falken

i have a 1969 L36 convertible 427/390 horsepower (a/c delete, power steering delete). is this a gross or net h.p. rating? some people say the horsepower on these cars were seriously under rated, why is this? Anyone know what a realistic horsepower # would be? has anyone dyno'd?

694speed350

Falken, the horse power rating was gross at the flywheel with out accesroies hooked up. but with that being said most of the high output engines were mostly underrated thanks to the feds and insurance cos.

Tom73

In '69 there were Gross rated. The Net rating did not come along till '72.

Under-rated??? Don't forget, the 427/390 was the base big block. In '69 you had three other 427s that were rated higher. If they were going to under-rate one it would be one of the high power ones. I would guess that in Net HP terms you are looking at low 300's.

tom...

capevettes

Are you confusing Net horsepower with Rear wheel horsepower? I agree that the 390 hp engine on a dynomometer will put 290 to 310 hp to the rear wheels. They do produce horsepower close to what it was rated at without all the accesories on.

IrishJoker

Don't forget GM "under stated" horsepower as well to keep the insurance and the feds happy!

Fevre

Techically GM never understated the hp, they just 'choose' to use the output reading at a rpm lower than the eng's max hp rpm, the hp they state I believe is accurate at the rpm stated but is not the eng's max hp

PRNDL

I have a 68 L-36. I don't think GM played any games with this engine. The 390 gross hp was probably accurate. That is the equivalent of around 300 net hp (today's ratings for an engine in a new car) so it would be similar to the LT1 vettes of 92 - 96, but produces more torque.
I would guess a factory fresh L-36 would produce around 270 rwhp on a dyno. With headers and exhaust and some carb and distributor (timing) tuning I got up to 300 rwhp.

Chilli

I've always been told that the difference between gross and net horsepower is about 20%. If that was totally accurate, you'd be lookin' at 312 net horsepower at the flywheel. Net flywheel to rwhp probably depends largely on your transmission (automatics use up more, I think), rear end, tires, etc.

Sayfoo

I have had my 69 427/390 mostly stock on the dyno. PRNDL is right on the money. Rear wheel ratings of 360 tq and 270 hp. I then switched to Hooker headers and side pipes. I believe I picked up noticable hp. Anyway, it really sounds cool.

Mr turbo rotary

That's probably about the norm. You are making ~69% of the claimed gross hp to the wheels.

http://www-unix.oit.umass.edu/~tcroy/horsepower.htm