G3i Ignition
Well Known Member
It may not be the ?actual? compression ratio that you are running.
I did a new cylinder and piston (stud assembly) change on my IO-540 parallel valve and kept the same compression ratio piston of (8.5 to 1). With the new assemblies installed, I later discovered the actual combustion chamber area was 10cc less than the stock Lycoming cylinder combustion area that I removed. This brought the compression ratio up to 9.1 to 1 ?actual? even with the correct piston part number for an 8.5 to 1 ratio. I feel that this matter is important to share. Having a higher ?actual? than thought compression ratio can cause serious engine problems with incorrect tuning and/or running a lower octane fuel.
One of the simple tests that I did to determine ?actual? compression ratio with the engine on the aircraft was to do a fluid volume test at TDC. This was done with a 100cc fluid graduated cylinder, a compression gauge hose with the one-way schrader valve removed and plastic quart cup.
With the lower spark plug installed and the intake and exhaust valve closed about 30-40 degrees BTDC. I added 300cc of ATF (Dextron Automatic transmission fluid). By bringing that cylinder to TDC will displace the combustion area and pump the excess fluid out through the top spark plug hole hose and into the plastic cup. The amount of fluid in the container will be measured with the graduated cylinder and subtracted from 300cc. This will give you your combustion area volume in cc?s. This number added to bore and stroke volume and then divided by combustion area (cc?s) will calculate ?actual? compression ratio.
Here?s one of my cylinders as an example:
Bore = 5.125 Stroke = 4.375 // area = 90.251759 cubic inches.
90.251759cu converted to cc = 1478.96 cc
Hose volume = 11cc
Pumped out 106cc @ TDC. Into container
Total pumped out = 11cc (hose volume) + 106cc =117cc
300cc ? 117cc = 183cc combustion area.
1478.96 + 183 = 1661.96cc divide by 183cc = 9.0817
Actual compression ratio = 9.1 to 1
Note: Margin of error testing compression ratio on the aircraft as to the cylinder removed and tested on the bench was less than 1.5%
Fyi
Thomas S.
I did a new cylinder and piston (stud assembly) change on my IO-540 parallel valve and kept the same compression ratio piston of (8.5 to 1). With the new assemblies installed, I later discovered the actual combustion chamber area was 10cc less than the stock Lycoming cylinder combustion area that I removed. This brought the compression ratio up to 9.1 to 1 ?actual? even with the correct piston part number for an 8.5 to 1 ratio. I feel that this matter is important to share. Having a higher ?actual? than thought compression ratio can cause serious engine problems with incorrect tuning and/or running a lower octane fuel.
One of the simple tests that I did to determine ?actual? compression ratio with the engine on the aircraft was to do a fluid volume test at TDC. This was done with a 100cc fluid graduated cylinder, a compression gauge hose with the one-way schrader valve removed and plastic quart cup.
With the lower spark plug installed and the intake and exhaust valve closed about 30-40 degrees BTDC. I added 300cc of ATF (Dextron Automatic transmission fluid). By bringing that cylinder to TDC will displace the combustion area and pump the excess fluid out through the top spark plug hole hose and into the plastic cup. The amount of fluid in the container will be measured with the graduated cylinder and subtracted from 300cc. This will give you your combustion area volume in cc?s. This number added to bore and stroke volume and then divided by combustion area (cc?s) will calculate ?actual? compression ratio.
Here?s one of my cylinders as an example:
Bore = 5.125 Stroke = 4.375 // area = 90.251759 cubic inches.
90.251759cu converted to cc = 1478.96 cc
Hose volume = 11cc
Pumped out 106cc @ TDC. Into container
Total pumped out = 11cc (hose volume) + 106cc =117cc
300cc ? 117cc = 183cc combustion area.
1478.96 + 183 = 1661.96cc divide by 183cc = 9.0817
Actual compression ratio = 9.1 to 1
Note: Margin of error testing compression ratio on the aircraft as to the cylinder removed and tested on the bench was less than 1.5%
Fyi
Thomas S.
