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Wisconsin Engines

Differences in Engine Compression Readings


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  #1  
Old 08-18-2011, 04:29:15 PM
Kane Kane is offline
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Default Differences in Engine Compression Readings

Hey there all,

I'm looking for a couple opinions on whether I should do the valves or not on my Wisconsin THD. When i bought the engine, I had compression readings of 90psi in #1 cylinder and 75psi in #2 cylinder with an OLD head gasket. I've since replaced the head gasket, and now I'm consistently getting 110psi in #1 cylinder and 90-95psi in #2 cylinder.

At any rate, the fellow at the small engine shop is ushering me to do the valves because of the differences in the compression readings. Should I be concerned about the differences in psi? I was planning on rebuilding the entire engine in the near future, but at this time, I'd just like to fine tune it. The engine is mounted and far too difficult to get to the engine repair shop just to do the valves. So, any opinions if I should grind the valves? Should I just lap them or something?

Thanks so much for any thoughts! Cheers
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Old 08-18-2011, 05:18:32 PM
xplor xplor is offline
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Default Re: Differences in Engine Compression Readings

Variation between cylinders of 20% is normal on a compression test. If you feel you have a problem then you need to do other tests like a cylinder leak down or running compression test.
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Old 08-18-2011, 05:30:38 PM
Dave L M Dave L M is offline
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Default Re: Differences in Engine Compression Readings

Kane I dont think the differance is enough to worry about I would just run it like it is
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Old 08-18-2011, 05:55:23 PM
oldschoolwisconsin oldschoolwisconsin is offline
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Default Re: Differences in Engine Compression Readings

I wouldnt worry about it too much. 90-95 psi is good for a gasoline engine. It most likely is valves if you wanted equal compression in both cylinders, but honestly, i wouldnt mess with it unless you really start to loose power
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Old 08-18-2011, 06:21:03 PM
Kane Kane is offline
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Default Re: Differences in Engine Compression Readings

Fair enough! Thanks for the thoughts fellows. I was gonna squirt a little 10w30 on top of the lower-psi cylinder to rule out rings on that piston. But otherwise, I'll probably just grind the valves when I disassemble the engine. Cheers for the thoughts.

On a side note, what are my options for dealing with a stripped cylinder head bolt? Its the thread-tap in the cylinder block that is stripped, not the bolt itself. Is there any recourse for me?
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Old 08-18-2011, 06:25:14 PM
aircoolengine aircoolengine is offline
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Default Re: Differences in Engine Compression Readings

Heli-coil it Kane.
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Old 08-18-2011, 07:41:20 PM
Kane Kane is offline
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Default Re: Differences in Engine Compression Readings

Alright, so I poured a tablespoon of oil onto the questionable cylinder head and got a consistent compression reading of 100psi. Does that improvement in compression signal worn rings on that piston? This is for curiosity's sake when I do a complete engine job in the future.
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Old 08-18-2011, 08:26:36 PM
Windables Windables is offline
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Default Re: Differences in Engine Compression Readings

make sure the throttle is wide open when you do the test just sayin
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Old 08-18-2011, 10:14:49 PM
Kane Kane is offline
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Default Re: Differences in Engine Compression Readings

Hummm, I was doing the test with the entire intake/exhaust manifold removed. Are the results I'm seeing not correct then? I'm always learning with engines...
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Old 08-18-2011, 11:11:03 PM
Bill Sherlock Bill Sherlock is offline
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Default Re: Differences in Engine Compression Readings

Having the intake/exhaust manifold removed while doing the compression test should make no difference to results. Having the throttle closed would restrict air flow to cylinder, might make a difference, but throttle should normally be open anyway while cranking the engine over, if governor is set up properly. Choke should be off.

Bill
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Old 08-19-2011, 02:10:50 AM
Old Style Old Style is offline
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Default Re: Differences in Engine Compression Readings

Quote:
Does that improvement in compression signal worn rings on that piston?
If you do the same to the other cyl you will see a improvement to that compression also..... Should be fine as is; especially if you are going to rebuild it later.
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Old 08-19-2011, 12:27:25 PM
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Default Re: Differences in Engine Compression Readings

A difference of 5-10 PSI after adding oil is not significant. Bad rings would be say going from 60-70 PSI to 100 PSI after adding some oil.
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Old 12-15-2011, 04:03:38 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Differences in Engine Compression Readings

Have you had the heads off? Difference in compression could be carbon in the cylinder. Check the bores for wear and scuffing/ vertical lines and ring lands. If OK, deglaze with a ball hone and install new rings. If worn or noticeable ring lands, de-land and bore to next oversize. Check valve guides for wear (valve heads should not move more than about 1/16" in any direction. Lap and clearance the valves, if guides are OK, decarbon heads and blocks. If you do the engine right the first time, you won't have to go back in a second!
Andrew
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Old 12-15-2011, 05:06:53 PM
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OTTO-Sawyer OTTO-Sawyer is offline
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Default Re: Differences in Engine Compression Readings

There's a very good chance you will still see that amount of difference between the cylinders AFTER a rebuild too. Could be the same amount or bigger or smaller difference or even end up with the currently low reading ending up higher than the current high reading cylinder.

