Engine Conversions Discuss all aspects of engine conversions in this forum

compression testing

Reply
Old 04-16-2003, 06:40 PM
  #1
jwv3
Thread Starter
 
Join Date: Jan 2003
Location: Laurel, DE
Posts: 80
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default compression testing

How do you guys check compression on a stripped down engine. I have a couple used engines with no housing for mounting a pull starter. I'd like to test the compression before going any further, but I have two questions. 1. If I use a starter motor to turn it over, will higher rpm's result in high/misleading numbers? 2. If the engine has not run in some time, the rings may not have the right amount of oil on them to create a seal. I could lube the cylinder walls, but would that also cause a wrong reading?
jwv3 is offline  
Reply With Quote
Old 04-16-2003, 08:50 PM
  #2
w8ye
Moderator
 
w8ye's Avatar
 
Join Date: Dec 2001
Location: Shelby, OH
Posts: 37,535
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default compression testing

Compression reading by itself is only a relative reference mainly because of the factors you mention.

You would have to make as many factors the same as each other and compare too engines (A new one and a old one of the same type) for it to have a concrete comparison.

People that mess with them all the time develop a pretty good feel for an engines worth by putting a little oil in the cylinder and pulling it through compression several times. I try it fast and slow so I can feel the "leak - down" as well as the over all compression. While I'm at it, I feel for the piston - rod fit as well as the main bearings. I don't bother with the compression gauge itself.

Good luck,

Jim
w8ye is offline  
Reply With Quote
Old 04-16-2003, 09:35 PM
  #3
Dr. Wine
 
Join Date: Jun 2002
Location: Luling, LA
Posts: 121
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default compression testing

Ditto.

However, I wouldn't put oil in the cylinder. This would result in a higher reading, unless of course you need oil to turn the engine over, then why would you test it anyway. To see what I mean, try it both ways on the same engine. If there is no change in compression, and your sure the cylinder was dry before hand, then the engine is no good. The engine is worn enough so that the viscosity of the oil will not seal off the wear. Not sure if I worded that correctly. Anyway you get my point.

All non-RC engines are tested without oil, even two cycle outboard engines and motorcycles. If someone puts oil in the cylinder(s), before testing, of something you are going to buy, they are trying to put one over on you.

Perry
Dr. Wine is offline  
Reply With Quote
Old 04-17-2003, 10:46 AM
  #4
jwv3
Thread Starter
 
Join Date: Jan 2003
Location: Laurel, DE
Posts: 80
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default compression testing

Thanks, pretty much what I have thought. Just a point about the oil; I wasn't thinking a heavy oil. That would surely raise the compression reading. I was thinking there is a certain amount of oil in the fuel mix and if an engine is tested after having recently run, it could have more oil in the cylinder than one that has been sitting on a shelf for a few years.
jwv3 is offline  
Reply With Quote
Old 04-17-2003, 12:32 PM
  #5
w8ye
Moderator
 
w8ye's Avatar
 
Join Date: Dec 2001
Location: Shelby, OH
Posts: 37,535
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default compression testing

Oil might be 'OK' if you always use the same kind of oil.

Judging an engines condition by feel and not necessarily by a compression Gage, is sort of a 'practice' that you build up the technique by 'feeling' of every engine you come across. Books and spec sheets cannot give you 'feel' so they have to rely on specifications.

Enjoy,

Jim
w8ye is offline  
Reply With Quote
Old 04-17-2003, 01:59 PM
  #6
captinjohn
 
Join Date: Jan 2002
Location: Hesperia Michigan, MI
Posts: 12,889
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default compression

I would just take spark plug out and pour in a ounce of the fuel mix you are going to run the engine with. Crank it over several times to get mix all through engine parts. Put compression gage in place....crank engine like you would to start it. That should give you pretty accurate reading. Put plug in and check compression by hand propping engine. Remember oil /fuel mix you just used is the same mix you are going to use to start engine. The use of any oil with high viscosity will give false compression readings. Good luck Capt,n
captinjohn is offline  
Reply With Quote
Old 02-12-2013, 10:40 AM
  #7
captinjohn
 
