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Thoughts on intake plenum size?

Old 10-17-2018, 09:32 PM
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Default Thoughts on intake plenum size?

So I saw a post where optimal Plenum size is 1.5x displacement. 2.0 for high end set ups. So on a 428, that is 14 liters of intake manifold.

So if that is a legit number, that helps explain why ls7 stock manifold is so poor.

But also, does this get to a place where ITB just makes sense because the earth is your plenum and you really cannot make a sheet metal intake large enough?

Or is that 1.5x - 2.0x displacement BS?

Or to ask it differently - is this part of the reason changing runner lengths shifts power around but rarely just raises the curve unless plenum volume increases, such as msd or fast vs stock?
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Old 10-17-2018, 11:04 PM
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As far as runner length goes even with individual runners, do you remember the Hilborn-injected '60's funny cars and altereds with those TALL stacks? Had to be a reason for them...
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Old 10-18-2018, 01:03 AM
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i've seen posts saying using a certain multiple of single-cylinder displacement (3? 6? i don't recall), or 80% of total displacement for n/a engines, etc etc.

if it's not a significant restriction at high rpm, it isn't interfering with the helmholtz effect of the runners, provides even distribution to all cylinders, and isn't slowing air velocity to the point that the engine has poor response, the plenum is doing it's job. and it's always a compromise for fitment and overall engine performance.

as far as runner length, IIRC, it's usually tuned to a harmonic frequency, not the actual fundamental frequency that matches the RPM where you want the pressure wave "boost." otherwise, you'd have stupid sized intake runners.

For intake manifolds that are mass produced, even performance manifolds, its a "few sizes fit most" sort of deal. swapping out to different different length runners without really spending the time to really match stuff to valve events and take in consideration the entire combo is kind of silly, if you ask me. there's more to that than a lack of plenum volume as a reason for it not making more power--the engine needs to be able to utilize that type of runner design. if your cam and heads aren't designed to for making very high rpm power, your setup won't like short runners.
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Old 10-18-2018, 01:16 AM
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Originally Posted by Darth_V8r View Post
Or to ask it differently - is this part of the reason changing runner lengths shifts power around but rarely just raises the curve unless plenum volume increases, such as msd or fast vs stock?
there are a lot of factors to consider, not just plenum size or runner length. not only runner length but their shape plays a significant role, as well as the shape of the plenum. it determines how air flows inside the intake manifold and is distributed to the various cylinders. Valve events and head flow, exhaust design, many other things play a role. it's a concert of physical effects.

i mean you could drill a bunch of holes and plastic weld 2L pop bottles on top of your manifold to increase your plenum size, i doubt it would get you much.
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Old 10-18-2018, 08:23 AM
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ITB is very regarded

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Old 10-18-2018, 10:48 AM
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Great article here that covers the question at hand very well.

https://www.enginelabs.com/engine-te...ake-manifolds/
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Old 10-18-2018, 01:55 PM
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Cool posts. And great article.

I do know that most runner lengths are designed for a third or fourth order harmonic pulse. The crazy long runner commend is correct. To catch that primary pulse, which would be high pressure, the runner length would be close to the header primary length.

I like to consider both where a cam wants to peak AND the runner length that peaks in the same place to try to match them up. I first ran into that on cathedral fast vs cathedral msd on my old set up. Msd was a better match for the cam so it ran smoother.

But you compare almost any "short" runner manifold to the stock ls7 they all make more torque and power. They raise the whole curve. Which is where I think if you keep the overall intake the same size and shorten the runners you gain plenum (?).

