So this is a question I've had looking at the various GM muscle car HP ratings in the 60's and 70's...what was the difference between the Corvette engine rating and other GM car ratings for the same engine? For instance, the LT-1 was advertised as 370 HP for the 1970 Corvette, but 360 for the Z-28. The Chevelle L-79 was rated at 325 HP while the Corvette one was 350. I know GM didn't want any other product having more HP than the Corvette, but was there an actual difference? Maybe timing, exhaust setup, carb tune, or was it just advertising nonsense?

 

exh manifolds from what I understand. Vettes used the ramhorns, the others got the log type

 

that was a big reason, but also look at the RPM reading they advertised it at.

 

Advertised power ratings in the 70's was a cr@pshoot.... The EPA and the insurance industry put the screws to higher powered cars; so the car manufacturers 'fooled' with the ratings for a multitude of reasons: some for marketing purposes, some to restrict sales to the mfgr's benefit, but mostly to help buyers be able to afford car insurance.

The primary method of doing so was to make HP/torque ratings at a much lower RPM setting than the actual redline for that engine. Just pick a point on the dyno curve that keeps that engine's power below a specific 'trigger point' and all is good. The extreme example for that method is the L-88 engines. When they became available in 1967 (and thru 1970), they were rated at LOWER power than the tri-power L-71 engine. L-71: 435 hp @ 5800rpm; L-88: 430 hp @ 5200. And, of course, the L-88 would pump out about 100 more 'horses' as it ran up thru 6500 rpm. This limited the sales of L-88's to many hot-rodders, as they would prefer to pay LESS for an L-71 that would produce more? "advertised" power (5 hp).

Many other engines used that approach, especially with GM. Most of the late 70's C3s were rated at lower than redline rpm to stay below some insurance limits for "higher HP cars": same L-48 & L-82 engines, but rated at hundreds of rpm lower than for earlier C3 years. Everyone thinks those engines were "dogs"; but they were really nearly the same...just rated at lower rpm.

There was a substantive change in engine ratings in 1972. That's when SAE required engines to be rated for "net" power, rather than "gross" power. What's the difference? Net power ratings subtract the power consumption of the accessories mounted to the engine in the vehicle (alternator, transmission, etc). The 1971 L-48 engine was rated at 270 bhp; the nearly identical 1972 engine was rated at 200 bhp. Quite a drop!! But, it was only rated differently; the engines performed exactly the same.

 

Yup ... also, as vettes were one of the most expensive cars to purchase ... for marketing purposes ...
... it makes sense the Hi-$ cars would have big HP numbers ... even if that HP number was simply derived differently.

Thus ... the old Wow, that Chevelle's got a vette motor in it

 

It was exhaust to a point....but 50% of power rating back then were created by the bean counter and the lawyers.
I can tell you 100% that not one LT-1 out of the box made 370 horsepower from GM. I have built an LT-1 to bone stock and a DZ 302 and both made 360hp.....that was through long tube dyno headers. This was the absolute power limit of the heads that were bolted to it. If you take either engine and install a pair of Dart 180 heads and a pair of headers....with no other changes, they will make 420.
The stock head unit will throw about 300 to the tire with headers.......and with a deep gear and slicks would run about 13.1-13.2.....this is right in line with a 2002 Camaro 5.7l SS which threw 300 to the tire and ran low 13's at 3400 pounds on Drag Radials.
Ok....now fire up the flame thrower! LOL!

Jebby

 

Thanks guys, so it appears that the exhaust manifold played a part in the HP rating differences. I guess if you were going to shell out big bucks (for 1969) you'd want top HP rating even if it was only for show.
Although I would think the stock 8" rims might also give you a traction edge over the other cars which used 6-7 inch rims as well.

 

It was here or on the chevelle site a link to a 70? article in hot rod showed a 70 LT1 Camaro running 12s

Recurved ignition
4.56?? gears
headers, no other changes save for some slicks. Not bad
My 70 z was a pretty strong runner..sure didnt run 12s but was no 14 sec slouch
Many cars like that esp BB cars took nothing mroe than a recurve and headers to really run.

302s didnt do too bad either but just werent good st engines period just made 0 torque.

 

Dont forget the a lot of the motors that had the 10 hp difference had a change of just the lifters. The Chevy 2 had an L79 that was rated at 365hp, the L79 vette had Hydraulic lifters and rated 350. I rebuilt my motor to L79 specs which had a 10.5:1 compression ratio, the solid lifters and ramshorns through 2.5 inch full side exhaust. My best in the quarter was a 13.7 at 100mph. About the same a 435hp big block was clocking in 69 by Car&Driver (13.8) records. Then a bigger cam and saluminum heads got me down to 13.5. With headers and my new cam this year I should get lower into the 13's.

The LT1 had solid lifters as well so it may have had a 10 hp boost at the top end. Remember all the higher HP numbers are at the top, everything else being the same, they probably were the same below 5500 rpm.
https://www.corvsport.com/corvette-quarter-mile-times/
As a bonus for stories, my high school buddies blue printed a motor to the 70 LT1 specs and put it in a shortbed cheyenne C10 if I remember correctly. I wish they had cell phones then. THeres a railroad bridge in front of my house with a short but steep hill thats great for burnouts. Well with little effort that nice little mouse had all 4 tires just boiling smoke and when he was done he drove up the street to my driveway as we watched the smoke just sit in the hollow down there. Then there was the super bright search light that cut through it looking like a laser beam, then the blue strobes. The cops must have just come across it and thought the bridge was on fire. We had a good laugh about that

 

The Chevy II NEVER had a solid lifter L76 327/365. they did have a L79 327/350

 

I read that one....they worked hard for a 12 and I even think they unbolted the front sway bar....such as it was back then

Jebby

 

365hp L76 was what I was thinking, who had that baby? Wasnt the difference between the L76 and the L79 just the solid lifters. I swear I read that in motortrend or hotrod a couple of years ago

heres one of the stories I had read but all the horsepower numbers from it where in chart forma dn all the pictures are cprrupted. But the info is still pretty cool. I guess thats the proble with digital media, it can be changed or just go sway. Paper isnt going to run out of batteries....
http://www.superchevy.com/how-to/pro...lock-testing#/

 

Only the 64 and 65 Vette got the L76

 

really.....I didnt know that

 

L-76 in the 1963 Corvette, also.

 

if you retard your engine from 36 degrees total to 34 degrees total you will lose about 20 hp. THAT is how GM effed with hp numbers. you have an employee pick thru a whole bin of centrif advance weights. all are in "spec" pick out the smallest and you get more centrifugal advance and power at the same initial advance setting.

 

Glad to know more than before I read this. Thanx.