[Gary Drexler]

If I remove my mufflers, and replace them with straight pipes will I sacrifice low end performance for better top end?

[mchell]

on a stock type motor you may well loose some low end and gain no top end also.......combo??

[Jack Gifford]

Okay... let's get this shouting match under way! Is beneficial back-pressure merely an old wives' tale or not?

[Laguna454s3]

No, you need velocity.

Smaller diameter and longer length pipe will aid in increasing velocity. This can help to scavange the combustion chamber and suck in the next cycle of fresh air and fuel.

Unfortunately there is no one correct size exhaust system for a street car as it is required to operate at RPM's of 800-5000. A race car which will operate in a specific RPM range can have it's exhaust 'tuned' for optimum volume and velocity to aid in scavenging the cylinders.

Newer engines are designed with variable valve timing to open the valves wider/longer with increased RPM but an exhaust system doesn't have that capability.

John

[Jack Gifford]

Not enough SHOUTING yet! But let's confine it to the old-favorite controversy regarding BACK PRESSURE.

[67 455 Bird ragtop]

Let's throw a wrinkle into the Back Pressure issue. With an x-pipe. Can you start small and get bigger as you head back ??? Say 3" out of the collector then to a 2.5 x 4.5 oval x-pipe and the rest of the way out ???

While I feel some back pressure is necessary how can you really tell what is enough, too much, or too little without a lot of dyno time on your specific combo ??

[David Holmberg]

Well as the exhaust gases cool they contract and take up less space and in turn don't need as large a pipe to flow thru.

If you want to keep velocity up normally you would step down in size the farther you get away from the motor on a street car. I have 2 1/2 in and 2 1/4 out at the mufflers on my BBC suburban tow vehicle.

[GTOGreg]

This subject is documented in Jim Hand's book on building hi perf Pontiac engines. Homberg is right, as gases cool they require less diameter. You are ok with running a smaller tailpipe than headpipe, not the other way around. I have 2.5 into 2.25 tails for a stock appearance (used with 2.5" RARE manifolds)

[Jack Gifford]

I guess I need to throw the first punch to get this argument really going-
I can't swallow the notion that "back pressure" can improve any 4-stroke engine's performance, if back pressure is defined as a time-averaged pressure that exceeds atmospheric.

[BruceWilkie]

Jack, I'm in your corner on this too. ZERO backpressure improves mechanical efficiency! Quite simply the piston doesnt have to push against anything. Anti reversion devices try to create a vacuum to pull exhaust out. Low overlap cams as found in stock type vehicles allow little to no intake charge to be pulled into the exhaust. Back pressure slows or stops the intake charge and can even cause the intake charge to be diluted with exhaust. (We know how great EGR is for ineficiency). Even some intake charge coming by the exhaust is beneficial in that it helps cool the exhaust valve. Very liitle fuel is wasted. Switching to dual exhaust for example has always been a benefit even in Grandmas car. It is true that if you dont tune for the loss of backpressure your mileage might suffer or an exhaust valve may suffer excess heat damage. Backpressure is not desirable in a 4 stroke engine though you can use backpressure for sound control or as a govenor but its not an efficient use. With "tuned" exhaust you have a reflected sonic wave arrive at the correct time to bounce off the closed exhaust valve in time to help pull the next exhaust pulse out of the cylinder. It is NOT a back pressure its a sound wave! TAG

[Gary Drexler]

Can the angle, size, or shape of your tailpipe increase sound waves or velocity?

[Jack Gifford]

Originally Posted by Gary Drexler

Can the angle, size, or shape of your tailpipe increase sound waves or velocity?

Wave motion comes into play in any moving column of compressible gases. But any "tuning" of a tailpipe wouldn't travel forward through the muffler, so would have a negligible effect on engine performance.

[TCSGTO]

I always cringe when I here people say you need backpressure for torque without even taking velocity into consideration. I always tell them running a single exhaust or just stuffing a potato up one of the taipipes should be worth 15-20 Ftlbs or so then.

[BruceWilkie]

READ David Vizards article in May 2005 issue Popular Hot Rodding "Exhaust Science Demystified" page 56 for additional insight. NO BACKPRESSURE.

[BruceWilkie]

http://www.superchevy.com/technical/...exh/index.html Here is the article!