understanding compressoin readings

[rwarre]

Recently checked the compression on my Lycoming O320 engine. Two of the cylinders read 82/80. Does that sound accurate or do I need to adjust the compression tested?

[Walt]

Compression tests are normally accomplished at 80 psi and usually shown as "78/80" for example. In your case 80/82 is very good and if adjusted to 80 would likely be 78/80.

[PCHunt]

Ummmm, he said: "82/80". The only way that could happen is if the gages aren't properly calibrated.

With the compression tester outlet blocked off completely, both gages should read the same number. If not, you can make a "correction" table.

When testing the leakage on a cylinder, after opening the air line, you should adjust the regulator so that the reference gage reads exactly 80 while the air is flowing through the tester.

[aerhed]

Most testers now have plastic gages. As stated, when disconnected with air on, both gages should read equal. If not, you can usually just "bend" the gage a little and the pointer will move. Pretty cheesy way to calibrate a gage, but it works.