L98 has dish pistons???????
#1
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Car: 91 Z/28
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L98 has dish pistons???????
I just removed the heads from a fresh, used L98 motor from GM. I was looking for flat top pistons and found dish pistons. I checked SDPC and the specs for L98 said dish.............. so all TPI L98 350s that came in Thirdgens have dish pistons?
Last edited by Fbody fan 35yrs; 02-29-2004 at 06:46 PM.
#3
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From: Palm Bay, Florida, USA
Car: 95 E-150 & 07 Kawasaki ZX-6R
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That's awfully weird... This site's tech specs says the L98's have 9.3:1....which isn't exactly low compression. So I imagine (assuming the site is right) that either stock back then they didnt come with dishes, or the combustion chambers were super-small or something and provided a very high compression number, so they used dished pistons to lower it down.
#4
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From: Illinois
Car: 1988 GTA Notchback
Engine: 5.7 TPI
Transmission: TH-700R4
ALL F-Body L98's have dish pistons with 4 valve reliefs. They measure out at roughly -12cc.
The only L98's that get true flat top pistons is the Corvette version with aluminum heads. They also have 4 valve reliefs.
The only L98's that get true flat top pistons is the Corvette version with aluminum heads. They also have 4 valve reliefs.
#5
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From: Loveland, OH, US
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9.3 is mighty low compression. It would be up around 10-10.2 with flat-tops.
They're defintely supposed to be dished, to lower the compression to such a cheap-pump-gas level.
They're defintely supposed to be dished, to lower the compression to such a cheap-pump-gas level.
#6
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From: Palm Bay, Florida, USA
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Axle/Gears: A weak one & a chained one
Originally posted by RB83L69
9.3 is mighty low compression.
9.3 is mighty low compression.
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#8
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From: Palm Bay, Florida, USA
Car: 95 E-150 & 07 Kawasaki ZX-6R
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Transmission: A bad one & a good one
Axle/Gears: A weak one & a chained one
Lemme guess......you've owned some mighty fast N/A motors before huh?
#9
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From: The Heart Land!
Car: 91 Z/28
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Well, I thought that TPI motors being "hight output" would have flat top pistons to get to the 9.3 to 1. I checked a ZZ4 short block with flat top pistons at SDPC, and it raises the ratio .5 over the L98 with the dish. They show 9.1 with 64cc heads on the L98, and 9.6 on the ZZ4 with the flats and 64cc heads. This worries me as I am changing to vortec heads with 68cc chambers. I thought vortec heads had smaller or at least the same size chambers as stock heads?? Thoughts???
Last edited by Fbody fan 35yrs; 02-29-2004 at 07:46 PM.
#10
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From: Palm Bay, Florida, USA
Car: 95 E-150 & 07 Kawasaki ZX-6R
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Axle/Gears: A weak one & a chained one
Lol....larger cc's equals better performance but lower compression. You'll definitely have some pretty mild compression with dished pistons and 68cc chambers... You could put some different pistons in....or consider that as preparation for forced induction...
#11
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From: The Heart Land!
Car: 91 Z/28
Engine: 5.0
Transmission: T-56 6 speed
I just ran the #s on my L98 heads, and they do have 64cc chambers. I hate it when you think you know how something is, and it .............. isn't. I'm sure there is someone out there that knows what you come up with when you use vortec heads on a stock L98 block?
Last edited by Fbody fan 35yrs; 02-29-2004 at 08:00 PM.
#13
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actually the Vortec heads are rated at 64cc as are the stock L98 heads. The aluminum vette heads are rated at 58cc.
I have heard that the vortec heads actually cc a little less than 64, the set I cc'd came out to the advertised 64.
I used a .015" thick head gasket to give me a couple of points more compression. I should be at the 9.5 - 9.7:1
I have heard that the vortec heads actually cc a little less than 64, the set I cc'd came out to the advertised 64.
I used a .015" thick head gasket to give me a couple of points more compression. I should be at the 9.5 - 9.7:1
#14
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From: The Heart Land!
Car: 91 Z/28
Engine: 5.0
Transmission: T-56 6 speed
I am getting my #s from this web site: http://www.mortec.com/castnum.htm Vortec casting # 12558062 L98 casting # 14101083
#17
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From: Loveland, OH, US
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Lemme guess......you've owned some mighty fast N/A motors before huh?
#18
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From: Palm Bay, Florida, USA
Car: 95 E-150 & 07 Kawasaki ZX-6R
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Axle/Gears: A weak one & a chained one
I consider N/A 'mighty fast' to be 10's or quicker pretty much. Quicker than most street machines will ever see.
#19
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From: Illinois
Car: 1988 GTA Notchback
Engine: 5.7 TPI
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Originally posted by smithtc
My L98 pistons are -7cc 4 valve relief
My L98 pistons are -7cc 4 valve relief
I doubt this, but, with GM anything is possible.....LOL
#23
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From: The Heart Land!
Car: 91 Z/28
Engine: 5.0
Transmission: T-56 6 speed
The block in the photo has flat top pistons with valve reliefs. If what has been discussed earlier is true that isn't a stock L98 block. ???
#24
Originally posted by TA-Wizard
I have heard of this claim before....if true it would mean that the factory shipped this motor out with flat top pistons with -7cc valve reliefs, the compression ratio would be over 9.9:1.
I doubt this, but, with GM anything is possible.....LOL
I have heard of this claim before....if true it would mean that the factory shipped this motor out with flat top pistons with -7cc valve reliefs, the compression ratio would be over 9.9:1.
I doubt this, but, with GM anything is possible.....LOL
Stock 89 L98
They have a "dish" and 4 valve reliefs that measure out to about -7cc.
