Engine Low Compression

hotstang

Founding Member
May 18, 1999
602
0
17
Tennessee
I just put some comp cams xe268H's in my 02 GT and after all 8 cylinders read 120 across the board. A couple of months ago I ran a test and all 8 were at 190. Can a larger duration cam lower my compression reading that much?

 
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Thank you for the reply.

I understand but 70 psi I guess I thought that was too much. It has a nice lope but a rough idle shakes the car. At about 1500 rpm it bucks like a bull. I don't have a tune for the cams yet but am hoping and was thinking I may somehow be a tooth off on both cams.
 
Aren't those NPI cams?

Anyway, unless I'm mistaken the intake valve opens 5 degrees BTDC and closes 45 degrees ABDC. At cranking speeds you're effectively pushing a good deal of charge back out the intake port. This is one reason your compression readings are so low. It's also, FWIW, the reason why cams generate lumpy idles. It's not a sign of power per se but rather that the engine is basically struggling to remain running at low RPM because it's efficiency is so crappy with race-oriented cams.
 
Thanks both of you!

Yes they are NPI cams but the only difference is they have a bit lower lift than a stock pi cam but much greater duration.

I know I am a fool but I did not degree them or even change the chains and guides (just did head with chains about 4k ago). I did it the cheater way and left the front cover on. But I did change my springs at least so I won't have to worry about valve float.

I have an AM tun with tunes for life but their fine print says they will not to for cams. Am I wrong will they?
 
Only reason I put in the NPI cams is they were free. I got those brand new and a set of patriot stage 2 heads brand new for free with some trading on Craig's list. Know anyone that wants a set of patriots for a good price :)