REAR MUFFLER CONSIDERATIONS

[Y-PIPE] [BACK-PRESSURE] [NOISE] [HORSE-POWER]

When I blew out one of my mufflers, I started thinking about the trend toward a single rear muffler. I asked the following :

DESIGN OF EXHAUST SYSTEMS
             required back pressure for the engine
             required back pressure for the valves
             if this is so good why didn't GM do it ?

Here are some of the answers

BACK PRESSURE
1. The engine doesn't "require" ANY back pressure. The less you have, the more power you can make. Only caveat is that significantly lowering the back pressure tends to make a carburetted engine run lean (unless you rejet the carburetor). This can lead to burned valves and detonation.

2. Required back pressure for valves: see above. The only other problem I know of regarding free-flowing exhaust, relates only to those racing engines with very short (usually individual) exhaust stacks. With these, if the engine is shut down immediately after a hard run (ie: very hot), cold
air travelling back into the exhaust stacks can warp the hot exhaust valves. (That's why drag racers cap their exhaust pipes after a run.) This is not a problem for us.

3. Why didn't GM do it? Well, there have been some improvements in exhaust design over the past 25 years.... Perhaps interference with the holding tank dump tube was a problem. (I believe GM routed it straight out the rear.)

If you do it, be sure to check your fuel mixture and rejet as necessary. RICK

INCREASED HORSE POWER OVER MUFFLERS
Since we GMC guys are not doing the dyno testing. This is hard to evaluate. But I did find this reference to a big motor application that says straight pipes would add only 5 to 8 HP.

http://www.c5-corvette.com/ubb/Forum1/HTML/000011.html

Since we are addding a single muffler in the rear we might not even gain this.

HEAT AND NOISE MOVED TO THE REAR
This might be good or bad. Have to make sure there is protection.

Y-PIPE EXHAUST

There is more than one way to make the Y-Pipe exhaust. The following pictures from Heinz and Steve show two of the possible configurations.

Heinz-1