuned length in order to
amplify the pipe pressure wave.
.

Hiya David,
I do appreciate all your knowledge and respect your opinion.

What I think we all would like to know is a trend of knowledge. A particular plenum size per rpm is helpful but it has little to do with the "ticks" per cycle, and the theory of how the "ticks" of the cycle matter vs the amount of volume available. Wet flow is extremely critical in this type environment as you know.

We can stop with static flow. Dynamic flow is what hopefully "sims" will be able to predict better. CFD is the future, what a huge endeavor.

I think you will all agree, the "tuning point" will always be changeable to get a decent average. There is a reason why GM and Ford have coincided to the same 10 speed transmission.

[SchmidtMotorWorks]

If you have seen the animation of the fluid motion of the plenum in a tunnel-ram, you will know that the pulses are quite local to each port.
That is part of why shape matters.
You can add all you want to the end of a tunnel ram plenum, it means very little to the other end.
Even on a singe plane, it is far too complex to define by a volume dimension.
The way a pulse reflects off the walls of a box is very different than how it reflects in a sphere.

I think plenum volume is one of those things that became a dimension to measure and discuss, because it is an easy dimension to measure, more so than because it is the most important.
The frequently used formula for plenum resonance don't relate well to common plenum shapes.