Using your coke bottle analogy, on a 7-liter engine, that is 5-7 two liter bottles. Like you said it will not fit without a stupid hood. In the past I referenced the map. If map was falling off, plenum or throttle are a restriction. If map not falling off, induction is fine (?).
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Old 10-18-2018, 04:42 PM
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That is because the ls7 intake is already a short runner intake Much shorter than any of the other stock LS intakes. I believe the runner length is 7", but it has been awhile since I was able to find that information posted somewhere.
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Old 10-18-2018, 05:32 PM
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Way back the rule of thumb for the starting point on velocity stacks+air boxes was the stack needed to have the same volume as the cylinder it fed, and if you had a upper plenum box it had to be 50% the CI of the engine..
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Old 10-18-2018, 07:33 PM
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Originally Posted by Darth_V8r View Post
Using your coke bottle analogy, on a 7-liter engine, that is 5-7 two liter bottles. Like you said it will not fit without a stupid hood. In the past I referenced the map. If map was falling off, plenum or throttle are a restriction. If map not falling off, induction is fine (?).
yeah, but air would have to be drawn in and out of the tiny little 2 liter bottle necks, which would make them all fairly useless as a reservoir for air at high rpm. Hence the reason for my analogy, lol.

honestly, in lieu of fancy math, i would imagine some dyno time with intake manifold designs is the way to go. You'd really have to design a whole intake, though.

just for example, let's say you have a 14 liter plenum (yowie). You'd need to fill it when you went off idle to WOT. Unless UGOTDATBEWST, air will be forced in via (atmospheric pressure - manifold pressure). It ain't just sucked in there bu magical engine suxxing force. It's also traveling through a comparitively smaller intake tract and throttle body.

a small plenum will fill fast, a mega-bul00ga-whale because-racercar plenum, not so fast.

then you have to make sure the shape of the plenum and doesn't effect anything adversely, such as one runner getting more pressure than the other 7.

there is a good compromise somewhere, and like i said before, a lot of other factors that matter more. And, honestly, at what point do you start getting minimal gains from increasing plenum volume?
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Old 10-20-2018, 02:39 PM
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​​​​​​Darth, this Gen 3 info might be of interest on plenum size.

GregGo on Performance Truck forum posted this plenum volume information.

https://www.performancetrucks.net/fo...s-truck-74122/

1997-2000 LS1: #12556333
-5.06 liter Plenum vol
-262mm runner
-.536 liters runner vol

2001+ LS6: # 12573572
-5.19 liters Plenum vol
-262mm runner
-.541 liters runner vol

2002 Truck (LR4,LM7,LQ4 # 12574009)
-4.0 liters Plenum vol
-263mm runner
-.513 liter runner vol

I haven't seen info for other intakes.

Given that the plenum volume & runner volume of the LS1 & LS6 are so similar, my guess is the difference in power mostly comes shape instead of volume, with the LS6's flat manifold floor being key to making more power.

The LS1 & LS6 plenum volume appears to be about 89 percent of stock displacement for the LS1 intake and about 92 percent of stock displacement for the LS6 intake.

​Resonance might so be a factor as as much as shape and volume.

​​​​​​
​​​​I wonder what the plenum volume is on a Super Vic (smaller than 5 liters?) or Fast LSX 92mm ( bigger than the factory intakes?) or LSXR (bigger still?).

One could seal off the intake runners and use a burette to measure how much it took to fill up the intake and runners of other popular intakes. Then one could use the burette to measure the runner volume for one side of the intake. Take that total volume for the four runners and double it. Subtract that number from the total volume of the intake and runners to get a fairly good estimate.
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Old 10-21-2018, 12:31 AM
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Originally Posted by 99 Black Bird T/A View Post
Resonance might so be a factor as as much as shape and volume.
i have wondered about this, and wondered if the pillars in the stock LS6 and later intakes was an attempt to cancel out lower frequency resonances inside the plenum that would interfere with the tuning of the runners.

i had read that they were there to control resonance some time ago, but wasn't sure what detriment that resonance caused. i had imagined at one time it messed with MAF readings because the resonance caused some reversion past the MAF sensor, but after some discussion on my other forums and some thought, concluded that wasn't the case. NVH control had also occurred to me as a possible reason, but maybe GM engineers were more concerned about power (for once).


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Old 11-26-2018, 02:35 PM
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Here is some plenum data I collected previously for modeling with Engine Anaylyzer Pro.

