Plenum Theorum

[F6 UTE]

Just thought the other thread was starting to detract from the purppose of it's incarnation, so started this one.

Plenum design discussion..

No where do I expect someone to disclose there volumes, dimensions or the like, but an open technical discussion should be an interesting read.. Thoughts and opinions are free.

© Helmholtz resonance effect. This is a phenomena which occurs when tuned length runners are used to increase the volumetric efficiency of the engine under NA conditions to over 100%. This produces a very strong pulsing effect within the plenum.

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Interesting Robin. I have nowhere seen anyone relate to Helmholtzs' theory openly in a Ford Forum, or relating to these cars. I see Simon G alluded to it, without actually mentioning it one time. It makes me think he used the formula to some degree, with engine dyno testing to come to a final spec on his plenum design.

I have seen no real literature to support the use of the formula in boosted applications. Plenty for N/A, as you mention.

One reason being, will the return pulse from the intake valve be enough to overcome the ~20psi in the runner, and the forces directed against it?? I am not sure, but every reaction has an equal and opposite reaction, so maybe, atleast untill the pressure in the runner is equal, then yes, maybe a return pulse?? I assume the benefit would be small, but worth looking into. Not that hard to work it out..

Also, tuned length gets bandied about alot. Well, tuned to what length?? Bottom end, midrange or topend??

Alot of guys pride themselves on there 'tuned length' manifolds, or extractors, (which are more commonplace) they have bought, but to what rpm range are they tuned to?? Are they aware if they even meet there own requirements? Does the 'tuned length' of the intake runners match the exhaust, if it's been done??

Personally, I have accepted a couple of things from my own - uneducated observations.

1.) Nizpro make very good, efficient power with there induction setup.

2.) APS make very good power with there Intercooler/Exhaust setup, sans Unichip.

So, we can summize a couple of things.

1.) There's more than one way to make good power, to a point

2.) The ultimate result will see all parts changed. aka Brian.

But, to what extent does the plenum play in this??

I am currently in the process of getting my own plenum tigged. Nothing fancy, and it's not as flashy looking as most on the market.. But I hope it works, and provides the result I am after.. Quick response, with good fill time and good power support.

Based on my own experience from playing with various cars, and the factory 'enhanced' models that come with them. Unlike Mitsubishi, or Subaru, Ford took the easy way out and simply bolted a turbo on the motor. The intake plenum, designed for NA use, does ok.. But it's a workaround..

I'm actually going to try and record what effect, if any the plenum I am fitting makes, whether it be possitive or negative. It's a starting point, and I don't plan on it being perfect.. Perdominaltly, the sole purpose of my plenum will be to shorten the intake piping and combined with a bigger intercooler, drop pressure loss across the entire system. I am not looking at the plenum as a power adder, but I don't want it to be detramental either..

Volumes of plenums, or theories of such are as varied as choice of car. Equal to engine displacement, Double the size, As big as possible etc are popular.. What exact size works, I assume Simon G has a better understanding than most. He's dabbled with plenums on various cars and would have an understanding as to what actually works.

I assume it also has some correlation to which turbocharger/s is being used, it's compressor map and what the end user wants.

Runner Length. Short runner are good for top end, longer runners give excellent torque down low, this is fact, not an opinion.. I am waiting to see a Nitto plenum back to back on a Ford Lower half and aftermarket upper half, of any brand.

If the XR6T revved to 8500rpm even 9k, as Jap 6's do, I would call it a winner. There powerband is alot higher in the rpm range than the Barra engine, and they predominantly boost late. They also launch VERY hard at the drags, generally on antilag.

But the design, in my opinion is based on upper rpm, which some people will be looking for, so it will fill a market neiche..

My only arguement to this, I've experienced better results looking at peak torque, and where it occurs ( lower rpm ), as opposed to peak power high in the rpm.

A diesel dozer needs to shift weight, it doesn't rev, and makes massive torque from idle. There's something in that, the XR6T is a heavy car, by any comparrison..

The sooner you get weight moving, the easier it is to then propell.

Go push a car, see how much effort is required to get the weight moving an inch, then see how much effort is required to keep it rolling, or even push it faster....

So, I've rambled, it's late and I'm probably wrong on a lot of things, but I highly doubt if I asked people who knew, they would tell me anyway, and I don't expect them too either.. I'm hoping this will somewhat help other DIYers in an attempt to throw some handywork the way of the engine bay..

I'll get some piks of my plenum up in the next couple of days, should be ready for an install end of the week. ( cooler woggie ) and also some negative feedback/banter, which I'm looking for.. so all should be good for some dyno numbers in a couple/few weeks.

[SavageF6]

Good post. Certainly raises some questions (and makes some points) that I'd be interested to see responses to. Can never have too much info on a subject like this.

[Ford_Power]

Certainly an interesting topic Ben, and it is something I have been wondering about myself for a long time. (The whole concept of "tuned" intake/exhaust and what are the exact tuning paramaters that they are so called "tuned" to. Eg Nizpro gear, APS gear, etc)

[Paradoxx]

The tuned runners will have to be setup to be useful in the rev range you want your most "NA" power to be. The valve opening and closing at a certain frequency will determin the length of the runner. Say the engine is at 2500rpm, the valve will be opening and closing at half this rate 1250 times per minute... this gives us a frequency of 1250/60 = 20.83Hz

The length of the pipe required to give you a resonant frequency of 20.83Hz is derived by working out the wavelength... for a closed pipe the length is 1/4 times the wavelength. Now this is how it works for sound waves, which is basically compression of air particles. Is this also true for compression of air in a "tuned" runner?

