I'm going to look at a 72 SS with a 402 (LS3) with the Muncie 4-speed. It's a matching numbers engine and was advertised at 240 hp net.

I know that in 72 the HP ratings had to be expressed as net. my question is, what hp rating would this translate to using the method used prior to 72?

thanks,

dan

1970 402: 330hp @ 4800; 410ft lbs @ 3200; 10.25 CR
1971 402: 300hp @ 4800; 400ft lbs @ 3200; 8.5 CR
1972 402: 240hp @ 4400; 345ft lbs @ 3200; 8.5 CR

Glenn,

a couple of things:

1) if 72 was the transition year to expressing engine HP as "net", then are you saying that the 240hp is equal to the 300hp rating in 71 using the different method used in 71 to measure HP? (This would assume that the engines are exactly the same).

2) if yes to the above, and 70 and 71 used the same method to rate HP, then how could the 240hp equal the 330 hp in 70 since they are different engines (at least the CR is higher in 70)?

I just want to clarify what I am trying to find out here so there isn't any confusion: i want to know what the 240HP rating in 72 would have been expressed as using the method to calculate and express HP in 71 and previous years. I am not looking for the actual "advertised" HP ratings.

Thanks again,

Dan

Dan,
Those specs came from a Chilton manual. I know nothing about that engine as far as head combustion chamber volume, etc. The book didn't give any valve lift figures, but I did find one thing regarding the intake valve opening. The 70 and 71 were at 28* while the 72 was 30*.

All I can do with this is guess. I'd guess the 70 and 71 had the same cam, but the 71 had either dished pistons and/or larger combustion chambers to lower the CR from 10.25 to 8.5.

I'd also guess that the 72 had the same piston head arrangement as the 71 because of the CR. It looks like a different cam was offered in 72 since I don't know of the factory advancing or retarding camshafts. With no lift and duration specs it's impossible to tell.

Maybe someone else has more info on the 402.

Glenn,

thanks for you response. this does seem to be a conundrum but I believe there is an answer somewhere. Since the Feds required HP to be expressed as "net" from 72 and on, there must be a way out there to calculate the HP rating as it would have been expressed in years prior to 72, and,

Remember, the "net" rating takes into account all the other components of the engine that draw on HP, and which, by extension, lowers the amount of HP available to the wheels. Prior to 72, the HP calculation did not take into account things like air conditioning and anything else that saps HP.

So, it's possible that the 71 and 72 engines are exactly the same, or close to the same, but that their HP ratings are simply expressed differently. If we knew they were exactly the same, we could conclude that the 72 net HP is really the same as the 71 non-net HP.

If the engines are different, in any way, then it would require going to GM. I guess this is what I'm asking: did GM ever publish what the HP would have been using the method employed prior to 72.

Regards,

Dan

Since the rpm that peak HP was reached at changed by 400 I think there might have been something slightly different from '71 to '72. The chassis service manuals have a lot of good info in the back of them that may help...

FYI,All low compression 402's still have small domed pistons,.Its the large open chamber heads kill the compression(119cc's).Bolt on a pair of closed chamber heads (98-101 cc's) and a nice cam,and good things happen graemlins/thumbsup.gif

Bolt on a pair of closed chamber heads (98-101 cc's) and a nice cam,and good things happen

mr 4 speed,
Is that possible? I thought open chamber pistons with closed chamber heads meant interference problems. True?
Dennis

Nope,not a problem.Been there done that,and it works.

Won't raising compression result in detonation on pump gas?

im sure raising octane and/or retarding timing would fix that

Originally posted by Danoprk:
Won't raising compression result in detonation on pump gas? Compression works out to be about 9.5 to 1 with a .039 head gasket and a 101 cc chamber head.Far from detonation territory.I've run an LS6 cam with a stock 71 402 with 1968 #215 closed chamber heads before..ran pretty damn strong.

May I point out that just maybe GM lowered the hp point by 400 rpm between the two years so that they COULD derate the engine for insurance purposes much like they did with the L-88 and the little Z-28 engine...

They may in fact be identical engines...

Please think about it... pdq67