This is the first of two rather lengthy posts I prepared about measuring and comparing aftermarket HP additions in a no-BS manner. ((Posted here and on the Corvette Forum, too). 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 Iā€™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 ā€œstockā€ 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%

Iā€™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 theyā€™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 ā€˜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.

 

Very nice post! Informative

 

Good stuff. What are some #s for stock LT1s?

 

I haven't seen this myself, but I tend to believe what I was told in this case.

335 HP LT4 in a 96 Corvette M6: about 295 RWHP
285 HP LT1 in a Camaro M6, about 255-260 RWHP (don't feel bad about this, it's about what a 300 HP rated '05 Mustang GT puts down).

 

Not bad. Most stock LT1s I've seen are normally putting down about 255-265 rwhp, so I guess that's right on par.....

 

Dynos vary. 260 one place might be 265 another. I've always felt that the dyno I use (Carolina Auto Masters in Durham - J. "the Milkman" Creech) was a bit conservative versus others, but what is important is the relative increase you get from mods. I think most LT1s with an M6 do in the 260-265 range. People tend to forget them since the LS1 came out but they have a lot going for them, not the least of which is the gobs of performacen parts available and the cast iron, bulletproof block they have.

 

True. Nothing against the LS1, but you couldn't pay me enough to get rid of my LT1 for one. I love the LT1.

 

I love them both. I had a new 1968 327 Camaro -- all I could afford at the time but new, when I was 20, which I did a lot of budget work on (again, all I could afford). Never got that car any better than the very high 14s, but I had fun.

The Chevy "mouse motor" in all its iterations is one of the greatest. I'd love to have an LT4 in something like a '96 vette or a 1968 Camaro.

That said, I think the [LS1-2-3 -6, and C5R-LS7] is perhaps the best engine design ever made: compact, lightweight, durable and they just breath well (although I must admit the heads on the new Hemi SRT are impressive).

GM can't concieve of or market cars well, or run a corporation profitably, but damn they design fine engines.

 

GM can't find their *** with both hands. They just laid of 30,000 people, undoubtedly due to their bungling, and stupid marketing decisions. Let's be honest. GM's "affordable performance car" is a FWD Cavalier replacement. It can be argued that the GTO is their "affordable performance car," but it doesn't make much sense to dump a car that stickers for the mid $20k range that isn't selling as it is, and then crap out one that stickers for $32,000 and expect people to purchase that one instead.

Let's take a look at Chevy's lineup. We've got a bunch of boring family sedans, and econojunk.

Aveo, Cobalt, Malibu/Maxx, Impala, and Corvette. Then of course, the basic SUVs. The ONLY vehicle in Chevy's lineup that evokes any performance is the Corvette. Sure, there's the FRONT WHEEL DRIVE Impala SS with 303 hp, but c'mon now. Who does GM think they're fooling? Or the Cobalt SS with FRONT WHEEL DRIVE, and 205 hp. Again, who do they think they're fooling?

Buick? Not even worth a response.

Pontiac?

Vibe, Solstice, Grand Prix, GTO, and the "crossovers." The GTO is a rebadged Monaro, and mighty plain looking, even with its dual hoodscoops. Not to mention, it carries the GTO name, which it definitely doesn't live up to. If GM wanted to capitalize on the GTO name, and be successful, they should've designed the car here in America, and not rebadged a Holden Monaro. Don't get me wrong, Holdens are awesome cars, but not with classic names like "GTO." The GTO should've been called the "GTP." If the GTO wasn't called a GTO, it would sell.

The Solstice? Lovely. A secretary's hot rod.

Grand Prix? Again, nice car, but 303 hp and FRONT WHEEL DRIVE do not mix. It's simply a band aid for an otherwise boring FWD sedan.

GM needs to pull their head out of their asses, and get on the ball.

 

I agree. Further, they do really dumb things: create Saturn but then dump Olds. I won't go into all of the why here, but Saturns remind me very much of Oldsmobiles back in the 70s, when Olds was the #3 selling brand in the world and could do no wrong - the products are pretty sound (I don't like them, but not everyone wants a performance car) but GM is mismanaging it badly. The whole Hummer division creates interesting vehicles but is a business disaster.