Plenum vs trumpets for ITBs

[WP0ZZZ]

Has anyone ever posted a comparison of power/torque curves of an engine using ITBs with a plenum vs ITBs with trumpets?

Unless I'm mistaken, conventional wisdom says that a plenum helps in the mid-range, but by how much?

[winders]

I would love to see A to B overlays...especially on high output high RPM engines. I am sure the torque increases in the midrange are real....I want to see what happens on the top end.

[pampadori]

Do the GT3 plenums that are popular right now include velocity stacks on the runners on the inside? Never seen one up close. How about 3.2 and 3.6 manifolds for that matter?

[winders]

No, the GT3 intake manifolds do not have velocity stacks on the inside. The only "stack" is what you see in the external view. The same is true for the 3.2L and 3.6L air-cooled engine intake manifolds.

[IAMDJGO]

I too looked into this recently while trying to decide which way to go on an induction system for a recent street/canyon engine rebuild. The post below put things in perspective for me in addition to several additional posts.

GT3 crank on 3.2/3.4

Rsscotty, Rothsport, Steve Weiner, jamesjedi were names that appeared frequently as well that gave me a better understanding of expected results. Dyno charts appeared in many of these posts as well as recommendations for plenum and or ITB sizes based on displacement.

[Jeff Higgins]

You guys are confusing a "plenum" with an "air filter housing". By their very nature, ITB's cannot use a "plenum". A "plenum" sits behind an individual (or sometimes multiple) throttle body and acts as a common chamber into which air passes from the throttle body, to be subsequently distributed down individual runners to each cylinder. ITB's, by definition, obviously do not do this. One throttle body for each individual cylinder.

Plenum size does have a profound affect upon where a motor makes its power. Larger plenums produce power lower down, smaller - or no - plenums produce power farther up. Swapping from trumpets on top of ITB's to a common air filter housing (whether it actually has a filter in it or not) absolutely does not have this same affect. It may affect how much air it will flow, but it will not change the character of the power band as a true plenum chamber will.

[WP0ZZZ]

Originally Posted by IAMDJGO

The post below put things in perspective for me in addition to several additional posts.

GT3 crank on 3.2/3.4
.

This was an interesting read in its own right. I'm not quite sure what conclusions to extract from the plenum vs velocity stacks comparison though. It looks as if several engine components were changed at the same time.

[WP0ZZZ]

Originally Posted by Jeff Higgins

A "plenum" sits behind an individual (or sometimes multiple) throttle body

I don't think that's the definition most experts are using these days. You can see, for example, that both Rothsport and Rasant refer to plenums when talking about the resonance chambers before ITBs:

https://rothsport.com/Products/Engine/Fuel-Injection-Systems/FI-Systems-Low-Butterfly.htm

https://rasantproducts.com/products/rasant-products-intake-system-is-6-individual-throttle-bodies-gt3

[Jeff Higgins]

Originally Posted by WP0ZZZ

I don't think that's the definition most experts are using these days. You can see, for example, that both Rothsport and Rasant refer to plenums when talking about the resonance chambers before ITBs:

https://rothsport.com/Products/Engine/Fuel-Injection-Systems/FI-Systems-Low-Butterfly.htm

https://rasantproducts.com/products/rasant-products-intake-system-is-6-individual-throttle-bodies-gt3

Yes, I have seen that. Perhaps they should assign these a different name, rather than reuse a decades old, traditional term that applies to something else entirely. Maybe something like, um... "resonance chamber"...

In all seriousness, though, there is no shortage of older terms being miss applied these days. All it does is lead to confusion. A common plenum intake system is a far different thing than an ITB system with a big air chamber over the top of it. They behave far differently from one another. More precise terminology would lead to a bit less confusion. Maybe that's just me being picky, though...

[Bill Verburg]

Originally Posted by Jeff Higgins

You guys are confusing a "plenum" with an "air filter housing". By their very nature, ITB's cannot use a "plenum". A "plenum" sits behind an individual (or sometimes multiple) throttle body and acts as a common chamber into which air passes from the throttle body, to be subsequently distributed down individual runners to each cylinder. ITB's, by definition, obviously do not do this. One throttle body for each individual cylinder.

Plenum size does have a profound affect upon where a motor makes its power. Larger plenums produce power lower down, smaller - or no - plenums produce power farther up. Swapping from trumpets on top of ITB's to a common air filter housing (whether it actually has a filter in it or not) absolutely does not have this same affect. It may affect how much air it will flow, but it will not change the character of the power band as a true plenum chamber will.

