The engine for my '70 coupe 350/350 4speed has been rebuilt, now being reinstalled. The car has a 2" pipe diameter exhaust system, but the mechanic feels that 2 1/2" pipes should be used with this engine. Which is correct, and will I benefit from a change? Thank you in advance.
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Re: Exhaust pipe diameter
Scott, since I can't answer your question from a technical standpoint, let me answer it from a judging point of view. According to the 70-72 Juding Manual, all small block cars, except LT1's got 2" exhausts. I would ASSUME that the GM engineers did testing and engine performance to ensure that their engines could breathe right. With that in mind, I would stick with the stock setup of 2" pipes. In addition, years ago I read an article about bigger pipes on cars and there was a discussion about a certain amount of exhaust back pressure being a required ingrediant in proper engine performance. Don't know why, don't really care why, just want the beast to run well.
Gary -
Re: Exhaust pipe diameter
Scott, since I can't answer your question from a technical standpoint, let me answer it from a judging point of view. According to the 70-72 Juding Manual, all small block cars, except LT1's got 2" exhausts. I would ASSUME that the GM engineers did testing and engine performance to ensure that their engines could breathe right. With that in mind, I would stick with the stock setup of 2" pipes. In addition, years ago I read an article about bigger pipes on cars and there was a discussion about a certain amount of exhaust back pressure being a required ingrediant in proper engine performance. Don't know why, don't really care why, just want the beast to run well.
GaryComment
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Re: Exhaust pipe diameter
Scott------
For 1970, Corvettes with 300hp 350 and 350hp 350 used 2" exhaust systems. Corvettes with LT-1 and LS-5 got 2-1/2" exhaust systems. Is 2" adequate for your engine? Well, let's put it this way: for 1969, ALL engines used 2" exhaust systems, including L-88 and ZL-1.
To be honest, for 66+ small blocks, I really don't see what advantage that a 2-1/2" system offers, although many had them. That's because ALL 1966 and later Corvette small blocks used exhaust manifolds with 2" outlets. So, following the theory that no chain is stronger than its weakest link, I really don't see the advantage of the 2-1/2" system. For 1962-65, special high performance small blocks received exhaust manifolds with 2-1/2" outlets. So, for use with these manifolds, I can see where a 2-1/2" exhaust system might have some real benefit.
All big blocks used 2-1/2" outlet exhaust manifolds and, with one exception, all big blocks also used 2-1/2" exhaust systems. The exception: as I previously mentioned, ALL 1969 big blocks used 2" exhaust systems. So, for these systems, the 2-1/2" outlet manifolds dumped into necked-down exhaust pipes.In Appreciation of John HinckleyComment
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Re: Exhaust pipe diameter
Scott------
For 1970, Corvettes with 300hp 350 and 350hp 350 used 2" exhaust systems. Corvettes with LT-1 and LS-5 got 2-1/2" exhaust systems. Is 2" adequate for your engine? Well, let's put it this way: for 1969, ALL engines used 2" exhaust systems, including L-88 and ZL-1.
To be honest, for 66+ small blocks, I really don't see what advantage that a 2-1/2" system offers, although many had them. That's because ALL 1966 and later Corvette small blocks used exhaust manifolds with 2" outlets. So, following the theory that no chain is stronger than its weakest link, I really don't see the advantage of the 2-1/2" system. For 1962-65, special high performance small blocks received exhaust manifolds with 2-1/2" outlets. So, for use with these manifolds, I can see where a 2-1/2" exhaust system might have some real benefit.
All big blocks used 2-1/2" outlet exhaust manifolds and, with one exception, all big blocks also used 2-1/2" exhaust systems. The exception: as I previously mentioned, ALL 1969 big blocks used 2" exhaust systems. So, for these systems, the 2-1/2" outlet manifolds dumped into necked-down exhaust pipes.In Appreciation of John HinckleyComment
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Exhaust back pressure is a myth!!!
For every psi exhaust back pressure, all other things being equal, an engine will loose about one psi BMEP. There are some engines, particularly carbureted, that may show less horsepower with lower backpressure because the fuel metering can be upset by less back pressure making the mixture lean.
This is what happens when you have guys with journalism degrees writing about engines in comsumer car magazines. Another myth is "premium gas burns slower (or 'cooler')", but I'll save that explanation for the next time this myth comes up.
Regarding the exhaust system, remember that the '70 power ratings were SAE Gross, which was the engine tested on a laboratory dynamometer, without the vehicle exhaust, any accessories other than the water pump, and the fuel mixture and spark timing set to optimize torque/power readings at every test speed, which did not necessarily represent their calibration on the car.
The smaller exhaust pipes will cost some power. I can't quantify it based on any data I have, but I'll take a WAG at 5-10 horsepower at the rear wheels at the power peak on a L-46. If you stick with 2" pipes for judging, it may not make much difference unless the pipes are THE originals because most replacement systems are different enough in configuration to loose some points for being "non-original", anyway. If it were my car, I would want to maintain general original appearance, but not necessarily worry about losing points because I had the wrong manifold castings. In that case I would BACKDATE the manifolds to the SHP 327 2.5 inchers (assuming I could fit all the accessory brackets without Mickey Mouse or aftermarket parts) and go with a 2.5 inch exhaust system.
