Ok, I've seen a few threads where people say the drivetrain loss on our cars is ~20%

I've heard people say Crank numbers are around 20% more than the Numbers at the Wheels

So:

How would you switch from WHP to Crank?

/0.8

Or

x1.2

I know it's still not very accurate, but I'd be interested to hear which calculation people use

 

Hi Ant_8u,

I can tackle this one since you were so nice to post all the links regarding the rear spoiler hydraulics. So the equation is assuming drivetrain losses are 20%.

Assume Crank HP = X. Then the equation is X - 0.20 * X = RWHP. Performing the subtraction for X yields. 0.80 * X = RWHP.
Solving for X yields X = RWHP/0.80 or 1.25 * RWHP.
So Crank HP = 1.25 * RWHP.

Hope my math is correct!

 

Thanks
And yes, your maths is correct

I'd just heard a few people talk about things differently, and wanted some clarification

My assumption was......

WHP is 80% of Crank
So that's the same as Crank x0.8

So, to switch from WHP to Crank = /0.8

(Which is the same as WHPx1.25)

 

There is a ton of speculation on this subject. As soon as you throw in different types of dynos, then it's like pulling a grenade...

An example is my recent trip to a superflow eddy current dyno, where they applied a load to the wheels so we could best simulate laps on the track. There was another shop owner (who reads this board btw) there and he commented that if I was on his dyno, my V8 Datsun Zcar would pull 420WHP, but on this dyno, I'd probably pull 320 with the load. I pulled 323WHP and 325#torque. The same dyno generates 374-385WHP for brand new Z06s, to give you a general idea. The calculations they used for my Zcar show it to make about 450-470BHP on an engine dyno. Their Calculation showed a TWENTY EIGHT percent power loss. Having two different shop owners stand there and tell me the same thing, two competitors shaking heads in concurrance told me that there's more to this whole "A dyno isn't a dyno isn't a dyno" thing.

It really does boil down to things not related to HP ratings... What does the car do from 0-60? 60-130? These are much better indicators of true power.

Mike

 

BTW, The guys at the shop said they have matched their numbers to the calculators on this website...

http://www.wallaceracing.com/Calculators.htm

They swear that their Superflow numbers are dead on with the HP calculator on that site...

And just so we're all straight, that 323HP number? Damned fast from behind the steering wheel on a 2700# car... Feels more like 450HP... A LOT more like 450HP...
Mike

 

You cannot use a set equation when the variables are so great. The only way you could possibly come close, assuming the increased ratios are constant, is to dyno a stock car on the same dyno for a modified car with the exact same atmospheric conditions and using the same car. Otherwise it is a constant guestimate.

 

Sure, but the shop owner's point was that he could get very close and that his dyno is constantly reading the conditions for heat and humidity. By capturing that data and storing it for his customers, when they return he can have a better understanding of the previous testing conditions and account for them.

I've been to a number of dyno shops over the last 20 years, with a number of cars. This guy was one of the better, if not the best, I've worked with.

Mike