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Relative compression test & Cam timing [message #334152] Thu, 28 June 2018 23:15 Go to previous message
GMC2000   United States
Messages: 193
Registered: March 2018
Location: Georgia
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Senior Member
cam chains on the Olds 455 seem to have a pretty bad reputation as well as the plastic gear. many GMC folks suggest replacing them at 75 - 80,000 miles.

most of the GMCs Ive looked at have more miles and some the engines do not run at all.

Ive been pricing out common replacement parts and whats involved in the replacement of them. ran across a tidbit about replacing the timing chain with a cloyes racing t-chain set that has a multi slotted lower gear so you can advance or retard the camshaft upon replacement.

someone somewhere (lol) mentioned that someone else (Dick Paterson) said (all hearsay lol) that he recommends using the slot for four degrees advancement on the 455 in the GMC. its actually widely recommended just about everywhere for V8s with stock camshafts but not aftermarket because the 4 deg advance is built into them. many say 2 of those degrees is nullified almost right off the bat upon t-chain break-in.

but generally, advancing the camshaft moves the torque down lower in the rev range which seems like a good idea for a GMC but then I thought is it really a good idea because of the heat generated by the 455 and a 12,000lb load?

so Ive been searching for info about the heat effects of altering the camshaft timing. I havnt found any but it seems to me it would have to? at least the combustion temps anyway? but no luck, nothing about it that I can find but I did find some other very interesting things..

the first one is that you can experiment with the v8 cam timing (if you have an adjustable gear or gear pin) by doing several compression tests starting with no advance, then use the first advance setting, do another compression test, then the first retard setting and a compression test and then depending on which setting had the highest readings (only have to test one and the same cylinder BTW)

go to the next adjustment in that direction and retest and so on until the reading start to drop again. then go back to the cam timing adjustment that gave you the highest compression reading.

I thought about this because Ive never heard about doing this. it seems to me it would be great for RVs and low rev engines but for high rev engines and camshafts with lots of overlap, I dont think it would be a good idea.

anyone here tried this? seems like a good idea if you dont have a dynomometer handy.

also while seeking out info in this area, I discovered the new schools "relative compression testing" I think some of y'all are gonna like this:

https://www.youtube.com/watch?v=BEklqMPkAbQ

a way to test compression (and other things) without removing sparkpugs! check it out.

still havent found any insights into how, if at all, cam timing on a V8 effects combustion temps either way, anyone?

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