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-   -   High altitude compression figures? [80-series] (http://www.risingsun4x4club.org/forum2/showthread.php?t=21281)

Zero1 05-13-2014 06:37 PM

High altitude compression figures? [80-series]
 
Knowing full well I won't be close to FSM spec with over 200k on the odo, but I also know a wide variance exists based on ambient air temperature and barometric pressure relative to what the FSM specs tell us at . What range of #s do y'all see that you deem urgent? Low but bearable? Decent? (taking into consideration the FSM #s of 171+ on a perfect engine [tested under perfect conditions] to a minimum tolerance of 128 (w/ a max cyl-to-cyl variance of 14psi). As I recall the FSM testing procedure is an engine up to operating temperature with [at sea level] with the throttle open, correct? I'm going to be testing on a cold engine and want to not panic with expected lower-than-typical numbers.

Thank guys![& gals]

Squishy! 05-13-2014 07:35 PM

If the engine has sat for a while it can throw off your numbers due to the rings losing oil therefore compression. The oil helps the rings seal. I HIGHLY recommend a warm engine. That said, a drop of oil down the tubes can help the rings seal, but you may get funky numbers. I've found 150-160 psi to be a decent range. Remember you don't want too much variance cyl to cyl. I think it's like 5% or 10%. I can't remember which. Hope that helps. I'm sure a cascade of info is gonna follow.

Zero1 05-13-2014 07:59 PM

Quote:

Originally Posted by Squishy! (Post 250551)
If the engine has sat for a while it can throw off your numbers due to the rings losing oil therefore compression. The oil helps the rings seal. I HIGHLY recommend a warm engine. That said, a drop of oil down the tubes can help the rings seal, but you may get funky numbers. I've found 150-160 psi to be a decent range. Remember you don't want too much variance cyl to cyl. I think it's like 5% or 10%. I can't remember which. Hope that helps. I'm sure a cascade of info is gonna follow.

HG concern is keeping me from warming her up. Oil does help but it can also give false #s... to a degree; some could argue they're simply closer to realistically accurate. Got milkshake out of the oil pan, yet against a test sample of new oil + 50/50 that separated pretty cleanly my milk had at most a skin on top. Still far from bueno. Burned enough coolant off to leave little less and whoever chose where the drain-**** went could receive some harsh feedback from me, since today was my day to do my oil pump cover gasket and front main, and thus everything in that vicinity is smothered in oil leakage and everything collected was well contaminated before it hit the pan. Hardly enough was left to even say how much oil was in the coolant; I've yet to do the block. Feeling stupid, or optimistic I ran compression tests as I had a few minutes left on my lunch. I fully expected no6 (and perhaps 5 with it) to be shot

@ ~45F cold engine, no throttle :: 145, 140, 150, 160, 102, 150 (and I don't fully trust the Cylinder 5 value to even be that high. Damn near thought I wouldn't be able to get the f'n plug out it was so corroded in, PB and all. Feared stripping it, and perhaps may even have a bit, considering it seems no small coincidence that its the odd man out of the #s. Definitely looking to be the culprit.

Obviously I'll know more when I find time to do a proper leakdown, but was just curious since the #s can vary so much by altitude, engine temp, etc. what folks up in this neck of the woods see.

Squishy! 05-13-2014 10:11 PM

There's a lot of variation there excluding #5. 10% is pretty high. If you're losing coolant #5 looks to be the culprit. Steam cleaned threads are very difficult to pull.

Zero1 05-13-2014 11:31 PM

My thoughts indeed and I concur. I'll confirm this week with a leak-down before deciding whether to pull the head and hope the block surface is decked well enough, or look for a short-block, or more diesel-y options. Such crappy timing, but I suppose we never can really name what would constitute "good timing" for an engine to give ;-) But never when getting the truck ready to sell and move on from 4-wheeling.

SteveH 05-14-2014 09:04 AM

I think my '95 FZJ (tested at 7000' el) with 190K on it, showed 145 as about the max, and a few were lower - 135-ish. This engine did burn some oil and IMO was a bit down on power. It tested the same for compression at 75K, so it just wasn't one of Mr. T's tighter engines.

Other than your #5, I think your numbers are pretty good, if not terribly consistent.

Zero1 05-14-2014 11:34 AM

yeah, i'm dreading to go out and run the leakdown on #5 over lunch. as they say, ignorance is bliss :D

Rzeppa 05-14-2014 12:03 PM

As alluded to briefly in the initial, post, elevation (altitude) has a huge effect on your compression numbers. If 171 is considered perfect, then at 5,000 feet the best you could get is 142.

The formula is pretty simple, just take your barometeric pressure at your elevation (lots of online charts, such as this one), divide by the pressure at sea level and multiply by your target reading.

Thus, 12.2 PSIA / 14.7 PSIA * 171 PSIG = 142

Alternatively, to see what your reading would be if you were at sea level, reverse the order of the division, to see what that #5 would read at sea level:

14.7 PSIA/ 12.2 PSIA * 102 PSIG = 123

Ambient temperature has some effect, but not as much as some people think because you use Kelvins to do the computation, so for example the difference between freezing and room temp would be:

273 / 298 = 92% - In other words the difference between freezing and 77F is only 8%

As a practical matter, compression gauges are only accurate to around 5% of reading if you're lucky. To measure pressure much better than that you're talking some serious coin.

Zero1 05-14-2014 12:14 PM

But you've also got to take into some consideration an engine at operating temp (~ 355K) vs. 280K (45F). I once was helping on an engine with a cracked head that when cool didn't leak but when warmed up the aluminum expanded enough. Was a bitch to diagnose, and not an everyday scenario.

I think its most of the cyl vs. cyl difference that's most concerning at the moment, plus oil that would've made chocolate milk look dark. I'd rather have oil in my coolant than coolant in my oil before its caught and the engine turned off.

About to head out and run a proper leakdown (ha, I can't even say that without chuckling since I'm stuck with a HF unit at the moment). Will report what I find!

Rzeppa 05-14-2014 08:51 PM

Quote:

Originally Posted by Zero1 (Post 250583)
But you've also got to take into some consideration an engine at operating temp (~ 355K) vs. 280K (45F).

No, not really. Compression tests are not done at engine operating temperatures, they are always done at ambient temperatures, in order for the results to be meaningful for comparison to specifications. Diagnosing a fault is a completely different scenario and as such testing while at operating temperatures is perfectly acceptable. Most of us do compression tests to see whether we're due for a rebuild or how well our rebuild went! Also, the old "put some oil in each cylinder" test is to see if our low compression is due to worn rings or a valve not sealing well.

Totally agreed about the expansion causing different conditions in compression, including exposing a fault that might not be observable at ambient temperature.


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