Tin Indian

I know there are calculators to find out your CR. You will need to know piston bore size, piston stroke length, combustion chamber volume, compressed head gasket thickness, and piston deck clearance. Lots of info!

Anyway, I did a calculation a little differently. This will only be used on a stock block 5.7L LS1 engine with stock flat top pistons. You will need to know your head chamber volume in CCs.

Known:

A 5.7L LS1 head has a chamber volume of exactly 66.67cc and 10.1 CR

A 5.7L LS6 head has a chamber volume of exactly 64.45cc and 10.5 CR

That means a decrease of chamber volume of 0.56cc = an increase of 0.1 CR

66.67cc = 10.1 CR (stock LS1 spec)
66.11cc = 10.2 CR
65.55cc = 10.3 CR
64.99cc = 10.4 CR
64.43cc = 10.5 CR (very very close to the actual 64.45cc LS6 spec)
63.87cc = 10.6 CR
63.31cc = 10.7 CR
62.75cc = 10.8 CR
62.19cc = 10.9 CR
61.63cc = 11.0 CR
61.07cc = 11.1 CR
60.51cc = 11.2 CR
59.95cc = 11.3 CR

Now, these numbers are based on a linear set of numbers. I can't be sure, but I would think that a change in head chamber CCs vs it's corresponding CR would be linear.

What ya think?

Ron,

99 Black Bird T/A

iTrader: (4)

Sounds like a reasonable rule of thumb. However the valves can also effect volume. Brian at TEA told me that a dished exhaust valve can usually change the compression by .1, so on that note it's not just chamber volume etc.

Tin Indian

99 BlackBird T/A ...

Thats true, but they CC the heads with the valves installed.

Although a bone stock 5.3L head has a chamber volume of 61.15cc (stock valves installed), my ported TEA 5.3L heads have a new chamber volume of 63cc due to the material that is removed to unshroud the valves and of course, the new valves. They are minumally milled .003" to insure straightness.

Anyway, looking at the above chart, that 63cc head would yeild a CR of right between 10.7 and 10.8 to 1

Tin Indian

As you can see, if you bolted up a bone stock set of 5.3L truck heads to a stock displacement 5.7L LS1, the 5.3L 61.15cc heads would yeild approx an 11.1 to 1 CR.

Again, with the material removed to unshroud the valves, and the bigger 2.02" Ferrea intake valves, it brings the un-milled 5.3L head to 63cc

(approx 10.78 to 1 CR)

10secondZ

iTrader: (24)

What Gasket thickness is that based on? ...056"?

99 Black Bird T/A

iTrader: (4)

Looks like I pulled a Homer Simpson

Phil99vette

iTrader: (11)

Is that a .056 HG? How about creating a spreadsheet or table with HG options and all combustion chamber sizes from 56cc to 67cc

Tin Indian

The .56 number I mention is CCs, not head gasket thickness.

The figures I posted above would have to be accurate using a stock GM graphite head gasket.

Again, all I did was to assume a chamber CC of 66.7 for a stock LS1 5.7L head (this is what GM publishes). And that this chamber volume equals exactly 10.1 CR

Then I assumed a chamber CC of 64.45 for a stock LS6 head (again, this is what GM states it is). And that this chamber results in a CR of exactly 10.5 (what GM states the CR is on a LS6 motor)

The math shows that for every .56cc decrease in actual chamber volume, you get a 0.1 increase in compression ratio (on a stock displacement 5.7L LS1 motor)

Make sense?

Ron,

BergerMan

Great job Ron, I'll bet this thread will be referenced every time someone asks "what will my compression be with this setup????"

We all know that there as certain variables in every setup that will affect the final c/r, but this will give everyone a basis to go by.

Tin Indian

Thanks BergerMan ...

Unfortunatly, not everone knows what the CC volume there heads are. Even if you use the Compression Ratio Calculators that you find on the internet, you still must know your chamber volume in order to plug in the numbers and come out with a CR.

blk~2000~Z28

TTT
Just thought that this should come back since there are new LS6 type castings that can be whatever chamber size you need.
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2001CamaroGuy

iTrader: (6)

That seems about right (at least a good basic idea of what your CR will be).