Compression comes out to 9.47:1 using a 0.039 compressed gasket thickness. Believe the GM gaskets on my motor were a little thicker.
Last edited by smithtc; 03-01-2004 at 09:20 AM.
#25
Originally posted by Fbody fan 35yrs
The block in the photo has flat top pistons with valve reliefs. If what has been discussed earlier is true that isn't a stock L98 block. ???
The block in the photo has flat top pistons with valve reliefs. If what has been discussed earlier is true that isn't a stock L98 block. ???
#26
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From: Starkville, MS
Car: 1991 Camaro Z28
Engine: 350
Transmission: 700R4
Originally posted by TA-Wizard
Black,
I am curious.....did your motor have a steel shim or composition head gasket when you first tore it down??
Black,
I am curious.....did your motor have a steel shim or composition head gasket when you first tore it down??
#27
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From: The Heart Land!
Car: 91 Z/28
Engine: 5.0
Transmission: T-56 6 speed
I have heard that about production. Automatic cars got one cam, and manual cars got another for the most part, but some times you can find either cam in either car. The same is probably true of pistons. It helps explain why some cars just run better than others of the same year with the same equipment. The new L98 I have looks just like the one in the link in smithtc's post. I hate to give up that compression, but I got a good deal on the motor and it, as always, comes down to "how fast do you want to spend"
#29
Back from the dead...
Seems to have been a lot of debate back in '04 over whether the pistons were -7cc or -12cc. Anyone confirm anything yet? I'm in the same boat...building an engine with a stock L98 bottom end, and some hard info on the piston CC could really help me out.
#30
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From: The Heart Land!
Car: 91 Z/28
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I can't tell you exact CC,s but I do know that at least some L98's especially in Corvette's had flat top pistons, and get you close to 10 to 1. Many if not most of the Fbody L98's had a dish in the top of the piston, and that puts you somewhere in the 9's to 1. Both pistons have valve relief's but the ones in the flat top pistons are a little deeper since they are physically closer to the valves. I sold L98 block I had and went with a Vette motor. It came from the factory with flat tops.
#31
I've heard the 87-89 L98's had 9.3:1 compression and the 90-92's had 9.7:1 because the 90-92's switched to speed density computers that gm felt would run more precise for more compression. That could explain why some L98's have dished pistons and some have flat tops.
#32
Awesome, thanks guys! The engine I have has the dished pistons. I did some calculations this afternoon, and it seems it would make sense that the dished pistons are -12cc and the flat-tops are -7.
#33
This topic has been gone thrrough several times on this board before. Around 1990 (some say as soon as 1989 in some motors) the factory switched from -12cc dished pistons to -6cc flattops. GM never changed the "official" compression ratio of the motor in the published specs but it was indeed higher (~9.8:1) on the later speed density motors.
I didn't want to believe it at first, but the vast direct and personal experience of memebers on this board has convinced me it's true. My experience is mostly with the 87-88 motors and they were definitely dished pistons every time I tore one open.
I didn't want to believe it at first, but the vast direct and personal experience of memebers on this board has convinced me it's true. My experience is mostly with the 87-88 motors and they were definitely dished pistons every time I tore one open.
#34
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From: Pittsburgh PA
Car: 89 Iroc-z
Engine: 555 BBC Turbo
Transmission: TH400
Axle/Gears: Strange 12 bolt 3.42
Re: L98 has dish pistons???????
back from the dead....
check my motor pic.... does this look like -12cc dish? i guess it does block was cast in late 88 i think for the 89 model year. car is a 89 irocz L98
check my motor pic.... does this look like -12cc dish? i guess it does block was cast in late 88 i think for the 89 model year. car is a 89 irocz L98
#35
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From: Woodland, CA
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Re: L98 has dish pistons???????
I would think so. I also had an l98 in my 89 iroc-z, look to the right on the photo, you can see the same dish.
#37
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From: Cypress, California
Car: 1989 GTA
Engine: 369 TPI
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Re: L98 has dish pistons???????
My 1989 had the same pistons from the factory as posted in the above picture. However as was posted above starting maybe in 1990 GM in fact did put in pistons with less of a dish. So the compression ratio was larger but not advertised.
#38
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From: Pittsburgh PA
Car: 89 Iroc-z
Engine: 555 BBC Turbo
Transmission: TH400
Axle/Gears: Strange 12 bolt 3.42
Re: L98 has dish pistons???????
that may be why SD cars ran faster than previous L98s. But i know they got a slightly weaker camshaft as well tho.
#40
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From: Orange, CA
Car: '90 Trans Am-12.45@110.71
Engine: 355 w/AFR 195's Elem. 400/430 HP/TQ
Transmission: Tremec T-56
Axle/Gears: 12 Bolt 3.73
Re: L98 has dish pistons???????
So far all of the '91-92 350's I have taken apart have had flatops w/4 reliefs.
#41
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Car: 1982 Trans Am & 1982 Corvette
Engine: L-98 with LO-3 induction. 350 CFI
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Re: L98 has dish pistons???????
I have an 89 L-98 with flat tops and my 90 vette L-98 had flat tops
#42
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'87-'89 L98 had 9.3:1 CR. '90-'92 had 9.7:1 CR. 12cc dish+reliefs vs. 5cc reliefs account for that.
Having reliefs is not proof of flat-tops. You can see both the dish and reliefs in the photos above. Look just like my Speedpro hypers, in fact.
Having reliefs is not proof of flat-tops. You can see both the dish and reliefs in the photos above. Look just like my Speedpro hypers, in fact.
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