Plenums.... why bother?

[spectral]

Throttle jammed open at WOT, and then when i shut off the engine it must have died with both intake and exhaust valves open on cyl 3 which backfired and spat the fuel air mix out the intake and the supposedly non-flammable Unifilters caught on fire.

So in your opinion, the main function of the plenum for ITB setups is safety, and the ability to route cold air from behind the headlight (in your car).?

I've being thinking of doing a plenum for my Clubman and was concerned about plenum size and shape.... actually, when I think about it, using single throttle body logic... from your posts I'm guessing that it isn't really critical to get it right?

[Ben Wilson]

ok.. so you think we are all young people, and that is the ultra basic way to explain it (althought i would have phrased it as, "when valve opens, a negative pressure (relative to average) goes up the runner, and is reflected as a positive pressure pulse, which you time to hit the open valve as it is closing, giving a final shove to the air going in)

i would LOVE to hear your advanced theories. i would highly appreciate if you would take the time, and type out a response bit by bit between taking care of kids (a handful i can only imagine ) and explain why the normal theories of reflection of pressure wave, combined with port velocity, are only a basic starting point. yes there is a lot of experimentation required in terms of shape and length of bellmouths etc to actually achieve the harmonics and the best compromise of port velocity, but why is the theory incorrect?

Siding with Mick here (now there's a change ), I did oversimplify it as I'm sure you are well aware. For the purposes of the discussion, anything more than the basics just overcompilates matters. We are dealing with a wide spectrum of ages and knowledge levels on this forum and I was trying to find an explanation which while still technically correct, was easy for the layman to understand.

Have you ever been to the NSTC (Questacon) here in Canberra? I used to work as an explainer there many years ago. The big thing I picked up there is that when you break down an idea to its most basic level, most people can understand even remarkably complex concepts fairly easily.

Looking at your explanation, at no point do you explain how the wave is reflected as a positive pressure pulse. Nobody is going to question you on this forum, and some people are going to just think "I'm never going to understand this" and stop trying.

You mention timing the resonant pulse to arrive at the end of the inlet event, but don't explain why. You've got lower velocities at the beginning and end of the stroke, but you've also got lower lift. I'd be thinking you'd want the pulse to arrive somewhere a little after max lift, when the velocity is starting to drop, but there is still heaps of valve area.

[Ben Wilson]

I've being thinking of doing a plenum for my Clubman and was concerned about plenum size and shape.... actually, when I think about it, using single throttle body logic... from your posts I'm guessing that it isn't really critical to get it right?

Wow, spend a while making a big post and you get left behind

Any plenum which flows decently shouldn't hurt power too much, try to avoid flat areas directly opposite the bellmouths as that can theoretically cause some nasty issues* and you should be good to go. Try to leave at an absolute minimum half your trumpet diameter between the end of the trumpet and the roof of the plenum.

* - most aftermarket air filters have a flat plate directly over the trumpets and they seem to work OK....

[oldcorollas]

Siding with Mick here (now there's a change ), I did oversimplify it as I'm sure you are well aware. For the purposes of the discussion, anything more than the basics just overcompilates matters. We are dealing with a wide spectrum of ages and knowledge levels on this forum and I was trying to find an explanation which while still technically correct, was easy for the layman to understand.

fair enough.
heck, i'm busy preparing for international conferences, but at least i usually take time to post
Mick knows he has more knowledge than anyone on toymods, but rarely actually posts information about topics like this? less talk, more teaching please

as for harmonics and things
here are some pretty diagrams
http://www.phys.unsw.edu.au/~jw/pipes.html
http://hyperphysics.phy-astr.gsu.edu...es/clocol.html

and helmholtz resonance
http://physics.kenyon.edu/EarlyAppar...Resonator.html
http://fergusmurray.members.beeb.net/resonata.htm
http://www.phys.unsw.edu.au/~jw/Helmholtz.html

i also think KM would be surprised at the amount of good knowledge able to be found in tah intahweb.

