L72 vs L71 which had a the stronger motor

[Jack M.]

Re: L72 vs L71 which had a the stronger motor

One GM dyno sheet had the 1966 L72 @ 464HP... and still climbing:

L72-Gross-&-Net-Power (800)a.jpg
L72-Gross-&-Net-Power (800)b.jpg

[Steve B.]

Re: L72 vs L71 which had a the stronger motor

Danny:Not so. One must remember all of the COPO Camaro's/Chevelles engines where all L72's. Also there was a fair
share of RPO's in the Full size Chevrolets that where L72"s. So if you add all the availability of the L72's from the factory
there was more then you think. Thanks>Larry

Exactly right, that's what makes the L71 special to me since it was a Corvette only motor.

To Mike's point, the 71 LS6 is a great motor. Even though the compression was only 9 to 1, it used the aluminum open chamber heads from the second design L88s/ZL1. I remember reading somewhere that it would make about the same HP as the 70 LS6 that was used on the Chevelles. Those cars had a more restrictive exhaust manifold too so it makes sense.

[Danny P.]

Re: L72 vs L71 which had a the stronger motor

Danny:Not so. One must remember all of the COPO Camaro's/Chevelles engines where all L72's. Also there was a fair
share of RPO's in the Full size Chevrolets that where L72"s. So if you add all the availability of the L72's from the factory
there was more then you think. Thanks>Larry

Was general talking about 1966 Corvette L72. but Larry most of the 69 COPO Camaro used the 427 ZL1 the rarest COPO and the more rarest Chevelles SS were the LS6 454 were used.

[Duke W.]

Re: L72 vs L71 which had a the stronger motor

The 1971 425 HP LS6 rating was the old SAE gross. LS6 was essentially a stroked L-72 with a 9:1 CR so it could run on 91 RON (same as today's 87 PON), aluminum heads, and a lazy emission control oriented spark advance map. For that year GM advertised both the old SAE gross rating and the new SAE net, and LS6 was rated at 325 SAE net Horsepower.

For 1972 only SAE net power and torque were advertised across the entire GM product line.

Duke

[Danny P.]

Re: L72 vs L71 which had a the stronger motor

thank you Jack for sharing the L72 dyno sheet, very impressive stats.

[Mark F.]

Re: L72 vs L71 which had a the stronger motor

I thought torque and horsepower always crossed at 5,252 RPM ?
If I'm understanding the axes correctly, I don't see that in Jack's chart...what am I missing ?

Never mind - I just figured it out...

[Duke W.]

Re: L72 vs L71 which had a the stronger motor

You are correct, but on this graph torque values are the left hand scale and power is the right hand scale.

Torque in lb-ft and horsepower are always the same at 5252 RPM, but the torque and power curves will only cross at this point if both parameters use the same scale.

Note that there are actually two left hand scales. The one occupying the top third of the graph is for spark advance, however, no values are plotted.

One other observation. The 2.5" exhaust system is very low restriction for a small block and only generates about 3 psi at close to 300 SAE corrected RWHP, about what the 327 LT-1 massaged head configuration makes, which yields a net gross ratio at the flywheel of 0.89 that I derived empirically from my database of various dyno tests, both lab and rear wheel, that I've collected over the decades and used this data to refine my Engine Analyzer simulation models so the simulations result in very good correlation with actual test data.

Big Blocks are a different story because the higher exhaust flow generates up to about 6 psi exhaust backpressure, which really takes a toll making the net/gross ratio more like around 0.80. For a given exhaust system configuration the larger the engine displacement the higher the pumping power required to push the exhaust out. A large displacement engine with headers and open exhaust that makes over 500 HP on a lab dyno may barely be 400 SAE net at the flywheel with manifolds and the OE exhaust system.

The above is why the 427 CID LS7 has a 3" exhaust system.

Duke

[Mark F.]

[Duke W.]

Re: L72 vs L71 which had a the stronger motor

I've written up the results of my research for the for the "327 LT-1" and "Special 300 Horsepower" configurations (A Tale of Two Camshafts, Corvette Restorer, Corvette Forum, and Web). I didn't go into much detail about the models I used, but have discussed details with other Engine Analyzer users.

There's nothing "secret" about my EA work, but unless one is an EA user, the details would likely be of little interest, but the results are useful to anyone who is interested in improving engine performance (more top end power and useable revs) without affecting visual appearance or operating characteristics such as idle behavior.

Duke