Stock L98 piston question
#1
Member
Thread Starter
Join Date: Jul 2001
Location: Brooklyn, NY, USA
Posts: 234
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1991 GTA
Engine: L98
Transmission: 700R4
Axle/Gears: 3.23
Stock L98 piston question
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.
Last edited by Fred91GTA; 11-09-2005 at 05:44 PM.
#3
Supreme Member
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.
#4
Supreme Member
Join Date: Jul 1999
Location: Mims, Florida
Posts: 2,149
Likes: 0
Received 1 Like
on
1 Post
Car: '87 IROCZ
Engine: 395 ZZ4
Transmission: ProBuilt 700R4
Axle/Gears: 9 bolt 3.70s
Damon is right, I'm wrong. I went to the trouble to find my factory pistons and the are dished with 4 eyebrows.
#5
TGO Supporter
Join Date: Jul 2003
Location: Grand Rapids, MI
Posts: 4,803
Likes: 0
Received 0 Likes
on
0 Posts
Car: Z28
Engine: Sb2.2 406
Transmission: Jerico 4 speed
Axle/Gears: Ford 9" 3.60
Originally posted by Damon
SCILLIONS
SCILLIONS
#6
TGO Supporter
Join Date: Oct 2002
Location: Saskatoon, SK, Canada
Posts: 9,067
Likes: 0
Received 1 Like
on
1 Post
Car: '83 Z28, '07 Charger SRT8
Engine: 454ci, 6.1 Hemi
Transmission: TH350, A5
Axle/Gears: 2.73 posi, 3.06 posi
Originally posted by Stekman
What would be the numerical value on that?
What would be the numerical value on that?
Trending Topics
#8
Senior Member
iTrader: (5)
Join Date: Sep 2006
Location: Austria
Posts: 951
Likes: 0
Received 2 Likes
on
2 Posts
Car: 84 TA / 89 Formula
Engine: LS1 / L03
Transmission: T56 / 700R4
Axle/Gears: 3.73 / 3.27
Re: Stock L98 piston question
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
#10
Member
Join Date: Apr 2012
Location: Walla Walla,WA
Posts: 226
Likes: 0
Received 3 Likes
on
3 Posts
Car: 91 Firebird, 87 E'LS' Camino
Engine: 5.7 TPI, 5.3
Transmission: 700R4,200R4
Axle/Gears: 4.10 Lock Right locker/2.43
Re: Stock L98 piston question
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).
#11
Supreme Member
iTrader: (13)
Join Date: Dec 2002
Location: Bowdon, GA.
Posts: 2,535
Likes: 0
Received 16 Likes
on
14 Posts
Car: 1988 Camaro
Engine: 355, 10.34:1, 249/252 @.050", IK200
Transmission: TH-400, 3500 stall 9.5" converter
Axle/Gears: Ford 9", detroit locker, 3.89 gears
Re: Stock L98 piston question
'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
'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
#12
Supreme Member
iTrader: (1)
Join Date: Sep 2005
Posts: 26,051
Received 1,672 Likes
on
1,269 Posts
Car: Yes
Engine: Usually
Transmission: Sometimes
Axle/Gears: Behind me somewhere
Re: Stock L98 piston question
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.
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.
#13
Member
Join Date: Apr 2012
Location: Walla Walla,WA
Posts: 226
Likes: 0
Received 3 Likes
on
3 Posts
Car: 91 Firebird, 87 E'LS' Camino
Engine: 5.7 TPI, 5.3
Transmission: 700R4,200R4
Axle/Gears: 4.10 Lock Right locker/2.43
Re: Stock L98 piston question
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
#14
Supreme Member
iTrader: (1)
Join Date: Sep 2005
Posts: 26,051
Received 1,672 Likes
on
1,269 Posts
Car: Yes
Engine: Usually
Transmission: Sometimes
Axle/Gears: Behind me somewhere
Re: Stock L98 piston question
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?
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?
#15
Supreme Member
iTrader: (13)
Join Date: Dec 2002
Location: Bowdon, GA.
Posts: 2,535
Likes: 0
Received 16 Likes
on
14 Posts
Car: 1988 Camaro
Engine: 355, 10.34:1, 249/252 @.050", IK200
Transmission: TH-400, 3500 stall 9.5" converter
Axle/Gears: Ford 9", detroit locker, 3.89 gears
Re: Stock L98 piston question
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.
GM used .018" steel shims on a lot of the 305s and some 350's, Most got the .028" gaskets though.
#16
Supreme Member
iTrader: (1)
Join Date: Sep 2005
Posts: 26,051
Received 1,672 Likes
on
1,269 Posts
Car: Yes
Engine: Usually
Transmission: Sometimes
Axle/Gears: Behind me somewhere
Re: Stock L98 piston question
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?
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?
#17
Supreme Member
Re: Stock L98 piston question
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.
Thread
Thread Starter
Forum
Replies
Last Post
84z96L31vortec
Tech / General Engine
7
08-20-2017 12:16 AM
84z96L31vortec
North East Region
1
08-10-2015 08:27 PM