Intake Plenums, does size matter

[BrianRA23]

1.5 x engine capacity sounds familiar. It makes sure there is enough air in there for all the cylinders. Otherwise one cylinder might starve since the previous one just took all the air.

Great care is taken to make sure all injectors flow the same amount and it is then assumed that all cylinders get the same amount of air. As stated above, changes in the design have reaped great results. I would remove the manifold and put a fan on the inlet (or a large volume of water for a less acurate but more visual understanding) and measuring the amount of air (or water) going out through each port. At the MAF you are looking at surely the standard manifold will show inconsistency between 1 and 6. One end will start to flow less. To fix the problem the throttle body will need to be moved out/up and away from cylinder no 1. In doing this you may well end up with a nice big shiny plenum.

Unless you are interested in off boost response I would not get too excited about runner lengths and bellmouths.

I have seen a chart showing the mixtures and amount of air for each cylinder on an old V8 across the rev range. At certain rpm points you can seen that certain cylinders would be lucky to contribute 3% to the total output of the motor. I have seen a 308 with a custom inlet manifold (100s on man hours worth) on LPG, with cooling system mods so that the mixtures and engine temperatures were consistent. It was powerful and very smooth. After an impressive run he sat a 50c peice on edge on the top of the engine parallel with the crank, then reved it up, let it drop back to idle and 5 minutes later it hadn't moved - that was a well tuned engine.

If the aeorspace guys are doing it on turbo engine design that would be new but for na engines it will just be pressure pulses bouncing of the end of the runners and utiilising the harmonics. Longer is better, reduced diameter gives better velocity etc

[SLO-055]

a theory would be that INCREASING plenum size wouldn't help performance unless the engine is able to ingest more than the OE plenum can hold, but does this theory hold up with forced induction, the plenums job doesn't change, it's still acting as a reservior (or surge protection device) but i can't see how the engine would have a surge problem like that in FI form, wouldn't it be more likely to see a drop in manifold pressure (i.e. drop from 1.4 bar to .8??) if the engines abilty to ingest air exceeds that of the plenums holding volume???. possibly a larger throttle area (tb) and big plenum will give a better result at lower 'boosted' rpm but after then wouldn't it come down to manifold pressure and flow to dictate how much power will be made. all this is just theory, one can only wonder and, in my case, confuse yourself and get a big headache in the process.
in your case 1JZ.747 your equally as fast as some with 150hp more than you, showing your running system is quite effecient right now, but you outline the jump to larger capacity an increase the VE of the old girl in the future, i'd just run with it, test your new setup then ask the question of whether the redesiging the manifold is worth it.
*my apologies if this post makes no sense*

[EldarO]

simple, the bigger it is, the more ait it will flow.

of course, thats the short, crude, and shockingly simple interpretation of it.

of course youve got to deal with flow characteristics etc. etc.

obviously, aswell, going too big, like having intercooler piping etc. too big, would be pressure drop.

Id imagine though, that, it would get to a point where youd reach the "maximum" effecive size, where it wont matter how much the plenum itself can flow, but, rather the intake ports etc.

Eldar.O.

[AndyTTR]

I'll get onto the aero boys (and girl) next time i see them.

i've noticed that a lot of turbo engines have much smaller plenums than the equivalent turbo models (ie 3SGE/GTE, 2JZGE/GTE). Perhaps when the intake air is pressurised and being forced into the engine having a larger 'reservoir' in the plenum isn't as important. Having a large plenum in addition to an intercooler would increase the turbo lag effect wouldn't it?

[Big rob]

Think about the total volume of air is in your intake plenum and intercooler and piping etc. THen think about how long it would take for your engine to ingest all that air. Correct me if i am thinking about this wrong but say your at 1000rpm, on a 3lt thats 3* (1000/4)= 750 litres of air every minute = 12.5 litres of air every second. So when your gunning it (generally when turbo lag is a factor) it revs are usually much more than 1000rpms. So lauching it at 4000rpm is 50lt every second. Evacuating the air in the piping and such really isnt going to contribute to too much lag really.

