[srickss]

Did a compression test on my 68 GTO 400 and found some disturbing numbers. I rebuilt the motor myself last winter and have noticed that it seems to shake more than usual. The motor is by no means radical or anything. it has the xe268 comp cam with compression right at 10:1, other than that it is pretty much stock. The compression numbers are as follows;

#1-200psi
#3-185psi
#5-145psi
#7-110psi
#2-190psi
#4-170psi
#6-190psi
#8-180psi

I have read that they should all be about 10% of each other or so. This is probably going to be another tear down, my question is whether any of you out there have any suggestions before I procede. # 5 and 7 are obviously the two lowest numbers and with them being next to each other could a head gasket be a place to look? Any help or ideas would be greatly appreciated. By the way I did the test with the motor cold and with all of the plugs out as well as a battery charger on. Thanks.

[Matt Meaney]

you probably know already, but for best results, the engine should be near normal opperating temp and the throttle wide open.

the next step, if possible, is to do a leak down test. it'll show you where your loosing compression, before you take it apart.

when my 70 400 was running poorly, i did a compression test. the low cylinders ended up being a rounded off cam lobe (didn't make noise ???) and a sunk exhaust valve.

from what i've read, 175psi seems to be the max allowable cranking pressure for pump premium. yours being higher, maybe some piston damage from detonation. the xe series of cams are known for building cylinder pressure and are mostly recomended for low compression engines.

you might try posting this in the street section for a greater exposure to the engine builders that frequent.

best of luck, hope it's not too bad.