I posted the following to another thread, but the question gets asked enough I think it deserves its own thread.
I've attached (with permission) three test sheets from George's rebuilt '65 L-84 back in 2001. There are three tests, and the engine is basically a "stock rebuild" - .060" overbore, with OE replacement 30-30 cam and forged, 5.3cc dome pistons. The heads received a valve reseat, but no "massaging" for improved flow, so they are representative of typical Flint-machined heads. Gasket thickness is not verified, but as is typical I am currently assuming that it has a thick composition gasket which would place the CR at something close to 10.25:1 assuming nominal deck heights, which were not measured.
Download the pdf and we can compare. Look at page three, and ignore the data about headers and mufflers. This configuration had the OE manifolds routed into the 3" dyno exhaust system with no mufflers. Correction is STP so this engine is representative of the configuration and SAE gross test procedure that Chevrolet used to establish the advertised SAE gross ratings.
Note that the 321/332 torque/power ratings are 92/89 percent of the advertised 350/375 HP ratings that can also be expressed as actual SAE gross torque/power being 8/11 percent lower than the advertised ratings, which is typical of the era. Sorry guys, that's the way it was back then. Honest ratings did not come into effect until the new SAE net procedures were first implemented in 1971.
Massaging the heads would have improved torque and power by close to ten percent, which would about the achieve the advertised ratings.
Now let's look at the two tests on page 1 and 2. These were back to back tests within about five minutes and the exhaust system consisted of the OE manifolds and full OE sidepipe system that routed to the dyno cell exhaust system with no downstream mufflers.
Sheets 1 and 2 torque/power average about 291/293 or about 91/88 percent of sheet three and the only difference is the addition of the sidepipes, so sidepipes alone knock down torque/power by 9/12 percent.
Now we can get some insight into how restrictive the sidepipe system is compared to the under-the-car exhaust. I have dyno data for two "327 LT-1" configurations, both tested with OE exhaust manifolds and measured CR averaging about 10.4:1. Both have the ...461 OE aluminum manifold, OE Holley, and LT-1 cam with nicely massaged heads. One was tested on a lab dyno at 356 GHP and the other on a chassis dyno with under-the-car exhaust at about 270 with SAE air density correction. Using 0.85 driveline/tire efficiency that would be about 318 SAE net at the flywheel, which yields a 0.89 net/gross conversion factor.
About half of this net-gross difference is the lower air density correction of SAE net compared to STP used for SAE gross, and the rest is the exhaust system and front end accessories. Of the latter, only an alternator and clutch fan that does not tighten absorbs only a couple of horsepower, so from a practical standpoint we can say that half the net/gross difference is air density correction and the other half is exhaust pumping loss.
Given the above the under-the-car exhaust system pumping loss is about 19 HP or about 5.3 percent.
Sidepipes ate up 39 GHP on George's L-84, which is about 11.8 percent, so we can conclude that sidepipes cost about double the parasitic pumping power as the under-the-car system, which is significant!
Duke
I've attached (with permission) three test sheets from George's rebuilt '65 L-84 back in 2001. There are three tests, and the engine is basically a "stock rebuild" - .060" overbore, with OE replacement 30-30 cam and forged, 5.3cc dome pistons. The heads received a valve reseat, but no "massaging" for improved flow, so they are representative of typical Flint-machined heads. Gasket thickness is not verified, but as is typical I am currently assuming that it has a thick composition gasket which would place the CR at something close to 10.25:1 assuming nominal deck heights, which were not measured.
Download the pdf and we can compare. Look at page three, and ignore the data about headers and mufflers. This configuration had the OE manifolds routed into the 3" dyno exhaust system with no mufflers. Correction is STP so this engine is representative of the configuration and SAE gross test procedure that Chevrolet used to establish the advertised SAE gross ratings.
Note that the 321/332 torque/power ratings are 92/89 percent of the advertised 350/375 HP ratings that can also be expressed as actual SAE gross torque/power being 8/11 percent lower than the advertised ratings, which is typical of the era. Sorry guys, that's the way it was back then. Honest ratings did not come into effect until the new SAE net procedures were first implemented in 1971.
Massaging the heads would have improved torque and power by close to ten percent, which would about the achieve the advertised ratings.
Now let's look at the two tests on page 1 and 2. These were back to back tests within about five minutes and the exhaust system consisted of the OE manifolds and full OE sidepipe system that routed to the dyno cell exhaust system with no downstream mufflers.
Sheets 1 and 2 torque/power average about 291/293 or about 91/88 percent of sheet three and the only difference is the addition of the sidepipes, so sidepipes alone knock down torque/power by 9/12 percent.
Now we can get some insight into how restrictive the sidepipe system is compared to the under-the-car exhaust. I have dyno data for two "327 LT-1" configurations, both tested with OE exhaust manifolds and measured CR averaging about 10.4:1. Both have the ...461 OE aluminum manifold, OE Holley, and LT-1 cam with nicely massaged heads. One was tested on a lab dyno at 356 GHP and the other on a chassis dyno with under-the-car exhaust at about 270 with SAE air density correction. Using 0.85 driveline/tire efficiency that would be about 318 SAE net at the flywheel, which yields a 0.89 net/gross conversion factor.
About half of this net-gross difference is the lower air density correction of SAE net compared to STP used for SAE gross, and the rest is the exhaust system and front end accessories. Of the latter, only an alternator and clutch fan that does not tighten absorbs only a couple of horsepower, so from a practical standpoint we can say that half the net/gross difference is air density correction and the other half is exhaust pumping loss.
Given the above the under-the-car exhaust system pumping loss is about 19 HP or about 5.3 percent.
Sidepipes ate up 39 GHP on George's L-84, which is about 11.8 percent, so we can conclude that sidepipes cost about double the parasitic pumping power as the under-the-car system, which is significant!
Duke
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