Horsepower ratings??

[utvolman99]

I thought that since the early 70s auto makes advertised rear wheel HP? I am in a discussion on another board and everyone is talking about ~20% loss from HP ratings. I could of swore that all changed from gross to net in 1972?

 

Please fill me in!

[Guest greimann]

Car makers have never rated output in rear wheel HP. NET HP is determined with the motor on a dyno stand and all the accessories installed and operating, including stock exhaust. It duplicates the motor being installed in the vehicle. Gross HP is just the motor with no accessories like alternator, PS pump, etc. and the standard dyno cell exhaust that was more than likely headers and no mufflers. Rear wheel HP is measured on a chassis dyno like a Dynojet and is the most convinent way hobbiests can rate their cars.

[Mudge]

12-15% loss from manual trans, and 15-20% loss from slush boxes is common.

 

The way cars used to be dynoed is removing water pumps etc (accessories), and GM supposedly also put race headers on the car, not a full exaust. Yet people still believe that some cars like the Z28s of the era were severely underrated, but who knows.

[johnc]

Horsepower is calculated number derived via torque with this calculation:

 

bhp = torque x rpm/5252

 

Being an derived number there are various methods of calculating it, with some methods more liberal and more accurate then others.

 

Until 1972 most US auto manufacturers used an old SAE standard method to develop the so-called "gross" horsepower numbers. This standard was designed to measure the horsepower output of the basic block, cylinder head, and internals. The standard did not specify intake, exhaust, and accessory configurations.

 

In 1972 the state of California, as part of their Clean Air act implementation, required all auto manufacturers to specify horsepower ratings using the SAE J245 (now SAE J1349 and J1995) standard. Somewhere the term "net" was thrown in by the automotive marketing folks, but there's nothing "net" about that standard. It measures crank horsepower with the engine as configured in the car including complete exhaust and intake systems.

 

"Rear wheel horsepower" is yet another marketing term that has no basis in SAE (or JIN, DIN) standards. Wheel dynos measure the ability of the engine, driveline, and rear suspension of a vehicle to accelerate against some type of resistance (usually just the mass of the rollers). The result of this measurement is a number that has no meaning until its run through a calculation.

 

These calculations are generally proprietary to each dyno manufacturer and can be adjusted at whim by the dyno operator to account for external conditions. The result of this calculation is a torque number that's again run through another calculation to get a horsepower number. Because of the proprietary nature of some of these calculations, comparisons of "rear wheel horsepower" between wheel dynos are probably inaccurate at best.

 

The current overall, accurate, and comparable measurement systems are SAE (the most liberal), JIN (the moderate), and DIN (the most conservative). So, when you see the horsepower ratings for an American car you can assume they are a little optimistic, for a German car they are a little conservative, and for a Japanese car probably exactly correct (until you get to the mandated 280hp limit imposed by the Japanese government.) When you see "rear wheel horsepower" numbers, be skeptical if they are used to compare cars against each other from different dynos.

[Phantom]

Bottom line -Datsun Z cars were rated at "gross" HP until the Zx model in 1979 when they went to "net". For what it's worth, the 1977Z was rated at 170 HP gross but was actually 149 net. Of particular interest was the HP & torque comparisons between it and the 83ZX

The 77 was 149 HP at 5,400 & 163 lb-ft at 4,400 while,

the 83 was 145 HP at 5,500 & 166 lb-ft at 3,000.

Want to take bets on which one has the best quater mile times with all else even?