I have a feeling that when you throw boost into the equation you are effectively shortening the length of the runners by creating a pressure barrier... just a theory I dont know for sure.

Edited 05/09/06 09:33 AM by XXXR6T

[F6 UTE]

You are on the right track steve. Yes, the correlation can be made with sound waves.

The air as it enters the intake chamber ( valve ) is almost, if not, sonic.

Once the chamber volume is known, a runner length can be assertained to take advantage of Helmholtz. In NA anyway. Boosted, I have no idea if it can, or is used..

Boosted apps have shorter runners in general than NA, and when looking at the factory plenum, the extra length is derrived from the air travelling the outside perrimeter of the plenum. ( that circular section visable.. ) for lower rpm.

In boosted form, I have noted the circular runners have no sign of 'use' compared to the shorter section of the plenum. - Path of least resistance, as is gramps' favourite saying.. The shorter section is 'wet' and the longer section ( NA ) bone dry..

My theory is that the lower runner design/length has more impact on performance than the holding chamber (plenum)

I agree, a well flowing, perfectly matched plenum would be beneficial, but how far outside ideal can we go before performance is detramental. Even effected??

Looking a Fords attempt, a fair way I think..

Mick at Ovaboost has just done a tune for a guy with an EB Falcon.

Bone stock EF motor, Ford XR6 turbo bolted on it, chinese intercooler and air pod. 3" piping. It has a cam of unknown origin ( for now ) in it. Autronics.

The car made 420rwHP ( over 300rwkw ) with just 6.5 psi. Mick formed up an extractor manifold and they are using an external gate. Factory EL- read BA plenum. just dicky spagetti intake piping. Yes the fatory CR will benefit the power/boost ratio, untill detonation comes into it, but it's a blooody good result from the amount of boost used.. To the point they were bagging my car

So, what about that plenum?? How much benefit is made from the design of it alone?

[Paradoxx]

I think benefits can be obtained from it's design by considering entry point, uniform shape, internal protrusions, any areas of uneven pressure (ie does the furthest runner get the same pressure/flow as the closest?) and I think the rest would he aesthetics. I'm surprised NIZPRO didnt factor the battery location in more carefully when designing their entry point. Sure there is an issue with the thermostat housing and direction through the front of the car, but if the angle was reconsidered maybe the TB can be a bit straighter and the plumbing itself can negotiate the new battery location in the air box spot?

But for the plenum shape itself, maybe some research into proven methods and designs over time. Have a look at the way Nissan and Mitsubishi do theirs (ie GTR and EVO)... what differs between their shapes? They have been producing these things for a long time so you would think they have found a good way to do it.

[F6 UTE]

Steve, you are governed by the entry point from under headlight. Nitto get around it by the fact they change the bottom runners.. Which compramise is better, I have my opinion.

Yes, look at WRC and see what plenums they use, some very interesting designs, with the 'best' one I have seen a conical section mounted parallel to a fairly standard rectangle box section.. Kinda like laying an icecream cone alongside a 1Litre milk carton.

I look at wrc more-so than other forms of motorsport, as there power delivery is more akin to what I'm after.. Also the power achieved from such small displacement is very impressive.

[Paradoxx]

I agree totally. I used to own a Starion with a VR4 motor (basically the original EVO engine) and the potential that thing had unopened was phenomenal! It had the Intake plenum I like and from memory it had quite long runners... I'll have to dig out the pictures.

[driftpig01]

I have had with a couple of plenum design on turbo rotars and have come up with the following:-

Large plenum- suitable for big Hp applications, no good for torque. there is a direct correlation between the CFM capable of the turbo the percentage drop from turbo to plenum and power outcome. If a large plenum is used the trubo will seem to spool up slower upto the point in which the the set boost level, however at this point there is a large reserve of air to create additional power.

Small plenum - my favourite, can also support big Hp however offers greater driveability. This is why the Nizpro works well on the T it has a greater volume than stock however the total volume doe not exceed the CFM capable of being produced at low engine RPM.

I had a plemum on a 13B which was double the capacity of the engine roughly 2.5l however it ran 18psi and rev to 9000.

I think it is best sumed up as light cars big plenum, heavy cars small plenum if turbo is involved money is better spent on other mods as plenum will only give minor gains.

Sorry about the long post and my 5.5 cents damm GST

[F6 UTE]

I agree totally. I used to own a Starion with a VR4 motor (basically the original EVO engine) and the potential that thing had unopened was phenomenal! It had the Intake plenum I like and from memory it had quite long runners... I'll have to dig out the pictures.

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I was running a VR4 bottom end, EVO head and manifolds, E3 cams, big 16G and 20psi. It used to chirp into 3rd - AWD.. Brilliant engines. I actually got a pik of it in a Fast Fours mag at a motorkana day I went to with the All4& scrotaries 's club. Drifting in an awd..lol.