HP Calculation ???

[CitySnake]

Just put down my $2,000 deposit with Sean Roe (actually Becky Roe who's about as pleasant as they come) and began trying to calculate the extra HP with the S/C. Since my experience with motorcars/racing came via Jaguar XKE's, HP, though always important, was seldom the top priority (suspension and braking were always the bigger issues <FONT size="1">not to mention the overall infamous British electronic system</FONT s>).

(Kindly excuse my ignorance but) How do car manufacturers calculate their marketed HP ratings (Viper 450/490) when the only real measurement, that I am aware of, is via a dyno at the rear wheels. Is it just the simple calculation of adding (+/-) 13% to account for some typical loss of HP through the drivetrain? If so, what's the point? Most Vipers seem to dyno at greater than the 450/490 (Gen 2) numbers if you add the 13%, so what was/is the purpose of keeping the marketing numbers low if you wanted to sell more high HP cars? I have some memory of a past system and new revised system of calculation but can't recall the details (getting old). Sean's S/C system will add a minimum of 135 HP as measured on a dyno at the rear wheels. So an average '01 with 415 dyno'd HP should rise to 550 REAL HP. Is this right? Can someone explain (as in HP for idiots) (Tired and confused now, taking a nap... read any replies later... zzzzzzzzzz).

 

[ACR Joe]

Manufacturer's typically specify brake horsepower (bhp). This is determined by running a fully dressed engine (all accessories, belts, exhaust, etc.) on an engine brake (aka dynamometer) to establish horsepower and torque curves and of course peak power. Rear wheel horsepower is either determined via actual testing on a chassis dynamometer or estimated by assumming a driveline loss value (~15%), the latter being a far less precise method.

 

[CitySnake]

Thanks Joe. Your explanation zapped a few long inactive neurons. A few follow up questions if I may:

Physically, when the factory determines Brake HP, to what engine part/drivetrain do they attach the dyno (I presume it is tested while not yet installed in a car)? And what is the purpose of determining Brake HP rather than determining only real rear wheel HP?

If one were interested in determining Brake HP after the factory (for comparative purposes) it would seem that the only way it could be determined is by adding ~15% to the actual rear wheel dyno, i.e. if my car (apres S/C) dyno's at (hypothetically) 550 HP RWHP then one could figure that the BHP is 632. Correct?

Did I say thanks?

 

[onerareviper]

Citysnake,

If your going to use 15% as the drivetrain loss, your formula would look like this: 550 / .85 = 647 HP. Not 550 * 1.15. There is some long winded explanation that I can't remember, but most people agree on dividing, not multiplying.

Now here is the kicker: Some people feel the drivetrain loss in a Viper is only 10%, others 12%, others 15%. Although, 15% is usually the standard many people tend to use. Here would be the differences:

1.) 550 / .90 = 611 HP
2.) 550 / .88 = 625 HP
3.) 550 / .85 = 647 HP

Later

 

[joe117]

15% of 500 HP is quite a bit of heat. Where does it go? I always wondered. Do the math on how many BTUs you get from 75 HP. What's the answer?

 

[CitySnake]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by onerareviper:
Citysnake,
If your going to use 15% as the drivetrain loss, your formula would look like this: 550 / .85 = 647 HP. Not 550 * 1.15. There is some long winded explanation that I can't remember, but most people agree on dividing, not multiplying.

Now here is the kicker: Some people feel the drivetrain loss in a Viper is only 10%, others 12%, others 15%. Although, 15% is usually the standard many people tend to use. Here would be the differences:

1.) 550 / .90 = 611 HP
2.) 550 / .88 = 625 HP
3.) 550 / .85 = 647 HP

<HR></BLOCKQUOTE>

One Rare:

Maybe I slept too long, and regardless of which divide / % add formula you use (you've got me here???) your examples baffle me. If there is only a 10% drivetrain loss wouldn't the HP be higher than if there were a 15% loss, i.e. examples 1 & 3 would be reversed? If you use only the % add formula then the loss factor would correctly relate to the end BHP.

HUH???

Anyway, somebody please tell me why there is any need to quote Brake HP at all?

 

[onerareviper]

Citysnake,

Let me explain it another way. You go to the dyno, and the graph shows you are making 550 RWHP (Rear Wheel Horsepower). Now you want to calculate the brake horsepower (bhp), that the manufactures use in there literature. So, are you losing 10%, 12%, or 15% through the drivetrain? The higher percentage of drivetrain loss, the more HP your car has, AT THE MOTOR. For example, if you dyno at 500 RWHP, and god-forbid 50% of your horsepower is being lost through the drivetrain (friction, heat, etc...), your motor would be making 1000 HP. Therefore:

1.) 550 RWHP / .90 = 611 HP (at motor) (10% of HP is lost in drivetrain = 61 HP)
2.) 550 RWHP/ .88 = 625 HP (at motor) (12% of HP is lost in drivetrain = 75 HP)
3.) 550 RWHP/ .85 = 647 HP (at motor) (15% of HP is lost in the drivetrain = 97 HP)

Got it?

Later

P.S. - You are correct, though. RWHP is the only true measure of a cars overall power. A car could make big-horsepower, but if it has a inefficient drivetrain, and is losing 25% of its engines power before it can be tranferred to the rear wheels, it could lose to a less powerful car with a more efficient drivetrain.

 

[KIHMO]

I've always been curious to know what the actual (stock) bhp is versus the rated bhp (450) for the Viper. I'm always reading that the manufacturers underate the bhp for whatever reason.

 

[CitySnake]

One Rare:

Actually now...I got it! I was "thinking" on the wrong side of your equation. I was fixing on the 550 RWHP and not the BHP side. Anyway, I now understand. Thanks for the explanation (twice

).

I'll just stick with RWHP and just smile alot.