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Discussion Starter · #1 ·
I've seen before that a way of guessing how much WHP a RWD car is making is to take HP and multiply by 15% or .15 to factor in drivetrain loss.

Example: Stock Twin Turbo Supra is rated @ 320hp x .15 = 48. So 320 HP - 48 = about 272 WHP. correct?

However is their a formula for the opposite of this, if you know how much WHP a RWD car is making on the dyno. Then can you calculate how much HP it would be making at the crank?
 

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You didn't pay attention in math class!

And probably only because I had four beers for lunch I'm going to reply.


If A X B = C then C divided by B = A, and C divided by A = B

If you assume driveline losses are 15%, you can take crank hp and multiply by 0.85 to get whp. Or take wheel hp and divide by 0.85 to get crank hp.
 

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You didn't pay attention in math class!

And probably only because I had four beers for lunch I'm going to reply.


If A X B = C then C divided by B = A, and C divided by A = B

If you assume driveline losses are 15%, you can take crank hp and multiply by 0.85 to get whp. Or take wheel hp and divide by 0.85 to get crank hp.
What he said. :)

Later, Steve
 

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adammackintosh.net
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wtf.

if you lose 15% from crank hp... imagine adding 15% TO the wheel hp.

see math above. we'll pretend this never happened.
 

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agm,

Let's assume you have 100 crank hp and use your math. Sorry, I forgot for a moment that this is the Supra forum. Let's assume 1,000hp. :)

1,000hp * .15 (15%) = 150hp
1,000hp - 150hp (15%) = 850rwhp

Now let's do the reverese

850rwhp * .15 (15%) = 127.5hp
850rwhp + 127.5hp (15%) = 977.5hp

As you can see the math doesn't work because you are taking 15% of two different numbers.

Let's use this math.

1,000hp * .85 (assumes a 15% loss) = 850rwhp

Now let's do the reverse

850rwhp / .85 (assumes 85% of the original number) = 1,000hp

Now that works.

Later, Steve
 

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His math is correct.
You are correct, sir. I meant my math comment to apply to his question below.

...However is their a formula for the opposite of this, if you know how much WHP a RWD car is making on the dyno. Then can you calculate how much HP it would be making at the crank?
 

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Blood for Blood
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lol... man we need a thread of the day award...
 

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His location says he's from Malvern which is like 20 minutes west of me. So if he shows up again I offer to buy him a couple beers and we can go over this math stuff while enjoying the brews.
 

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Discussion Starter · #14 ·
Haha guys thanks for anwsering my question. I had a feeling there was a simple solution to it. im glad i provided some humor to everyone by asking this question!

Nick i might take you up on your offer for those brews haha, where are you located?
 

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I'm just west of City Line and just south of I-76, in Narberth which is between Bala Cynwyd and Ardmore. Straight shot down Rt 30. I often go through/by Malvern while cruising around. My Supra is down now waiting on a couple parts but will be up and running in a couple weeks or so.
 

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adammackintosh.net
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wow i definitely failed math too then. excuse me while i backpeddle as much as possible.
 

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I have an Excel spreadsheet which calculates HP based on acceleration and vehicle weight, and since these are usually used for highway pulls, you have to take in account for drag, so if factors in drag coefficient and frontal surface area and adds the corresponding HP it takes to overcome those forces. It's essentially how a dyno calculates the HP/torque, but you have to know the vehicles weight since that is the mass you are accelerating, not the known rotational mass of a dyno. Its a good tool to use if you have the ability to datalog your vehicle speed vs. time at a rapid rate.
 
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