DukeComment
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Exhaust back pressure is a myth!!!
For every psi exhaust back pressure, all other things being equal, an engine will loose about one psi BMEP. There are some engines, particularly carbureted, that may show less horsepower with lower backpressure because the fuel metering can be upset by less back pressure making the mixture lean.
This is what happens when you have guys with journalism degrees writing about engines in comsumer car magazines. Another myth is "premium gas burns slower (or 'cooler')", but I'll save that explanation for the next time this myth comes up.
Regarding the exhaust system, remember that the '70 power ratings were SAE Gross, which was the engine tested on a laboratory dynamometer, without the vehicle exhaust, any accessories other than the water pump, and the fuel mixture and spark timing set to optimize torque/power readings at every test speed, which did not necessarily represent their calibration on the car.
The smaller exhaust pipes will cost some power. I can't quantify it based on any data I have, but I'll take a WAG at 5-10 horsepower at the rear wheels at the power peak on a L-46. If you stick with 2" pipes for judging, it may not make much difference unless the pipes are THE originals because most replacement systems are different enough in configuration to loose some points for being "non-original", anyway. If it were my car, I would want to maintain general original appearance, but not necessarily worry about losing points because I had the wrong manifold castings. In that case I would BACKDATE the manifolds to the SHP 327 2.5 inchers (assuming I could fit all the accessory brackets without Mickey Mouse or aftermarket parts) and go with a 2.5 inch exhaust system.
DukeComment
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Fine Advice *NM*
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Fine Advice *NM*
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Re: Exhaust back pressure is a myth!!!
Dyno runs without exhaust? I'll bet there were a lot of carbon monoxide fatalities!
Gain horsepower---Lose torque. For a street driven Corvette I'd opt for torque! Go with STOCK!
Dale.Comment
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Re: Exhaust back pressure is a myth!!!
Dyno runs without exhaust? I'll bet there were a lot of carbon monoxide fatalities!
Gain horsepower---Lose torque. For a street driven Corvette I'd opt for torque! Go with STOCK!
Dale.Comment
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Re: Exhaust back pressure is a myth!!!
Duke-----
For most 1970 and 71 small blocks, the 62-65 2-1/2" outlet manifolds CAN be used with no problem, whatsoever. That's because 1970 and 71 small blocks (except LT-1) did not use the air injection reactor system. So, no manifold fittings for same are required. GM never made a 2-1/2" outlet small block manifold with AIR fittings, so folks needing that system have no 2-1/2" outlet manifolds available to them. But, folks without AIR do. For 1970 and 1971 here's what's needed:
left side manifold----GM #3846563---This manifold is a direct "bolt-on" and has the correct bracket on the front for the 70-71 style alternator mounting.
right side manifold----GM #3814970 or GM #3797901, depending upon whether one desires the unused early style, side alternator bracket on the front or rear, respectively.In Appreciation of John HinckleyComment
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Re: Exhaust back pressure is a myth!!!
Duke-----
For most 1970 and 71 small blocks, the 62-65 2-1/2" outlet manifolds CAN be used with no problem, whatsoever. That's because 1970 and 71 small blocks (except LT-1) did not use the air injection reactor system. So, no manifold fittings for same are required. GM never made a 2-1/2" outlet small block manifold with AIR fittings, so folks needing that system have no 2-1/2" outlet manifolds available to them. But, folks without AIR do. For 1970 and 1971 here's what's needed:
left side manifold----GM #3846563---This manifold is a direct "bolt-on" and has the correct bracket on the front for the 70-71 style alternator mounting.
right side manifold----GM #3814970 or GM #3797901, depending upon whether one desires the unused early style, side alternator bracket on the front or rear, respectively.In Appreciation of John HinckleyComment
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Re: Exhaust back pressure is a myth!!!
SAE gross ratings were obtained with the production manifolds, but attached to them
were generous sized pipes that led the exhaust gases out of the dyno cell, and dyno
cells typically also have an evacuation pump downstream to ensure that exhaust gases
are purged. Thus, there is "zero backpressure" - actually a slight negative pressure due
to the evacuation pump, and if that pump has enough capacity to maintain a couple psi
depression in the exhaust that could make for a good three percent extra horsepower at the
brake.
DukeComment
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Re: Exhaust back pressure is a myth!!!
SAE gross ratings were obtained with the production manifolds, but attached to them
were generous sized pipes that led the exhaust gases out of the dyno cell, and dyno
cells typically also have an evacuation pump downstream to ensure that exhaust gases
are purged. Thus, there is "zero backpressure" - actually a slight negative pressure due
to the evacuation pump, and if that pump has enough capacity to maintain a couple psi
depression in the exhaust that could make for a good three percent extra horsepower at the
brake.
DukeComment
Comment