L-81 Performance Specs/Compression Ratio?
Thread Starter
Instructor
Joined: Mar 2010
Posts: 129
Likes: 1
From: Fort Worth Texas
L-81 Performance Specs/Compression Ratio?
I just purchased an 81 and was looking at the L-81 spec plates. All of them I have seen show compression ratio at 9:1, which is the same as the L-82. The plate shows the L-81 horsepower at 190 and the torque at 280. The L-82 is 230 horsepower and torque 275. I was under the impression the L-81 had low compression like the L-48. What is the difference between the L-81 and L-82 to warrant the 40 horsepower increase and loss of 5 ft/lbs of torque?
Thanks, Zak.
Thanks, Zak.
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
My books show an 8.8 compression ratio and 190 hp. Welcome to the world of low power Corvettes. I own an '81 as well. I just pulled the worn out factory engine yesterday. I've got a fresh roller cammed 355 in the garage. It should roughly double the horsepower of the stock motor. It's been a long time coming, believe me.
Race Director
Joined: Oct 2000
Posts: 12,710
Likes: 3,161
From: Shamong, NJ
The 81 L-81 has an 8.2:1 compression ratio, the same as the 80 L-48. The 80 L-82 had 9.0:1, like all L-82's. The L-81 actually produced more torque than the L-82 (280 lb-ft vs 275 for the L-82).
The besides the higher compression, the L-82 used different heads and camshaft than the L-81 (and L-48's), which was where the horsepower difference came from.
The besides the higher compression, the L-82 used different heads and camshaft than the L-81 (and L-48's), which was where the horsepower difference came from.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Rated "torque" for two different engine is NOT comparable, unless those torque readings were taken at the same rpm's for both. To 'step-around' insurance company requirements (for higher HP engines), GM and other car companies stated engine specs at reduced rpm levels...just so those specified figures would be lower than insurance limits for performance engines. The L-88 engine was actually specified with LOWER horsepower (430) than the L-71 engine (435) in 1969. Of course, the spec for the L-71 was at 5800 rpm, while the L-88 was spec'd at 5200 rpm....and we all know the L-88 would easily wind to more than 6500!
At 5800 rpm, the L-88 would likely have been putting out close to 475 hp, with well over 500 hp being made at 6500 rpm.
You have to "read between the lines" to understand what the advertised power figures really mean.
At 5800 rpm, the L-88 would likely have been putting out close to 475 hp, with well over 500 hp being made at 6500 rpm.
You have to "read between the lines" to understand what the advertised power figures really mean.
