i know ford is rear wheel hp.how about chevy,like on a 94 lt 1 says 300 hp is that crank or rear wheel hp.......and what does hp realy stand for,300 hundred horses [animal] put into one motor or 300 horses running in a pact? we were having a discussion with my buds,one says one thing i say another.i say lets get some oppinions. thanks guys or gals.



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If I remember correctly in the late 60's hp was rated at the crank and then where forced to change to rwhp.

 

First of all, Fords are not rated at their rear wheel HP. Wherever you heard this, you were miusinformed. All manufacturers rate their engines at flywheel HP according to one of several standards (definitions) in effect at the time as defined by SAE (Society of Automtoive Engineers) or DIN (Deutsche Industrial Normale - Germany) or other (Japan and China have theirs, too). All are similar in that they rate Hp at the flywheel, but not exactly the same - DIN and SAE differ by about 1% : the engine in my Porsche is rated at 320 under one system and 316 under the other.

And a HP bears little if any relationship to what a real horse could or would produce: it is a unit of work defined by engineering formulae: 33,000 ft-lbs minute, which means the ability to lift 330 lbs 100 feet in one minute (imagine you hook the horse with pulleys to a crak to lift the weight). Supposedly, some time in ancinent history, someone actually tested a horse using weights and pulleys like this and figured this was what an average horse would do. Now, it is just a unit of power output measure and it doesn't matter, you can compare engines and cars as long as everyone uses the same 33,000 ft lb deinition.

If it matters, the way to interpret HP rating is that a 300 HP car would produce 300 times as much power as one of those mythical perfect horses that could pull excatly 330 lbs exactly 100 feet straight up in exactly one minute.

Cars produce less HP when measured at the rear wheels than at the flywheel because of drivetrain losses: about 5-8% through the tranmission, 1-2% in the driveshaft U-joints, 3% in the differential, and 1-2% in the tires. Generally, only the manufactuer can measure HP at the flywheel: once you have the car you can only measure HP at the rear wheels.

No manufacturer rates his car in ads and brochures with the rear wheel ratings because teh rear wheel rating would vary too much depending on not just the motor, but the type ofdrivetrain, transmission, and rear axle (axle ratio should not make a difference, but limited slip or not does, as does the type of axle gear design and the type of limited slip) and if it has four wheel drive, etc. They would end up having to have nearly a dozen ratings for the very same engine in the same car, just with different equipment. Thus, Ford, Chevy, Chrysler, etc., all do this consistently: HP advertised and listed in brochures and specifications is net flywheel rating for the basic engine.

But what you measure when you put your car on a chassis dyno is at the rear axle, that is really all that matters, and that will always be less than their flywheel rating. Typically, you turn the AC, etc., off, and test the car:
- Two wheel drive Chevy V8s with the M6 transmissions lose about 11% to 12% in the drivetrain and tires (a 400 HP C6 registers about 350 at the rear axle, orabout 13-15% with an automatic (same engine, same car, with an automatic, about 340 HP rear wheels).
- Your 300 Hp 'vette would put down about 265 HP at the rear wheels with a manual, maybe ten less with an automatic.
- Two-wheel drive Ford V8s (Mustang) lose about 12-13% with a manual (I saw a stock 300 HP 2005 5-spdd GT put down about 261 HP at the rear wheels) and about 15% with an automatic. (( An exception is the 2003 Cobra, which although rated at 390 HP tested at about 350 rear wheels - as if it was rated at about 410)).
- My stock 320 HP Porsche (automatic) put down 265 at the rear wheels (actually, it followed that Mustang GT that did 261 HP on a dyno the same day)
- Four wheel drive cuts HP measured (at all four wheels now, difficult to find dynos that can measure this but they are available) but about another 3-5%.

History
There is a term invented prior to WWI, and sometimes seen in brochures and car manuals/specifications up into the 1960s, called "taxable horsepower." This was a formulae based on stroke, bore, etc., that estimated HP for taxation purposes: early on 20 Hp cars were taxed like 20 horses, literally, and this was the legal formulae. Not used much anytime recently, but you might see it in old car manuals.
In the muscle car era through to 1972, automakers advertised what is now called a "SAE gross HP" definition that was very optimistic. It has been described as an "at the brochure" standard -- HP ratings that were just pure fantasy. Still, every manufacturer's fantasy was based on the same BS set of rules, so a 1966 Corvette rated at 425 HP definately produced more power than a 'vette rated at 390 HP, and probably more than a Ford rated at 410 HP, etc. The problem is that you cannot really compare them well to today's ratings (see below)
- In about 1973 the industry changed from that gross to a net rating that was about what the engine produced, under the best of circumstances, at the flywheel. That 425 Hp 'vette engine was suddenly a 325 Hp engine (same engine, just a more realistic rating). This was still a bit optimistic, but close. Initially (1973) it was real close, but recently, manufacturers, [articularly some Jpanese brands, discovered how to "game" the ratings tests to squeeze more Hp out of an engine than it really had on the street.
So, last year, the industry went to a slightly tighter "net" standard that corresponds almost exactly to what the engine will actually produce is tested on an engine stand but with all the right accesories (power steering pump, belts) and full exhaust and air cleaner.
One big change with this recent new standard, is that it much harder to underrate a car engine (label it as having less than it does), which has been done on occasion: the 2003 Cobra (390 rating, about 410 real) was underrated (probably so as not to raise a red flag to insurance companies), and the Camaro Firebird V8s from '98 on were rated at 305 or 310 HP but tested as if they had about 335 HP flywheel, probably because GM wanted to keep a bigger gap in advertised HP for the 350 HP C5, which shared the same engine.

