L98 Heads
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
L98 Heads
Hi guys, I need help once again. I picked up a pair of L98 Aluminum heads and need help figuring out the compression ratio of my engine if I install them. They're for my '74 L82 Vette and the bottom end is all original. I was thinking about using head gasket Part#NAL-12557236 to try to keep my compression ration down to a 91 octane friendly ratio. What will the compression ratio be with the L98, stock L82 bottom end, and .051in. gasket?
Link to gaskets:
http://www.summitracing.com/parts/NAL-12557236/
Link to gaskets:
http://www.summitracing.com/parts/NAL-12557236/
Drifting
Joined: May 2005
Posts: 1,258
Likes: 38
From: springtown Texas
i came up with 10.2:1 comp.
the heads are 58 cc and if i remember right, the pistons are 3.5cc. i'm not sure on the pistons though . i guessed your pistons are @ .025 down in the hole. thats about normal. so it came to 10.2 comp.
i run the same heads with a .015 gasket and have 11.1:1 comp and run 93 pump with no prob. but my quench is .038 which is the diff.
here is the website for the calc.
http://www.csgnetwork.com/compcalc.html
the heads are 58 cc and if i remember right, the pistons are 3.5cc. i'm not sure on the pistons though . i guessed your pistons are @ .025 down in the hole. thats about normal. so it came to 10.2 comp.
i run the same heads with a .015 gasket and have 11.1:1 comp and run 93 pump with no prob. but my quench is .038 which is the diff.
here is the website for the calc.
http://www.csgnetwork.com/compcalc.html
Last edited by GS977; Jan 9, 2012 at 11:27 PM.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
i came up with 10.2:1 comp.
the heads are 58 cc and if i remember right, the pistons are 3.5cc. i'm not sure on the pistons though . i guessed your pistons are @ .025 down in the hole. thats about normal. so it came to 10.2 comp.
i run the same heads with a .015 gasket and have 11.1:1 comp and run 93 pump with no prob. but my quench is .038 which is the diff.
here is the website for the calc.
http://www.csgnetwork.com/compcalc.html
the heads are 58 cc and if i remember right, the pistons are 3.5cc. i'm not sure on the pistons though . i guessed your pistons are @ .025 down in the hole. thats about normal. so it came to 10.2 comp.
i run the same heads with a .015 gasket and have 11.1:1 comp and run 93 pump with no prob. but my quench is .038 which is the diff.
here is the website for the calc.
http://www.csgnetwork.com/compcalc.html
.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
The stock L82 is "9:1CR" with a 64cc head. Figure 58CC will take you to at least 9.6:1 with the stock head gasket. A shim gasket will get you right around 10:1, probably a hair under. If you're running the stock L82 cam it should work out pretty good.
*edit* Just read you're using 0.051 gasket. You won't need to worry about detonation.
*edit* Just read you're using 0.051 gasket. You won't need to worry about detonation.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
The stock L82 is "9:1CR" with a 64cc head. Figure 58CC will take you to at least 9.6:1 with the stock head gasket. A shim gasket will get you right around 10:1, probably a hair under. If you're running the stock L82 cam it should work out pretty good.
*edit* Just read you're using 0.051 gasket. You won't need to worry about detonation.
*edit* Just read you're using 0.051 gasket. You won't need to worry about detonation.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
I live in Los Gatos. CA. I have a stock L36 (factory 10.25 C/R) and a 65 442 (10 C/R). 91 wanna be "Premium" works OK, just watch your low load advance. I know factory ratings were not true C/R... Plus aluminum is easier on pinging. I say run 'em.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Just looked it up, you're right.
Head CC 75.47
Gasket thickness 0.021
Piston deck height 0.020 below surface
Don't have a value for the piston volume above surface.
http://www.csgnetwork.com/compcalc.html
Using this calculator, and I guessed -5CC for piston volume to get 9:1 on the original setup, I get 10.2:1 for your setup with L98 which should be perfect!
Be mindful of the timing curve, of course.
That should give you a great running motor.