WHY ???

Because there are other variables too that haven't been mentioned yet including cam lobe wear, valve stem clearance, lift, duration, and overlap all of which will change by adjusting the clearance and will affect the cranking compression which is different than the calculated compression ratio. Cam shafts can have a slight mismatch in lift, duration, & lobe seperation from the factory too even without the wear factor added to it.

If the valves on one cylinder have .008 - .010 clearance and the other cylinder has .020-.025 clearance it would give different compression readings because of the difference in lift , duration, & overlap.

One cylinder could have better airflow through the intake & exhaust ports than the other one.

As others have already noted, your readings aren't that far off from each other to begin with, and all of the things they've already mentioned will affect it, whether it's worn rings, carbon build up, worn valve guides, etc. They're close enough that it's not worth worrying about unless you're building a hot rod out of it.

---------- Post added at 05:06 PM ---------- Previous post was at 04:57 PM ----------

Rereading your original post, I find it interesting that both cylinders raised 20 pounds (or 15-20 to the lower reading cylinder) with the new head gasket.

I assume you cleaned the head good, removing any carbon build up which would have been raising the compression and thereby should have lowered it a little once removed, so I'm guessing your new head gasket is quite a bit thinner than the original one giving you a higher compression ratio which raised the cranking compression.

Either that, or did you possibly do the original test with the carburator & muffler still on it and then do the second test with the new head gasket with them removed , which could be showing the difference in the breathing ability with and without those restrictions. Obviously, you can't run the engine without them, but it would be interesting to see what the readings are with them attached again if that's how the original testing was done.
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Old 12-15-2011, 05:21:41 PM
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Default Re: Differences in Engine Compression Readings

Manifold on or off is not going to produce a significant difference.

Increase in compression after head gasket job was either a slight leak at gasket, valve settings, or some cleaning you may have done during the job. Also, is new gasket thinner?

Pressure differential between cylinders is pretty nominal, I would not pay too much attention to it.

Just read post by OTTO. He is 100% on target.

Run it....
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Old 02-14-2012, 10:50:19 PM
Kane Kane is offline
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Default Re: Differences in Engine Compression Readings

The gasket that was installed (and now replaced) when I bought the engine was very deteriorated! The new gasket is much thicker, and I cleaned off a lot of "gunk" on the head which I can only describe as "dried liquid sealant" of some sort. The compression readings were both conducted with the manifold installed, but the head, 3 manifold, and carburetor gaskets were all replaced... but I don't know if those will affect compression readings (I'm assuming they did).

I'm currently trying to work out why my carburetor is running so rich at the moment, but this engine is coming together slowly. Thanks for all the help folks (I really appreciate everyone's thoughts)!

Cheers
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Old 02-14-2012, 11:02:34 PM
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Default Re: Differences in Engine Compression Readings

It's a Wisconsin, Buddy! If you have fuel, air, and spark, it'll do what you want it to do. A few lbs difference in compression won't make any difference in performance.

Just for fun, after you've used it a while, run another test and see if anything changes in the compression readings!
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Old 02-15-2012, 01:33:23 AM
coyote62ny coyote62ny is offline
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Default Re: Differences in Engine Compression Readings

we used to have one on a john deere 14t hay baler. 2 cylinder that thing would run any time you needed it. the only problem we ever had with it was the head got warped and we had it planed new head gasket good as new then. ive seen that engine so hot in the summer when running and baling you could see the flames coming out of the exaust ports through the exaust manifold the manifold would run a almost transparent cherry red. you know its hot when you idle the engine 5 minutes shut off the switch and its still runing 5 minutes later. run it . dont sound like you need to rebuild it any time soon remember sae-30 or sae-40 oil no synthictic sh** its got a lot of life left in it
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Old 02-15-2012, 01:36:22 AM
Straight Arrow Straight Arrow is offline
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Default Re: Differences in Engine Compression Readings

IT doesn't sound like you have a problem. On any compression test, be sure all readings are equal.I was taught to disable ignition, block throttle open and turn over same #of compression strokes on each cyl. We have always used 15% variation as a rule of thumb.
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