Anyway back to the topic

[oldcorollas]

simple, the bigger it is, the more ait it will flow.
of course, thats the short, crude, and shockingly simple interpretation of it.
of course youve got to deal with flow characteristics etc. etc.
obviously, aswell, going too big, like having intercooler piping etc. too big, would be pressure drop.
Id imagine though, that, it would get to a point where youd reach the "maximum" effecive size, where it wont matter how much the plenum itself can flow, but, rather the intake ports etc.
Eldar.O.

oh dear. bigger plenum DOES NOT mean more flow into the engine from the plenum. look at the TRD 3SGTE for the JGTC (photos here somewhere) there is almost no plenum...

bigger intercooler and piping means LESS pressure drop. less restriction = less pressure drop.

it is almost ALWAYS how much the ports and engine can ingest, as opposed to what the plenum can flow freely.... i would suggest that the volume of airflow through a plenum is the very least of the issues here.

it is EVERYTHING about providing the best volume, pressure, and velocity to each port as it needs it. i thought that is what was being discussed? not the outright airflow capability of a big box?

[ed]

what ^^^^ he said

ports/runners determine the airfow of the engine - the plenum should be designed to guide incomming air equally to the runners.

air box:
- provides air equally to all runners
- provides air flow with minimal turbulance artefact
- tries to dampen pulsations/harmonics

its not about how much a plenum can 'flow' at all

cheers
ed

[YelloRolla]

what ^^^^ he said

and again

- tries to dampen pulsations/harmonics

Going bigger acts like a big shock absorber and better reduces the "pulsations". According to my engine tuner it is quite possible to expand the power curve by having a larger plenum.

It is definitely possible to make a lot of power through a well designed small plenum (efficient runner to throttle placement), but as pointed out earlier, if you have a big throttle (or if the throttle was just after the turbo), the whole intercooler and piping literally becomes part of the plenum.

One theory told to me that I like (because it is simple) is to make the plenum at least equal to the volume of the runners that it is feeding.

hope that helps (it is past my bed time)..

[BrianRA23]

And how do you determine the volume in the runners - another simple rule, you make it at least equal to the volume of the cylinders. This is intake design for na engines, it is based on one full unit being sucked into the cylinder and having another move in behind it hopefully with enough momentum to help push itself in the next time the valves open.

But I think we are of the opinion that manifold design for na is not directly transferable to turbo engines (unless worried about off boost response).

[ed]

And how do you determine the volume in the runners - another simple rule

ok - thats news to me...

figuring out runner diemnsion has, to my knowledge, nothing but co-incidental relationship to cyl volume. that model of intake harmonic resonance is grossly incorrect/incomplete

[1JZ.747]

hmmm

some very interesting stuff here lads.


im paying more attention to those with more thn 15 rep points.

what Jase says would appear to make sense.

i have always be puzzled by the fact the japs seem to keep the factory plenum where as here in australia most workshops want to changes it first.

[takai]

And how do you determine the volume in the runners - another simple rule, you make it at least equal to the volume of the cylinders.

That relationship is completely coincidental, and usually completely wrong. From memory the Honda GSXR and Kawasaki Ninja engines actually have the best flow when the plenum is smaller than the swept volume of the engine.
A lot of that is due to the compactness of the head, and the fact that the port spacing is close together, with narrow dia runners.

Most important is the reflection off the walls of the box causing turbulence, although as some adept people have noted that is more important for NA applications.
For FI applications, where the limitation isnt so much how much the box can flow (as its all pressurised) a whole different set of flow dynamics comes into play.

I came across an interesting article last night about maximum flow trumpet and plenum floor design the other day, and i have it in PDF at home. Ill upload it to my webspace soon.

EDIT: Some more good theoretical physics to look at is Helmholtz resonance, as the elasticity of air pulsations has a lot to play with design (especially with reflections off the "side" (opposite runners) wall.

[oldcorollas]

although only 4cyl...

2L
470ps @ 6000
650Nm @ 4500
buggerall plenum... how transferrable this example is??
http://www.materials.unsw.edu.au/~sf...lenum1-sml.jpg
http://toymods.net/forums/showthread...ht=amlux+3sgte

[takai]

The rest of those pics are hosted here:
http://gallery2.plebeians.net/v/cars/engines/trd3sgte/
Thanks to the Japanese mechanical doctor again

[ReQuieM]

Above 15 rep? I never get rep points. boo hoo