GD70

I searched the archives but didn't come up with anything. From what I've read, is it true as a rule of thumb subtract about 1/3 off gross rating to get net hp? 350 hp actually 250? I'm sure someone has come up with a conversion table of some kind. Interesting to compare todays performance hp figures with older engines.
Thanks, Glenn

JACKAL0PE

There's not any sort of conversion table.

Those old engines were rated based on seat-of-the-pants and corporate dictates. A good example is the "425 hp" L88 when everyone knew it was about 100 hp more. What you're asking for is a formula to calculate something based on a specific guesstimate.

GD70

so... if you put a 350 hp on a dyno, what would rear wheel hp be, if the motor is in top running condition?
I read an article in Corvette Fever where a guy had his 350-350 dynoed and it actually put out something like 160-170 hp at the rear wheels! Even after they did some engine work, tune-up, carb adjustment etc...it still pulled only around 190 hp or so. Shocking the desparity!

FKING1

Rear wheel HP vs flywheel has nothing to do with gross vs net.
Gross ratings were made on an engine dyno with no accessories, while Net are as the engine is the vehicle ( still on an engine dyno). Corvette went to SAE net figures in 1972.

Godfathers Ghost

there is no specific conversion factor because every setup is different. Automatics use more power than manuals, 9 in. ford rearends use more power than GM 10 bolts, driveshaft (one piece, two piece,ect.), liveaxle or IRS, condition of these parts...too many variables. Some say rule of thumb for average, rear drive car is 20%, but, what is an average car? The drivetrain doesn't care what type of engine is in front of it (all though engine speed acceleration does affect it). It doesn't care if it's Kia 120hp four banger or a 600hp BBC, if it takes 75hp to turn the trans, driveshaft, rearend, ect. you're gonna have either 45hp or 525, assuming same drivetrain and similer engine acceleration of the engines themselves. What this all means is that in order to make a good estimate, you have to know how much power you're setup takes. I can tell you this much (these are average numbers for the items listed), a TH400 trans can rob as much as 75 hp, but usually only around 50-60, the TH350 is about 10-15 less, and a manual trans is also another 10-15. One piece driveshafts, maybe only a few, virtualy none if it is perfectly straight (u-joints). A C3 IRS can be as little as 10hp for a really well setup, proper ride height adjusted car(half shafts staight, again, the u-joints, but this changes as soon as you hit a bump), and up. Brake drag can be 5 hp or way up, also depending on the quality of the parts and setup, usually 10-15 with our integral drum parking brake.
If you can't test your own setup, the best is to ask around for those who have the same setup and have some test results, if they have flywheel numbers and dragstrip results, you can use a formula (sorry, I don't have it, but it does exist) to calculate their wheel hp and subtract from the dyno number to give you their drivtrain loss.

NHRalph

Manufaturers used to use Gross horsepower ratings up until around 1970 or 71, then they went to net rating, using air cleaners, exhaust, ect. A 300 horse became 270, then in a few years with the gas crisis, they de-tuned the engines even more, so the old 350/300 horse was now 190.

mstanton

For what it's worth, in 71 Chevrolet advertised both gross and net hp.

engine gross/net

Base 270/210
LT-1 330 /275
LS-5 365/285
LS-6 425/325