[lotuseatsvipers]

This is something I have been wondering about quiet a bit for a while now, how is backpressure good in some cases?

I've even read that not being able to produce backpressure at the low rpm range is one reason four cylinders have very little low end power (and of course the other obvious reasons added to that).

anyways, it doesn't make any sense to me, and if any of you know whats up with how an engine exhales and how it affects performance let us in on the insider tip.

also if anyone knows any good, not too boring books on the details of how cars engines do what they do (I know the basics fairly well but I'm obviously missing the nitty gritty and I'd like to know!) then please tell me.

[JiminVirginia]

I'm interested, too.

It seems to me that there is such a thing as "optimal" back pressure: an exhaust system can have too much or too little, given the configuation of the engine. This means that if back pressure is just right or too little, there is no point in installing an aftermarket system that reduces back pressure. But if there is too much....or if you're doing other engine mods....then maybe....

Most manufacturers probably do a decent job of optimizing their exhaust systems, but they have cost and space and sound considerations driving them, too. Did MINI get it just right on the Cooper and MCS, or not?

Any thoughts out there?

[Grinder]

Thanks for asking the question i've been meaning to for so long!! Obehave....that made *some* sense to me, but i think i need it dumbed down a little more than that even. I dont understand the internals of an engine and valve positions and scavenging and all that. Maybe i need to read one of these books. What book on suspension tuning are you reading?

[jlm]

the key to understanding the relationships among valve overlap, rpm and torque is to keep in mind that the gas/air movement is affected by sonic pressure waves which travel at a more or less fixed speed: 1100 ft/sec. At open ends of pipes, pipe junctions, etc, you get pressure pulses (positive and negative) that may produce scavenging effects, but it takes them time to get back to the intake track base on the length of pipe from the joint to the intake valve. this time is fixed; it does not depend upon rpm. What does depend on rpm (and cam timing, valve overlap) is whether the valve is opening at that time or not. As you increase rpm's, the time between intake cycles gets smaller, so you would need a short pipe for the negative pressure pulse to get there on time. Conversely, at lower rpm, you need a longer pipe.

If you can get more mixture into the cylinder, you will have a more forcefull explosion and more torque. Tuned exhaust and intake lengths are really only tuned effectively for a limited rpm range. Other effects (like the momentum of a moving column of gas, the speed of the moving gas column, etc.) will also complicate the matter.

[PigLick]

Grinder, for a good place to start, check out www.howstuffworks.com. It's is a pretty cool website, and they have a whole section on automotive issues, and have pretty good, simple, but thorough descriptions of most of the major components/systems present in automobiles. Obviously not extremely in-depth info, but it will give you a good basis, then if you still want to know more you can think about spending money on books and such

PigLick

[friedduck]

I know enough to be dangerous.

I think when people talk of back pressure, they're really referring to exhaust velocity. Think of water in a wide, slow-moving river vs. a fast-moving stream, if you will. I know that exhaust velocity helps with scavenging. Too wide a pipe width and I don't think you get the velocity for optimum scavenging. The trade-off is, narrower pipes can't flow at high rpm. Thus things like Yamaha's EXUP valve in the header that changes at different rpm.

I also know that exhaust pulses travel out from the port (obviously) and bounce around in there, and that if you tune the length right, you can take advantage of the reverse bounce to help scavenge.

I don't know how these two factors play against each other. Search for stuff written by Kevin Cameron. Sometimes a little dense, but he knows his stuff.

Jeff

[Bilbo Baggins]

Quote:

Originally posted by friedduck
I don't know how these two factors play against each other. Search for stuff written by Kevin Cameron. Sometimes a little dense, but he knows his stuff.

Jeff

Kevin Cameron is a great writer. He has a way of explaining things that make them very understandable. I've been reading his stuff for years. Best thing------it gets you thinking along different paths.