Plenums.... why bother?

[RAd28]

something that's had me wondering for a while now... is why bother with a plenum at all??
if you have 6 runners, why not use something like an exhaust collector around the other way? 1-2-6, or 1-3-6?

is it a performance thing? plenum provides better results?
OR
is it a convenience thing? plenum provies simpler way to divide boost?

just picked up an old issue of ZOOM, #87 (SR20 powered KE20 on the cover) and there's a pair of old valliants in there, one has a turbo setup, with triple throttle bodies and no plenum as such...



now clearly this isn't the best example, 'cause each throttle body won't get the same flow, BUT if you were to use a 3-1 merge collector you could...

comments/ideas?

[Stu]

My opinion would be....

When the car is running on boost, then the plenum isn't really doing much, but under off boost situations, the plenum acts as a storage of air. So rather than the cylinder having to suck some of its air from the open end of the inlet tract, the plenum already as enough in it. This makes sense when you imagine all of the cylinders requiring about 500cc of air at a time (2L 4 cylinder). If each cylinder is trying to suck its required air though a single pipe, they'll never get enough. Imagine 4 people trying to drink a glass of milk through a single straw, they'll struggle and the strongest sucker will get the most at the expense of everyone else. If each person had their own straw, they would likely get the same amount. I hope that makes some sense......

Of course under a boosted situation, this is just about flawed.

I think....

[RAd28]

well that's a theory... but under off boost cruising, the plenum is only a few liters, it's still drawing air in through the throttle body, throught the intercooler, all the pipe work, turbo, and filter... so i really don't think it's gonna make any difference whether the plenum is there or not... according to your theory...

i think...

[nb86]

the physics is to get a volume big enough to damp the ocillations in the inlet tract and provide a sort of reserviour of air for the inlet. you need to damp the ocillations in the inlet runners so that the scavenging effect can operate properely, otherwise you just get all sorts of cross cancellation in the inlet.
this is all well proven with single throttles, but with individual throttles and inlet tracts it's totally different. as youve seen on NA engines, the ultimate inlet is individual matched, tuned length runners. i'm just spurting idea now (i've never seen it done) but maybe individual throttles/runners with some sort of resonating chamber before the butterflies would serve the dampening pourpose and allow the sort of setup you're talking about to work effectively.
it's a lot of work, but if someone gives it a go, i would love to see the results.

[RAd28]

what about all the pipe work between the turbo and the throttle body? that's a fair volume of air when you consider it...

[Stu]

But wouldn't each cylinder be helping to fill the plenum. As each cylinder only takes a small portion of the air in the plenum, the plenum itself sucks in the air required to fill it. If it does this then each cylinder is doing less work to draw air from a further distance.

[RAd28]

stu - the answer is no... i just can't think of a way to describe what i mean each cyclinder is drawing air ,which creates low pressure, (know as vacuum) this low pressure draws more air in, but the effort taken to draw the air in the first place robs power in the end.
each cyclinder is drawing air from the port, right back to the air filter, everything in the way is just an obstruction, until the turbo comes on boost, then the fun begins...

i think what NB86 said has the best merit, reversion in the inlet tract could be a problem, but i don't think there's any pulses from the turbo,

[Stu]

Yep, these subjects are trickey.... I definitly agree with NB86, I just think that theres more tunning in a plenum than just the pulsing. But I have also been known to be wrong before......

[thechuckster]

pulses come from intake valves opening suddenly and the engine sucking in air - or during boost, the pressure in the plenum modulates as intake valves open up and allow compressed air to enter a cylinder.

Air being drawn into a cylinder is not a continuous process - hence the pulsing.

[willwal98]

Not the easiest concept to explain but this is my take on it (hopefully correct).

If you look at a standard motor with stock runners and plenum, say for example an 18r-g. It's a 2ltr, each cylinder is 500cc, each runner is 500cc and the plenum is 2ltr.

So, the piston goes down and begins to create a vacume, valve opens,
* 500cc from the plenum goes into the cylinder and creates a vacume behind it
* 500cc from the plenum then fills the runner and creates a vacume in the plenum,
* Leaves a vacume and air comes in through the intake pipe.

If you look at this with the engine running you can see that the constant vaume pulses in the plenum creates a nice constant suck throught the intake pipe. This builds on itself (for lack of a better term) causing the air to move in better to get the maximum ammount of air in. A poorly designed intake will not create this cycle and will end up havin a lower pressure in the cylinder than a well designed one.

So really it is doing what extractors do but in reverse, just takes a different design to do it. In extractors the high pressure pulses leaving create a vaume behind them, making it easier for the next pulse to move out.

This is looking from an NA poing of view. Turbo may be a little different.

[RAd28]

yes... to a point though... just look at the effort that some people go to, to get tuned length runners, there's gotta be more science then just having the volume of the runner = the capacity of the cyclinder...

i guess the question is, if the science can desing a plenum properly, why can't you design a runner system like the one shown, using the same concepts?

what benefit will a properly designed plenum have over a properly designed runner setup?

[oldcorollas]

i guess the question is, if the science can desing a plenum properly, why can't you design a runner system like the one shown, using the same concepts?

tuned reflection of a positive pressure pulse toward the valve, due to the presence of an "open end" in the runner at the plenum.

edit: and if you look at the pic above, they have 1-2, 3-4, and 5-6 cylinders paired, whioch would also change the effectiveness of pulse tuning.
no doubt you CAN do it this way, but you are increasing the length for both positive and negative pulses to bounce around., not to mention the difficulties in getting even flow between all the cylinders with such a log style manifold (looking at the joins)

[M.J.H]

As the others have said the plenum is there to provide a source of air for each cylinder.
Have a look at a 5m or 7m cressida or Crown or Supra and you will see what is effectivly a resivour for air in the intake piping for when the throttle is cracked right open. Has a supply as such.

the setup in the first pic would also work well on boost i think as it is all under pressure.

But just maybe he went with that design to be different to everyone else.


But when it is all said and done if there was a better setup then a plenum style then surely a car maker would be using that yes.

[river]

Hi,

So, the piston goes down and begins to create a vacume, valve opens,
* 250cc from the plenum goes into the cylinder and creates a vacume behind it
* 250cc from the plenum then fills the runner and creates a vacume in the plenum,
* Leaves a vacume and air comes in through the intake pipe.

If you look at this with the engine running you can see that the constant vaume pulses in the plenum creates a nice constant suck throught the intake pipe. This builds on itself (for lack of a better term) causing the air to move in better to get the maximum ammount of air in. A poorly designed intake will not create this cycle and will end up havin a lower pressure in the cylinder than a well designed one.

You mean 500cc, don't you?

I can't see how the plenum provides an air storage resevoir that makes it easier for the piston to suck in. As soon as air is being sucked into the cylinder from the plenum, then air from outside the plenum is being sucked into the plenum - and all this sucking/work is done by the piston.

The more pipes and the more length the air has to travel increases the resistance of the incoming air, and therefore affects engine performance.

The best performance for an NA engine would be to have the shortest possible route, with the least amount of bends for the air to get into the cylinder - hence the side-draught carbs that are bolted onto the engine in the 18R-G, and 2T-G and a host of other engines. No plenum, no pipes, no bends... just straight in with the shortest path of the least rresistance.

seeyuzz
river

[thechuckster]

river: at maximum power output - yes, a short, straight distance between throttle and valve is ideal. But at off-throttle/idle this setup is less than ideal.