Ford tells us our cars are 300 hp at the flywheel. Most have dyno'd at 265 or so. So what is a good way to back figure our HP if we know the Dyno RWHP?

I have heard that 15% is a good estimate.

 

265/300=.8833333

If you wanna use 265 as a benchmark. Problem is every engine will yield different baseline hp numbers at the flywheel, auto transmissions will lose more power than a standard transmission, and of course different dynos will yield different rwhp numbers.

But for the sake of argument my car made 479 rwhp on the dyno. If you take my 479/.88333=542 hp at the crank.

Of course you could just multiply by 1.132 to get the number too.

And all this is just for arguments sake because there are so many variables.

 

I know most produced at dyno stock around 260 to 270 (some as low as 255 and as high as 275) depends on what day it was built, etc also. I know around 1972 Feds mandated net HP after all the accessories were added. Thanks

 

The estimate I always use is RWHP/.85 = ~FWHP
In my case:
283/.85=332

 

That's what most of us use for manual transmissions. For automatics (with their inherently bigger drivetrain loss) it would be RWHP/.80 = ~FWHP.

My GT automatic, for example, would be 364/.80 = 455 flywheel hp.

 

So my estimate of 310 RWHP should be around 365 (5 speed)

 

Excuse my ignorance in this matter but where does the .80 and .85 figure come from? I just got home a few minutes ago from having my Automatic '05 GT Dynoed on a DynoJet. It yeilded 299.26 RWHP so at the .85 figure it would be 351.76 FHP or at .80 it comes to 373.75 FHP. I seriously doubt both of those figures for my car. I am not suggesting that either of you guys don't know what you are talking about but it just doesn't make sense to me.

 

I thought that in the s197 a 10-12% increase for 5 speed manual and 15% for auto was more likely.

 

That sounds right for a Dynojet dyno. A Mustang chassis dyno tends to read slightly lower and 15% for manuals and 20% for automatics is the generally accepted drivetrain losses for those. My car was dyno'd on a Mustang chassis dyno. The operator told me (as many others have told me) that cars typically dyno 5-10% more on a Dynojet vs a Mustang dyno.

 

It is generally accepted (do a google search or pick up any auto magazines with dyno testing) that a late-model Mustang loses about 15% of the flywheel horsepower through a manual transmission on the way to the tires. An automatic transmission (with it's fluid coupling torque converter) loses 20%.

IMHO, a Dynojet dyno (read my post above) reads higher RWHP due to the unrealistic way it measures power (Mustang dyno's are loaded with resistence to better emulate the real world). A 12% loss for manuals and 15% loss for automatics is probably as good a guess for a Dynojet as any (as 2k7gtcs pointed out above).

And if Ford rates an S197 GT Manual at 300 flywheel hp, and they put down 255 rwhp (on a Mustang dyno), that's a 15% drivetrain loss which supports the generally accepted figures. I've seen Mustang dyno sheets for stock GT automatics in the 240-245 rwhp range, which is right about at the 20% drivetrain loss bogey, too.

 

Older cars are much less efficient, by the way. I used to own a 1968 Pontiac GTO with a turbo 400 automatic transmission. Factory rated at 350 flywheel hp, and modified to around 425 flywheel hp (according to the shop that balanced, blueprinted, and built it for me), it only put down 283 rwhp - a 33% drivetrain loss.

 

Ditto - my numbers are from a Mustang Dyno, and I also have 4.10 gears that skew the number somewhat.

 

No talk so far of whether the driveshaft is stock or upgraded to a one-piece unit. That matters.

 

stock drive shaft, but 4.10 gears, 5 speed