'71 vs. '72 LT-1 Differences

[Mark K.]

I've decided to postpone my '67 BB purchase until I am more knowledgeable and stick to what I know a bit better - '70-'72 cars. Are the '71-'72 motors identical except for the TI ignition on the '71's? Is the hp difference just due to gross vs. net hp methods and not due to any mechanical differences. Thanks in advance, Mark.

[Ed Jennings]

Re: '71 vs. '72 LT-1 Differences

Mark, I owned a 71 BB in the early 70's and had several friends with 71-72's. I believe the only significant difference is the gross vs net rating method. The 72's also did away with the fiber optic system on the console. Ohter than that, the cars are nearly identical, I believe. May have been some minor changes in the pollution controls on some engines.

[Duke W.]

Re: '71 vs. '72 LT-1 Differences

From the carb inlet to exhaust manifold outlet the '71 and '72 LT-1s are the same. In '71 GM quoted gross power with net power in parentheses; '72 and later they only quoted net HP.

There may be some changes in the emission configuration, which could include carburetor and distributor calibration in addition to the various add-on emission control systems.

Duke

[Mark K.]

Thanks guys! No text. *NM*

[Warren F.]

Re: Thanks guys! No text.

I believe there are some minor differences between the '71 & '72 LT-1 engines.

First off,there is the net figure ratings.....for '72 it is 255hp, for '71 it is 275hp. If I remember correctly, there is a difference in timing settings between the two.

[Dave S.]

72 was 255 HP vs. 71 was 330 HP *NM*

[Patrick H.]

Re: Thanks guys! No text.

Warren,

I believe that timing settings between the two are identical.

There is a difference in the vacuum hose routing and emissions solenoids for the two years. Of note, 70 and 72 are more alike, and 71 is different.

Patrick

[Duke W.]

255 was net, 330 was gross *NM*

[Dave S.]

Re: 255 was net, 330 was gross

I realize that. Where did 275 HP come from.???

[Duke W.]

Re: 255 was net, 330 was gross

Good question! I checked the NCRS spec guide and the '71 LT-1 was rated at 330HP gross/275 HP net. In '72 the rating was 255 HP net, and no gross rating was quoted.

Another example I can give is the '71 Vega 2bbl. (L-11)engine. The '71 was rated at 110 gross and 93 net, but in '72 the net rating was reduced to 90 HP.

As far as I know, both the LT-1 and L-11 Vega engines were the same internally both years and only varied in carb and distributor calibration and external emission control sustem configuration, which usually doesn't affect peak torque and power.

All I can figure is that the '71 net ratings may have been estimated or not
tested to full accuracy. It could also be that the SAE specification that defined net power was not fully developed and/or changed in that era.

In any event, I don't think the actual output of the LT-1 was substantially different between '71 and '72 if measured by a consistent method.

Duke

[Dave S.]

Re: 255 was net, 330 was gross

Duke,
Thanks for the expanded response. I own a 71 LT-1 and collect/read everything I find on them and to my recollection I have never seen the 275 HP in print. The NCRS listing must have come from somewhere as they are usually careful what they publish. I'll keep my eyes open in the meantime.

[Duke W.]

Re: 255 was net, 330 was gross

The 275 HP net rating is shown in parentheses on the '71 powertrain chart - page 51 of the '68-'82 NCRS Spec. Guide. It was probably similarly listed in the '71 sales brochure.

Duke

[Warren F.]

Re: 255 was net, 330 was gross

The '71 LT-1 engine spec shows both gross and net(in parenthesis)in the factory brochure, dealer technical poster, dealer sales album for sure, as I have them.

The different hp rating between the '71 & '72 I believe is that the 1971 engine rating is as installed with all accessories, 1972 rating is taken at rear wheel instead of flywheel.

[Duke W.]

Re: 255 was net, 330 was gross

No, no, no! "SAE net horsepower" is measured at the flywheel "as intalled" with all engine accessories and the vehicle exhaust system and corrected to a specific set of "standard" atmospheric conditions that have changed since 1971. GM has never published rear wheel horsepower numbers

As I stated previously, the difference between the '71 and '72 SAE net ratings is probably tied up somewhere in the test procedure/standards as the SAE net rating system was new at that time and the standard or test procedures may not have been fully developed or was in a state of flux.

There certainly was not sufficient difference between the '71 and '72 LT-1 engine configurations to account for 20 HP.

Duke

[Warren F.]

Re: 255 was net, 330 was gross

Duke:

Thanks, for the correction. I had heard from other Corvette owners that this was the reason for the discrepency. My error. Warren