Plenums.... why bother?

[ed]

river, the other part of your question relates to 2 other often confused terms...

the first is "ram tubes" ie harmonic tuning of intake runners and trumpets, and the 2nd,to wich you refer, is an "air ram"

the early f1 cars relied soley on ram tubes and induction armonics to achieve good VEs. later came the development of the air ram/air box which took air at speed and pressurised the plenum (note: pressure, not velocity) thus increasing the port/plenum diferental and increasing filling (ala supercharger)

[o_man_ra23]

Lets consider just how much a plenum will help at high rpm. first a few equations
V = F Lambda
V = velocity, with sound waves its 1000kph, which is 278m/s
F = frequency in hertz, lets take the redline of an 18RG (I want RIVER to experiment at this rpm and find out what fun his car can be), which is 117Hz
Lambda is wavelenght in meters, which can be calculated to be 2.4m

Assume individual runners.

This would mean you would need a 1.2m runner to get a reflection of positive pressure back at BDC next cycle... BUT being a 4 stroke, it only opens every second cycle, a runner of 2.4m will be needed.

Seeing as this is a wave, we can use harmonics to shorten the runner length. If we bring the runner back to 1.2m, the positive pressure will have reflected 3 times and arrive back at the valve with less force than if it had only reflected once. Next we can go down to a 600mm runner, now reflecting 7 times. still a bit long. bring it down to 300mm and it reflects 15 times before returning with positive pressure. This is a more realistic runner length, but the pressure wave has reduced in magnitude by a great deal. But again being a wave, it gives off harmonics. The 15th harmonic will be the one that arrives back with positive pressure after reflecting once, but the 15th harmonic usually has a magnitude in the order of about -60dB or so, and can be neglected for this purpose. Each time there is a reflection, energy is transferred into the metal, and your magnitude reduces, so at 7000rpm or below, in a practically lengthed runner, the reflections really seem to be quite insignificant.

HOWEVER, add a plenum, and you can quarter the lenght of runners required, as there is a valve open once every half cycle. A 600mm runner then will give a reflection from piston 1 to 3 (and so on through the cycle). As the intended cylinder will be the only one with a valve open and air flowing, the majority of the wave force will go down that cylinder, and help. This is why plenums help NA motors. ITBs really come into play on the ol school carbies, to feed fuel to each cylinder in a more efficient manner, and get better fuel vapourisation. In this case, the butterfly then creates enough of a disturbance in the air path to spread the wave out into its harmonics, and it then becomes significantly less useful, hence the reason people opt for trumpets and individual filters.

Thats all theory though, and there are other practical considerations to be able to tune the system properly, but that will get you a rough start. If you can get your head around that straight away, then go do an engineering degree in RF systems, you will probably do quite well.

Cheers, Owen

[Adash=P]

What do you guys make of this?

[RAd28]

what the hell is going on there??? looks like an early incarnation of the FJ20, with extractors bolted on the intake side... i'd suggest that perhaps they were trying some kinda intake manifold setup like i've discussed... but the pipe ends near the bell housing, doesn't make sense to me...

if it is an FJ, then that is the intake side, and those definatly look like injectors strapped in... any more pics? story? anything?

[RAd28]

anyone care to add their opinion?

[Screamn_Sleeka]

i dont see an advantage of that FJ setup...

But, can i jsut throw in an idea,
ITB's with an airbox and pod filter, isnt that the same as a plenum?

[thechuckster]

built on a budget?

doesn't have a 'factory' feel about it

[Adash=P]

lol, yeah I know. I was thinking WTF when I first saw it. Looks like they are pretty close to being tuned lenght runners but why they used this set up is beyond me.

Those wondering what motor this is, its a Nissan KA24 but it looks like this one came froma van of some sort (due to mount on the top of trans)

Pretty sure this is how it came out of the factory.

Stupid Nissans

[RAd28]

Those wondering what motor this is, its a Nissan KA24 but it looks like this one came froma van of some sort (due to mount on the top of trans)

lol... i had a flick through wikipedia last night, and that would have been my second guess... though it does look like the pic on wikipedia, but i dunno why the factory would bother with a setup like this, seems like alot of effort for factory, why didn't they just make up a cast item?

[Mr Revhead]

toyota use a similar set up on the 5ehfe in the cynos
cant recall the exact name but it has acoustic in the title...

