jordan89

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/

jordan89

Anybody know the dome volume on the stock pistons on the L82 engine? I need to figure these things out as soon as possible. Thank you.

GS977

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

jordan89

Unfortunately, here in California we only have 91 octane. We are limited on where we can find any higher. Does anybody know/confirm the dome size please? Thanks GS977.

Shark Racer

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.

jordan89

Mines a '74 vette. Aren't the stock heads like 76cc?

jordan89

Ttt

Jeff_Keryk

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.

Shark Racer

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.

GS977

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

jordan89

What head gasket do you guys recommend/should be used with these aluminum heads and iron block? I was thinking about using gm part#12557236 sine they are for aluminum heads.

billla

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.

63mako

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.

jordan89

Can you please outline these two procedures for me Billa? I'm also still debating on what head gasket to use, the thickness.

GS977

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

scottyp99

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

63mako

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.

jordan89

What was your compression ratio? We are limited to 91octane fuel out here, so it's important that my CR not be too high.

jordan89

I have jegs cam part#555-200107, so I was planning on using that. I'm on a budget so there's no hope of getting any others pair of aluminum heads right now.

63mako

Well just checked duration numbers on the factory L82 cam. Came up with
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.