1991 Z28 shortblock vs. 1991 Corvette Shortblock
08-18-2002, 09:33 PM
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Car: 1987 Corvette
Engine: Modified L98
Transmission: DN 4+3
Axle/Gears: 3.07
1991 Z28 shortblock vs. 1991 Corvette Shortblock
Does anybody know of any difference between the two?
Both 2-bolt mains.
Both Block Casting # 14093638
Both 10.0:1 Compression, correct?
The reason i ask is because I spun a connecting rod bearing in my '87 Corvette. I have a person willing to sell me 1 91 Z28 bottom end for a fair price. The only difference between my 87 and the 91 that I know of is the different piston causing a 1 point bump in compression (which is fine!).
Thank you!
Both 2-bolt mains.
Both Block Casting # 14093638
Both 10.0:1 Compression, correct?
The reason i ask is because I spun a connecting rod bearing in my '87 Corvette. I have a person willing to sell me 1 91 Z28 bottom end for a fair price. The only difference between my 87 and the 91 that I know of is the different piston causing a 1 point bump in compression (which is fine!).
Thank you!
08-18-2002, 10:18 PM
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the only difference that I can think of off hand would maybe be the cam, and if you wanted you could swap it out, or hell put a good cam in it, other than that if it is the same block casting number then it is the same.
08-18-2002, 10:23 PM
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Car: 1987 Corvette
Engine: Modified L98
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Axle/Gears: 3.07
Originally posted by mechanicalmehem
the only difference that I can think of off hand would maybe be the cam, and if you wanted you could swap it out, or hell put a good cam in it, other than that if it is the same block casting number then it is the same.
the only difference that I can think of off hand would maybe be the cam, and if you wanted you could swap it out, or hell put a good cam in it, other than that if it is the same block casting number then it is the same.
Block
Crank
Rods
Pistons
Bearings
My current heads, intake, cam...., will all be transferred over. Thanks!
08-18-2002, 10:44 PM
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if i remember right, the corvettes were 4 bolt mains, and the F-bodys were 2 bolt mains. only difference in the blocks. The heads were aluminum instead of Iron. I think the cams were the same. Just what i remember, i may be off.
08-18-2002, 10:47 PM
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Car: 1987 Corvette
Engine: Modified L98
Transmission: DN 4+3
Axle/Gears: 3.07
Originally posted by Gruvin84SS
if i remember right, the corvettes were 4 bolt mains, and the F-bodys were 2 bolt mains.
if i remember right, the corvettes were 4 bolt mains, and the F-bodys were 2 bolt mains.
I've had my oilpan off before and my '87 vette is a 2-bolt. Introduce confusion.
08-19-2002, 12:54 AM
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Re: 1991 Z28 shortblock vs. 1991 Corvette Shortblock
Originally posted by scorp508
The only difference between my 87 and the 91 that I know of is the different piston causing a 1 point bump in compression (which is fine!).
Thank you!
The only difference between my 87 and the 91 that I know of is the different piston causing a 1 point bump in compression (which is fine!).
Thank you!
Different Piston? Could you please elaborate. I have a '87 set of vette heads and a '90 350 TPI setup on a 1/2 ton chevy pick up, I am wondering if you know what my compression would jump up to if I swapped on these heads? I would plan on running the thickest headgaskets available but w/ the standard size any idea of what compression I'd be running? I ask as I am adding the same heads to the same block as you are. My guestimate w/ a std head gasket is gonna be about 10.7:1.
I thought corvette L98's also came w/ 4bolts, but you can always splay a 2bolt block to a 4bolt and it will be stronger than the stock 4bolt.
Last edited by PonchoDoc; 08-19-2002 at 01:08 AM.
08-19-2002, 04:43 AM
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Car: 89 formula
Engine: 383
Transmission: 700R4
Scorp I have read your posts on the corvette forum this will only a temp till you get the old motor redone right? In that case the just get it.The Z28 has about 9.5-1comp W/64cc heads so with your 58cc vette heads you should get around 10-1comp.
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08-19-2002, 12:06 PM
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Originally posted by e-man
The Z28 has about 9.5-1comp W/64cc heads so with your 58cc vette heads you should get around 10-1comp.
The Z28 has about 9.5-1comp W/64cc heads so with your 58cc vette heads you should get around 10-1comp.
What are the good corvette boards? I have questions about the L98 aluminum heads, specifically the 1987 ones and really do not see a proper place to post my questions here. I am curious if all 87's use hardened guideplates or not, as if I use these heads I would like to not have to buy any more stuff, other than hardened push rods and gaskets.
Last edited by PonchoDoc; 08-19-2002 at 12:23 PM.
08-19-2002, 04:13 PM
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Car: 89 formula
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This is a good vette forum http://corvetteforum.com/
08-20-2002, 06:59 AM
#11
ive seen his name on that board for years, i think hes beat you too it. the rotating assembly will be fine with your vette heads and cam of choice. btw, my L98 vette is a 2 bolt also.
