E-mail me offline with your complete motor/head and cam specs for assistance.

 

Alot has to do with the camshaft, duration LSA and ICL all have a big part in how much compression you can handle. Basically we are talking about the Dynamic Compression Ratio. I've run 11.5:1+ with a small camshaft in a SBC on 92 octane with a good amount of timing it it with no problem. So with a different cam your 12:1+ might be just right.

OTOH, going to a bigger gasket is just as likely to cause more of a problem with the detonation since you are making the quench area larger which is not the best way to go. Smokey Yunick found this out years ago, adding gaskets to a detonation prone motor will not help the problem.

If anything take some material out of the chambers and lower the compression that way. You can also cut some bigger valve reliefs in the pistons and you can find 2-5 more cc of volume that way too.

Bret

 

Ok, well I worked out the math on your setup. I'm assuming that you're using the lunati ls11 flat tops, with releifs, 6.125 rods, and 4"crank stock bore. You and your machinist are correct, the .060" cometics would bring you down to 12.2 There is a little more that you would want to consider as well. Your piston, crank, rod combo would put the piston OUT of the hole .015" Then you had .020" milled off the deck of the block. This puts your piston quite a way out, a total of .035" out of the hole. You should have at least .035" of piston to head clearance. This would actually require a .070" gasket to be safe. A .070" gasket in your setup wiuld give you 11.9 to one CR. Since your piston is so far out of the hole, you could even run an .080" gasket, and it would give you a .045" piston to head clearance which would still be fine. That would give you 11.6 to 1 which should just be doable with good tuning. Here's how I see it:

.060" gasket= 12.27 to 1 and .025" piston to head clearance=piston to head contact likely, and too high of CR

.070" gasket=11.94 to 1 and .035" piston to head clearance=tight but OK, CR more reasonable

.080" gasket=11.64 to one and .045" piston to head clearance= good clearance and good compression ratio... just don't know if you could find this thick of a gasket.

Bryan

 

Thanks for the advice fellas but im going to have him do another set of heads. I have some that were on my orginal motor that i will bring him.

Any one wanna buy some heads milled 40 set up for dual springs and 2.02 1.60 valves?

They flowed 305 @ 600 intake
and 227 @ 600 on the exhaust.
?

 

Why not just swap out to a cam with more overlap to bleed off some cylinder pressure?

 

I'm running 11.2 to 1 on 93 octane; no knock - zero, according to an Atap!

Before I installed the new heads that are shaved .055, I asked lots of questions here about high CR. One option I considered was to reduce CR by installing a thick Cometic gasket.

I think the overwhelming majority here said a thick gasket is a bad idea since you INCREASE the chances of knock...

Some of the turbo and blower guys use thick gaskets but it is not an ideal solution... try posting this question on the blower section, however...


The best solution would be to increase the size of the combustion chamber of the heads followed by installing dished pistons...

 

I think you could make it live with 12.2:1CR, if you put 93 in it, and then add Hexane or some other form of octane booster when you gouge the throttle.
Just stay outta the bitch when you don't have good fuel.
I had a formula for homeade Octane booster that was made up by some really smart *************- let me see if I can round it up and I'll post it for you.

 

Here ya go-

Sunoco Ratings
SUNOCO "Standard"
Color...Purple
Research Octane...115
Motor Octane...107
R+M/2 ...111
Specific Gravity...0.725
Reid Vapor Pressure... 8#
Distallation temp, degrees F
Initial...90
10%...160
50%...220
Final...360
SUNOCO "GT Unleaded"
Color...Natural
Research Octane...105
Motor Octane...95
R+M/2 ...100
Specific Gravity...0.760
Reid Vapor Pressure... 8#
Distallation temp, degrees F
Initial...90
10%...150
50%...210
Final...230

SUNOCO "Supreme"
Color...Blue
Research Octane...116
Motor Octane...109
R+M/2 ...112
Specific Gravity...0.715
Reid Vapor Pressure... 8#
Distallation temp, degrees F
Initial...90
10%...155
50%...215
Final...260
SUNOCO "Maximal"
Color...Red
Research Octane...118
Motor Octane...115
R+M/2 ...117
Specific Gravity...0.700
Reid Vapor Pressure... 6#
Distallation temp, degrees F
Initial...100
10%...150
50%...220
Final...240


Richard Lassiter's "How to Mix Your Own Brew"
FORMULA 1
Toulene
R+M/2...114
Cost...$2.50/gal
Mixtures with 92 Octane Premium
10%...94.2 Octane
20%...96.4 Octane
30%...98.6 Octane
Notes: Common ingredient in Octane Boosters in a can. 12-16 ounces will only raise octane 2-3 *points*, I.e. from 92 to 92.3. Often costs $3-5 for 12-16 ounces, when it can be purchased for less than $3/gal at chemical supply houses or paint stores.
FORMULA 2
Xylene
R+M/2...117
Cost...$2.75/gal
Mixtures with 92 Octane Premium
10%...94.5 Octane
20%...97.0 Octane
30%...99.5 Octane
Notes: Similar to Toulene. 12-16 ounces will only raise octane 2-3 *points*, I.e. from 92 to 92.3. Usually mixed with Toulene and advertised as *race formula*.

FORMULA 3
Methyl-tertiary-butyl-ether (MTBE)
R+M/2...118
Cost...$3.50/gal
Mixtures with 92 Octane Premium
10%...94.6 Octane
20%...97.2 Octane
30%...99.8 Octane
Notes: Oxygenate. Very common in octane booster products. Has lower BTU content than toulene or xylene, but oxygenate effect makes the gasoline burn better and produce more energy.

FORMULA 4
Methanol or Ethanol
R+M/2...101
Cost...$0.60 - $1.75/gal
Mixtures with 92 Octane Premium
10%...94.3 Octane (Methanol)
10%...94.7 Octane (Ethanol)
20%...Not Recommended
Notes: Methanol is wood alcohol. Ethanol is grain alcohol and found in Gasohol in 10% ratios. Both alcohols are mildly corrosive and will eat gas tank linings, rubber and aluminum if used in excessive ratios. Main ingredient in "Gas Dryers", combines with water.

FORMULA 5
Isopropyl Alcohol and Tertiary Butyl Alcohol
R+M/2...101
Cost...$0.60-$1.50/gal
Mixtures with 92 Octane Premium
10%...94.5 Octane
20%...Not Recommended
30%...Not Recommended
Notes: Similar to Methanol/Ethanol. Isopropyl Alcohol is simply rubbing alcohol.


MAKE YOUR OWN OCTANE BOOST
How to make your own octane booster (this is the basic formula of one of the popular octane booster products). To make eight 16 ounce bottles (128 oz = 1 gal):

100 oz of toulene for octane boost
25 oz of mineral spirits (cleaning agent)
3 oz of transmission fluid (lubricating agent)

This product is advertised as "octane booster with cleaning agent *and* lubricating agent!". Diesel fuel or kerosene can be substituted for mineral spirits and light turbine oil can be substituted for transmission fluid. Color can be added with petroleum dyes.


http://www.gnttype.org/techarea/misc/octanebooster.html

 

Overlap doesn't bleed of cylinder pressure, that deals with the EVC and ICO points compression deals with the IVC. Basically more duration or a higher LSA(or ICL) will help you run more compression on a engine.


Bret

 

I am running ~12.2 with a small quench and works fine here in MD on 93 octane. I think with an all aluminum motor and a cool T-stat you'll be fine.
Phillip

 

so which of these formulas does this mix your own correspond to??