Variations in compression checkers affect readings

[fearofpavement]

This all started with a saw I have for sale. I posted up a compression reading of 170 psi and nmurph said that was too high. I have confidence in my high quality USA made gauge so I decided to re-check now that the saw had cooled and the shop had warmed to room temperature. So I did another test and got the same reading of 170 psi. So now I'm curious and pull a Stihl 026 off the shelf and tested that. 190 psi. Hmmm, that seems out of range. So I decided to try one more thing and using the same gauge, I tried another hose with shorter threads on it. (more closely matching the length of the spark plug threads). With the different hose, I got a reading of 151psi on the 026. That sounded about right so I retested the Jonsered 2172 again (twice) with the new hose and got 140 psi both times. WOW! Whooda thunk that the tiny difference in displacement by the thread length variations on a compression checker could make that much of a difference. In the pic, it shows the plug from a 2172, the "long thread" checker and the "short thread" checker. Both compression hoses are the same diameter and length. Note that all readings were taken with the same gauge.
Just thought that was really interesting info and could account for some of the variations in readings people get from different saws.

 

[Chris-PA]

Funny, I just wrote this in another thread: These are small displacement engines, so any additional volume you add to the combustion chamber - like removing the plug and adding some adaptors - will significantly reduce the compression pressure reached. Since the combustion chamber is so small it would not take much to have a big effect.

I really think that too much is made of static/low speed compression readings. They are useful in a general sense to diagnose issues, or to look for relative changes that might indicate problems, but otherwise I think the readings get misinterpreted.

The idea is that you can infer the (much higher) pressures in the cylinder at higher rpms while the engine is running by measuring the static/low speed pressure. I don't really think it's a very reliable yardstick. My McCulloch has a notch in the top of the exhaust port, which bleeds off cylinder pressure at low speeds but does not really effect cylinder pressure during operation. It's not too hard to imagine some cylinder or ring defect that would do the opposite - bleed off at high pressure but not show up at low pressure.

 

[newmexico]

Good info, and I might be wrong but I think another thing to take into account when looking at compression readings is the altitude that the test is performed at. I believe it's a rule of thumb that for every 1000' in elevation atmospheric pressure decreases about 3%.

if this is the case then a 150 psi reading at sea level might drop to 136 at 3000 feet.

 

[fearofpavement]

We're at 800' elevation here. I was just amazed at the spread between the two readings. On the small 026 cylinder the spread was 39 psi. And on the larger 2172 cylinder the spread was 30 psi. I bet on an automotive cylinder, it wouldn't register any difference at all.

 

[newmexico]

yep, preaty neat to see the difference between those two saws. Now too get an 009 and an 090 stihl and compare those two spreads.

 

[nmurph]

Interesting, but not surprising. You can remove .010" of squish and increase compression by 10psi. I'm sure that the longer foot on the L gauge would significantly change compression. I 140psi is more in line with my experience. I don't have nearly the experience as JJ, but mine does jive with what I have seen some pretty experienced members report.

 

[Lignator]

The good news is that as long as your pressure gage is repeatable, checking compression does still have value. If you start with a stock saw and start modding the cylinder you will still see a change in compression which is really what you are interested in.

 

[Chris-PA]

The good news is that as long as your pressure gage is repeatable, checking compression does still have value. If you start with a stock saw and start modding the cylinder you will still see a change in compression which is really what you are interested in.

I sort of disagree, which is what I was attempting to say upthread. I think it is possible to make modifications to the porting that might decrease cylinder pressure at a hundred rpm but that would increase it at 10,000rpm. Raising an exhaust port would be one example, where the intent is to get more power (which probably = higher cylinder pressure) at high rpm, but that would reduce a low speed compression reading.

So I don't think you can always use a low speed compression reading to gauge the effects at 10,000rpm. Now if you do something like reducing the combustion chamber volume that should have an effect at all speeds, but it may not be the same at all speeds.

 

[Terry Syd]

Interesting how much extra volume one tester had over the other. One of the 'Red Neck' compression mods I make is to use a Champion RCJ6Y plug with the gasket removed. The plug is a half range colder, but has never fouled in any saw I've used it in. It has a thicker wall for the threads and because it is a colder plug the electrode doesn't go as deep as other plugs. Plus the electrode is longer and sticks out further. (check out the volume difference with your standard plug) I only had a problem in one saw where the side electrode would just touch the top of the piston, so I had to find a plug that would index away from the piston.

I doubt if it got me an extra 39psi in my 50cc saw, but from your results it may have gotten me an extra 10psi over the stock plug.

 

[wyk]

The good news is that as long as your pressure gage is repeatable, checking compression does still have value. If you start with a stock saw and start modding the cylinder you will still see a change in compression which is really what you are interested in.

Exactly. The same thing with the chainsaw dyno thread. The actual HP, corrected or not, isn't the important thing. It's the difference between mods - assuming everything is within a safe range.

 

[wyk]

There's a whole lot more goin on in there than simply a higher exhaust port. You have air(in a strato), mix, and exhaust all fighting it out. It's a musical chair game until the piston covers that exhaust port. All we can measure with instruments is the static pressure. This gives us an idea how the saw will handle, and this is all we have to go off of as far as instrumental testing goes.

I sort of disagree, which is what I was attempting to say upthread. I think it is possible to make modifications to the porting that might decrease cylinder pressure at a hundred rpm but that would increase it at 10,000rpm. Raising an exhaust port would be one example, where the intent is to get more power (which probably = higher cylinder pressure) at high rpm, but that would reduce a low speed compression reading.

So I don't think you can always use a low speed compression reading to gauge the effects at 10,000rpm. Now if you do something like reducing the combustion chamber volume that should have an effect at all speeds, but it may not be the same at all speeds.