This thread also raises some questions:
http://www.c5-corvette.com/ubb/Forum1/HTML/000011.html
They say if they do the dyno work,(which we don't do), about the best they
can expect is to gain 5-8 hp. Not much! They go on to say, if you have
free flow mufflers they are not sure if there is a gain and might even
loose some hp.
I sort of liked reading this because it was on Corvettes which are heavy
breathers, but then they go on to talk about you might need a better intake
manifold that breathes better (gag-- wonder if they ever heard of a Toro
intake manifold.)
Must admit I am sort of leaning back to the Y-pipes since I have the Howell
TBI which would correct the lean condition that could be generated.
gene
>
> 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
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/
http://www.c5-corvette.com/ubb/Forum1/HTML/000011.html
They say if they do the dyno work,(which we don't do), about the best they
can expect is to gain 5-8 hp. Not much! They go on to say, if you have
free flow mufflers they are not sure if there is a gain and might even
loose some hp.
I sort of liked reading this because it was on Corvettes which are heavy
breathers, but then they go on to talk about you might need a better intake
manifold that breathes better (gag-- wonder if they ever heard of a Toro
intake manifold.)
Must admit I am sort of leaning back to the Y-pipes since I have the Howell
TBI which would correct the lean condition that could be generated.
gene
>
> 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
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/