For internet knowlege it was an intresting read as i said. Not much like it on the internet i wouldnt imagine so it is good internet knowledge

so you think internet knowledge is lower than yours (and you may be right) but if you never share/teach, you may as well not (and may not) know anyway

Anyway KM you race sports sedan so you must be doing something right ! Are you going to be racing down at Phillip Island in October?

maybe Tony Axisa will be driving Fred Axisa's sports sedan?

[Ben Wilson]

as for harmonics and things
here are some pretty diagrams
http://www.phys.unsw.edu.au/~jw/pipes.html

Damn you

You've got my brain running in circles again with some of those diagrams.

Hmm - Need to build a machine to generate and measure harmonics.....

[oldcorollas]

Damn you
You've got my brain running in circles again with some of those diagrams.
Hmm - Need to build a machine to generate and measure harmonics.....

time based finite difference pressure analysis... corrected for velocity of the molecules, and focusing on reflections from each surface/change of direction in the intake tract?
edit, i realise this is beyond the scope of most people.

so, a few approaches. thinking first anout ITB.
1) velocity. just make the pipe the right size and don't worry about harmonics. the 5-10% difference from using 1st or 2nd order harmonics can be equally negative to performance if not tuned right. ie, make the length of pipe such that harmonics don't come into it much.
1a) use the charge stop/start theory (i forget the name), where you use looong runner to maximise the kinteic energy of the air column in the runner to have like a water hammer effect.. when the velocity slows down, the backing up air increases pressure at the valve to force more in.
2) get the pipe relative close in size for velocity, but focus on the length of pipe for harmonics. choose the rev range for assistance, and the width of desired powerband to coincide with the order of harmonics you want to use (and work out how much you lose between the "good rpm" as well)
3) combine the processes such that either you increase the effect of both at a given rpm range, OR, you use them to complement each other to extend the rev range (ideal velocity up high, more pulse tuning lower)
then with plenum, you need to take into account more about the reflections of waves, and turning/sucking/blowing of air into the runners. starts to get very complex, and difficult to take everything into account. some basic resonance theory can be used, but finite difference type calcuations may need to be used in order to account for local changes in geometry.

lastly. an engine relies on sound to increase it's efficiency, both for exhaust and intake.
F1 engines are loud because they maximise the amplitude of the sound waves to increase the pressure pulses and increase cylinder filling.
an F1 engine is a true musical instrument.
as rpm decreases, the available energy for harnessing decreases (as the square of rpm?) and so the effect of tuning gets less, but still worth playing with.

You mention timing the resonant pulse to arrive at the end of the inlet event, but don't explain why. You've got lower velocities at the beginning and end of the stroke, but you've also got lower lift. I'd be thinking you'd want the pulse to arrive somewhere a little after max lift, when the velocity is starting to drop, but there is still heaps of valve area.

ok, to answer this directly. when the valve is open, the cylinder is an open container. if you time the positve pressure pulse to arrive just after max lift, then the next negative pressure pulse will arrive just after the valve closes. meaning that there will be decreasing pressure as the valve is closing, decreasing the amount of air that stays in.
the idea to have a positive pressure pulse hit the valve just as it's closing is to push as much into the cylinder as possible.... or more importantly, since, as you say, the velocity is decreasing, and you have already forced a lot of air in... it is just as possible for air to escape from the cylinder.
think of it this way. you are trying to increase VE over 100%. ie, you want a positive pressure in the cylinder relative to the atmosphere. if you could instantly stop the process after you reached 110% VE (at BDC), the air will escape the cylinder and reduce to 100% VE. to keep the extra air IN the cylinder, you need a "pressure plug" of sorts.

anyway, i'm rambling, back to presentation

[kingmick]

Anyway KM you race sports sedan so you must be doing something right ! Are you going to be racing down at Phillip Island in October?

