miller3

Hello, I'm having hard time understanding how l82 compression ratio is 9.0-1. Here are the specs I have found

Bore:4.00
Stoke:3.48
Head chamber 76cc
Piston volume: 6cc
Gasket thickness 0.04
Deck Height 0.025

This comes to cr of 8.5-1 not 9.0-1.

Do I have something wrong here? Let me know, trying to understand effect of parts changes to cr but I need to know what I should be starting with.

jim2527

I've noticed different calculators give different results. You'd think they'd all be the same....

Did you enter +6 or -6 for the piston volume?

Kacyc3

sounds right to me, the 76cc heads provide more room to for the combustion providing less compressed pressure so it reduces the ratio. On 350s the 76cc head was usually a low performance head from the factory, using a 64 cc head would up the ratio of the same setup to almost 9.6:1.

REELAV8R

The head gasket used was a .017 shim gasket. Using that it comes out to 8.89:1 and then the .11 boost from marketing and you get 9.0:1.

However your 76 cc heads are more like 79-81cc's. At least they were on my L48.

miller3

So then if I swap head would using a 0.028 head gasket from GM or the felpro 0.015 head gaskets work without any resufacing because the original head gaskets were so thin?
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HeadsU.P.

You only use a negative number when using a "domed" piston. Because you are subtracting volume. Which I doubt is the case here.

REELAV8R

Yes you can. Prep to the block side is important though. I used a scraper then fine sand paper wrapped on a flat block of wood on the deck surface to prep it. Absolutely seal the cylinders off so as to not get any grit into the cylinders or any of the coolant holes. It's a pain, but important.
It comes in a spray or a liquid. I use the liquid. Not too much though you don't want it squishing all over into your water passages and cylinders.
I have used the .015 Felpro shim gasket on two different aluminum heads (9.9 CR and now 10.6 CR) along with the K&W copper coat sealant. You can get it at Oreilly's auto parts. I have not had any leaks or problems using this combo.

Using the .015 combined with the piston being .025 in the hole will give you a great .040 squish distance, perfect for detonation prevention.

NewbVetteGuy

What a great thread! A couple of questions:

How do you seal the cylinders while you're trying to clean/sand the block surface to get all the old gasket bits off? I've seen youtube videos of people just throwing shop towels in there and calling it "good enough" -is it?

What are you referring to above (spray or liquid?)?


Did the distance of the piston in the hole vary over the years? (I thought so for some reason...)


Given the above, why would anyone deck the block of a Vette AND possibly lose the block #'s? (If you can achieve an ideal squish distance of 0.040"; isn't that all you need?



Adam

NewbVetteGuy

Wow, great factoid! This is the first that I'd ever heard of the factory gaskets being a thin .017".


Adam

dochorsepower

If you are going to go to the trouble of replacing head gaskets, get something better than those 882 series heads.

REELAV8R

That's weird. I had put in it was the K&W copper coat that comes in liquid or spray and somehow it got edited out.


To seal off the pistons I masked them off with tape and a circle of cardboard for each cylinder.. The green painters tape. I have seen pics of rags in the cylinders too. I guess that may be sufficient if vacuumed out before you pull the rag as I did before I pulled out the cardboard circle. I just didn't want any sand paper grit in my cylinders so I masked them off the way I did to be sure. Had to clean the inside of the cylinder with carb cleaner first or the tape wasn't going to stick. For the water jacket holes I just used bits of rag, not paper rag, cloth. That way I could remove it after I was done with pointed nose pliers. They'll get wet, if you use paper rag you may not be able to pull it out without it disintegrating.

The standard distance down the hole for GM on the 350 was .025". No variation as far as I know. It's possible since I'm no expert on the matter but that is the ONLY dimension I've read for a stock 350 (not LS1).
You can see here in a Lunati chart that the 9.025 height is standard for many GM motors.
http://www.lunatipower.com/Tech/Pist...ionHeight.aspx

The block is often decked to make it flat again. Often the deck is not perfectly flat from one corner to the other. Mine wasn't. It was .032" difference from one side to the other. I felt that it was not enough to warrant milling material off the deck at my power level. The deck could also have corrosion or detonation damage that may need to be removed. Mine had a bit of detonation damage on the top of cylinder #8. I just smoothed out the rough texture so as to not promote detonation in the future and I was able to do so without really enlarging the hole in any significant manner. Not decking it is a plus in my mind as the deck surface is thicker and stronger this way.

I measured the gasket that came off of my factory built OEM motor on my 77 corvette, it was a .017 shim gasket. Being a shim it doesn't crush. For the L-82 or L-48 for that matter, it's the only dimension that makes sense to even get close to the 9.0:1 CR or 8.5 CR advertised. I may have made a bit of a leap guessing the L-82 is the same, but I'd be willing to bet it is.