08-20-2002, 08:07 AM
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As pointed out, the ONLY difference between 91 F-body and your 87 is the pistons AND the head gasket (GM used a .018" shim type in 1991). The pistons are flat tops with 9cc valve releives. I just swapped the heads on my 1991 L98 and I can confirm the above.
The compression ratio of a stock 1991 L98 F-body was 9.75:1 with the .018" shim head gasket. I swapped to 58cc ported Aluminum L98 heads and used a .029" head gasket. This yield around 10.15:1. With a standard .038" head gasket the compression ratio drops to 10:1. That is why I chose the thinner .029" head gasket to get the little extra bump in compression ratio.
One thing to do while you have those heads off, is to clean up the combustion chamber. I found the Aluminum L98 heads that I picked up had a LOT of sharp and rough spots. After I cleaned up the combustion chamber, I was able to run much more spark advance than my buddy that use to have them on his ZZ4 block (until he went to AFR 195 Competition Ported heads with 2.05 intakes). Yeah, I drolled over those puppies too.
I even offered to "season them" on my engine for awhile, but he refused my offer for some reason.
The compression ratio of a stock 1991 L98 F-body was 9.75:1 with the .018" shim head gasket. I swapped to 58cc ported Aluminum L98 heads and used a .029" head gasket. This yield around 10.15:1. With a standard .038" head gasket the compression ratio drops to 10:1. That is why I chose the thinner .029" head gasket to get the little extra bump in compression ratio.
One thing to do while you have those heads off, is to clean up the combustion chamber. I found the Aluminum L98 heads that I picked up had a LOT of sharp and rough spots. After I cleaned up the combustion chamber, I was able to run much more spark advance than my buddy that use to have them on his ZZ4 block (until he went to AFR 195 Competition Ported heads with 2.05 intakes). Yeah, I drolled over those puppies too.
I even offered to "season them" on my engine for awhile, but he refused my offer for some reason.
Last edited by Grim Reaper; 08-20-2002 at 08:11 AM.
08-20-2002, 10:28 PM
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And just to add my highly-inflated 2 cents.....and because I've done it before......
You might want to rethink using the 58cc heads on the lower compression ratio short block AND using a THICKER head gasket.
If you increase the quench too much it will drive you crazy with pinging and the ease in which it will detonate with even stock amounts of spark advance. Qunech is essentially the piston to head clearance. While you don't want them to meet, too much quench is BAD as well. The generally accepted/ recommended quench for a street driven ( pump gas ) SBC is .040"... tighter is better than bigger if it comes down to it..
I slapped a set of iron L98 heads on a Vette L98 Stock shortblock with the F-body gasket and got BAD pinging and detonation because the quench increased too much, even though the compression ratio DROPPED with the 64cc heads.......
Hell, you're using alum heads already ( they are much more tolerant to higher compression ratios on pump gas as they transfer heat much better than iron.....I think I meant to type dissipate... not transfer
)
So the GENERAL rule of thumb is that for every 1 CC increase on the cylinder head, you'll LOSE .1 point off the compression ratio...ASSUMING the head gasket compressed thickness and deck height stay the same........
SO.......IF ( nice word huh? ) the Vette L98 compression in 87 was 9:5 to 1 ( I think I'm close at least ) and the '91 Z29 L98 rated at 9:3 to 1 then there were OTHER differences ( pistons, head gasket thickness etc ) than JUST the 58cc Vette heads vs. the 64cc iron F-Body L98 heads. Reason being, everything else being the SAME, if you just swapped to the 58cc Vette heads, the Z28's compression ratio should jump from 9:3 to 1 to 9:9 to 1.
The ONLY way to know for sure is to use a compression ratio calculator by measuring the following values........
Head CC size ( Only TRUE way, especially if you cleaned the ports up at all ) is to measure them with fluid. Either pay big bucks for a "kit" to do this or use my ******* trick learned from better gearheads than I.....
Get a glass burette marked in CC's.
Get a bottle of alcohol ( not the drinking kind...but hell why not )
Get something that will keep the head level while you have it upside down, chambers facing up.
Get a piece of glass or plexiglass if you KNOW it's perfectly LEVEL.
and then drill a HOLE into the glass.
The head should have the valves installed.
Pour a NOTED amount of alcohol into the burette. Say 100cc.
Place the glass over the combustion chamber ( it must cover it fully to work right ) and put something heavy to keep it sitting level and STILL over the chamber.
Now pour the alcohol from the burette into the chamber through the hole, being careful not to SPILL ANY. Fill it up until the fluid is LEVEL with the bottom of the glass ( not coming out of the hole )
Now look at the burette. IF there is 42cc of alcohol left then the head's CC size is 58cc... Doing each chamber will open your eyes to GM quality control
Now you need to measure the deck height of the piston(s). At TDC, measure from the top of the piston to the leading edge ( FLAT ) of the block. In other words, if your piston is FLUSH with the deck, it's called ZERO deck.
You'll also need to know if the pistons have valve reliefs /dish and what cc they are if they do. Or if the are domed or flat tops. ( -cc for reliefs or dish, +cc for dome )
The next measurement is the COMPRESSED thickness ( when torqued to spec ) of the head gasket.......GM gaskets will list this value up front.
