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Originally Posted by olddog
I have read on here many times about someone very confused at how they lost so much HP from the engine dyno to the chasses dyno. When you consider the large inertia of a 4+" stroke FE, large diameter flywheel and clutch, large wheels and tires, and heavy duty rear end and tranny, a 3 second chasses dyno run is going to measure much lower numbers. This is due to so much power going into accellerating all this enertia. Let's face it your car is not going to go from 40 mph to 120 mph in 3 seconds, at a 1:1 gear ratio.
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The length of a dyno run is based on the vehicle itself, and not so much the dyno. A more powerful vehicle will accellerate faster on a dyno, just like it would on the road. It also depends on what rpm the dyno operator starts the dyno at. If you start the dyno at 4500 and the motor only goes to 6500, that's going to be a very quick dyno. I usually start about 3000 rpm, and pull to redline, which varies with vehicle. But the average time for a pull on my dyno, I would say is about 6-7 seconds. It also depends on the TYPE of dyno. I use an inertia dyno. So the vehicle is moving a 3000 lb mass on my dyno. Obviously a more powerful car will spin the mass faster. Some people use eddy current dynos, that "load" the dyno with a specifc load, set by the operator. With these dynos, you can adjust the amount of load, which naturally will affect the numbers.
The one thing I always suggest to people, is to not get caught up in the peak numbers. The dyno is a tool, used for tuning. Where you finish hp wise is not as important as how much you gained from the tuning.
Cobras have a LOT of parasitic loss, much more than any other vehicle. IRS eats up power, and so do Cobra side pipes. I have dynoed vehicles with the same motors as in some Cobras and they don't have anywhere near the loss.
Big block, small block, it doesn't matter.