yeah, some are large, some a small, some are wide but short others long but narrow, whats the math/physics behind it?

thanks

 

its all dependant on how much air the motor needs and has readaily available at any given point when the throttle is opened. as for the physics. its more than id care to explain. maybe someone else will wite you a huge essay on the subject.

 

by the book the scientific design of itnake and exhaust systems

 

do u mean "buy" not "by" ??

 

lol arere you seriously going to corect hm lke tht. wel thn im shure yo know wht he ment.

 

to tell you the truth i didnt really understand him at all

 

I think he's suggesting you read that book.

Here's my dum version... An infinitely large plenum would be good for running at a steady throttle. It won't get 'depleted' by each intake stroke. But then when you change the throttle it'll take forever to reach a new equilibrium pressure. So a really small plenum is better for quick throttle response. So you pick a reasonable compromise between those 2 extremes.

 

so putting aside throttle body size, runner width and length, the actual plenum size ( assuming its same size or more than the throttle body) will only affect the response of the engine?

and can u expand a little more how a car would behave with an infinatly large plenum?

 

The infinitely large plenum is a stupid extreme example, just to explain the idea.

Each time a cylinder sucks in a charge of air, it depletes the pressure in the plenum during the intake stroke. If the plenum is really big, the plenum pressure doesn't change 'much' during the course of the intake stroke. This makes more sense if you think of a 1-cylinder engine, so the intake strokes are farther apart.

Say you're driving along with just enough throttle to go a steady 40 mph. Now you want to nail it because you're about to be passed by a geo metro... So you open the throttle wide, your intake sucks in tons of air, but it takes a long time before the manifold pressure gets any higher.

Runner length is a different story. That's about pressure pulses reflected back from the plenum, to reinforce the next intake stroke. You pick the length based on rpms.

 

so what are the benefits of having a larger plenum as opposed to something like ITBs which have none

maybe i should read that book, haah, i have all summer


Modified by 7thGear at 5:44 AM 6/3/2004

 

You mean ITBs?
Individual throttle bodies sorta have the advantages of both. The 'plenum' is really upstream of the throttle, so it's always at atmospheric pressure. I guess that would be some kind of air intake chamber? But with ITBs I think(?) it's difficult to get a stable pressure measurement downstream of the throttle. Normally your ECM wants to have a manifold pressure measurement. So if you are a company like Bosch you can re-work your entire engine-management scheme so you don't need a MAP sensor. Trying to make ITBs work with a Honda ECM is harder.

There's probably lots more to the story that I'm not thinking about right now.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">You mean ITBs?
Individual throttle bodies sorta have the advantages of both. The 'plenum' is really upstream of the throttle, so it's always at atmospheric pressure. I guess that would be some kind of air intake chamber? But with ITBs I think(?) it's difficult to get a stable pressure measurement downstream of the throttle. Normally your ECM wants to have a manifold pressure measurement. So if you are a company like Bosch you can re-work your entire engine-management scheme so you don't need a MAP sensor. Trying to make ITBs work with a Honda ECM is harder.

There's probably lots more to the story that I'm not thinking about right now.</TD></TR></TABLE>

well said...actually both responses were well said...but i'm too lazy to put both responses in quotations.