Fred91GTA

Hi, I wanted to ask, the stock cast L98 pistons are dished pistons right? If so, how much is the dish? I'm trying to figure out compression ratios for different head and head gasket combos. Also, does anybody know how far down the cylinder the piston is from the top? Thanks.

doc

Stock '87 L98 pistons are flat top with 4 "eyebrows" which I believe account for 7cc when figuring CR.

Damon

L-98 pistons are all slightly dished with 4 valve relief eyebrows and have about 12cc worth of total dish volume. They are the same basic piston design that GM put into SCILLIONS of 350 motors over the years.

doc

Damon is right, I'm wrong. I went to the trouble to find my factory pistons and the are dished with 4 eyebrows.

Stekman

What would be the numerical value on that?

Air_Adam

Lots.

84z28350

3,465,894

ownor

iTrader: (5)

so 12cc or 7cc now? went to tear down my L98 to the shortblock yesterday and noticed they are slightly dished with 4 reliefs as described here

Dyno Don

iTrader: (7)

12cc is correct, the piston is .025 down the hole stock.

nuggie

sorry to revive a dead thread here but can this really be accurate according to http://www.csgnetwork.com/compcalc.html that would put the stock CR at 12+:1... or is it suppose to be negative 12cc for pistons there? That puts it down at a much more reasonable 9.15:1 but I though those motor were advertised at a 10:1 CR (-7 comes to 9.6:1 close enough to say 10).

Night rider327

iTrader: (13)

'87 spec'ed at 9.3
'90 spec'ed at 9.7

And GM has always fudged the compression (and HP) numbers

Yes with that program you use a negative number for any dish volume.

Using that one I get 9.2175

4.000 bore
3.48" stroke
-12cc pistons
4.100 head gasket bore
.028" head gasket thickness
.025" deck clearance

sofakingdom

iTrader: (1)

Nuggie, if you're coming up with 12:1 kind of numbers, you're entering the piston volume as a dome rather than a dish.

Night's numbers are about the same as I would use to estimate a factory CR, except that I wouldn't quote it out to 4 decimal places. There's WAY too much uncertainty in everything to be that precise about it. Deck clearance in factory blocks has AT LEAST .005" of difference from least to most, and sometimes closer to .015", which can be due to the block decks not being the same on both sides, the decks not being perfectly parallel to the crank centerline, rod length variation, and so on. The stroke is ALMOST NEVER "perfect"; most stock cranks have at least a couple of .001"s of variation from one journal to another, and the spec is the UPPER LIMIT, not the "exact" stroke anyway (i.e. "not to exceed" 3.48", NOT 3.48" on the nose). Chamber volumes vary dramatically from one to another, and ESPECIALLY from one head to the other, since they're basically as-cast with no machining... expect AT LEAST 2cc of random variation among cyls IN EACH HEAD from casting, plus 2cc of common-cause offset ON EACH HEAD due to head deck machining. And so on... in reality, even trying to "estimate" your CR out to ONE decimal place is probably fantasy land.

The choice of "calculator" is irrelevant as long as it allows you to enter all the pertinent numbers. It's a CALCULATOR. Just like, if I want to know what 2 + 2 is, I don't feel obligated to check it with every calculator at my disposal... as long as it lets me type in 2 + 2, I'm pretty sure I'll get the same answer from all of them.

GM's numbers are largely meaningless. They're a somewhat reasonable approximation of their production target, and it's generally safe to say that an engine model with a bigger number probably had "more" than another model with a smaller number, but that's about it. Their QC is MUCH better nowadays than it was in the 70s when it was so bad that I'm surprised in retrospect today that as many cars actually ran as did back then (of course, customers' standards were much lower as well); but it wasn't all that much better in the 80s and early 90s when these cars were being built. There's ALOT LOT LOT LOT of variation that you'll see in EVERYTHING about them, even with the same part numbers, just from sloppy manufacturing and lax QC and "process drift" over time.

nuggie

I am running freshly decked heads and probably block plus I am not really after specifics, I am just aiming for a ball park of 11:1. I thought however that stock head gasket was .040

sofakingdom

iTrader: (1)

Don't "think"; measure.

Measure your head cc's as well. You'll probably be surprised. Especially if they've had a valve job done to them, or valves replaced; just the difference in thickness, tulip/non-tulip, etc., as well as how far they're sunk into the seats, is worth SEVERAL ccs.

Measure with micrometer, mark with chalk, cut with axe. How precise is the finished product?

Night rider327

iTrader: (13)

No GM never used .040" gaskets. That is cheap "rebuilder" gasket thickness.

GM used .018" steel shims on a lot of the 305s and some 350's, Most got the .028" gaskets though.

sofakingdom

iTrader: (1)

On the other hand...

What difference does it make how thick the stuff the garbage man hauled off is? Will your block, heads, air/fuel mix, cam, remember it?

ASE doc

11:1 on pump gas is pushing it, even with aluminum heads and ideal circumstances, including blueprinting of the engine. Even when looking for 600hp from 400+cid, we don't go over 10.5. Too much possibility of trouble, and it can really put a cramp on your tuning capability.