Horsepower and Torque Measurements

The are few things that get Corvette enthusiasts more fired up than horsepower. Its a sort of Holy Grail and yet it is one of the most misunderstood measurements in existence.

Horsepower Measurements:

At the simplest, if you multiply a torque rating by the RPM where the rating was measured and then divide the result by 5252, you get the gross horsepower---horsepower measured at the engine flywheel for all practical purposes.

As an example, the webmaster's 1989 Corvette torque specification is 345 ft pounds of torque measured at 4400 RPM. If you multiple the torque by the RPM and divide that number by 5252, you get 289 HP as the result.

But wait, there's more (as the commercials say):  The published horsepower for the 1989 'Vette is 245 so what gives?

Well, the Society of Automobile Engineers have established a standard called "SAE Net Horsepower"  and that standard calls for horsepower measurements to be made with certain items known to restrict horsepower (such as air cleaners, certain emission equipment and so on) in place.

In the case of the 1989 Corvette, those items are responsible for lowering the horsepower rating by about 15% hence the 245 HP published rating.  The 289 HP measurement is gross HP and the 245 number is net HP and all manufacturers use SAE Net HP, something that has been required since the mid 1970s.

In the good old days of 1960 hot rods, the horsepower specifications were always gross HP and the Stingrays of that era are rated higher than some of the C4 cars that came later but in reality, an Apples to Apples comparison shows that there wasn't that much difference, particularly in the case of the later year C4 vettes. (Truth in publishing requires me to state that some of the big block 'Vette engines of that era were more powerful than any of the C4 small blocks except the ZR-1---don't want to offend any of the C2 and early C3 guys).

HP measurements versus Torque measurements:

Sometimes you will see two specifications quoted:  A torque rating at one RPM and a HP rating at another.  What gives in that case?

It is just marketing taking over the "specmanship"

Using the '89 'Vette again, due to the shape of the intake runners and their length, the torque peaks at 4400 RPM.  Right after 4400 RPM, the torque falls off drastically so in that case, both the HP and the torque are quoted at 4400 RPM because both peak at the same point.

But lets say we are looking at one of the newer generation engines where the torque peak is flatter and does not fall off as rapidly.

In that case, you might have something like this:

Torque peaks at 4400 RPM but it does not appreciably fall off:  rather, it slowly decays so that at 5000 RPM, it is almost as high as it was at 4400 RPM.

If you are a marketing type, you will want to hype the torque number that is highest and the horsepower number that is highest so you publish two different engine RPMs and life is grand:

Torque peaks at 4400 RPM and is 350 Ft Pounds.  The gross horsepower at 4400 would be 293 HP in this case.  But lets say the torque is still running 340 Ft. Pounds at 5000 RPM.  The torque peak is definitely at 4400 RPM so that rating is true but the horsepower is 323.6 at 5000 RPM so that one is true also.   You are multiplying a higher RPM with a slightly lower torque number so the resultant is higher horsepower. See how it works?

Now, you deduct whatever HP the SAE forces on you for this particular engine and you are done.


Rear Wheel Horsepower

Now we come to the really painful part:  You never get anywhere near the net or gross HP rating in actual practice because you lose a tremendous amount of horsepower in the drive train.

Some of you have placed your pride and joy on a chassis dyno and nearly puked when you saw you were supposed to be making 300 HP from an LT-1 C4 and yet you were only measuring 225 HP according to the dyno.  Again: what gives?

Unfortunately, an automatic transmission 'Vette loses about 24% of the available HP enroute to the pavement.  A manual transmission is a bit better but not much: maybe 22% is lost in that case. When you take into consideration wear and tear, poor tires and so on, you can experience even greater losses.

Friction and drivetrain inefficiencies, tire efficiency, temperature, humidity---in other words everything but the phase of the moon (and maybe that too if you could measure things that accurately)---all conspire to limit the amount of horsepower available at the rear wheels.

So what can you do?

If you have lots of money, you could pay someone to develop roller bearings for everything in the drive train, maybe come up with a great grease for the rear end and special fluids for the transmission.

Special tire compounds would help and then you could run only on days when the air was dense and humidity high (and there was a full moon).

In other words,  there is nothing you can do but accept the physics of the thing and pretend that the horsepower measured at the flywheel of the engine really means something.

Thats what I do.