Compression question / Pinging

[sccabrian]

Last night, my friend and I pressure tested our Z cars. Both are '72s and both have L28 engines. The only difference is that he has flat top pistons and an N42 head, and I have dished pistons with the N47. From front to back cyl, his pressures were all exactly 175psi (12:1 more or less) and mine were 130, 130, 135, 140, 125, 130.

The problem is that when we put the flat tops into his car, it would ping no matter what octane gas was used. The timing was backed down to 7 deg (stock?) and now he must use 93 octane, or it still pings. Before the pistons, my 240 was actually faster, but with the pistons, we both run a low 15 second quarter.

So my questions are, shouldn't there have been more of a performance gain with the flat tops? Why would it ping when flat tops came stock in some L28? (I've heard it's the shape of the combustion chamber in the head?) And what can we do to compensate so that he gets added performance, but no pinging? It is already running incredibly rich. So far, we have considered an MSD to get a better burn, maybe different carbs (more air?), or backing the timing down even more. I don't think the timing is a good idea. Also, is the 125-140psi range on my cyl normal?

Thanks for any help, Brian S

[Ed]

Why would it ping when flat tops came stock in some L28? (I've heard it's the shape of the combustion chamber in the head?) Thanks for any help, Brian S

The flat tops came stock in the later L28's when they had different heads like the N79. Which had larger combustion chambers than the N42. The N42 head was designed for the earlier L28 with dished pistions. Performance gains with just higher compression would be minimal, you have to complete the package. Intake, Ignition, Exhaust. I would use the MSD. And upgrade the cam. Another option your friend has is to reduce the compression by using a thicker head gasket.

[zguitar71]

His compression should be closer to 10:1 if the N42 head is stock and the pistons are stock flattops.