Will the Real Compression Test Please Stand Up!

[HarleyT]

So did my teachers......

 

[Deleted member 117362]

But, did Ed say you are special?

 

[rwoods]

But once you are above that pressure it's open. At spring pressure and above, is it not open? No problem here, enlighten me.

Because you asked I will wade back in, but please understand that so long as a gauge has a valve reasonably close in operation to the one it was designed to have and all else is working you just read the gauge. Further I don't have a clue as to the amount of the spring tension in these tiny valves - it might just be some fraction of a pound for all I know. I am also assuming that the valve is simply a little plunger with a spring to automatically close it. I am no engineer and may be as wet as a bull frog in the rain, but assuming it is important or just for science sake or just to carry on - the best illustration I can think of is a door with a spring assist to keep it closed. Let's say 50# of spring assist. If the homeowner is on the same side of the door as the spring and is exerting 200# of pressure against the door how much pressure would a thug have to exert to open the door? A tad more than 250# (homeowner pressure plus spring pressure plus enough to overcome the equilibrium). The homeowner is the trapped air between the gauge and the valve. The valve is the spring assist door. And the thug is the actual engine compression. The gauge reacts off the trapped air only so unless the dial is calibrated to compensate for the spring it is not reading the actual pressure produced by the engine. Wait you say the valve is opened so shouldn't the pressure on both sides of it be the same? No, the gauge side air pressure of 200# when coupled with the 50# spring can close the valve if the engine side pressure drops below 250#. What is likely confusing folks is the pressure is built up a stroke at a time. What happens is pressure from each stroke is transferred in part as air is moved from the piston side to the trapped air side (the valve closes after each stroke) until the piston is no longer able to produce enough pressure to overcome the trapped (stored) air pressure and the valve spring pressure. At which time it can no longer open the valve (or open the door in our home intrusion example). As I said before I don't know the actual spring pressure (or any other factor that increases the pressure necessary to open the valve - I believe these are referred to as the cracking pressure by those smarter than me) but I doubt it is much. I also don't know the cracking pressure of a tire valve but I seriously doubt those tiny springs exert any significant resistance. If they did then the gauge should be calibrated to take it into account. Just repeating in a different way I seriously doubt the valve used is really important to an accurate reading beyond matters related to its chemical resistance to the mix or closing speed (the slower the valve closes the more pressure that will bleed off when the piston goes past TDC; if the valve deteriorates and leaks due to the mix you will have a false and ever changing reading as the pressure bleeds off). If tire valves in fact have a significantly higher cracking pressure then you could check what I have said by simply checking the compression reading on a engine with the proper valve and then with the tire valve. If the readings are the same then I am all wet or the difference in cracking pressure wasn't really significant. I don't know how to measure the cracking pressure - I assume with another gauge of some sort.

If you weren't good guys I would really be regretting I ever posted. I could have sharpened a chain or two by now.

Ron

 

[HarleyT]

That is a Texas size paragraph!!

 

[Deleted member 117362]

Because you asked I will wade back in, but please understand that so long as a gauge has a valve reasonably close in operation to the one it was designed to have and all else is working you just read the gauge. Further I don't have a clue as to the amount of the spring tension in these tiny valves - it might just be some fraction of a pound for all I know. I am also assuming that the valve is simply a little plunger with a spring to automatically close it. I am no engineer and may be as wet as a bull frog in the rain, but assuming it is important or just for science sake or just to carry on - the best illustration I can think of is a door with a spring assist to keep it closed. Let's say 50# of spring assist. If the homeowner is on the same side of the door as the spring and is exerting 200# of pressure against the door how much pressure would a thug have to exert to open the door? A tad more than 250# (homeowner pressure plus spring pressure plus enough to overcome the equilibrium). The homeowner is the trapped air between the gauge and the valve. The valve is the spring assist door. And the thug is the actual engine compression. The gauge reacts off the trapped air only so unless the dial is calibrated to compensate for the spring it is not reading the actual pressure produced by the engine. Wait you say the valve is opened so shouldn't the pressure on both sides of it be the same? No, the gauge side air pressure of 200# when coupled with the 50# spring can close the valve if the engine side pressure drops below 250#. What is likely confusing folks is the pressure is built up a stroke at a time. What happens is pressure from each stroke is transferred in part as air is moved from the piston side to the trapped air side (the valve closes after each stroke) until the piston is no longer able to produce enough pressure to overcome the trapped (stored) air pressure and the valve spring pressure. At which time it can no longer open the valve (or open the door in our home intrusion example). As I said before I don't know the actual spring pressure (or any other factor that increases the pressure necessary to open the valve - I believe these are referred to as the cracking pressure by those smarter than me) but I doubt it is much. I also don't know the cracking pressure of a tire valve but I seriously doubt those tiny springs exert any significant resistance. If they did then the gauge should be calibrated to take it into account. Just repeating in a different way I seriously doubt the valve used is really important to an accurate reading beyond matters related to its chemical resistance to the mix or closing speed (the slower the valve closes the more pressure that will bleed off when the piston goes past TDC; if the valve deteriorates and leaks due to the mix you will have a false and ever changing reading as the pressure bleeds off). If tire valves in fact have a significantly higher cracking pressure then you could check what I have said by simply checking the compression reading on a engine with the proper valve and then with the tire valve. If the readings are the same then I am all wet or the difference in cracking pressure wasn't really significant. I don't know how to measure the cracking pressure - I assume with another gauge of some sort.

If you weren't good guys I would really be regretting I ever posted. I could have sharpened a chain or two by now.

Ron

Thanks, I get what you are saying and appreciate your response.

 

[crotchclimber]

Also don't forget to take into consideration the altitude it was tested at. Drops a little over 5psi for every 1000ft above sea level. A saw reading 160psi at sea level is going to read 132psi at my house.

 

[Baconaman]

Once the valve opens the gauge is going to see the full pressure.

Great videos in those links.

 

[Baconaman]

Also don't forget to take into consideration the altitude it was tested at. Drops a little over 5psi for every 1000ft above sea level. A saw reading 160psi at sea level is going to read 132psi at my house.

Good to know. I'm at 2000ft. Will 2000 ft have any effect on tuning?

Sent from my Cellular Moe-Bile phone using Tapatalk

 

[9050LX]

Revisited.I can attest to a low compression reading from a standard tire valve, and that is what led me here.I guess I love this place.

 

[pioneerguy600]

What would make it clear would be does a compression gauge give the exact correct numbers that are in the cyl?

No it does not, it will be robbed a little by that spring pressure, whatever it is. The comp readings are only comparable between the type of Schrader valve that is being used, regular red band tire valves will read lower than the white band valves meant for small engines.The white band valves are what is being used for factory published compression numbers. If a tire valve compression tester is used to take a reading then the psi number attained will be lower. Anyone can test this out by just switching valves and testing on the same saw, do it as quickly as possible betyween tests so the temps are close to same and the amount of oil is in the cylinder.

 

[9050LX]

I was getting 90lbs with a tire core, put the red one back in (was leaking down), and pulled 150.(echo 590 new piston, no base gasket)