This is the first of two rather lengthy posts I prepared about measuring and comparing aftermarket HP additions in a no-BS manner. this talks just about ratings and what "Rear wheel HP" means, etc. My second one talks about the success or failure of various mods and what it takes to get 500 RWHP (at least in my experience).
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Three types of HP ratings are often discussed and compared: engine dyno, flywheel/factory rating, and rear wheel dyno results. Here is my take and some data comparing them from what I’ve actually witnessed.

Engine dyno HP is measured when the engine is out of the car on an engine dyno. You rarely see this quoted outside of magazines like Hot Rod that actually build and test engines on engine stands. Such engine dyno #s are often 5-10% higher than the factory rating: the engine is out of the car, has no exhaust, and often is not running accessories such as the radiator fan or even its water pump.

Flywheel rating is with the engine in the car as driven (exhaust, accessories). This is almost impossible to actually measure unless you are a manufacturer. Most manufacturers quote/advertise this #. There are exceptions where they understate it (1998-2003 Camaro V8s and Ford GT being extreme examples) and somewhere they overstate it (some current Japanese cars have turned out to be overstated by 5%).

RWHP is measured at the rear wheels with a Dynojet, Mustang or other brand chassis dyno (the Mustang usually gives slightly lower numbers by about 1-3%).

Here are ten “stock” cars I have seen tested on the same Dynojet, and the corrected (for temp, altitude, pressure using SAE method) RWHP they recorded:
#s are, Engine dyno HP where available, factory rating, dynojet RW test, losses from rated HP to RW measured HP in %
1998 Camaro Z28 M6: --- , 305, 294, 3.6%
2001 Corvette M6: ---, 350, 309, 11.7%
2002 ZO6 M6: 425, 405, 357, 11.9%
2006 Z06 M6: ---, 505, 447, 11.5%
2004 Carrera (Auto): ---, 320, 265, 17.2%
2005 Mustang GT M5: ---, 300, 262, 12.7%
1996 Lamborghini Countach S M6: 476, 452, 390, 14%
2003 Lingenfelter SC Corvette M6: ---, 510, 455, 10.8%
2004 Glamour-Tuner Mustang M5: ---, 450, 365, 18.9%

I’m not going to identify the tuner for the Mustang (people are too litigious and owners take it too seriously), but it is a well-known company selling new modified and supercharged Mustangs through select Ford dealers. That car is typical in my experience with many tuner cars: some tuners exaggerate the HP they’ve added. The only tuner I completely trust in this regard is Lingenfelter, which typically rates its products conservatively.

RWHP is really all that matters, but one often wants to convert RW to compareable factory rating for comparison purposes. I use an 11.7% losses factor when doing so: the average for the three stock ‘vettes listed above. Thus, if mods add 89.5 RWHP, I say they added (89.5*111.7%) =100 at the flywheel . Many people use 15%, but as you can see from the tests above, this would tend to overstate the HP.

Anyone interested in this topic, and modifying your engines to get more power, might want to pick up the January issue of HOT ROD and look at the article on page 104: "LS1 Cookbook: Five Steps to 530 HP.

NOTE: they start with a stock GM LS1 crate engine with the original LS! (rather than later LS6) intake manifold.

It produces 366 HP on an engine dyno
It was rated by GM at 345 in the car.
It typically dynboed at about 295 at the rear wheels.

After modifications to "staage 5" the LS1 is putting out 531.6 HP on the engine dyno, which if you work the ratios, works out to about 428 HP on a chassis (rear wheels) dyno - just about what you'd expect.**

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** Calculation is:
(295RWHP/366 engine dyno) x 531.6 engine dyno==> 428 RWHP

Thanks for the heads up, I'm going to go out and buy that today.

my 2003 c5 coup is getting hot with only 36000 miles, any ideas
only have changed oil and filter so far..

Next time make a new topic godspeed. It could be your thermostat or low coolant.

Thanks for the info Lee. I don't mean to impose, but would you answer some additional questions?

On the losses:
1) Are the losses incurred through the accessories and drive train linear?
2) Are there ways to minimize the losses?

On the factory rated HP:
1) Would there be a variation in actual HP from engine to engine due to build process variations?
2) a)Will the engine HP go up as the internal parts begin to "wear in" and internal friction begins to decrease?
b) If so then how much would you expect to gain?

Again, don't feel obligated to answer these if you don't feel like it.

Thanks,

Dale

Thanks for the info Lee. I don't mean to impose, but would you answer some additional questions?

On the losses:
1) Are the losses incurred through the accessories and drive train linear?
Not exactly, but pretty much -- no accurate data that says otherwise; engineering analysis indicates most should be (except the Ac pump, which presumably is off during real attempts at speed); and assuming its linear works pretty well as a guideline in decisons about mods.
2) Are there ways to minimize the losses?
Use premium lubricants in the trans and diff. Install hard compound tires and inflate them to the maximum in the permitted range, check all u joints and constant velocity joints. Install an underdrive pulley on the engine. (Note that the tire reocmmendations fly in the face of getting great traction but they do reduce losses).

On the factory rated HP:
1) Would there be a variation in actual HP from engine to engine due to build process variations? Yes, definately some, maybe 1up to 3% variation there.
2) a)Will the engine HP go up as the internal parts begin to "wear in" and internal friction begins to decrease? b) If so then how much would you expect to gain?
Yes, cars typically gain about 3-7% HP as they wear in: test a LS2 vette with 5 miles on it and and it probably gives 335 RWHP, test it at 3000 miles and it will give about 350. Then, starting around 40,000-50,000 miles power slowly detriorates due to valves and valve seat wear so that by the time they reach 100,000 miles it is back down to about where it was when new and getting worse as time goes by. Usually a valve job will restore it. (An old trick to build a fantastic engine is: put it together well from preimum parts. Install it in a car and put about 40,000 miles on it, taking very good care to change the oil often, etc., while babying the engine, or better yet, set it up on a bench with cooling, etc., and run it for 1000 hours constantly at 2500 RPM. Then remove the heads, replace the valve guides, polish the ports, and do a really good three-angle valve job, etc.,then reinstall the heads. The engine will be just about as good as you can get.)



Dale