cohocarl

Just wondering if anyone knows what changes were done to the L98 when GM went to the smaller chamber aluminum head in mid-86. My 85 had "624" 76cc iron heads, and when aluminum heads were introduced on the L98 in mid-86, from what I've read, they are 58cc(?). Was a different piston or rod put in the aluminum headed Vettes to retain the 9:1 compression ratio?

Just curious.

Thanks,

Carl

Red85

I'll reference the casting numbers for you later this evening, but I can tell you something is amiss in the chamber volumes you mention assuming GM retained their tried-and-true 4 valve relief pistons on subsequent model years. To keep 9.0:1 c.r. AND reduce the volume from 76cc to 58cc, they'd either need to dish the piston or install a MUCH thicker head gasket on the same 4.00" X 3.48" engine.

ralph

yes i believe GM used a different piston. The 85 had TRW forged pistons with one large valve relief.

dgoodhue

The aluminum headed 86 had 9.5:1 compression ratio. They did use different pistons.

Juliet

I didn't think the heads were changed unti '87 - and then only partway into production. They held up convertible production for the Alum heads so all Converts in 87 had them, but some of the first coupes didn't in '87. Or am I thinking of another change altogether? ~Juliet

dgoodhue

Your think of 1986 ...

GRX

Ralph, are you completely sure the stock pistons where FORGED? :eek:
If so, supercharging would be easier on these pistons?

dgoodhue

I have heard a couple now tell me the stock 85 piston were forged.

Marcho Polo

When I had my '85 motor rebuilt, the block was original, verified by the vin stamp on the pad.... it had never been bored, verfied by my machine shop... and he also told me that there were forged pistons in there.

There were little valve reliefs in there. Here's a pic of the top of an '85 piston:




I have heard later years used a dished piston. Putting 58cc aluminum L98 heads on an '85 block would be around 11:1 compression I think.


[Modified by Marcho Polo, 2:33 PM 10/26/2001]

GRX

Craig, this may be a crazy question, but how can one tell if the pistons are forged? I mean, with a crank, it's easy to determine, but do forged pistons have a "part line"?


[Modified by GRX, 12:38 PM 10/26/2001]

Juliet

Ah, OK thanks. Must be a blonde day. :p: ~Juliet

Marcho Polo

Honestly, I don't know, but the machine shop owner has been around for 25 years or so and I just accepted that he knew his stuff.

ralph

grx, believe me if you have an 85, you have forged pistons. Marchos picture is right on...those are them. GMPP had them in the catolog...the numbers stamped on top of the pistons matched the number in the cat and they were listed as trw forged pistons. I couldn't get anyone to tell me the volume of the valve reliefs so i had to measure them myself (they are 4-5cc). The pistons are spec'd at .025 down in the hole although mine measured about .024. With 59cc chambers and a .026 gasket, i've got about 10.7:1 cr. The stock 85 gasket was a .020 steel shim (no good with Alum heads tho). .026 was the thinest gasket i could find that would work with the alum heads and my cr. And i did a lot of research.

Aggravated4life

For more info,
Late 86 Coupes and convertibles got the aluminum heads and new one piece main seal.Same heads were used in 87 but GM switched to a roller cam in the block in 87.

Then in 88,they took the aluminum heads and flatted the exhaust port to a D shape and cut back the valve a certain amount to increase flow.(aka D port heads)

Hence the 88-91 and up aluminum heads flow a little better then the first 2 years it was out.

89 cars didnt have the cold start injector the 85-88 cars had,and GM added Multec disc style injectors and stopped using the older Bosch pentel cap style.

85 years were said to have 24 lbs rated injectors while 86 and later was 22 lbs.
MAF was used for 85-89 and speed density or Map sensor was used in 90-91.

In 90 or 91, GM added lighter pistons and bumped the compression ratio around 9.8:1 or maybe 10.0:1...Im not exactly sure.
Maybe someone knows that one better.
But I did read that change once before.

So far I think that covers most of the changes.

dgoodhue

FYI I looked up piston part numbers in my parts and illustration manual. It appears 84 and 85 have the same part number and therefore have forged pistons. Early 86 psiton have a different part number so I assume they are cast pistons (judging by price, they are slightly cheaper than the late 86 which I know has cast piston.)

cohocarl

Thanks for all the replies. I was mainly wondering about the different pistons. From when the L-98 was introduced in 1985 till it's end in 91, there were quite a few changes to it. I'm amazed G.M. didn't change the name of it with all the changes through the years.

Thanks again,

Carl
:chevy