Hello
Is there a formula to look at horsepower ratings as they used to be rear wheel Horsepower and convert to what it would be today where they just measure it off the engine and also to go backwards from that?

Thanks

There's bit of gray area between how much loss you actually get from the drivetrain. From what I've heard it's about 17-20% for manual trans and 20-25% loss for autos depending on how tight of converter is used. It's really not black and white though. Lots of variables.

Thanks Firebob!!
so basically a 350HP 400 of first gen Era with a 4 speed being conservative at going plus 15% would be a 400 HP motor of todays rating.

Funny in talking to most of the rebuilders if I say I want a 350 to 400HP motor, all but one have told me to the flywheel when I tell them rear wheel HP the number jumps significantly.

Other than the fact that the 400 engine had high compression which sucks for todays gasoline an almost stock rebuild using a Pontiac cam spec would easily give me 400HP by todays standard I would think

My 68 428 is rated 375-390 HP depending on the cam. I got 335.1 rear wheel HP through a Muncie four speed. I haven't had my car on the chassis dyno since the rebuild but I think the 500 flywheel HP will net me just shy of 420 rear wheel HP.

Thanks for the input Bluebird!!

I had my old motor chasis dynoed in 2006 and it was 300RWHP. So if going with the 17% rule thru my M20 I get 351FWHP. Pretty close to the 360HP Pontiac said a stock 400 should be. I've since changed things very slightly and am about to get it pulled again before the summer is over so we'll see if it comes up about the same.

Sounds great!!

Hear is factory hp