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There have been numerous discussions about motor performance, motor swaps, car performance, and overall HP improvments in the last few weeks. I'm going to take a few moments to set the bench racing record straight for future discussions...
There are five real figures that need to be clarified when discussing vehicle performance. These are:
SAE Gross HP
SAE Net HP
Advertised HP
Dyno HP
and...
Power-to-weight ratio.
SAE Gross HP was used up thru 1971, and was quite literally a 'gross' exaggeration of engine HP. All accessories were off the motor during the test, and the test cell was set up to 'favor' the motor. In the case of one manufacturer, high static fans were installed on the exhaust side to remove spent gases - this acted as a kind of 'polish supercharging' to inflate the HP values.
SAE Net HP was initiated in 1972 after the abuses mentioned above. As a general rule of thumb, these SAE net values reduced reported HP by approx 27%, and the system is still in use today.
Advertised HP is the numer manufacturers use for marketing purposes. This is the SAE Net HP generated by a 'perfect' motor - balanced, blueprinted, and tweaked by the best of the best to get the highest possible HP rating for advertising puposes. Real-world motors make about 20% less than advertised HP.
Dyno HP is what you see with your own two eyes as YOUR motor is bolted up to a dyno stand. Assuming a stock motor, it is ALWAYS the lowest of the four HP numbers. Remember, figures lie and liars figure.
Power-to-weight is a ratio of the vehicle's weight divided by its HP. This is a leading indicator of its performance, and is quite reliable - I've won many a street race by estimating power-to-weight. Where folks get goofed up is when they use different HP figures to factor power-to-weight. Let's put this into practice...
a 1966 Mustang coupe with a 289 2 bbl makes 200 SAE gross HP weighs 2500 lbs (prox). The power-to weight ratio is 12.5:1. Conversely, an Opel GT in 1973 makes 88 SAE net HP and weighs in at 2100 lbs (prox.) for a power-to-weight of 23.8:1.
No contest, right?
Whoa, buddy! SAE gross to SAE net is an apples-to oranges compraro! To make it 'fair', the Mustang should be 'de-rated' to SAE net - more like 146 HP. This provides a power-to-weight of 17.1:1. But wait - we're not done...
If we were to hook both motors up to a dyno, we'd see that the 289 is actually closer to 115 HP, and the Opel 1.9 is around 70 HP. Solving for power-to-weight nets out 21.7:1 for the 'stang, and 30:1 for the Opel.
This is important, as the ratios are getting closer as we reflect reality. When we started, the Mustang's power-to-weight was 50% better - when we made it apples-to-apples, it was only 30% better.
Now...We add in Weber, some porting and a low restriction exhaust to pick up approx 20 HP. The power-to-weight of th GT drops to 23.3:1 - this is within spittin' distance of the 'stang. At this point, gear ratios and driver's reaction win/lose the race.
Soo...keep this in mind when yer friends start bandying about HP figures inna bench racing session, and keep a calculator handy!
There are five real figures that need to be clarified when discussing vehicle performance. These are:
SAE Gross HP
SAE Net HP
Advertised HP
Dyno HP
and...
Power-to-weight ratio.
SAE Gross HP was used up thru 1971, and was quite literally a 'gross' exaggeration of engine HP. All accessories were off the motor during the test, and the test cell was set up to 'favor' the motor. In the case of one manufacturer, high static fans were installed on the exhaust side to remove spent gases - this acted as a kind of 'polish supercharging' to inflate the HP values.
SAE Net HP was initiated in 1972 after the abuses mentioned above. As a general rule of thumb, these SAE net values reduced reported HP by approx 27%, and the system is still in use today.
Advertised HP is the numer manufacturers use for marketing purposes. This is the SAE Net HP generated by a 'perfect' motor - balanced, blueprinted, and tweaked by the best of the best to get the highest possible HP rating for advertising puposes. Real-world motors make about 20% less than advertised HP.
Dyno HP is what you see with your own two eyes as YOUR motor is bolted up to a dyno stand. Assuming a stock motor, it is ALWAYS the lowest of the four HP numbers. Remember, figures lie and liars figure.
Power-to-weight is a ratio of the vehicle's weight divided by its HP. This is a leading indicator of its performance, and is quite reliable - I've won many a street race by estimating power-to-weight. Where folks get goofed up is when they use different HP figures to factor power-to-weight. Let's put this into practice...
a 1966 Mustang coupe with a 289 2 bbl makes 200 SAE gross HP weighs 2500 lbs (prox). The power-to weight ratio is 12.5:1. Conversely, an Opel GT in 1973 makes 88 SAE net HP and weighs in at 2100 lbs (prox.) for a power-to-weight of 23.8:1.
No contest, right?
Whoa, buddy! SAE gross to SAE net is an apples-to oranges compraro! To make it 'fair', the Mustang should be 'de-rated' to SAE net - more like 146 HP. This provides a power-to-weight of 17.1:1. But wait - we're not done...
If we were to hook both motors up to a dyno, we'd see that the 289 is actually closer to 115 HP, and the Opel 1.9 is around 70 HP. Solving for power-to-weight nets out 21.7:1 for the 'stang, and 30:1 for the Opel.
This is important, as the ratios are getting closer as we reflect reality. When we started, the Mustang's power-to-weight was 50% better - when we made it apples-to-apples, it was only 30% better.
Now...We add in Weber, some porting and a low restriction exhaust to pick up approx 20 HP. The power-to-weight of th GT drops to 23.3:1 - this is within spittin' distance of the 'stang. At this point, gear ratios and driver's reaction win/lose the race.
Soo...keep this in mind when yer friends start bandying about HP figures inna bench racing session, and keep a calculator handy!