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.
I have found the thread on the mounting of the muffler in the rear quite
interesting as I have been intending to do this eventually on my coach.
However, I'm finding conflicting information. Rick (and I believe Steve
also) maintain that removing back pressure is "good". As far as I can
gather, this is almost a truism in racing circles. As I understand it, less
back pressure allows the gases to exit more quickly on a high cubic inch,
high horsepower engine, although even this is suspect if I can believe the
figures on the recent Dodge Vipers (recently changed from dual to single
exhaust, which the magazines say increased the horsepower . . . go figure).
But getting back to the GMC. It strikes me as a given that we would like to
have the torquiest (is that really a word?) engine possible at the lowest
possible RPM range. This would allow the engine to push this big bulky c
hunk of metal along efficiently, and thus increase the gas mileage by
having the oomph to do it at a lower RPM. Now to cut to the chase. I
received literature recently from SuperTrapp about their mufflers. As
posted, you quiet these mufflers down by removing discs. According to the
literature from the company, this creates more backpressure, quiets the
engine, enriches the carburetion and "decreases the powerband to give you
more low-end torque"; adding discs increases the exhaust opening, reduces
back pressure thus "increasing the powerband to give you more top end".
Seems to me that the top end is not where we want to be in the GMC but
hanging out at the torquey (surely this is a word) low end with the
diesels.
Please comment gentlemen.
Vic Marks
Vancouver, BC
75 transmode
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.
I have found the thread on the mounting of the muffler in the rear quite
interesting as I have been intending to do this eventually on my coach.
However, I'm finding conflicting information. Rick (and I believe Steve
also) maintain that removing back pressure is "good". As far as I can
gather, this is almost a truism in racing circles. As I understand it, less
back pressure allows the gases to exit more quickly on a high cubic inch,
high horsepower engine, although even this is suspect if I can believe the
figures on the recent Dodge Vipers (recently changed from dual to single
exhaust, which the magazines say increased the horsepower . . . go figure).
But getting back to the GMC. It strikes me as a given that we would like to
have the torquiest (is that really a word?) engine possible at the lowest
possible RPM range. This would allow the engine to push this big bulky c
hunk of metal along efficiently, and thus increase the gas mileage by
having the oomph to do it at a lower RPM. Now to cut to the chase. I
received literature recently from SuperTrapp about their mufflers. As
posted, you quiet these mufflers down by removing discs. According to the
literature from the company, this creates more backpressure, quiets the
engine, enriches the carburetion and "decreases the powerband to give you
more low-end torque"; adding discs increases the exhaust opening, reduces
back pressure thus "increasing the powerband to give you more top end".
Seems to me that the top end is not where we want to be in the GMC but
hanging out at the torquey (surely this is a word) low end with the
diesels.
Please comment gentlemen.
Vic Marks
Vancouver, BC
75 transmode
)
