View single post by Esprit2
 Posted: 12-13-2017 09:48 pm
PM Quote Reply Full Topic
Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 311
Status: 
Offline
Cylinder pressures vary with compression ratio, not stroke length/ displacement.

It's normal practice to check cylinder pressures on a hot engine, with the plugs removed, the throttle held wide open, and the engine cranking over at 200 rpm or better. Lotus' pressure specs are based on such a practice; however, Jensen-Healey specified checking pressures on a cold engine. So, on a low compression 907, we have the luxury of two compression pressure specs, one cold and one hot. Hot is the most indicative of the engine's operating condition, but cold may be all you can get if the engine isn't running.

Compression Pressure Specs:
907LC .... 8.4:1 .... 110-130 psi COLD = Jensen-Healey spec
907LC .... 8.4:1 .... 150-170 psi HOT.. = Lotus Spec
907HC .... 9.5:1 .... 165-185 psi HOT
912LC .... 9.44:1 ... 165-185 psi HOT
912HC .. 10.9:1 ..... 170-190 psi HOT

Regardless of the pressure, all cylinders should be within 10 psi of one another.

The modern (since Jan 1993) composite head gasket's compressed thickness is half a millimeter (0.020") thicker than that of the old steel-asbestos-steel gasket. With all else being the same, the result is about a half point drop in compression ratio... ie, 8.4:1 becomes 7.9:1.

Regards,
Tim Engel

Last edited on 12-14-2017 12:29 am by Esprit2