Gross vs. Net horspower--how big is the difference?

[Alan Davis]

Hi,
I'm currently looking to buy a big block Corvette. I read that Chevrolet
switched from gross to net horsepower ratings some time in 1971.
The most powerful Corvette made was a 1969 427 which made 435 horsepower
(gross).
By today's horsepower measuring standards, about how much net horsepower
would this 427 engine make?
Is there a conversion formula that can give an estimate?

Thanks a lot,
Alan

[Dan Carr]

ada...@cyberramp.com (Alan Davis) writes:

Alan,

Gross HP ratings were last used by GM in the 1971 model year.
The government mandated the switch from gross to net ratings.

I have one data point for GROSS vs NET comparison. The 1971 GMC Sprint
sales brochure shows both ratings for several engines. The two 454s listed
(the LS-5 and LS-6) were rated as :

HP TORQUE
GROSS NET GROSS NET

LS-5 454 365 285 465 390
LS-6 454 425 325 475 390

As I have said before on the net, auto makers have frequently inflated
and deflated the horsepower ratings of various engines due to marketing,
government, and insurance concerns. Only a dyno test can tell for sure !

I would guess that if the '69 Corvette 427 was rated at 435 gross HP,
it would equate to about 345 net HP.

Gross ratings were usually determined from engine tests where the engine
had no accessories (fan belts, etc.), and sometimes had no exhaust manifolds.
Net ratings are supposedly for engines as installed in a vehicle with all
equipment attached. However, both gross and net ratings are measured at
the FLYWHEEL, not the rear wheels. So, put a new 310 net HP (or whatever)
Z28-SS on a dyno and it would probably pull about 280 HP at the rear wheels.

Another thing to consider is that while these old high-horsepower motors
ran well straight from the factory, they were often not in the optimal
state of tune. An hour or so of fine tuning the carb and distributor
could ofter increase the power substantially.

BTW, the '71 LS-6 was originally planned to be available
in the '71 Chevelle, El Camino, and Sprint, but was pulled
from the option list before production began.

------------------------------------------------------------------------------
____ _____ _____ _
/ \ /\ | \ | \ (Dan Carr) - --==== ,---' `--.
/ /__\ |____/ |____/ (Research Systems) - --==== `--O---O-'
\ / \ | \ | \ (Boulder, Colorado) E_l___C_a_m_i_n_o
\____/ / \ | \ | \ (d...@rsinc.com) GMC SPRINT

1972 CHEVY EL CAMINO SS-454 (LS-5,at,ps,pdb,ac,cowl,tach,posi,buckets,etc.)
1968 CHEVY EL CAMINO SS-396 (L-34,at,ps,pdb,tach,posi,buckets,etc.) Hot Rod.
1971 GMC SPRINT SP-454 (LS-5,at,ps,pdb,cowl,tach,posi,buckets,etc.) Project.
1968 BUICK GS-COLORADO (350-4,at,ps,pb,ac) Next project.
1971 BUICK GS-350 (350-4,at,ps,pdb,ac,posi,buckets,etc.) Daily driver.
1971 BUICK SKYLARK CUSTOM (350-4,at,ps,pb,ac,buckets) Parts car.
------------------------------------------------------------------------------

[eugene]

Alan Davis wrote:
>
> Hi,

> I'm currently looking to buy a big block Corvette. I read that Chevrolet
> switched from gross to net horsepower ratings some time in 1971.
> The most powerful Corvette made was a 1969 427 which made 435 horsepower
> (gross).
> By today's horsepower measuring standards, about how much net horsepower
> would this 427 engine make?
> Is there a conversion formula that can give an estimate?
>

> Thanks a lot,
> AlanI don't have a formula for converting gross hp to net hp, but this might
help. The Plymouth 318 engine in standard form was rated about 200 gross
hp in 1973; the rating was 150 net hp in 1974. To my knowledge, no
change in the engine. Try to get some Motor's or Childens shop manuals
of that age; then you might be able to make a year-by-year comparison.
An automotive school might have back issues. Try writing GM customer
service; they should know.

[The Connell]

d...@rsinc.com (Dan Carr) got really drunk on 1 Jul 1996 16:23:52 GMT
and wrote:

-------snip------

>BTW, the '71 LS-6 was originally planned to be available
>in the '71 Chevelle, El Camino, and Sprint, but was pulled
>from the option list before production began.

>------------------------------------------------------------------------------
> ____ _____ _____ _
> / \ /\ | \ | \ (Dan Carr) - --==== ,---' `--.
> / /__\ |____/ |____/ (Research Systems) - --==== `--O---O-'
> \ / \ | \ | \ (Boulder, Colorado) E_l___C_a_m_i_n_o
> \____/ / \ | \ | \ (d...@rsinc.com) GMC SPRINT

> 1972 CHEVY EL CAMINO SS-454 (LS-5,at,ps,pdb,ac,cowl,tach,posi,buckets,etc.)
> 1968 CHEVY EL CAMINO SS-396 (L-34,at,ps,pdb,tach,posi,buckets,etc.) Hot Rod.
> 1971 GMC SPRINT SP-454 (LS-5,at,ps,pdb,cowl,tach,posi,buckets,etc.) Project.
> 1968 BUICK GS-COLORADO (350-4,at,ps,pb,ac) Next project.
> 1971 BUICK GS-350 (350-4,at,ps,pdb,ac,posi,buckets,etc.) Daily driver.
> 1971 BUICK SKYLARK CUSTOM (350-4,at,ps,pb,ac,buckets) Parts car.
>------------------------------------------------------------------------------

All the literature i've read seems to point out that the LS6 was
available for chevelles in '71.. Just a little note