How my compression? - JeepForum.com
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post #1 of 3 Old 04-07-2020, 03:31 PM Thread Starter
Maverickxeo
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How my compression?

On my 94 5.2L engine, with around 380k KM on it, the compression is ~130PSI. The lowest is 128 and the highest is 135. From what I gather, both the spread and the actual numbers show in spec, but what does this actually mean? I understand it is 'good' but the lower than max (150PSI) indicating wear? If so, can I assume that being in range of 120-150PSI, that my compression is on the lower end and my engine has about 1/3 of it's life to go - or is it not how that works? If I were to, say, add a power adder (supercharger, etc), would that put the engine at risk of failure or is it still okay due to being within the factory spec (not that I would on this engine, but in general)?

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post #2 of 3 Old 04-07-2020, 04:36 PM
Rallyrs
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I'm curious what the numbers are on mine too.

I have a 98 with a 5.2.

The 7 psi spread seems pretty good though. Hopefully someone will chime in to tell us if that is typical psi for an engine with your miles.

I only remember ever checking compression on 2 of my cars.

My Focus with 2.3 liter I-4. I don't remember the exact readings but I don't think they were hugely far off from yours.

And on my Friends 1970 (Torino)Ranchero with the regular old 429 4v (Thunder Jet)360 hp engine.
Engine had been rebuilt-stockish with cast pistons,but even stock 4 barrel model pistons gave 10.5 or 11.0 to 1 compression ratio in that year engine.

Plus the heads had been surfaced at least once, maybe more, and a hipo cam was in it.

Owners manual said don't run less than 94 octane in it,which was laughable because this was late 70's or early 80's and California had only 91 octane garbage at this point.

Car was pinging like mad so during a tune up we did compression test.

Averaging 210 psi per cylinder and we were like WTF?!

No wonder it was pinging.

We ended up trying h2o injection and that was the ticket.

When the Ranchero got totalled the engine went in my F100 and I used octane booster instead of the water.
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post #3 of 3 Old 04-08-2020, 12:03 AM
Tomcorr
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Quote:
Originally Posted by Maverickxeo View Post
On my 94 5.2L engine, with around 380k KM on it, the compression is ~130PSI. The lowest is 128 and the highest is 135. From what I gather, both the spread and the actual numbers show in spec, but what does this actually mean? I understand it is 'good' but the lower than max (150PSI) indicating wear? If so, can I assume that being in range of 120-150PSI, that my compression is on the lower end and my engine has about 1/3 of it's life to go - or is it not how that works? If I were to, say, add a power adder (supercharger, etc), would that put the engine at risk of failure or is it still okay due to being within the factory spec (not that I would on this engine, but in general)?
To me it means an engine with that kind of mileage on it, leave well enough alone and drive it as is and be happy it runs good.

If you want to beef it up, then rebuild it so it is race ready and runs a 3.5 quarter mile.
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