Head CC 75.47
Gasket thickness 0.021
Piston deck height 0.020 below surface
Don't have a value for the piston volume above surface.
http://www.csgnetwork.com/compcalc.html
Using this calculator, and I guessed -5CC for piston volume to get 9:1 on the original setup, I get 10.2:1 for your setup with L98 which should be perfect!
Be mindful of the timing curve, of course.
That should give you a great running motor.
Drifting
Joined: May 2005
Posts: 1,258
Likes: 38
From: springtown Texas
i did the math again and with a .051 head gasket and 76 cc heads. and i used a 5 cc dish piston. i think your stock pistons are less but just say it's 5 cc. you have 8.41
now with 64 cc heads and .051 gasket you have 9.46 comp.
now with 58 cc l98 heads and .051 gasket you have 10.11 comp.
and if the l98 heads need to be surfaced to get them perfectly straight they will be less then 58 cc. i ran into the same prob as you. i didn't want 11.1:1 comp but with the 58cc chambers you need a big dish piston to get the comp down in the 9s. i ran a .015 gasket and my pistons were .023 down in the hole so i had .038 quench and that along with the heads being aluminum keeps it from pinging. my total timing is 35-36 max at 2800rpms on up.
i'm not saying don't run them just be carefull and you have to find out what the cc is on your pistons and measure the piston height. it will probably be @ .022-.027 down in the hole. and watch your timing.
hope this helps
now with 64 cc heads and .051 gasket you have 9.46 comp.
now with 58 cc l98 heads and .051 gasket you have 10.11 comp.
and if the l98 heads need to be surfaced to get them perfectly straight they will be less then 58 cc. i ran into the same prob as you. i didn't want 11.1:1 comp but with the 58cc chambers you need a big dish piston to get the comp down in the 9s. i ran a .015 gasket and my pistons were .023 down in the hole so i had .038 quench and that along with the heads being aluminum keeps it from pinging. my total timing is 35-36 max at 2800rpms on up.
i'm not saying don't run them just be carefull and you have to find out what the cc is on your pistons and measure the piston height. it will probably be @ .022-.027 down in the hole. and watch your timing.
hope this helps
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
Just looked it up, you're right.
Head CC 75.47
Gasket thickness 0.021
Piston deck height 0.020 below surface
Don't have a value for the piston volume above surface.
http://www.csgnetwork.com/compcalc.html
Using this calculator, and I guessed -5CC for piston volume to get 9:1 on the original setup, I get 10.2:1 for your setup with L98 which should be perfect!
Be mindful of the timing curve, of course.
That should give you a great running motor.
Head CC 75.47
Gasket thickness 0.021
Piston deck height 0.020 below surface
Don't have a value for the piston volume above surface.
http://www.csgnetwork.com/compcalc.html
Using this calculator, and I guessed -5CC for piston volume to get 9:1 on the original setup, I get 10.2:1 for your setup with L98 which should be perfect!
Be mindful of the timing curve, of course.
That should give you a great running motor.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
General rule of thumb for a GEN I is that a decrease of about 10cc in chamber volume = about 1 point increase in compression. It's fairly easy to measure the piston dome volume in the car if the heads are off - happy to outline this if anyone's interested.
I'd be worried about that potential .071 - .076 quench - that's almost twice what's optimal and will increase detonation sensitivity. Always the issue with tops unless carefully planned - CR vs. quench.
Deck height can be all over the map on a production engine .020-.025 is common, but I've seen extremes as well. Also not difficult to measure with the engine in the car and the heads off.
I'd be sure to do some careful measurements during the swap to ensure you've got something workable.
I'd be worried about that potential .071 - .076 quench - that's almost twice what's optimal and will increase detonation sensitivity. Always the issue with tops unless carefully planned - CR vs. quench.
Deck height can be all over the map on a production engine .020-.025 is common, but I've seen extremes as well. Also not difficult to measure with the engine in the car and the heads off.