Also you'll need to know the bore of your engine ( 4" on a virgin 350 block ) and the stroke ( 3.48" for a 350 ) and the number of cylinders.
Go here http://www.rosspistons.com/CR%20Calculator.htm and use their cool calculator or do the listed math...your choice.........
HTH
Chris
Tell me to shut up anytime.....
You might want to rethink using the 58cc heads on the lower compression ratio short block AND using a THICKER head gasket.
If you increase the quench too much it will drive you crazy with pinging and the ease in which it will detonate with even stock amounts of spark advance. Qunech is essentially the piston to head clearance. While you don't want them to meet, too much quench is BAD as well. The generally accepted/ recommended quench for a street driven ( pump gas ) SBC is .040"... tighter is better than bigger if it comes down to it..
I slapped a set of iron L98 heads on a Vette L98 Stock shortblock with the F-body gasket and got BAD pinging and detonation because the quench increased too much, even though the compression ratio DROPPED with the 64cc heads.......
Hell, you're using alum heads already ( they are much more tolerant to higher compression ratios on pump gas as they transfer heat much better than iron.....I think I meant to type dissipate... not transfer
)So the GENERAL rule of thumb is that for every 1 CC increase on the cylinder head, you'll LOSE .1 point off the compression ratio...ASSUMING the head gasket compressed thickness and deck height stay the same........
SO.......IF ( nice word huh? ) the Vette L98 compression in 87 was 9:5 to 1 ( I think I'm close at least ) and the '91 Z29 L98 rated at 9:3 to 1 then there were OTHER differences ( pistons, head gasket thickness etc ) than JUST the 58cc Vette heads vs. the 64cc iron F-Body L98 heads. Reason being, everything else being the SAME, if you just swapped to the 58cc Vette heads, the Z28's compression ratio should jump from 9:3 to 1 to 9:9 to 1.
The ONLY way to know for sure is to use a compression ratio calculator by measuring the following values........
Head CC size ( Only TRUE way, especially if you cleaned the ports up at all ) is to measure them with fluid. Either pay big bucks for a "kit" to do this or use my ******* trick learned from better gearheads than I.....
Get a glass burette marked in CC's.
Get a bottle of alcohol ( not the drinking kind...but hell why not )
Get something that will keep the head level while you have it upside down, chambers facing up.
Get a piece of glass or plexiglass if you KNOW it's perfectly LEVEL.
and then drill a HOLE into the glass.
The head should have the valves installed.
Pour a NOTED amount of alcohol into the burette. Say 100cc.
Place the glass over the combustion chamber ( it must cover it fully to work right ) and put something heavy to keep it sitting level and STILL over the chamber.
Now pour the alcohol from the burette into the chamber through the hole, being careful not to SPILL ANY. Fill it up until the fluid is LEVEL with the bottom of the glass ( not coming out of the hole )
Now look at the burette. IF there is 42cc of alcohol left then the head's CC size is 58cc... Doing each chamber will open your eyes to GM quality control
Now you need to measure the deck height of the piston(s). At TDC, measure from the top of the piston to the leading edge ( FLAT ) of the block. In other words, if your piston is FLUSH with the deck, it's called ZERO deck.
You'll also need to know if the pistons have valve reliefs /dish and what cc they are if they do. Or if the are domed or flat tops. ( -cc for reliefs or dish, +cc for dome )
The next measurement is the COMPRESSED thickness ( when torqued to spec ) of the head gasket.......GM gaskets will list this value up front.
Also you'll need to know the bore of your engine ( 4" on a virgin 350 block ) and the stroke ( 3.48" for a 350 ) and the number of cylinders.
Go here http://www.rosspistons.com/CR%20Calculator.htm and use their cool calculator or do the listed math...your choice.........
HTH
Chris
Tell me to shut up anytime.....
08-21-2002, 08:10 AM
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Originally posted by ctandc
You might want to rethink using the 58cc heads on the lower compression ratio short block AND using a THICKER head gasket.
...
SO.......IF ( nice word huh? ) the Vette L98 compression in 87 was 9:5 to 1 ( I think I'm close at least ) and the '91 Z29 L98 rated at 9:3 to 1 then there were OTHER differences ( pistons, head gasket thickness etc ) than JUST the 58cc Vette heads vs. the 64cc iron F-Body L98 heads. Reason being, everything else being the SAME, if you just swapped to the 58cc Vette heads, the Z28's compression ratio should jump from 9:3 to 1 to 9:9 to 1.
You might want to rethink using the 58cc heads on the lower compression ratio short block AND using a THICKER head gasket.
...
SO.......IF ( nice word huh? ) the Vette L98 compression in 87 was 9:5 to 1 ( I think I'm close at least ) and the '91 Z29 L98 rated at 9:3 to 1 then there were OTHER differences ( pistons, head gasket thickness etc ) than JUST the 58cc Vette heads vs. the 64cc iron F-Body L98 heads. Reason being, everything else being the SAME, if you just swapped to the 58cc Vette heads, the Z28's compression ratio should jump from 9:3 to 1 to 9:9 to 1.
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