No prob wont be at phillip island as Fred has other things on at the moment. I was talking to Fred and his son Phil about this at the start of the week and prob do the Oran park national round.
I am suppose to be at Eastern creek right now as Fred is having his first race back after 3 years and is do the state round there this weekend, but i cant make it because of family matters, its killing me not being there. Hahaha i just got og the phone to Phil seeing how things where going and ill keep ringing constantly over the weekend

p.s missed most of this as the emails of the threads come and go and miss half of the messages, very strange how it works.

[kingmick]

maybe Tony Axisa will be driving Fred Axisa's sports sedan?

No tony isnt driving the car for the moment. Fred is back in it.

[Ben Wilson]

Just to add some interest, here is the graph from EAP comparing inlet flows and pressures at 8k (tuned length), and 4k. Bear in mind that cam size has a lot to do with the changes in flow rates as well as harmonics.

A little confusing, but interesting...

[takai]

So in your opinion, the main function of the plenum for ITB setups is safety, and the ability to route cold air from behind the headlight (in your car).?

I've being thinking of doing a plenum for my Clubman and was concerned about plenum size and shape.... actually, when I think about it, using single throttle body logic... from your posts I'm guessing that it isn't really critical to get it right?

For my car thats why i went the plenum route. That and it opens up the opportunity for me to go to Class C by simply strapping a turbo on the side. For many others its not such a big issue, and i know many who just run filtered trumpets or no filters at all. Mine was a fairly unlucky incident, but such things happen.
When i was doing mine i was also concerned about size and shape, but as i modelled a few there doesnt seem to be too much around that size which improves flow overly. Im mainly compromised by how close the clutch master is to the 4th cylinder runner (i actually have to grind the clutch master down 5mm), which changes the reflections off the back wall significantly and compromises both 3rd and 4th cylinders. In your case with the 20v and Clubman you have fewer packaging issues and would be able to create a plenum with a much better resonance package.

Ps. away this weekend, no more posts till Monday.

[RAd28]

WOW!!!!!

firstly.. you all suck! i had to turn my music off just to read this! and now my head is throbbing... due to a lack of sound pressure waves which have previously caused a resonance of heimlicks alpha particle super node homoligation on the trumpety bellmouth lobe of my ear drums... AAAARRRRRRGGGGGHHHHHHHHH

secondly, thanks to all who's so far contributed, HOWEVER, i still havne't solved the mystery so far we know that runner length in particular is a juggle game of variable such as cam timing, air speed, pressure, ideally though a harmonic pos pressure node hitting the valve when it's open is perfect,

we know that resonance is the devils breath, any air moving away from the intake valve is bad for business. i've seen fuel mist around the filters on my bike on cold mornings. though as far as i'm aware, it's only bad news if the reversion pulse doesn't leave the runner before the valve has opened again, once the pulse has left the building, then it leaves behind a lower pressure... thing... which will be filled asap.

i'm suprised that noone has mentioned anti-reversoin headers, 'cause the thory works the same here. even a number of single cyclinder (single exhaust port) motorbikes make use of a reversion type chamber since scavanging isn't possible.

so lets say we build an inlet system like that quad turbo V8, with anti-reversion headers, what then???

[thechuckster]

so lets say we build an inlet system like that quad turbo V8, with anti-reversion headers, what then???

and there's how much room left under the bonnet of your RA28 for this experiment?

[RAd28]

for a 6cyl? single throttle body? hmmm roughly a bit more then the stock plenum.

forgetting the practical side for a moment... it's the theory i wanna know

[oldcorollas]

to emphasise the point that F1 are like instruments
i know it's a youtube link , but it demonstrates part of the acoustic capability of motors, in terms of frequencies.
http://www.youtube.com/watch?v=MGEqlNU30Tg
(thanks to toycrash for link)

[ed]

For internet knowlege it was an intresting read as i said....
I will add when i feel the need,

mick - youve been neg repped mate! come on... quit being an armchair expert and actually contribute to the forums with information, or i will continue to neg rep you and start deleting your posts.

bullshit tease and carry-on dont hold water in this section any longer mate. pick up your game