Join Date: Jan 2002
Location: Hesperia Michigan, MI
Posts: 12,889
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: compression

I do not know how this laptop got me here....still a good subject ......
captinjohn is offline  
Reply With Quote
Old 02-12-2013, 11:01 AM
  #8
Propworn
 
Propworn's Avatar
 
Join Date: Jul 2002
Location: Canada
Posts: 1,470
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: compression testing

I presume gas two strokes are the topic of discussion. If you think about it a pull starter with a vigorous pull will flip the motor over quite fast so I wouldn’t worry about using an electric starter. If it has anything over 90 lbs compression it is run able. It will only get better as the engine becomes oiled up from running and the rings reseat themselves. Compression checks should be done with the throttle wide open to allow unrestricted air flow.

More important than compression is crank case sealing. If there are leaks the engine will become hard starting or unrunable. Using snoop or a soapy water solution, roll the engine over slowly dripping the snoop on the crankcase seals and joints. The odd small bubble is nothing to be concerned about but obvious bubbles or the snoop being sucked inside the engine is a definite warning sign. At this point minimum would be seal replacement most likely bearing and seal replacement.

If there is compression and no obvious air leaks from the seals or crankcase joints go for it the engine should run. OOPPS my bad meant 90 lbs above.

Dennis
Propworn is offline  
Reply With Quote
Old 02-12-2013, 11:45 AM
  #9
w8ye
Moderator
 
w8ye's Avatar
 
Join Date: Dec 2001
Location: Shelby, OH
Posts: 37,535
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: compression testing

With chainsaws, you screw in the compression gauge to the spark plug hole, Then pull the chainsaw through compression strokes until the gauge quits rising. This is usually a half dozen pulls.

Most good chain saws will pull 150 pounds. If they get below 120 pounds they are hard to start.
w8ye is offline  
Reply With Quote
Old 02-14-2013, 10:18 AM
  #10
av8tor1977
 
av8tor1977's Avatar
 
Join Date: May 2003
Location: Tucson, AZ
Posts: 7,100
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: compression testing

I often repair used yard machines, and of course, convert engines for airplanes. When I convert them for airplanes I just automatically use a new Frank Bowman ring because I want max power, easy starting, etc., and what's 11 bucks for a new ring in the grand scheme of things??

As for the yard machines, I cheat a little. I shoot some WD-40 into the spark plug hole, and then turn them over several times. Then I check the compression. In my experience, anything below 90 psi is probably not worth fixing. Anything below 60 or 70 lbs. probably won't even start. 100 or over still has life in it. This has proved out over numerous years of repairing and selling the machines.

The compression reading will vary according to the original compression ratio of the engine, the exhaust port timing, the amount of carbon build up on the piston and head, and of course ring wear. I figure that my WD-40 treatment leaves more or less the same amount of oil to help seal the rings as a normally just run engine would have for the compression test. The injection of premix (oil/gas) and then turning it over numerous times before taking the reading has merit as well.

As mentioned, be sure to block the carburetor wide open when checking the compression.

AV8TOR
av8tor1977 is offline  
Reply With Quote
Old 02-16-2013, 06:13 AM
  #11
1QwkSport2.5r
 
1QwkSport2.5r's Avatar
 
Join Date: Oct 2006
Location: Cottage Grove, MN
Posts: 9,080
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: compression testing

Other than what's been said here, the only thing I do differently to distinguish from an engine in decent shape versus trash heap is I pull the head off and turn the engine over swiftly. I stick my nose over the cylinder and note the 'pop' and air (fuel charge) flow from the transfer ports. If you get a good pop and can feel a little wisp of air coming from the transfers, then it should run at the very least 'O.K'. If it doesn't pop good and the air flow is minimal, it's at the very least in need of a new ring(s) and light honing.

1QwkSport2.5r is online now  
Reply With Quote
Reply


Thread Tools
Search this Thread
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -8. The time now is 05:37 AM.