sharrar wrote:Thanks Alan, I see some people attach a battery charger to stop the battery go flat - would you recommend?
Sharrar,
The compression test doesn't take long, so a strong battery with a full charge should have no problem out-lasting the test's duration. If the battery is weak, then use jumper cables to hook up the battery of a running car to bolster the test subject's battery.
Perform the test with...
1) The engine hot, at full operating temperature. A cold test will result in low readings.
2) Hold the throttle
WIDE OPEN while cranking the engine over.
3) Crank the engine at 200 rpm or better. That requires a strong battery with a full charge. If the car's battery isn't up to the task, then use jumper cables as mentioned above. No, you don't have to monitor the tach (can't, the coil is disconnected). The engine is clearly cranking faster than normal with the spark plugs removed. If it's not, then get out the jumper cables and give it some help.
The test results should be...
907LC (8.4:1 cr) = 10.2 - 11.6 bar (150 - 170 psi)
907HC (9.5:1 cr) = 11.2 - 12.6 bar (165 - 185 psi)
912LC (9.44:1 cr) = 11.2 - 12.6 bar (165 - 185 psi)
912HC (10.9:1 cr) = 11.5 - 13.0 bar (170 - 190 psi)
910LC (7.5:1 cr) = 8.8 bar (130 psi min)
910HC (8.0:1 cr) = 9.5 bar (140 psi min)
910LC/ HC (8.5:1 cr) = 10.5 - 11.8 bar (152 - 172 psi) with JE Forged 8.5:1 Pistons.
For the 907LC, Jensen-Healey recommended performing the compression test on a cold engine. In that case, the results should be...
907LC (8.4:1 cr) = 7.58 - 8.96 bar (110 - 130 psi)
COLD.
Only Jensen-Healey specified a cold test, and it's best to perform the test hot if you have the choice. But if the engine won't start and your forced into a cold compression test, then 907LC owners at least have an official results spec to reference. I don't have 'factory' cold compression specs for the Lotus engines.
Note: The current Goetze composite head gasket was introduced in Jan 1993. It's compressed thickness is 0.5mm (0.020") thicker than the old/ original gasket. That raises the cylinder head higher off the block deck by that amount, increasing the combustion chamber volume, and lowering the compression ratio by about half a point (9.5:1 becomes 9.0:1).
For all engines built prior to Jan 1993, but now assembled with a Goetze composite gasket, the resulting compression is down from original spec by half a point.
For engines built from Jan 1993 onward, Lotus machined the block deck lower by 0.5mm (0.020") to compensate for the thicker gasket. For those engines, the compression ratio remains per original factory spec with the new Goetze composite gasket.
If you're doing a comprehensive engine rebuild, including some machine work, you could also have the block decked 0.5mm (0.020"), and liners set deeper into the block by the same amount (ie, preserve the original 'nip' spec).
Anything that lowers the head (reduced center distance between the cams and crank) will slightly retard the cam timing. That includes milling a warped head to flatten the bottom, or decking the block.
Anything that raises the head (thicker head gasket) will slightly advance the cam timing.
With stock pulleys, there's nothing you can do about it. But if your cam pulleys used to have perfect dot alignment before any of the above changes, you'll notice a slight timing dot mis-alignment afterwards. It's small, live with it.
Regards,
Tim Engel
, hopefully it's relatively easy
