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Originally Posted by snakeeyes
Dynos are a great thing, but I think we all tend to place too much stock in the numbers and not enough in how the car actually runs. The post about exhausts and IRS robbing power are dead-on. Research on this site and elsewhere and you'll find that side exhausts, which we all love and believe to be much more efficient than an under-car setup, can actually be a detriment if not carefully designed, As far as your numbers' I can only offer those of my Beck Lister for comparison, The car has a Corvette IRS, which does rob power vs. a live axle because of the multiple u-joints, the angle of the driveshafts, etc. It also has side exhausts. The builder dynoed the engine on an engine dyno--542 hp/ 508 ft-lbs. A year or so later, after I got the car together, I had it dyno-tuned on a chassis dyno. I don't have the numbers in front of me, but I think it made around 417 hp and 395 ft-lbs.--losses of close to 25%. I had been expecting about 15%, which the figure you often see, so I was pretty shocked--until I drove it.
Get the car tuned, get it to a dragstrip, then decide if it's making adequate power (and getting it to the ground) or not.
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Obviously, your 25% loss is not only drivelline loss, but also loss do to the exhaust system, and maybe even the air cleaner as well, so it maybe it's appropriate. In addition, from your post, it appeared that the chassis dyno was not the same guy who built/dyno'd the engine on the stand, which may also enter some fudge factors in determining % hp loss.
Like you said, I think to get optimum performance, you need to have it chassis dyno'd and TUNED, as there can be large performance gains with the engine set-up right. It beats taking the car the strip, and repeatedly drag racing it, adjusting things to try to determine the best state of tune. Probably easier on the car as well.
By the way, I guess you're running a sbc? What is it's displacement ?