So just as a rule of thumb, how do you calculate BHP from your RWHP number? Do you take the RWHP number and add 15%? Am I far off on this?

 

Not sure but I just had my 05 LS2 on a dyno last month. 358 RWHP. My car is stock other than a Blackwing intake, Borla and X-pipe exhaust. So if you figure 15% loss, that means my car is putting 418HP from the Flywheel. I doubt my few mods make 18HP, but who knows!

 

Maybe i'm wrong but I think 18 hp is very realistic just from those mods.

 

BHP= Brake horsepower. That means the torque is measured at the brakes. (Basically, the only change is the tire/wheel is removed.)
RWHP = rear wheel horsepower. This means the torque is measured @ the tire contact patch.
BHP>RWHP because of the MOI effect of the tires and wheels. The percentage would change depending upon the weight/location of weight of the tires/wheels.

Now, if you want "crank" horsepower, then that's completely different.

 

Good point, let me correct then, crank hp vs rwhp, how is the difference calculated?

 

Different measurement points.

There is a 45-55 difference between net flywheel HP published by the manufacturer and actual rear wheel hp measured on a dyno jet depending on if its a manual or automatic transmission.

Lots of folks use 15% to 18% but that is not really make sense because why would an engine with 600hp measured at the flywheel lose more horsepower than a 300hp engine in the same drive train.

 

Because it takes more power to accelerate the drivetrain at a higher rate.

FWIW, I am not a believer in the set percentage method either. I think the more power you make the lower the percentage is.

 

I don't really know the answer to this question but the answer that I have read that made the most sense to me is very similar to the one posted by MEZ.

Some years back there was a thread on this forum that collected the stock RWHP readings on C6 LS2's with manual transmissions and 3.42 gears. I have forgotten the average but it was in the 340's. Let's call it it 345. Either in that thread or another one at the same time, it was suggested that the the difference of 55, between the 345 and Chevy's claim of 400 for the flywheel HP, was 13.8% which got rounded to 14%. These might not be the exact numbers from the thread but they are very close. Many others not connected with that thread also reported something in that ballpark for the manuals and a few more points for the autos. Thus, something in the low to mid-teens became casually accepted.

To paraphrase what MEZ said, however, why would the difference be any greater than the stock difference for a more powerful, modded engine operating through the same drive train?

One reason is that, if the modded engine makes it's power at higher RPM's than the stock engine, the percentage difference will be greater because friction loss is greater at higher RPM's. Friction increases at a greater rate than linear, maybe as the square of RPM's? I am sure someone here knows that rate.

I doubt this often make a big difference, though, because many modded engines peak only a little higher than stock. I use the 55 number for my limited purposes.

 

Where do you guys come up with some of this BS.

Brake horsepower is the measurment of an engines output by appling a brake to it, not the horsepower measured at the cars brakes.

You mount an engine on a test stand and connect a brake to it and you measure the horsepower. No car is required to measure "Brake" horsepower.

 

A C6Z makes 505 at the crank and lets say 430ish at the wheels which would be a 15% loss and 75hp. So are we saying if my car is putting down 640rwhp and we use the same 75hp loss it would be around 715hp at the flywheel?.

 

The C6Z makes its peak power at higher RPM's than the C6 LS2. I don't know what the C6Z peak power RPM is but the redline is 500 RPM's higher than the LS2, so that may be a good surrogate for the difference in peak power RPM's. Assuming that your 505/430 numbers for the Z are valid and your car makes its peak power at the same RPM's as the Z through the same driveline as the Z, then your 75 should be a reasonable estimate. If either your peak power RPM or drivelive are different than your subject C6Z, then the 75 is harder to justify.

 

Darn, im just trying to figure out what my 640rwhp Z is making at the crank. Who knew this would be such a difficult formula to figure out.

 

The difference between crank[shaft] horsepower and rear wheel horsepower is around 12% for a manual transmission car and 15% for an auto. Take the RWHP and devide it by either .88 or .85 and you will have an estimate of the horsepower at the crank[shaft], or as some call it, at the flywheel.

example...640 RWHP divided by .88(manual trans)= 727 crank[shaft](flywheel) horsepower. Remember that is an estimate.

 

The Dyno run I have for pure stock Z06 was 454 rwhp. Add 51 hp or 10% and you get 505. From what I have seen this is about average. So using % does not make sense.

I wish everyone would simply start thinking in terms of RWHP so this whole debate would be over with.

 

RWHP is not really an exact science either, if you go from dyno to dyno you usually get different numbers. In the ballpark but still different numbers.

 

Wheels/tires removed! It was funny though.

 

Yes. In fact, no car is allowed when measuring brake horsepower - by convention, bhp always means the power measured (using a dynamometer, to which "brake" refers) at the crank, with no driveline attached.

 

Some people have a very active imagination, I guess from playing to many video games and not enough studying.

 

Brake Horsepower does not mean the power is measured at the brakes.

 

There is no rule of thumb. Although you will find that a lot of people here like to multiply their RWHP number by up to 18% to stroke their egos, the reality is that first and foremost:
1- Every dyno reads differently.
2- Different tires, tire inflation pressures, wheels (due to mass) and transmissions will greatly affect how much power a car puts down at the wheels.

So your RWHP number in itself is not by any means a very accurate measure of how much power the car really has. Starting with that, if you multiply that number by a fixed amount, you are making the assumption that, as the car makes more power, the driveline eats more of it up. You'd have to believe that on, say, a 700RWHP car, the transmission is consuming 200 horsepower. Which is simply not true (nor possible).
The truth is that driveline loss is closer to a fixed number than to a percentage. You can get a reasonably accurate estimate of your crank HP by dynoing a bone stock car with the exact same transmission and tire/wheel combo as yours on that dyno, seeing what it puts down, and then adding the difference between that car's WHP number and its SAE-Certified crank HP to your WHP number.

E.G. I know a bone stock manual LS2 C6 advertized by GM to have 400crank HP makes 340RWHP on a dynojet. My manual LS2 C6 makes 600. My crank HP is somewhere around 660RWHP.
This in itself is not completely true because since my car accelerates much faster on the dyno it will suffer from more windage losses (a load bearing dyno such as a Mustang, a Dynomite, or a Dynapack will eliminate this variable and be more accurate in that aspect), and since it makes peak HP at a higher RPM than a stock car, it is also suffering from higher driveline losses than a stock car at the RPM it peaks out at (this could be corrected for IF you measured or had access to a plot showing driveline loss vs RPM; they exist). A closer estimate would probably be 680-ish crank HP, but now we're guessing. The only way you could know for sure is if you have your engine on an engine dyno...