Not true at all
since the 3.2s all 911s have used a plenum, most w/ variable geometry, the air filter box is a separate upstream assembly

on the variable geometry plenums there are several different strategies employed
from a single valve 2 stage up to a dual valve 4 stage


993 single valve 2 stage w/ the added variable of varying trumpet lengths, used w/ MAF air meter


late 964 single valve 2 stage, also used on early nvr 993, 964 use barn door air measuring w/ single throttle body, 993 hot wire MAF


964/993RSR single valve 2 stage, used w/ ITBs


997 RSR non variable used w/ ITB's, sometimes uses air filter box sometimes not





GT3 street 4stage used w/ a single throttle body



A buddy's 993, Gamroth built 4.0 w/ GT3 plenum and ITB's

[Bill Verburg]

The only dyno I have showing the effect of a non functioning resonance valve on a 964

[TimT]

Not true at all

Agree...

These are some crappy pictures, but they are of a Porsche Motorsports plenum from one of the Racers Groups Le Mans GT3s from '03? (ancient history)

It has some sort of conical thing where the air enters, and a vacuum (ecu?) operated flapper.. all before the ITBs

[Jeff Higgins]

Sorry Bill, but it's pretty clear you didn't understand a word I said.

Again, in the traditional use of the term "plenum" in an induction system, air flow into the "plenum" is controlled by an upstream throttle body or bodies. The "plenum" serves to distribute that common air supply to the individual intake runners. Your photo of the 964 system is the classic example of this.

In this newer use of the term "plenum", nothing is controlling airflow into this chamber - it is breathing freely through its inlet side, with no butterfly, slide, or anything else controlling that airflow. Control of the airflow happens after the "plenum" has distributed the airflow to the individual intake runners, via individual butterflies, slides, or whatever over each intake port. This is a fundamental difference in design, operation, and affect. The Gamroth system you show is the classic example of this.

With the respective functions, and affects, of these two vastly different "plenum" systems being entirely different from one another, my contention is that the newer system should have been named differently, to help differentiate those differences in design and functionality. Doing so would help to avoid confusion in discussions such as these.

[IAMDJGO]

Originally Posted by WP0ZZZ

This was an interesting read in its own right. I'm not quite sure what conclusions to extract from the plenum vs velocity stacks comparison though. It looks as if several engine components were changed at the same time.

Originally Posted by sp_cs

Guess which curves are for which setup (thin or thick lines?

Both 993 and velocity stacks were dyno’d with the 11.65:1 pistons, twin plug and 42mm Eisenmanns.

This was the dyno graph (same engine after modifications, different induction systems indicated by dashed and solid lines) that was telling to me

[winders]

Technically, the GT3 intake manifolds that people call "plenums" are not the plenums. They are intake manifolds. The plenum is the aluminum piece to which the throttle body is attached.

Here is a photo of the plenum next to the throttle body:

[winders]

Originally Posted by IAMDJGO

This was the dyno graph (same engine after modifications, different induction systems indicated by dashed and solid lines) that was telling to me

All that tells me is the 993 intake manifolds are too small for that engine. I would really like to see that engine using a set of 996.2 GT3 intake manifolds instead.

[WP0ZZZ]

Originally Posted by Jeff Higgins

With the respective functions, and affects, of these two vastly different "plenum" systems being entirely different from one another, my contention is that the newer system should have been named differently, to help differentiate those differences in design and functionality. Doing so would help to avoid confusion in discussions such as these.

I don't really see why you would like them to be called differently. The crucial air resonance effects are there regardless of the location of the throttle butterflies. Placing throttle valves close to the cylinder heads does not fundamentally eliminate resonances in the plenum.

Originally Posted by IAMDJGO

This was the dyno graph (same engine after modifications, different induction systems indicated by dashed and solid lines) that was telling to me

They are clearly different but what I meant is that I think it's not a clean comparison because other engine components were changed besides the plenum/trumpets.

[WP0ZZZ]

Originally Posted by Bill Verburg

The only dyno I have showing the effect of a non functioning resonance valve on a 964

It's interesting that at very high rpm, closing the flap would be better.

[skinnerd]

I may be wrong....but I don't think so.
Every time I've been to Rothsport's shop looking over there ITB setups, they invariably tell me that the 964 or GT3 air filter boxes (or misnamed plenums), that sit on top of the ITB's, add a lot of mid range torque that is absent if you go with ITB's alone.
They have countless dyno plots that show this....it's not just a theory.

[WP0ZZZ]

Originally Posted by skinnerd

They have countless dyno plots that show this....it's not just a theory.

I would love to see those plots