TurboSlacker
SINISTR
- Joined
- May 29, 2001
- Messages
- 1,139
We decided to do a compression and leakdown check on three 1987 Grand Nationals at the same time. The results are interesting and maybe insightful for others to see.
Typical numbers of a healthy engine is compression 150-160 with all cylinders within 10% of each other. Leakdown less than 10% leakage for each cylinder.
Josh's 20k mile stock 87GN
Compression on all cylinders was near 156. Leakdown was 4% or less with one cylinder at 90%. Excellent results that you hope for with a nice low mileage stock engine. Josh's numbers were slightly better than the higher mileage cars below.
Dan's 50k mile mildly modded 87GN
Compression range 150-154. Leakdown was 5% or less in all cylinders except #6. The #6 had 13% leakage that reduced to 9% after squirting oil into the combustion chamber to better seal the rings. Very noticable leakage could be heard out of the exhaust pipes which indicates an issue with the #6 exhaust valve. No leakage could be heard thru the throttle body (intake valve check) nor excess leakage thru the valve cover breathers (normal to hear a little).
Frank's 50k mile decently modded 87GN
Nearly identical to Dan's results. Compression 150-155. Leakdown was 5% or less in all cylinders except #3. The #3 had 24% leakage that could be heard thru the exhaust pipes.
We found the results interesting and it was really cool to do back-to-back testing of 3 GN's together.
Luckily none of results pointed to head gasket concerns. I was pleased to learn that my GN's #6 leakage was not too excessive and actually a little better than similar leakdown results done four years ago. This excourages me not to rip off the heads to fix any thing at this point. My stock headgaskets will remain intact!
The question now is...is there anything that can be done to reduce the exhaust valve leakage without removing the heads ? Any seals that can be replaced or internal additives or removing the valve spring to gain access to anything ?
Typical numbers of a healthy engine is compression 150-160 with all cylinders within 10% of each other. Leakdown less than 10% leakage for each cylinder.
Josh's 20k mile stock 87GN
Compression on all cylinders was near 156. Leakdown was 4% or less with one cylinder at 90%. Excellent results that you hope for with a nice low mileage stock engine. Josh's numbers were slightly better than the higher mileage cars below.
Dan's 50k mile mildly modded 87GN
Compression range 150-154. Leakdown was 5% or less in all cylinders except #6. The #6 had 13% leakage that reduced to 9% after squirting oil into the combustion chamber to better seal the rings. Very noticable leakage could be heard out of the exhaust pipes which indicates an issue with the #6 exhaust valve. No leakage could be heard thru the throttle body (intake valve check) nor excess leakage thru the valve cover breathers (normal to hear a little).
Frank's 50k mile decently modded 87GN
Nearly identical to Dan's results. Compression 150-155. Leakdown was 5% or less in all cylinders except #3. The #3 had 24% leakage that could be heard thru the exhaust pipes.
We found the results interesting and it was really cool to do back-to-back testing of 3 GN's together.
Luckily none of results pointed to head gasket concerns. I was pleased to learn that my GN's #6 leakage was not too excessive and actually a little better than similar leakdown results done four years ago. This excourages me not to rip off the heads to fix any thing at this point. My stock headgaskets will remain intact!
The question now is...is there anything that can be done to reduce the exhaust valve leakage without removing the heads ? Any seals that can be replaced or internal additives or removing the valve spring to gain access to anything ?
Did you fire your car up and go around the block getting on it hard and retest #6 to see if it improved?Carbon on the valve seats can do this and a good beating helps alot of times 
