# 1969 GTO detonation



## jimmyt (Apr 17, 2021)

Purchased this car early 2021. Do not know any history on the engine, supposed to be matching numbers. Changed the oil right away and put a magnetic drain plug in it. After less that 10 hours use driving to local shows the oil was dirty but not using any. Then detonation started under medium to heavy acceleration. Made sure the timing, dwell and plugs were to specs. Changed the oil again, the plug had attracted some metal residue. Spark plugs all normal and consistent in appearance but not sure if the Autolite #86 are the correct heat range. A/C R45S on order. Went through all vacuum lines to check for proper connections or leaks and proper advance etc. Always run 93 octane and don't believe I have bad gas. Now to the question......could the detonation be caused by cylinders 5 & 7 having low compression? 50 / 60 dry and 100 / 70 wet. All other cylinders are 145 to 155 dry. Thanks


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## PontiacJim (Dec 29, 2012)

jimmyt said:


> Purchased this car early 2021. Do not know any history on the engine, supposed to be matching numbers. Changed the oil right away and put a magnetic drain plug in it. After less that 10 hours use driving to local shows the oil was dirty but not using any. Then detonation started under medium to heavy acceleration. Made sure the timing, dwell and plugs were to specs. Changed the oil again, the plug had attracted some metal residue. Spark plugs all normal and consistent in appearance but not sure if the Autolite #86 are the correct heat range. A/C R45S on order. Went through all vacuum lines to check for proper connections or leaks and proper advance etc. Always run 93 octane and don't believe I have bad gas. Now to the question......could the detonation be caused by cylinders 5 & 7 having low compression? 50 / 60 dry and 100 / 70 wet. All other cylinders are 145 to 155 dry. Thanks


Compression numbers are not good - you have a problem. Being both cylinders,it could be a head gasket blown out between the 2 cylinders. 

I would check valve adjustment first to make sure you simply don't have a valve being kept open and lowering cylinder pressure or a worn/rounded cam lobe - which may be where the metal is coming from.

Detonation can also beat the bearing and damage them as well, so metal could be bearing material?


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