I'd be sure to do some careful measurements during the swap to ensure you've got something workable.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
That gasket is .051 compressed thickness. Combined with a piston .025 in the hole gives you .076 quench. Your better off using a .015 rubber coated steel shim gasket. This will increase compression but the .040 quench will offset the compession increase in reguard to detonation resistance. Check existing piston to deck clearance.
http://www.summitracing.com/parts/FEL-1094/
Edit: Billa beat me to it.
Edit #2. Found piston volume -3.5 CC
Calculated compression. using -3.5 volume for piston, .025 in hole, 4.100 gasket bore, 4.0 bore, 5.7 rod 3.48 stroke, comes up 10.2 to 1 with .051 gasket and 11.2 to 1 with .015 shim. Both are trouble. Disregard my post above. Look for different heads.
http://www.summitracing.com/parts/FEL-1094/
Edit: Billa beat me to it.
Edit #2. Found piston volume -3.5 CC
Calculated compression. using -3.5 volume for piston, .025 in hole, 4.100 gasket bore, 4.0 bore, 5.7 rod 3.48 stroke, comes up 10.2 to 1 with .051 gasket and 11.2 to 1 with .015 shim. Both are trouble. Disregard my post above. Look for different heads.
Last edited by 63mako; Jan 10, 2012 at 02:01 PM.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
General rule of thumb for a GEN I is that a decrease of about 10cc in chamber volume = about 1 point increase in compression. It's fairly easy to measure the piston dome volume in the car if the heads are off - happy to outline this if anyone's interested.
I'd be worried about that potential .071 - .076 quench - that's almost twice what's optimal and will increase detonation sensitivity. Always the issue with tops unless carefully planned - CR vs. quench.
Deck height can be all over the map on a production engine .020-.025 is common, but I've seen extremes as well. Also not difficult to measure with the engine in the car and the heads off.
I'd be sure to do some careful measurements during the swap to ensure you've got something workable.
I'd be worried about that potential .071 - .076 quench - that's almost twice what's optimal and will increase detonation sensitivity. Always the issue with tops unless carefully planned - CR vs. quench.
Deck height can be all over the map on a production engine .020-.025 is common, but I've seen extremes as well. Also not difficult to measure with the engine in the car and the heads off.
I'd be sure to do some careful measurements during the swap to ensure you've got something workable.
.
thats what i used. felpro coated .015 steelshim gaskets. and my pistons were .023 in the hole. i measured them. the tight quench is the key to no detonation. i also agree with billa
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
What are you running for a camshaft? Cam timing has an effect on Dynamic Compression Ratio (I know, I know, another variable thrown into the mix, that's all ya need, right?) and DCR is what will govern what your actual cylinder pressure will be, hence it is a factor in detonation resistance. At least that's what I heard.......
edit: You'd probably have to run a huge cam, and those just are not huge cam heads.
Scott
edit: You'd probably have to run a huge cam, and those just are not huge cam heads.
Scott
Last edited by scottyp99; Jan 10, 2012 at 02:11 PM.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
I can see those heads working great on a L48 short block with a cam upgrade but with cali fuel and an L82 piston your in trouble already. I would sell then to a forum member and throw some $ in to go with a 64 CC aluminum head. Even then your compression will be boarderline @ 10.45 with a .015 gasket depending on your gearing and cam with 91 octane.
Last edited by 63mako; Jan 10, 2012 at 02:09 PM.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
What was your compression ratio? We are limited to 91octane fuel out here, so it's important that my CR not be too high.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
What are you running for a camshaft? Cam timing has an effect on Dynamic Compression Ratio (I know, I know, another variable thrown into the mix, that's all ya need, right?) and DCR is what will govern what your actual cylinder pressure will be, hence it is a factor in detonation resistance. At least that's what I heard.......
Scott
Scott
312/312 advertised duration
222/222 duration @ .50.
450/460 lift
114 LSA.
This might just work as 312 advertised is really huge and the dcr is based on the intake closing point that is obtained from the advertised duration. Factory slow ramp just might make it work. Would need to do a little more research.