The LS1 has a plenum volume of 5.06 liters, runner length of 262mm and runner volume of .536 liters.
The LS6 has a plenum volume of 5.19 liters, runner length of 262mm and runner volume of .541 liters
The Truck intake has a plenum volume of 4.0 liters, runner length of 263mm and runner volume of .513 liters
* This is from Chevy LS1/LS6 Performance by Chris Endres.

Fast 92 25% increased plenum volume for less intake charge turbulence

The L76 manifold has a runner length of 12.5"

[Fast 102 LSXr] Port volume is about 830cc vs 600cc stock [L76]. Flow is about 10-12cfm over stock but port length is longer by about 1 3/4" due to the wrap-around, bell-mouth design. Just ruff data....

I got the answer from Comp, the [LSXr 102] plenum itself is 5.1 liters (5100cc) versus 3.7 (3700cc, 225.79ci) liters stock.
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Old 11-27-2018, 09:39 PM
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I've already forgot most of the detail stuff I learned when designing my cross ram dual plenum intake, which I never completed. Got technical info from one of GM's LS7 designers, which called it something to the effect of a penny pincher restriction. I forget how it's figured what's being called a liter here, but the performance design intake is listed to have 7.2 liters of plenum volume per side.
http://www.performancedesign.com/carbonxr
Plenum volume is the biggest thing lacking in most intakes, but plenty of other factors such as uneven distribution, which the taper of dual plenum designs address.
Never did really even try to wrap my head around all this 3rd & 4th harmonics & frequency type of stuff. Really didn't care either. With the intake I designed, things were in close range of what was known to be optimum & no doubt would work a crapload better than my embarrassing ls1 intake I still have in use. It's biggest fault is what racers call D.N.F.
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Old 11-27-2018, 09:59 PM
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DNF is a common issue with fourth gen F body
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Old 11-29-2018, 07:08 PM
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Grumpy Jenkins said.....Its going to be different for every engine. You have to tailor them in till you find it. I would think that would mean adjusting them to the track and weather conditions as well. I would think a custom adjustable set could be made so while at the track you go to and the DA you want to race in you could find the perfect length. And that's what she said!
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Old 11-29-2018, 07:34 PM
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Just as a comparative,, when Mazda started using active intake runners in the IMSA efforts,,
it got banned quickly,, was "too effective". Of course the rotary's got beat up every rules meeting for years..
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Old 11-29-2018, 11:03 PM
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I am surprised we dont see more active runner stuff. Seems coupled with vvt, you would have a very effective means of stretching the powerband and fuel economy
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Old 11-30-2018, 09:54 AM
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I experimented with active runners back in the 4-cylinder days.
I basically put a plenum w/ large throttle body on some ITBs that were way too large for the engine.
Rigged them up sort of like carburetor secondaries. They didn't start to open after quarter throttle or so, and reached full throttle at the same time as the throttle body.
The ITB blades had holes drilled in them that still made it possible to cruise around with very little throttle.
I ended up gaining back most of the low end that was lost running ITBS that were made out of 4 stock throttle bodies for that car.

Did the same exact thing for the header. I put throttle blades in the header. The blades were smaller than the primary tube size.
They had something like 1/8" clearance around the blades and they opened with the throttle cable.

Tuning was a catastrophic PITA. Was one of my first dives into megasquirt. Once I got most of it hammered out, butt dyno felt massive mid range and cruising gains over the plain ITB/header combo.
Btw, I'd recommend not to use junkyard throttle body blades in your header tubes. They didn't last very long before the started to look like burned exhaust valves.
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Old 11-30-2018, 12:19 PM
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IN my head, I've "designed" intake runners stacked like a telescope, if that makes any sense, with actuators to extend and retract the velocity mouth, making the runner longer and shorter with RPM. My problem is I lack programming talent. But even a simplistic model like 13" runner lengths at idle and 6.5" runners at 7000 rpm and a linear interpolation. Numbers made up for the purpose of describing what's in my head. But combine that with variable intake valve close, and I think you've got something.

Someone probably already does this that I'm not aware of or tried and it was too expensive
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