[thechuckster]

probably also done that way to keep it under the floorpan and front seats...

[YelloRolla]

RE - the reverse header type manifold -
Throttle to runner angle is about as efficient as you can get. No plenum chamber (of any size worth noting) I imagine would make it very throttle responsive as well as getting the intake runner length to better suit the rev range (how much time do we spend above about 4K?).

Plenum chambers are also helpful in getting even flow to the runners (has been mentioned), by dampening the oscillations. And they also can make the packaging quite compact while providing a nice head of pressure above the runner (pressure too refers to atmo engines, where there is 180 odd miles of air on top of the whole engine waiting to push air into it).

edit
Nice caclulation there o man ra23.
Will you please cite the source? I like theory and I like to know where it comes from.
I believe that the speed of sound is more like 340m/s.
Given 58.5 intake events per sec (although the 18RG's that I've had stopped pulling at 6000rpm) the sound wave will travel 5.812m in 1/58.5 sec. BUT the intake valve will only be closed for about 65% (assuming 252deg of vavle duration) of that time.
So - 5.812 div by 2 (because the reflection has to travel up and back = 2.906m (close to the theory outlined) 65% of 2.906 = 1.889m or for 3rd harmonic (6th reflection?) that would be 630mm (I think?).
Which (if the theory is correct) would probably be a desirable length for where one would like peak torque to occur (usually a couple of thousand rpm shy of redline on a nice engine)?

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

These are only my thoughts and I am by no means an authority on the subject (about 2c worth).

[o_man_ra23]

screemin sleeka - the butterflies disturb the sound wave too much, hence no airbox usually on ITBs

YelloRolla,
Speed of sound is roughly 1000kph (not trying to work out exact measures for people)
This gives 1,000,000 m/h
or 16,667 m/min
or ~278m/s
formula to work out Velocity comes from the equation for light, where C=FLambda. This equation holds true for all waves by substituting C for the relevant velocity.
You want the high pressure to be close to the end of the valve's opening time, as this is when the cylinder is sucking the least, and needs the most help. You also want the neg pressure wave in the plenum then, sucking as much air for the next charge as possible.
3rd harmonic has nothing to do with how many reflections take place. Its got to do with frequency. the 3rd harmonic is a pressure wave that is created at 3 times the frequency of the original wave. It starts out life at around 1/3 (works on a trigonometric function) and decreases from there.

If you want further theory, I can continue, but must get ready for work now.

Cheers, Owen

[Ben Wilson]

But, can i jsut throw in an idea, ITB's with an airbox and pod filter, isnt that the same as a plenum?

At WOT, there will be very little difference - In fact a plenum setup might flow a little more (turbulence from the butterflies can effect flow into the valves). The big difference is throttle response. With a decent size plenum and a single throttle body; when you open the throttle, you need to fill the plenum and each runner before the pressure inside the plenum rises.

With a ITB setup, you've only got the area between each butterfly and the valve to fill, so things happen a lot faster..

[YelloRolla]

Speed of sound is roughly 1000kph (not trying to work out exact measures for people)
This gives 1,000,000 m/h
or 16,667 m/min
or ~278m/s

The speed of sound is roughly 1100fps or 1225km/h or 340m/s. Making 278m/s not even close. http://en.wikipedia.org/wiki/Speed_of_sound

But that is not the question.

Your knowledge, or ability to work out the math is not in question - I like the formula that you expressed, but I want to know what text (in relation to intake theory) that you sourced it from - so everybody (well so I) can read it.

You want the high pressure to be close to the end of the valve's opening time, as this is when the cylinder is sucking the least, and needs the most help. You also want the neg pressure wave in the plenum then, sucking as much air for the next charge as possible.

The cylinder is sucking the least at the beginning of the valves opening (the valve overlap period), when pressure is high in the exhaust and low in the intake (depending on exhaust timing). Not that I disagree with the theory otherwise - which is why I pointed out that the valve is only closed for <65% of the time (meaning low lift opening is not far away).

3rd harmonic has nothing to do with how many reflections take place. Its got to do with frequency. the 3rd harmonic is a pressure wave that is created at 3 times the frequency of the original wave. It starts out life at around 1/3 (works on a trigonometric function) and decreases from there.

Thank you for that. I am/was obviously completely confused with the terminology.