Should I be able...

[lyeinyoureye]

to turn an engine w/ good compression via the passenger side wheel in first gear by hand w/ both wheels off the ground?

[hagar]

try turning each wheel in NEUTRAL ---then let us know what you find.

hagar.

[lyeinyoureye]

It's relatively easy in neutral, and requires a little more force, but still turnable in first. Could I just slap a torque wrench on the axle nut, and figure the approximate compression by recording the difference in torque required to turn it in different gears/neutral?

[tawney]

Took a break from working on my caddy to check the forum and was intrigued by your idea about checking compression by torque on the wheel: assuming you've got one wheel in the air, the other on the ground, right? I've got less than 200 miles on a re-build; haven't checked compression yet with a gauge, but in neutral it turns so easily it doesn't register on the torque wrench. In first and second gears it exceeds the 150 ft.lb. limit of the wrench; in third gear it's about 140 ft.lbs.; in fourth it's about 120; in fifth it's about 90. That's with the FF transmission, and a new clutch.

[lyeinyoureye]

I was thinking about both wheels in the air, however it seems like leaving one on the ground is good for measuring compression in 3rd-OD like you said, since those may be below the torque wrenches minimum setting. Or the wrench may have a range where it's the most accurate as well, probably not at the extremes. Since we're really just after the difference in compression,

If you look at the gear ratios for your FF, it seems like the difference in your measurements for 4th and 5th corresponds to the difference in gear ratios. Otoh, 3rd seems to be way less than what I would expect compared to 4th given how short it is, but this may be a byproduct of the extra torque from the other tire on the ground. I just tried it, w/ only two gears because it's hot as hell, and got results for 4th and third similar to your results. But, at leas I could feel the compression stroke, so I know it's not totally dead.

I think that in order to get a useful measurement, we need a consistent amount of peak torque opposing the torque wrench, which seems hard to do w/ a tire down because of the difference in tires/etc, or even with both up because the force required to turn the wheel seems relatively small.

[libbybapa]

If you have both wheels off the ground, the one wheel will spin one direction and the other wheel will spin the other way, while the engine does nothing.

Andrew

[A1-2-A3]

Now you want to check your compression this way? Its not a good idea at all to be accurate. Look at the variables, tranny oil temp, oil condition and kind of oil, gear ratio of your box compared to the many ratios they have, clutch condition, oil contamination on the clutch, the list goes on and on... Turning the engine over with a torque wrench on wheel nuts is for torquing wheel nuts period. I do not see any sense in this.. Thats my two sense.
I would remove injector, insert compression guage and turn engine over.

[A1-2-A3]

You could buy a electronic torque wrench like my snap on 500 dollar 1/2 inch one. It tells you digitally how much your turning in nm, in-lbs and ft-lbs. Also does torque angle to the degree. Your old clicker types would take days to figure out how much you are turning. Also I still think its not the greatest idea, it will not give you a accurate conclusion to compression at all.

[tawney]

Who's going for accuracy here? This is, (possibly,) a substitute method. Not everyone has a diesel compression tester, and even if you do it takes a bit of time and new heat shields. Someone asking why his engine won't start could be told how to at least get an idea if it is possibly due to low compression. What if you're looking at buying an old Rabbit or whatever: it could be a simple method to check the ballpark compression on each cylinder before you buy. No it's not going to be accurate, but it could provide some easy information at a time when pulling all the injectors and replacing heat shields just isn't practical. Even if there's no correlation at all to real compression, if three of the cylinders are at 90 ft.lbs. in fifth gear and one is at 50, you would have some meaningful information; well, at least maybe: by comparing notes here on the forum we could at least put it to the test.

[lyeinyoureye]

If you have both wheels off the ground, the one wheel will spin one direction and the other wheel will spin the other way, while the engine does nothing.

Andrew

Jeje. So the R&P is a ratchet in that respect? In any event, I'm looking for some way to gauge compression, even if it's just a loose comparison. I've seen that a good engine should start w/o plugs down to ~40F (based on this my compression is very low) and have asked about this in the past, relating cold start up temps to compression, but I haven't received any info so I'm looking for some other advice.

[Fatmobile]

Great topic.
I put cars in high gear and push them backwards when I'm setting the injection timing, instead of turning the crank bolt backwards.
There is a difference between pushing a fresh engine backwards and a something with compression around 340... or around 300.
My hillbilly method of checking compression is to turn the crank bolt, if it springs back near the top of the compression stroke it's around 400psi....
Turning the crankbolt can tell you alot about the compression and where air is going.... but I have never even though of using a torque wrench on the axle nut to get a rough numerical value.

[Vincent Waldon]

So what about a torque wrench on the crankshaft bolt.... eliminates all the vagarities of what gear, what ratio, slop it the diff, etc etc etc ??

I own a compression tester, but for getting a quick take on a prospective purchase in a parking lot somewhere I can see a use, if it's at all accurate.


Vince

[A1-2-A3]

Putting a torque wrench on the crank would tell you something. I thought there was a guy on here that put it in a certian gear and push it and said if you can't move the car then its fine if you can then the engine is worn.

I have my torque wrench at home today and I will let you guys know how much force it takes to turn a engine over past the compression stroke on a cylinder on a new 1.6 non-turbo engine at the crankbolt.

[hagar]

Hillbilly Compression test ---goes like this .

hagar did this since day one ---work's great. I lift one wheel off ground and turn by hand ---in gear.

If engine turn HARD-HARD-PLOP-HARD I know that one cylinder need's TLC.

Then I use a leakdown test on that cylinder --I do not worry about absolute compression.

hagar.

[tawney]

I like the torque wrench on the crank bolt idea; eliminates lots of variables plus you don't even have to lift a wheel. Maybe a couple of us should take torque measurements for each cylinder, then do a real compression test with gauge and see what kind of correlations we get. I'm wanting to do a compression test on my engine anyway, so when I do it will be simple to compare to the measurements of the torque wrench on the crank bolt. Next weekend maybe.