MYTURBOT/A

Why am i hearing alot that a 72cc head will make a 9.5:1 comp ratio on a stock ls1 when the write up says 9.1:1 ?

Bryan Wilkinson

iTrader: (1)

Ok great info, now, how do we calculate that for a 427 with a 4.125 bore.

Thanks
Bryan

1slowCamaro

MYTURBO T/A - I was wondering the exact same thing. Everybody says a 317 casting 6.0 head will put you at 9.6 - 9.7 CR with the stock 71 cc chambers. That doesnt match up with that table very well. Which one is correct?

Justin

MYTURBOT/A

Many people have bolted on stock 6.0 heads and all of them report a 9.5-9.6 comp ratio so I trust that to be acurate.

71CamaroLS1

Yes, I know this is my first post - I've just started reading these forums after buying an LS1 to put in my 71 Camaro. But I've got a lot of experience with the "old tech" motors

Just looking at the table at the beginning of this post, it seemed to me to exaggerate the sensitivity of CR to chamber volume. So I ran a quick calculation or two, using the basic formula I've always used to compute CR (lots of places to find this, the constants I use are from "Lingenfelter on modifying small block chevy engines"). The overall calculation is pretty straightforward:

CR = (SV + CV) / CV

where:

CR = compression ratio
SV = Swept Volume (displacement of one cylinder)
CV = Compressed Volume (volume at piston TDC)

To get the swept volume of one cylinder in CCs, the handy formula is:

bore x bore x stroke x 12.8704

For a stock short block, this works out:

3.90 x 3.90 x 3.62 x 12.8704 = 709 CC (rounded up)

Compressed volume is the sum of chamber volume, deck volume, gasket volume, and piston dish volume (including valve reliefs). For a stock LS1:

Chamber volume = 66.67
Deck volume = 3.9 x 3.9 x 12.8704 x (-.006) = -1.17 CC (negative because piston out of hole)

Gasket volume = 3.9 x 3.9 x 12.8704 x .060 = 11.7 CC (guessing on the gasket bore?)

Piston dish volume = 0 (true flat top piston)

Thus CV = 66.67 + 11.7 - 1.17 = 77.2 CC

Put all of this into the first formula:

CR = (709 + 77.2) / 77.2 = 10.18

Pretty close - my guess is that I've got gasket size slightly off, or that the factory is taking into account the piston/ring/wall volume, or something like that.

Anyway, the interesting thing is to run this with some different chamber volumes. Shrink the chamber by 8 CCs (to 58.67):

CR = (709 + 69.2) / 69.2 = 11.25

Expand the volume by 6 CCs (to 72.67):

CR = (709 + 83.2) / 83.2 = 9.5

So it seems to me that the table in this post does slightly exaggerate the changes in CR due to chamber volume. Or am I completely missing something?

BigTex

iTrader: (16)

Did you take into account that the Gen III motors have a negative deck height? About .008 I think. Also, depending on the type of GM gasket, compressed height is usually .054 or .060.

71CamaroLS1

I did account for the piston out of hole - I used .006, based on some other posts I'd read. I used a .060 gasket thickness. Fiddling with those two parameters will increase or decrease the individual CR calculations slightly, but the point about relationship between CR and chamber volume remains ...

1fastWS6

iTrader: (3)

I think it would be beneficial to add to the original post that if you're using the later GM MLS gasket, that will decrease your compression by .1 - .2.

Craig

Fireball

iTrader: (21)

making this thread sticky again

71CamaroLS1

If this is going to be sticky again, the calculations should probably be redone (see my post above).

Shinobi'sZ

I just used Ross Piston's Calculator and can not get to 9:1 CR using the following data.
Bore 3.898
Stroke 3.622
Head Gasket .060
Deck Height -.008
Piston Dish .00
CC Volume 72
Cylinders 8

I get 9.6:1

How do you get 9:1 using stock gasket thickness and 72 cc head volume????????

Fireball

iTrader: (21)

Something definately not right here. I'll unsticky this again until resolved

bad2000ss

iTrader: (6)

Using 3.898 bore, 3.622 stroke, -0.008 deck hight, 0.00 dish pistons, and 0.060 gasket thickness this is what i come up with...

