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L82 dynamic compression question

Old 08-14-2017, 02:19 AM
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miller3
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Default L82 dynamic compression question

Hello,

I'm trying to figure out what the dynamic compression ratio would be for this combo.

67cc head
6cc flat top piston
0.25 deck hight
0.28 head gasket (I'm not resurfacing block, only changing heads so I'm hoping to be able to use the Gm composite gasket to get it to seal.
4.0 bore
3.48 stroke
5.7 connecting rod length
Stock l82 cam, 222 @ 0.050, 114 love separation and .45/.46 lift
If I change the cam timing to 2 degrees advanced, where does it put dcr? With a quench of 0.53 will that be ok for 93 octane? If we went with 64cc heads would it still be ok?

Thanks for help,
Adam
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Old 08-14-2017, 09:18 AM
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resdoggie
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Google dcr calculator. You want a dcr below 8.5 for pump gas.

Last edited by resdoggie; 08-14-2017 at 09:19 AM.
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Old 08-14-2017, 09:26 AM
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miller3
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Originally Posted by resdoggie View Post
Google dcr calculator. You want a dcr below 8.5 for pump gas.

Understood, but they key piece of information I'm missing is when the intake valve closes, I can't find that spec. And then I'm not sure with a quench of 0.53 if 8.5 would still be ok?
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Old 08-14-2017, 09:40 AM
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United Engine Compression Calculator web site is a wonderful tool for crunching numbers.
But true, you can not do anything without the intake closing event. If you find the stock timing specs, add the two degrees of advance to see what happens to the DCR.
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Old 08-14-2017, 10:01 AM
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resdoggie
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If the cam is advanced, the dcr will go up.
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Old 08-14-2017, 10:01 AM
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I run a 64CC iron head with a very thin steel shim head gasket on my otherwise stock L-82 and it runs great. I run 93 octane without detonation issues. Better quench comes from thin gasket. (2) Fel Pro part 1094 would get the installed thickness down to .015"
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Old 08-14-2017, 10:32 AM
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Originally Posted by stingr69 View Post
I run a 64CC iron head with a very thin steel shim head gasket on my otherwise stock L-82 and it runs great. I run 93 octane without detonation issues. Better quench comes from thin gasket. (2) Fel Pro part 1094 would get the installed thickness down to .015"
The worry I have is that the thin head gasket might not seal on my original surface block. Which was why I was expecting to have to use a thicker composite gasket, of course we won't know till we open the motor up, but prepping for worst case.

Did you advance the cam at all?
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Old 08-14-2017, 11:21 AM
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REELAV8R
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Ok this took a lot of digging since the L-82 cam is always only shown @ .050 and not at .006, or advertised duration.

A previous forum member posted this in regards to the L-82 cam;

OEM #3863151, 350 HP, L-79 327

WG-996 (Cam only)
WG-996Kg
151H
Hyd C
222 222
306 306
.447 .447
110/118

OEM #3896962, 350 HP, L-82, 350

WG-1170 (Cam only)
WG-1170Kg
962H
Hyd C
224 224
292 288
.450 .459
115/113
The part number 3896962 matches the number that chevy performance uses for the stock L-82 cam. The duration @ .050 varies by 2* however. So a bit of a discrepancy there.

The duration that you usually can never find for this cam is the advertised (lash) duration. Here it states it's 292 for the intake.
I would say that is probably pretty close if not right on for a cam of this age and it's intensity.
So if I plug that duration into this calculator;
https://www.jeepstrokers.com/forum/calculator/

it gives me an intake valve closing angle of 76*.
It also gives me a DCR of 6.91 with the intake gasket and chamber size you quoted.

If you use the .015 felpro shim instead and drop the chamber size to 64cc's then you get a 7.35. A pretty good improvement.

I would (and have i have used) the .015 felpro with some K&W copper coat gasket sealer and with decent prep to the block you should not have any problems with sealing.
I'm using one now on a 10.6 CR engine with aluminum heads.
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Old 08-15-2017, 09:32 AM
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Originally Posted by miller3 View Post
The worry I have is that the thin head gasket might not seal on my original surface block. Which was why I was expecting to have to use a thicker composite gasket, of course we won't know till we open the motor up, but prepping for worst case.

Did you advance the cam at all?
No cam advance. I did use the copper spray gasket sealer.

Unless your block surface is trashed you should be just fine using steel shim head gaskets.
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