Plenums.... why bother?

[RAd28]

i'm with yellorolla about the speed of sound,


i think where we're going wrong in trying to understand why a plenum is used, is simply 'cause we're trying to over analyse the situation, if you really want you can calculate optimum runner length for each degree of cam rotation, taking into account the overlap, piston speed, bore, stroke, boost pressure, throttle position, volumetric flow rate, etc etc etc... but all i wanna know is a basic theory as to why a plenum is necessary...


O_man_ra23: you say velocity of light is C=F x Lambda. where C=velocty, L=lambda, then what is F? speed as i've always known it is C=Lambda/time

[Ben Wilson]

ibut all i wanna know is a basic theory as to why a plenum is necessary...

It's not necessary, but it's better. At the simplest level, it allows even air distribution between all the cylinders. For a more complex answer, see previous posts...

Looking at the picture you posted in the first post, getting even distribution with that sort of setup would be a nightmare. Even with a merge collector, you'd have to be 100% perfect with your fabrication and have the air entering the collector perfectly straight to ensure even distribution.

[RAd28]

fair point that, as i said in that first post i understand why that one isn't ideal, theoretically the first TB gets 50% of the air, second and third TB share 50%, (not taking into account efficiencies etc)

but the standard plenum doesn't do such a great job either, take the 1G for example the TB position means cyl 3 and 4 get first dibs on the intake charge, 2, 5 get next shot, and 1, 6 are left to pick up the scraps....same case as the cast manifold on the 18R... so i'm inclined to disagree that distribution is the reason for the plenum.

obviously a stock plenum is better then the pic in the first post, but a merge collector, even a half decent one could do as good a job, if not better then a plenum in terms of distribution

[Ben Wilson]

The bigger the plenum, the more chance of even distribution. Like everything with motors, it's a balancing act. Big plenum, nasty throttle response, but nice distribution. The standard plenum works fine at standard boost levels, once you start to bring inlet velocities up and get closer to the limit, you need a bigger plenum to compensate.

[o_man_ra23]

The equation doesnt come from texts relating to intake theory, it comes from texts relating to wave theory. Communication Systems 4th edition by Haykin is a good example, ISBN 0-471-17869-1. This level of wave theory is the same between pressure and light waves.

F is frequency in Hz. Hz = 1/s where s is seconds in time. so:
C = F*Lambda = Lambda/time

Hz is also known as cycles/second where cycles is unitless, therefore Hz = 1/seconds(restatement, but different explaination)

Sorry bout the speed of sound, I usually deal with light (C = 3 * 10^8) as that is the speed electrons travel at. Mech engineer will know sound better. But the increase in the speed of sound will give longer wavelengths anyways.

Exhaust theory is there to create a negative pressure wave (vacuum) during valve overlap, to extract the last amount of combusted gas and draw in new charge while the piston is slow. There is no point using the intake pressure wave to force air in during this time, if it will reduce to a negative pressure at the closing of the valve to extract the forced in gasses... it would work against itself. The end of the intake valve opening is the best time to force the last bit of gas in. This is why overlap in FI vehicles should be very limited, as fresh fuel/air charge will be forced straight from the intake into the exhaust during overlap, and large overlap in NA motors will allow the exhaust's negative pressure wave to extract every last bit of combusted material and suck in as much of the fresh charge as possible.

So in summing that last bit, the exhaust should have a negative pulse at the end of its cycle, and the intake should have a positive pulse at the end of its cycle... as it jams in more air than having it at the start of the cycle. Also having the positive pressure at the end of the cycle reduces harmonics and gives a much better pressure wave.

Cheers, Owen

[RAd28]

on the topic of the speed of sound... the numbers given are for sea level. what happens when you drop altitude (higher pressure)?? speed of sound increases, so when your running 1bar boost, that's twice the pressure at sea level... right? (1bar = 1 atmosphere approx)....

so i'd expect a significant increase in the speed of sound, and therefore increase in wavelenght.

make sense?

[YelloRolla]

I prefer simpler theories (because I am simple), because there are so many variables, like the speed of sound changing markedly with air pressure.

Exactly (10)

[RONA]

^^^ Why have you quoted yourself

[RAd28]

'cause it's more socially acceptable then scratching yourself?

[Ben Wilson]

Showing that he made the same point in an earlier post...

[YelloRolla]

Because I have an over inflated sense of self importance.

And what's more - the speed of sound travels markedly faster through steel/aluminium components.

[Rcubed]

what happens when you drop altitude (higher pressure)?? speed of sound increases, so when your running 1bar boost, that's twice the pressure at sea level... right? (1bar = 1 atmosphere approx)....

approx

so i'd expect a significant increase in the speed of sound, and therefore increase in wavelenght.

make sense?

no. pressure doesn't affect the speed of sound SIGNIFICANTLY. in an ideal gas NOT AT ALL. this is because as the pressure increases so does the density. the temperature is much more significant because if the temp increases then if the pressure is const density decreases or vice versa.
heating the air will increase the speed of sound and therefore the increase in speed of sound in the intake under boost is due to temperature rise not pressure.
the tuned wavelength doesn't change. it equals the length of the runner you are using. what changes is the frequency. so a runner which is tuned to 3501RPM [to be outside river's comfort zone] on a cold day [no boost?] may be tuned to say 3600RPM on a warm day [with boost].

http://hyperphysics.phy-astr.gsu.edu...wavrel.html#c1
http://en.wikibooks.org/wiki/Acoustics/Sound_Speed
http://en.wikipedia.org/wiki/Speed_of_sound

[ed]

jase, im about to bombard your email with a few very good articles... get you away from this simple C=FL bollox

cheers
ed

[takai]

Want to post them up for all of us Ed? I have hosting space if you need it, although i daresay v-eight.com has enough

[ed]

i cant

i promised the guy that downloaded them through the uni account that i wouldnt rehost and distribut them. rather i will email directly to those people who would be interested.

I will put up a list of available articles, library stylez, on v-eight if you want to have a look whats there