Compression Check

compression

The compression ratio (C/R) is the ratio between the area in the cylinder and the area remaining when the piston is at top dead center (TDC) to arrive at the C/R you would devide the cylinder volume by the combustion chamber volume.
An example would be: if the cylinder volume was 625 cc's and the combustion chamber was 64 cc's, you would devide 625 by 64 and would have a 9.76 to 1 compression ratio.
Problem is.... you can't just go by the volume of the combustion chambers. Keep in mind that if your pistons are dished or notched for valve clearance you would have to add the volume of that to the combustion chamber. There are other issuses like head gasket thickness and deck clearance to consider, as this could further lower the compression ratio.
So.....If the total volume of the dish/notches/head gasket was 5 cc's your total combustion chamber volume would now be 69 cc's (64 + 5). Now devide 625 by 69 and your actual C/R would be 9.05 to 1. with a 64 cc head.
Since changing your rocker arms wouldn't affect your combustion chamber volume it would not change your C/R.
On the other hand, changing the cylinder head could either raise or lower the C/R depending on what size the combustion chambers were.
If your GT 40 heads have 64 cc chambers, and the TW heads also have 64 cc chambers, your C/R would not change.
If the TW chambers were larger, the C/R would be lower. If the TW chambers were smaller, the C/R would be higher. Hope this helps.

Zig

Pretty much what zig said, what is the CC of the chamber of the old vs new heads? The TW looks to come in 61-65cc variety and the GT40 in 64. TW205s I think are 65 cc so you would be the same compression.

9-1 compression is no big deal, just cant go crazy with boost/timing or ya gotta run some race gas.

What are you thinking of running?

zigmont said:

The compression ratio (C/R) is the ratio between the area in the cylinder and the area remaining when the piston is at top dead center (TDC) to arrive at the C/R you would devide the cylinder volume by the combustion chamber volume.
An example would be: if the cylinder volume was 625 cc's and the combustion chamber was 64 cc's, you would devide 625 by 64 and would have a 9.76 to 1 compression ratio.
Problem is.... you can't just go by the volume of the combustion chambers. Keep in mind that if your pistons are dished or notched for valve clearance you would have to add the volume of that to the combustion chamber. There are other issuses like head gasket thickness and deck clearance to consider, as this could further lower the compression ratio.
So.....If the total volume of the dish/notches/head gasket was 5 cc's your total combustion chamber volume would now be 69 cc's (64 + 5). Now devide 625 by 69 and your actual C/R would be 9.05 to 1. with a 64 cc head.
Since changing your rocker arms wouldn't affect your combustion chamber volume it would not change your C/R.
On the other hand, changing the cylinder head could either raise or lower the C/R depending on what size the combustion chambers were.
If your GT 40 heads have 64 cc chambers, and the TW heads also have 64 cc chambers, your C/R would not change.
If the TW chambers were larger, the C/R would be lower. If the TW chambers were smaller, the C/R would be higher. Hope this helps.

Zig

Click to expand...

Well, that's a close description . . . the actual volume you would consider would include the volume of the cylinder at bottom dead center, the volume of the head gasket and the volume of the combustion chamber added all together to arrive at the total you would divide when the cylinder is at TDC.

For example, a 302 has a 4.0" bore and a 3.0" stroke; that means that you would take 4 divided by 2 squared, multiplied by 3.1416 to get the area of a circle, times 3 to get the volume of the cylinder, then times 16.387 to convert to CC's (it comes out to 617.78 cc's; there are metric conversion calculators all over the internet you can use though).

Next, you would find the volume of the head gasket; most of the oneswe would use would have a diameter of 4.125" and a compressed thickness of .040", so you would use the same formula as you did with the cylinder--Pi*r-squared (in this case it would be 4.125 divided by 2 times itself, which equals 4.25, and then multiply that by 3.1416, and then multiply that figure by .040, and finally, multiply that by 16.387; the total in this case would be a volume of 8.76 cc's).

The cc volume of a standard 302 piston is something like 8cc's (for the Speed Pro H273CP Hypereutectic piston).

Add them together:

617.78 - cylinder volume
8.76 - head gasket volume
8.00 - piston dish/valve relief volume
64.00 - combustion chamber volume
-----------
698.54 - total of all volumes

Then, you would divide that by the total volume that would be above the piston crown at TDC:

8.76 - head gasket volume
8.00 - piston dish/valve relief volume
64.00 - combustion chamber volume
-----------
80.76

Therefore, the compression ratio would be: 698.54 divided by 80.76, or 8.65:1

I just did a quick search on Google, and found this handy little compression calculator:

Engine Compression Ratio (CR) Calculator

as you can see, there are quite a few things to consider besides just the cylinder head combustion chamber volume . . .