9.1:1 = 77.27
9.2:1 = 76.21
9.3:1 = 75.16
9.4:1 = 74.15
9.5:1 = 73.16
9.6:1 = 72.19
9.7:1 = 71.24 (Approximate Combustion chamber of 6.0L heads +/-)
9.8:1 = 70.32
9.9:1 = 69.41
10.0:1 = 68.53
10.1:1 = 67.66
10.2:1 = 66.67 (Stock LS1 Head)
10.3:1 = 65.96
10.4:1 = 65.18
10.5:1 = 64.45 (Stock LS6 Head)
10.6:1 = 63.61
10.7:1 = 62.85
10.8:1 = 62.10
10.9:1 = 61.37 (Approximate Combustion chamber of 5.3L heads +/-)
11.0:1 = 60.66
11.1:1 = 59.96
11.2:1 = 59.27
11.3:1 = 58.60
11.4:1 = 57.93
11.5:1 = 57.28
11.6:1 = 56.65
11.7:1 = 56.02
11.8:1 = 55.41
11.9:1 = 54.81
12.0:1 = 54.20
15.4:1 = 38.00 (Stock C5R Head)

Although the stock ls1 compression figure may seem off, i have seen a lot of sites selling "stock ls1 long block" with a 10.2:1 compression ratio (i.e. pace performance). Perhaps 10.2:1 is the true compression ratio of a stock LS1 motor and GM says otherwise. GM has a nice way of making up their own horsepower and torque ratings for motors as they see fit, perhaps they figure 10:1 is just easier to say. The LS6 compression ratio seems right on according to the combustion chamber and the calculations. Since no one has ever been able to give a diffinitive answer on the actual size of a 6.0L combustion chambers (i've seen anywhere from 70cc-72cc) i think it is safe to assume that 6.0L heads would not lower a stock LS1/LS6 motor's compression ratio any more than 9.6:1.

If you notice on the original chart by the original thread starter, all combustion chamber sizes are exatly .56cc different per tenth from 9.1:1 to 11.6:1. That is obviously impossible. As the compression ratio increases, the combustion chamber does not decrease by equal incriments. A simple analogy would be swapping rear end gears without recalibrating your speedometer. If you swap from 3.42's to 4.11's your speedometer will read 15 miles per hour faster at 90mph and will read 5 miles per hour faster 20mph. It won't read a consistant increase/decrease across the board.

Hope this helps some people out.

tux2112

TTT

these calcs are w/0.00 dish pistons ... my pistons have 8cc reliefs cut in them... so could I just add 8cc to the Compress Volume?

Trying to figure out my CR ... 382 (3.898 bore, 4.000 stroke) using Felpro gaskets (so I heard they're .040" compressed?) w/unmilled TEA S2 5.3 heads (63cc chamber volume)

BigTex

iTrader: (16)

Felpro makes several LS1 gaskets. They have graphite and MLS I think. The 1041 MLS gasket has .043 compressed height and a 4.1" bore (from the package). Their stock replacement gasket probably has a stock compressed thickness of between .054 - .060".

bad2000ss

iTrader: (6)

With a 63cc combustion chamber, 3.898 bore, 4.000 stroke, -0.008 deck hight, -8cc dish pistons, and 0.040 gasket thickness i come up with 11.1:1

Shinobi'sZ

Stock 2002 MLS Crushed Head Gasket Thickness is .054 not .060

DownUnder

Guy's, are the MLS gasket's the later metal type? So if using these on a block that has the graphite gasket b4 you will loose .1 comp?

JZ'sTA

iTrader: (23)

You entered .060 when it would acturally be .052 This is what a stock head gasket is. With the same numbers you have and the different head gasket size the C/R comes out to 9.7:1, (this was with a .0008 deck height) still pretty close to where you were at. The only thing I could not do is add a negative .008 in the factor. The ross calculator dosen't allow a negative number in that spot. When using a .008 (instead of .0008) it come out to 9.55:1??????????????????????????

M6HuggerSS

i believe my chambers are coming out around 68cc in my 5.7 heads.
if i use a stock graphite gaskit how much do i need to mill the heads
to get a 10.75 to 10.8-1 CR....anyone???

or is there a gasket i can use to get that CR ???????