Originally posted by: Eli
eh.. Sorry, but you don't know what you're talking about.. or at least, you have a very poor grasp of it.Originally posted by: halik
Originally posted by: Eli
What's the proper term, then?Originally posted by: Howard
Backpressure decreases torque.
I know for a fact that a slightly more restrictive "can" type exhaust increases low end and decreases high end torque on a 2-cycle engine, while a free flowing expansion pipe decreases low end but increases high end.
because the way the carbs are calibrated.
In fuel injected cars, nothing like that applies. The more open the exhaust the better.
Only carbureated have the double-soaking effect calculated into the fuel mixture. IE when the intake valve opens the pressure from the exhust will push the air-fuel mixture thru the carb again, which mixes even more fuel into it. That way playing with the backpressure can make your car run richer or leaner and hence change the output![]()
To further explain, it has nothing to do with carbs. It's also independant of fuel injection... It doesen't really have anything to do with the fuel system, other than.. thats what the reaction effects.
Read my long post farther up. It does not matter that a 2-cycle engine has ports and a 4-cycle engine has valves, the same forces are still at work.
The bolded part is absolutely NOT a universal rule. If you use a pipe that is too big for your exhaust flow, when the exhaust gasses hit the huge tube, they lose velocity, and the negative pressure behind the positive pressure front dissolves.
To explain "scavenging"..
Like I said, when the exhaust valve opens, the hot exhaust gasses rush out and down the header. Behind this positive pressure front, is a negative pressure wave. This helps scavenge the leftover exhaust gasses out of the combustion chamber by pulling them out. In cases of extreme tuning, it can even help draw the fresh air(or air/fuel mix) into the chamber, due to valve overlap.
Of course, like I said.. it will only be operating synergistically at certain RPMs.
One of the first things I thought about was carbs. So many people will put an 800cfm carb on a 300ci engine, and it will barely idle, or it will run like crap because at low rpm the intake velocity is so low. But thats like the opposite thing from what the OP was asking.
Oh yeah, and that can be a problem on fuel injection too. People often use larger throttle bodies than they need because an advertisement somewhere said it would add 13HP.