compression test, what's good?

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compression test, what's good?

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compression test, what's good?

Postby UpstateSC » December 6th, 2017, 2:31 pm

Working on figuring out what need done on a engine.

Tested the compression. - Engine at the low edge of normal operation temp, 125 across all 4. Good? not good?
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Re: compression test, what's good?

Postby rickf » December 6th, 2017, 3:57 pm

Extremely good, almost too good. That kind of tells me there might be carbon on the pistons or more likely you did not have the throttle blocked open during the compression test.
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Re: compression test, what's good?

Postby UpstateSC » December 7th, 2017, 9:04 pm

oops check it again with the throttle open
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Re: compression test, what's good?

Postby njjeeper » December 8th, 2017, 9:33 am

Doesn't having the throttle closed result in lower numbers?
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Re: compression test, what's good?

Postby UpstateSC » December 8th, 2017, 1:10 pm

cold, throttle open.

now 112-115
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Re: compression test, what's good?

Postby Surveyor » December 8th, 2017, 2:28 pm

I started off with way worse and a couple cylinders pitted. Tore it down based on Rick's and others advice. Honed the cylinders (still minor pitting visible on 2), lapped the valves, set the valve clearances, new rings, etc.. Put back together. With throttle open/cold they ranged from 125 to 113. Runs great so far but idling fast and next chance I get I'll work on that (linkage needs adjusting). Looking at past posts, Rick stated TM shows 135 to 145 psi as spec. We are slightly lower so I suspect slight burning of oil and slightly lower power and fuel efficiency than one to spec.
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Re: compression test, what's good?

Postby rickf » December 8th, 2017, 5:19 pm

UpstateSC wrote:cold, throttle open.

now 112-115

Nothing wrong with those numbers. Surveyor, Are you sure it was me that said that about those numbers? I doubt I would ever say that a 7:1 compression engine would normally have numbers that high. I will look through the manual tonight and see what it says. Even if it says that I don't agree. Only with oil in the cylinders and brand new rings and brand new valves and valve job and even then I would question it.
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1984 M1008
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AN/TSQ-114A Trailblazer- Gone
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Re: compression test, what's good?

Postby rickf » December 8th, 2017, 5:54 pm

Well, I looked it up and they do say 135-145 at cranking speed. I would have to see that to believe it! First off is the normally slow cranking speed of these and second is the low compression ratio. Like I said, under idea circumstances maybe..............................
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Re: compression test, what's good?

Postby Surveyor » December 8th, 2017, 7:07 pm

rickf wrote:
UpstateSC wrote:cold, throttle open.

now 112-115

Nothing wrong with those numbers. Surveyor, Are you sure it was me that said that about those numbers? ....


Went back and looked. Looks like I was the one that brought up TM first in thread as we discussed what would and would not work. There was also the Army Ordnance Manual that stated anything 85 and above was good.... found it -

http://armyordnance.tpub.com/OD1001/Com ... ing-92.htm
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