What is the "real" reason for wanting exhaust backpressure? I understand that it is neccessary, as does my buddy but his answer was "you just need it". That doesn't really fly with me, so if anyone has the real reason I'd appreciate it. The only thing I can think of is to help close valves and prevent valve floating at higher RPMs.
Thanks in advance guys
Scott

 

i was looking through the message board and saw this, maybe it will help: https://www.ford-trucks.com/forums/s...d.php?t=309999

 

You dont need it. Backpressure is ALWAYS detrimental to engine performance. The more exhaust gasses you can get out of the cylinder the more fresh air/fuel you can put back into it.

My methodology is to match the exhaust piping size to the size of the manifold outlet or collector diameter. That way, you're not restricting it or letting it lag too long and / or tumble as it would escaping into a larger pipe. The way I see it, if a set of headers would have worked better with a bigger or smaller collector, it would have been produced that way.

As always, my $0.02

Edit: I have heard of needing backpressure on some air-cooled two cycle engines, but this was to prevent the engine from self-destructing. I think.

 

ahhh...excelente! Much more sense has been made....
Thanks for the link!
Scott

 

You want to have just a bare little amount of backpressure, so that the exhaust maintains its velocity as it moves down the pipe. On extreme race motors it doesn't need any help, but at low RPM it'll bog down and lose its scavenging effect. You'll notice this as lost torque at lower RPMs. Obviously you want little to no backpressure on a high-rpm motor.

 

You don't want any backpressure, you want flow velocity. This is achieved through proper sizing of the exhaust pipe.

 

Quite right, on a 2-cycle. Well mostly. They won't self detruct, really. Perhaps you would get some fuel being burned in the exhaust lines, which could damage them, naturally, but mostly it wouldn't run *well*.

By the nature of a 2-cycle, what with having no valves, and intake and exhaust ports opening and closing in the cylinder walls directly as a function of piston position, back pressure is required for any sembelance of economy.

High-performance 2-stroke engine exhausts are actually tuned accoustically to reflect a back-pressure wave to arive at the exhaust port at about the time the last of the true exhaust is exiting, bringing some of the intake charge with it. The tuning is the reason for some really funky 2-stroke "mufflers"... which don't, really. This wave forces a bit of that charge back into the chamber with the piston on the rise, and helps keep compression ratios up.

Of course, fluid dynamics being the beast it is, the timing of this wave is an art to tune, and works (almost) perfectly really at only one RPM, but this is usually meant to be full throttle. Handy, that.

Anyways, that's today's installment on 2-Cycle Theory.

Check out some really wild, riced-out 2-stroke chainsaws called "hotsaws" from logging shows. Very strange muffler designs. Some gotta be started externally (like with a power drill). www.hotsaw.com is a small site to start with. Most can cut 3 swipes all the way through a 22" pine log in under 4 seconds! Crazy, man....

'K. Later!
-Alpinwolf

 

and a lot depends on the motor.. If you put a larger free flowing exhaust on some motors, you will find yourself looking for ways to get the low end grunt back, particularly on a 4x4 or if towing (don't ask how I know this...). Now if the vehicle in question already produces tons of low end, then it may not be as noticable and you can get away with larger exhausts. It is definitely another phase of engine tuning, and not all respond the same.

--Mike

 

To expand a bit on what Mike said, a single 3" exhaust would be too big for a 4 cylinder Ranger but just about right for a big 460, providing both engines are stock. The HP/TQ on the Ranger would be moved up the RPM band too much to be usefull.

Here is a great thread-

https://www.ford-trucks.com/forums/s...essure+falcon+

 

well from what ive peiced together...uve seen on bull dozers and tractors that lil flappything on the end of the pipe....well the makes more back pressure at low r's, hence more tq and opens up the higher u rev giving more hp..at least thats what ive peiced... u also see it on older semi's

 

ooo and on another note... the backpressure helps the piston go down easyer at low r's.

 

I think the flapper thing is to keep rain out of the exhaust when the engine is off. And backpressure is bad. Always. Flow velocity is what decreases with too large an exhaust pipe, and is what decreases low rpm performance.

 

well ive always heard the low r back pressure improved tq...im gald i stand corrected