1995 vs. 1996 LT1 - drop of compression

[Oliver S.]

I'm currently looking to obtain a late C4 with LT1. Having compared the LT1 spec of 1995 and 1996 I was astonished that the compression rate was lowered from 10.5:1 to 10.4:1 in 1996. Peak power was the same but torque was marginally lower and at later rpm (different sources yield different numbers). Was there a reason for this? Any stricter emission regulations or to get a slightly bigger difference to the LT4 - however the LT4 was manual and a '96 LT1 was auto (some say there were some early '96 manual with LT1, though).

Oliver

[Duke W.]

Re: 1995 vs. 1996 LT1 - drop of compression

0.1 compression is pretty much noise level, and it was maybe respecified based on how the engines were manufactured. Also, CR varies cylinder to cylinder on any engine. It takes a lot of measuring and hand grinding to equalize them to no more than 0.1 maximum variation on a sixties vintage engine.

Back in the sixties, nominal small block deck height/clearance was 9.025/.025", but, as built, many decks were high by up to .015". which drops the actual CR by about 0.5. As a general rule, sixties vintage compression ratios, as built, were lower than advertised by up to about 0.5. Plus, advertised numbers tended to be "rounded-up" and production variation was not specified.

In 1968 the specified CR of the base 300 HP engine was dropped to 10.0 from the 10.25:1 '67 spec even though the engines used the same piston and had the same nominal head chamber volume; 10.0 was more realistic for the engines. When I ran all the nominal data through a CR calculator the result was 9.8, and, as built, may have been as low as 9.5.

Specs were a lot more honest in the nineties (and deck height machining tolerance was likely tighter), but slight changes don't mean there are different parts or different output. It's probably just a refinement based on how the average engine turned out.

You may be able to find all the relevent specs, including tolerance - deck height, piston compression height/volume, head chamber volume, compressed gasket thickness in the AMA specs. Then run them through a CR calcutor and see what you get including the maximum range based on min and max tolerance. You'll probably be surprised.

Duke

[Joe C.]

Re: 1995 vs. 1996 LT1 - drop of compression

My guess is a better head gasket for 1996. The advertised compression ratio difference is well within the range of manufacturing tolerance in any case, and unlikely to cause any measurable difference in torque between ordinary production engines randomly dyno tested.

[Joe L.]

Re: 1995 vs. 1996 LT1 - drop of compression

I'm currently looking to obtain a late C4 with LT1. Having compared the LT1 spec of 1995 and 1996 I was astonished that the compression rate was lowered from 10.5:1 to 10.4:1 in 1996. Peak power was the same but torque was marginally lower and at later rpm (different sources yield different numbers). Was there a reason for this? Any stricter emission regulations or to get a slightly bigger difference to the LT4 - however the LT4 was manual and a '96 LT1 was auto (some say there were some early '96 manual with LT1, though).

Oliver

Oliver-------

The components affecting compression ratio were mostly the same for 1995 and 1996 LT1. The heads, head gaskets, and pistons were the same. In fact, pistons were the same for all 1992-96 LT1. However, there was a difference involving the intake and exhaust valves between 1995 and 1996. It may be that the 1996 valves had slightly more concave to the valve heads and that's why the compression was marginally lowered for 1996. Otherwise, it may just have involved a re-calculation as Duke mentions.

The camshaft used for 1995 LT1 was different than the camshaft used for 1996 LT1. I do not know how they differed. The difference may or may not explain the difference in torque ratings. The 1996 cam did become SERVICE for 1995.

By the way, there were FOUR different camshafts used for the LT1 engine over the 1992-96 period. 1992 was unique to that year. 1993-94 were the same. 1995 was unique and 1996 was unique.