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      06-09-2019, 06:20 AM   #11
CSBM5
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Drives: 2019 M2 Comp, 2011 M3, etc
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Without question the imposition of ever stricter emissions standards took its tool on engine performance (both in terms of power output and driveability) with the worst of that period peaking in the late 70s. The first glimmer of hope appeared with Lambda sensors (O2) and 3-way cats, which for BMW were first seen in the 1979 528i.

However, don't forget that older car horsepower figures were all quoted in terms of SAE gross horsepower prior to the 1972 model year (SAE J245). This allowed automakers to measure the highest possible output from a bare engine. They typically had optimized tubular headers into an open exhaust; no air filter but nicely sculpted intake plumbing into the carb; no water pump, no power steering pump, no alternator; ignition timing was free to be set at whatever the automaker desired, etc, etc.

In other words, SAE gross HP was the maximum possible test stand horsepower output without any normal detractors (such as all the above plus mufflers and probably some stuff I'm forgetting).

SAE J1349 was introduced to measure SAE net HP which required the engine to be measured with all accessories, intake and exhaust systems as it would have installed in the car in question - essentially requiring engine output to be measured as it would actually be delivered installed in the car.

Overnight into the 1972 model year, HP ratings plummeted since California required only SAE net HP (J1349) to be used from '72 onward; hence that standard was adopted nationwide. The difference between SAE gross and net is not easily nailed down as one ratio but typically is in the 20-30% range (net lower than gross of course). During the 1971 model year some carmakers quoted both values, so for those models you have a direct comparison, and some were as large as 35% lower between the two ratings.
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Current Stable:
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2019 F87 M2 Competition 6MT, LBB, slicktop, exec pkg
2007 E91 328i Silver, slushbox, Eibach fr/E93 M3 rear sway bars, ARC-8
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