Basic piston head measurements 91 L98
Thread Starter
Melting Slicks
Joined: May 2001
Posts: 3,014
Likes: 18
From: Raleigh, NC
Basic piston head measurements 91 L98
I'm looking for some basic information for my 91 L98. I've searched on here and I can't seem to come up with a definite answer.
On my 91, I have seen 2 common measurements for piston dish volume, both 6 CC and 8.5 CC. Is either correct?
I have also seen .016 and .029 for deck clearance. Is either of those correct?
I have seen anywhere from 56-60 CC mentioned regarding the late model L98 cylinder heads. Does anyone know what the real measurement is?
I have seen a compressed head gasket thickness of .041 and .049. Is either of those correct?
The stock head gasket bore has been quoted at 4.003, 4.006, etc... Anyone know the real answer?
The most common compression ratio I have seen regarding my 91 L98 is 9.5:1. When I run the above numbers through a compression ratio calculator, the lowest compression I can achieve is 9.7:1. That leads me to believe some of, if not all of, the numbers above are incorrect.
If someone could shed some light on what the real numbers are that would be great.
On my 91, I have seen 2 common measurements for piston dish volume, both 6 CC and 8.5 CC. Is either correct?
I have also seen .016 and .029 for deck clearance. Is either of those correct?
I have seen anywhere from 56-60 CC mentioned regarding the late model L98 cylinder heads. Does anyone know what the real measurement is?
I have seen a compressed head gasket thickness of .041 and .049. Is either of those correct?
The stock head gasket bore has been quoted at 4.003, 4.006, etc... Anyone know the real answer?
The most common compression ratio I have seen regarding my 91 L98 is 9.5:1. When I run the above numbers through a compression ratio calculator, the lowest compression I can achieve is 9.7:1. That leads me to believe some of, if not all of, the numbers above are incorrect.
If someone could shed some light on what the real numbers are that would be great.
Team Owner
Joined: Sep 2002
Posts: 22,209
Likes: 12
From: OBAMA IS HITLER
the 90 (and 91) are factory rated at 10.25:1. this is a fact.....so says the supplemental to the 90 shop manual. this 9.5:1 business is perpetuated by idiots who don't know what they're talking about.
as for deck clearance, stock (untouched) is supposed to be .025". the combustion chambers are 58cc for your year. the rest i don't have handy.
as for deck clearance, stock (untouched) is supposed to be .025". the combustion chambers are 58cc for your year. the rest i don't have handy.
Last edited by Red Tornado; Jan 2, 2007 at 09:08 PM.
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
the 90 (and 91) are factory rated at 10.25:1. this is a fact.....so says the supplemental to the 90 shop manual. this 9.5:1 business is perpetuated by idiots who don't know what they're talking about. as for deck clearance, stock (untouched) is supposed to be .025". the combustion chambers are 58cc for your year. the rest i don't have handy.
Thread Starter
Melting Slicks
Joined: May 2001
Posts: 3,014
Likes: 18
From: Raleigh, NC
Awesome, thanks guys. I have the service manual for my 91 and have read the section on the pistons a couple times and haven't seen any mention of the dish volume. Anyone know that one for sure?
Pro
Joined: Nov 2002
Posts: 737
Likes: 11
From: Charlotte MI
I'm looking for some basic information for my 91 L98. I've searched on here and I can't seem to come up with a definite answer.
On my 91, I have seen 2 common measurements for piston dish volume, both 6 CC and 8.5 CC. Is either correct?
I have also seen .016 and .029 for deck clearance. Is either of those correct?
I have seen anywhere from 56-60 CC mentioned regarding the late model L98 cylinder heads. Does anyone know what the real measurement is?
I have seen a compressed head gasket thickness of .041 and .049. Is either of those correct?
The stock head gasket bore has been quoted at 4.003, 4.006, etc... Anyone know the real answer?
The most common compression ratio I have seen regarding my 91 L98 is 9.5:1. When I run the above numbers through a compression ratio calculator, the lowest compression I can achieve is 9.7:1. That leads me to believe some of, if not all of, the numbers above are incorrect.
If someone could shed some light on what the real numbers are that would be great.
