Pistons

[david]

Does any one know what year GM lowed the compression on 6.5.
And how low did they go????
I think i will just go those!!!
David
:confused:

[billg]

This is from memory, but I believe they went from aprox. 22 (early 90s) to 21 (mid 90s) to 20 (late 90s). I think using the late 90s pistons is the way I would go, if I had to rebuild mine right now. The 18s are nice, but there

[cruzer]

You can also get .010 reduced compression height
pistons and offset piston pin bushings for the connecting rods to reduce compression. When I rebuilt my engine I wish I knew about the offset bushing. I think it reduced another .010. Also
I think the felpro gaskets are 0.010 thicker than stk GM. The .010 reduced pistons were the same
money as stk thru mahle.

[richard7]

My engine has 20.2 to 1 CR. From the GM powertrain website, I know that 2000 engines had already this compression ratio.

I beleive it was used when they increased the HP/Torque rating in the mid-1998 model year.

[Jim P]

I am not positive but I think the pistons are all the same on all the years. The difference is the heads. Atleast thats what my local engine rebuilder told me. He said that they cc'd the combustion chambers on a 95 and a 99. The 99 has a bigger combustion chamber and lowers the compression. When I refer to combustion chamber, I am talking about the hole in the head under the precup. Even if both heads have the same precup, The newer combustion chamber holds more volume.

[david]

Richard7 ,
Where did you find that info???
I went to the site and could not find it!!
David :confused:

[cruzer]

I think JimP has got it right. When I rebuilt my engine there was no ID for yrs, just compression.
The diff in compression among pistons is where the hole is drilled for the wrist pin. Higher in the piston lowers compression. Kennedy's are not like that though. His have stk height and are dished in a way to control the combustion burn.
He had them specifically designed and casted.

[billg]

Wow, seems that the cylinder head rebuilders just specify one head for the 6.5. If this is true, your truck may never be the same just from replacing the heads. Not to mention a side-to-side difference. I doing a head gasket job right now and I called on rebuilt heads; no one mentioned a difference between years.
bg

[tom.mcinerney]

Maybe check with Avant Salvage or John Kennedy?

[cruzer]

I was reading the 1999 Feature Articles & Product review, and somewhere in the back of the book it breaks down the comp ratios of the different yrs.
Comppression ratios where changed by the pistons.
Not by the heads. I can't recall the yrs but there were 3 different ratios w/ the latest model being around 19:1. I believe the info was in the More Power article. Sorry for any confusion.

[Kennedy]

If anything, the CR on the late engines is HIGHER than the earlier ones. There was a head gasket shift (mid '90's?) somewhere though that dropped thing a bit...