I used the CARMATH.com calculator to estimate comprseeion ratio for the following setup:


6 liter iron block Gen III with a stock 4" bore
Stroker crankshaft with a 4.125" stroke
Heads with a 60CC chamber
flat-top pistons

The computation showed 414 CI and a compression ratio at 12.67 to 1

Obviously, this CR is too high for a street car on pump gas although I assume it would kick serious butt on aviation fuel.

Will dished pistons fix this?

 

1. Cannot run avgas in cars.

2. Why not run a 64-67cc head to compensate?

 

Hey John:
My relatives are pilots and they buy high octane avfuel for their planes (102 Octane no lead) ...so why can;t u use this in high compresssion cars ...obviously, it is prohibitively expensive but its more available than race gas...those who have high compression big-blocks that run like **** on low 0ctane unleaded used this trick...my old vette (70 454) did not have the same problem....

...anyway, this is not the point....if a set of Gen iii heads is milled .055 to yield a chamber size of 60cc can you increase the chamber size later to yield 67cc chambers?

...is a dished piston another alternative?

 

I doubt you can gain back 7cc by opening up the combustion chambers on a LS1 head, anyone know?

I have read before that avgas is formulated differently and should not be used, let me see if I can find the link.

 

Don't run Avgas in a car engine. Avgas is designed for use in airplanes because the engines run at levels where oxygen is less. You will also end up ruining your O2 sensors.

I have a 418 with a 63cc chamber, .054 head gasket and a -12cc dish. This yields approx. 11.4:1 with the piston .010" out of bore.

Like John stated, you should stick with a larger chamber and you will still need a dish.

If you stick with a 60cc head you should use a 8 cc dished piston. It will get you 11.6:1. This is with a .054" gasket and zero deck. Very good for pump gas and will make great power.

 

Thanks...

...the 11.6 CR seems really high; are you shure this will run on pump gas. I'm at sea level.

...I assume I can go with a bigger dish ..say 12CC and lower the CR some more...i thought the max would be about 10.75?????

Thanx for the info...

 

If I keep the 60cc chambers and order -12cc dished pistons, I assume I will have 11.22 CR -according to the CarMath.com calculation.

To perform this calculation in CarMath, I added 12cc to the chamber size (72CC [60CC + 12CC] since there is no mathematical perameter for the variable "dished/dommed" piston.

Does this sound correct?

Another question: The 4.125 crank is big; will this fit in a stock LQ4 block (6 liter iron Gen III block)? WIll I need to "clearance the block" for an oversized crank or does this depend on whose crank you use?

 

Robert,

I have a 418 motor that is closer to what you are trying to do. If you already have the 60cc heads I would do the following;

6.125" rod
1.0425" compression height
12 cc dish
.054" head gasket
.010" out of bore

This will yield you approx. 11.46:1 No problem with pump gas with the right tuning.

Should make over 500 in both HP and TQ.

 

great! thX...