On my 91, I have seen 2 common measurements for piston dish volume, both 6 CC and 8.5 CC. Is either correct?
I have also seen .016 and .029 for deck clearance. Is either of those correct?
I have seen anywhere from 56-60 CC mentioned regarding the late model L98 cylinder heads. Does anyone know what the real measurement is?
I have seen a compressed head gasket thickness of .041 and .049. Is either of those correct?
The stock head gasket bore has been quoted at 4.003, 4.006, etc... Anyone know the real answer?
The most common compression ratio I have seen regarding my 91 L98 is 9.5:1. When I run the above numbers through a compression ratio calculator, the lowest compression I can achieve is 9.7:1. That leads me to believe some of, if not all of, the numbers above are incorrect.
If someone could shed some light on what the real numbers are that would be great.
http://www.nhra.com/tech_specs/engin...ts/CHEV-91.rtf
Thread Starter
Melting Slicks
Joined: May 2001
Posts: 3,014
Likes: 18
From: Raleigh, NC
That is an awesome sheet, but I don't think I am smart enough to read it properly. According to the way I am reading your sheet:
This piston is not dished, it is a flat top with valve reliefs cut into it. The volume of the reliefs measures 4.5 CC.
With the piston at TDC, there should be .013 between the top of the piston and the deck surface.
The head gasket on my 91 is .016 thick.
Your sheet also claims that in order to have the 113 cylinder head, you must have 12.26:1 compression.
I must be reading the sheet incorrectly, is there anyway you can take the time to explain it? Thanks for posting that though, it looks like it contains all the information I am looking for if I can just figure out how to read it.
This piston is not dished, it is a flat top with valve reliefs cut into it. The volume of the reliefs measures 4.5 CC.
With the piston at TDC, there should be .013 between the top of the piston and the deck surface.
The head gasket on my 91 is .016 thick.
Your sheet also claims that in order to have the 113 cylinder head, you must have 12.26:1 compression.
I must be reading the sheet incorrectly, is there anyway you can take the time to explain it? Thanks for posting that though, it looks like it contains all the information I am looking for if I can just figure out how to read it.
Racer
Joined: May 2006
Posts: 293
Likes: 0
Those specs are allowed by NHRA for their stock or Super Stock class cars. Those are not factory specs. I only know a few specs for the 1991, for sure. From the center of the main bearing bore to the top of the deck, the block has a blueprint dimension of 9.025". The deck clearance is .025" down the hole, and the heads have 58 cc combustion chambers.
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Burning Brakes
Joined: Sep 2001
Posts: 1,013
Likes: 9
the 90 (and 91) are factory rated at 10.25:1. this is a fact.....so says the supplemental to the 90 shop manual. this 9.5:1 business is perpetuated by idiots who don't know what they're talking about.
as for deck clearance, stock (untouched) is supposed to be .025". the combustion chambers are 58cc for your year. the rest i don't have handy.
as for deck clearance, stock (untouched) is supposed to be .025". the combustion chambers are 58cc for your year. the rest i don't have handy.
10.25:1, 58cc heads, .0025 down in the bore (I've measured), .049 head gaskets.
Thread Starter
Melting Slicks
Joined: May 2001
Posts: 3,014
Likes: 18
From: Raleigh, NC
Thanks a bunch guys!
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
I expect this to be common sense...
Blueprint specs are not the same as what your engine currently has if never been apart.
Only sure way is to physically measure it yourself/machine shop.
IE my LT4 engine's spec for piston to deck height ranged from .030 to .050!
Chamber size varies significantly too. More than likely on rebuild (if that is what you are doing) you will not used a .016 gasket-more like a Fel Pro that sits at .039 compressed thickness and doesn't fit the bore as closely so you have more crevice volume.
Good Luck!
Blueprint specs are not the same as what your engine currently has if never been apart.
Only sure way is to physically measure it yourself/machine shop.
IE my LT4 engine's spec for piston to deck height ranged from .030 to .050!
Chamber size varies significantly too. More than likely on rebuild (if that is what you are doing) you will not used a .016 gasket-more like a Fel Pro that sits at .039 compressed thickness and doesn't fit the bore as closely so you have more crevice volume.
Good Luck!