 

THANKS LEE. HOW DO I DO A SLIDE SHOW LIKE YOU HAVE.I WANT TO PUT SOME OF MY RIDES ON MY PROFILE.I SEE U HAVE A CAMERA ICON AND U CLICK ON IT AND GET YOUR SWEET RIDES. I LOVE THAT Z06

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I was going to try and answer your question earlier, but I knew Lee would be able to answer it a lot better than I ever could. Thanks Lee, I enjoyed reading that, I definitely learned something I didn't know.

All you have to do is click on "Gallery" at the top and then upload your photos onto the site.

 

In the muscle car era, horsepower was measured by a different standard. Not only was it flywheel horsepower, but the vast majority of ratings were of motors with no accessories or other factors that may impose drag on the motor.

For example, to obtain a 450 HP rating in the 60's, Chevrolet would test the engine on an engne dyno, but remove the AC compressor, PS pump, etc. It was common to use an extremely high octane (or leaded) fuel as well.

 

That was what the SAE gross standard permitted at the time, you put the engine on an engine dyno without all the accesories even attached (i.e., you didn't even have to spin an idling AC compressor) and at one point the standard did not even require you to run the water pump!. In addition all manner of pure BS things could be done, such as you not only did not have to have the stock exhaust you were going to have in the car, but you needed no exhaust, and could actually put fans on the exhaust ducts in your dyno room to "suck" the exhaust out of the headers, etc.

When you take all the BS of those gross ratings (about 20-25%) away, and drivetrain losses, the real HP you get is still pretty good for cars from the muscle era, particularly considering they had carburators, not fuel injection, and points and ondensors, not digital ignition. I learned to drive in a 425 cubic inch, 365 HP rating 1965 Olds 88, a real rocket Oldsmobile (0-60 in the high sevens, good back then for a full size - 4800 lb -- sedan). I saw a restored Olds with this same engine and transmission tested on a RW dyno at a classic car show about five years ago: it produced about 235 HP to the rear wheels. I also recall seeing a 435 HP big block vette, stock but restored, do 290.

 

That is correct, in my opinion, muscle cars from the 60's are over-rated. Aside from the 427 Corvettes, or the ZL-1 Camaros, the majority of muscle cars made at that time were not terribly fast.

"400 HP" at that time *might* net you a 14 second pass. For a variety of reasons: Horsepower rating, tire technology sucked, drivetrain efficiency, etc.

Anyone remember the article, I believe in GrassRoots Motorsports where they took a 50's Corvette and matched it against a Honda Odyssey Minivan? The van beat the vette in acceleration, cornering, braking...basically every test that was performance oriented.

 

Yeah, the article was in the 50th anniversay edition of Motor Trend. They had a good article about "the good old days" in which they said, yes, they were great cars, but fast? Not by today's standards. That Honda Minivan killed the 57 vette in every performance category they tested - acceleration, breaking, and around a road course. I also remember the other milestones: the Buick Century was so named because it would just do 100. The first Olds 88 would do . . . 88, and the fastest stock car sold in the US in 1953 (a year before the first V8 Crovette or the T-bird came out) was a Triumph TR-3 with 0-60 in 12.3 seconds.

And 14 seconds was about all the majority of muscle cars could do in the 60s. I cruised a lot never saw a solid-lifter 426 Hemi or a tri-power 427 vette on the street. At that time, I considered anything "fast" if it would do 0-60 in under 7 seconds and the quarter mile in the mid 14s: a 375 Hp SS396 Malibu, etc. I don't think we have minivans that can quite do that yet (but of course we have SUVs that can), but there are plenty of BORING sedans today that can do that with no drama.

 

In the height of the Muscle Car era, my PARENTS were 10-15 years old, so I can't comment on those times

I wonder if I can find an electronic copy of that article?? I would love to re-read it.

With the sudden surge of high horsepower cars recently (Ford GT, Z06, Viper, Charger SRT-8, etc) I wonder what everyone will be talking about in 20 years? Will the horsepower race continue? Guess we'll see.