AT this weekends show, there was a chassis dyno available (three pulls for $75.00). I understand that there is a certain percentage of power loss due to transmission, differential and suspension affect, but the numbers I have heard is between 18% and 30%. I am trying to find out what the correct (or closest to correct) percentage. My chevelle is a 69 SS, 325hp 396, M-20 4-spd, 3.73:1 non-posi, p235/60 r14 tires
The dyno run sheets show:
(a) absolute barmoetric pressure - 29.98 in.Hg
(b) Intake air temperature - 67.0F
(c) Vapor Pressure - 0.3 in.Hg

Run #1
at 4600 rpm my max power was 245.9
at 3200 rpm my max torque was 336.2
Run #2
at 4600 rpm my max power was 248.5
at 3200 rpm my max torque was 341.7
Run #3
at 4500 rpm my max power was 249.8
at 3300 rpm my max torque was 341.6

Thanks.

That all looks just about right...

The loss in your drivetrain (4 spd & 12-bolt) should be around 20-22%. So that works out to your crank HP being around 310-320 HP, which is about typical for those.

Don't forget, the factory's "ratings" from the 60s are a total joke; don't believe that your motor, as installed in a chassis, tuned for normal fuel and driving conditions, would produce that laughable number they gave it back then. That one isn't near as silly as some of them though.... the 327/275 comes to mind.

Just for reference, I ran my 83 Z28 400 on a dyno; it was a SBC 400, std bore, 186 heads with a little gentle massaging, Comp XE274, 1-5/8" headers, Edelbrock Performer not RPM, Holley 800, the stock HO air cleaner. It gave 274 HP / 373 ft-lbs at the wheels, at similar RPMs to yours, I think it was 4500/3400. It has a 7.5" 10-bolt though, and a 5-speed, ao somewhat less drivetrain loss.

Your losses are probably more like 15%. We have chassis dynoed a couple of 4-speed cars with engine dyno sheets and came within about 15% of the FWHP.

oneofakind67 is right as most stick cars only lose from 13-15 percent through the drivetrain. By the way 245-249 HP at the rear wheels is pretty good for a (325 H.P. "396"). All of these cars are rated as gross horsepower and are generally 50-70 lower when net ratings are used.