Compression tester

[WV Mountaineer]

View attachment 296811

 

[dboyd351]

lets say the displacement of the piston is 10 and the volume of the combustion chamber is 1 and the volume of the hose is 1. With the valve at the spark plug hole we have a 10-1 compression ratio with atmospheric pressure of 14 we end up with 140 pounds of compression. If we move the valve to the other end of the hose and allow that volume to become part of the combustion chamber it drops our ratio to 5-1 resulting in 70 pounds of compression.

What about the fact that you keep pulling the starter cord until you reach a peak compression reading?

 

[Majorpayne]

the longer the hose the more you have to pull the starter cord. You have to compress the volume in the hose until it reaches the pressure of the combustion chamber, then it will stop going up.

Then why does it matter which end the valve is on?

 

[WV Mountaineer]

lets say the displacement of the piston is 10 and the volume of the combustion chamber is 1 and the volume of the hose is 1. With the valve at the spark plug hole we have a 10-1 compression ratio with atmospheric pressure of 14 we end up with 140 pounds of compression. If we move the valve to the other end of the hose and allow that volume to become part of the combustion chamber it drops our ratio to 5-1 resulting in 70 pounds of compression.

Only when the valve is closed all the time!

I give up lol

 

[MechanicMatt]

Why do you guy think compression goes up when you reduce the squish?? Your lowering the volume of the head. Adding the hose adds volume.

 

[MechanicMatt]

Have you ever had heads cc'd on a V8? Ever have the heads milled?? why?? Less volume equals a higher compression. So did the OP ever get his poulan running? Want another? I have one in craftsman grey Ill send ya for some orange parts.

 

[dl5205]

I think the point wvmountaineer is trying to make is when the cylinder end schrader core is Open (when you're cranking the saw), the Hose Is effectively Open.

I don't really know for sure if either of you are totally right or not, but when I buy, I want to make sure it has a schrader or some other kind of 'foot valve' at the cylinder end, just because.

I'm trying to justify a new OTC 5606 to myself.

 

[WV Mountaineer]

A car with a 18 inch outside diameter tire is driving down the road at 100mph. At what speed is it moving forward where it impacts the road?

 

[WV Mountaineer]

I think the point wvmountaineer is trying to make is when the cylinder end schrader core is Open (when you're cranking the saw), the Hose Is effectively Open.

I don't really know for sure if either of you are totally right or not, but when I buy, I want to make sure it has a schrader or some other kind of 'foot valve' at the cylinder end, just because.

I'm trying to justify a new OTC 5606 to myself.

Exactly! If what Steven said was accurate then a smaller hose and gauge would also give you a higher result. That simply isn't the case.

It's a nice gauge for sure. Well worth the money. Thanks to those who told me before I ordered it.

 

[WV Mountaineer]

Have you ever had heads cc'd on a V8? Ever have the heads milled?? why?? Less volume equals a higher compression. So did the OP ever get his poulan running? Want another? I have one in craftsman grey Ill send ya for some orange parts.

Yes, less volume will give you higher compression. The volume of the Gauge, tube and cylinder never changes when you are testing regardless of the position of the valve.

No i didn't get it running. I have no need for that saw was just having fun.

 

[WV Mountaineer]

If the valve was closed all the time it would have the same result but the gage would not be able to display those results.

Lets go through this one step at a time. We know the valve is open to transfer pressure and move the gauge. What is the total volume when this occurs?

 

[dboyd351]

It you pull it over 6 times and it is pumping 150 pounds it is not going to register 900 pounds it is going to register 150 pounds.

And if you pull it over enough to reach maximum pressure, it will be the same reading regardless of volume, assuming you don't have a leak.

 

[MechanicMatt]

P=F/A Is the equation used to measure pressure. Do you know what "A" stands for? Area. This is my last post in this thread, I promise. If you increase the area (longer hose before valve/compustion chamber) your going to directly affect the "P" (pressure). Have a good night fellas. Oh Im serious on the offer, if you want another lil poulan to play with, just PM me, I promise I wont argue compression testers with you

 

[WV Mountaineer]

that is not true.

There is nothing short of reducing the combustion chamber volume that can give you a higher reading. It you pull it over 6 times and it is pumping 150 pounds it is not going to register 900 pounds it is going to register 150 pounds. It can't register more than result of the compression ratio minus leakage.

Who said anything different? Please don't start stating obvious facts to prove a point about something totally different.

BTW, it's not compression ratio minus leakage. It's "compression". Compression ratio is determined by the min and max volume of the cylinder. Leakage could mean a bunch of different things but is usually measured by loss of pressure over time.

I don't want to offend anyone here but I also don't want to leave false information on a forum. I think we have each explained our points enough that people can read each argument and draw their own conclusion.

I graduated from WVU in 1999 with a degree in Electrical Engineering. Electricity values can directly translate to fluids and air is a fluid. Before someone says "NO IDIOT, AIR IS A GAS" you are correct but both Gasses and Liquids are considered a fluid. Imagine the check valve is a switch that can either be on or off, open or closed. The PSI or pressure is Voltage. When the switch is closed, the voltage is going to be the same on both sides of the switch. Much the same way the pressure is going to be the same when the valve is open.

Your argument is true before before the valve is opened but by design the valve opens at whatever the sensitivity is set to, and that equalizes the pressure.

Bottom line is, my saw needs parts that are worth more than a running saw. :yoyo:

 

[WV Mountaineer]

o.k. I want to understand. The check valve is there to trap the compressed air, to hold the needle position steady so we can get a gage reading.

If you really want to understand then just stop right there and think about what you said as you are correct. The check valve "traps" the compressed air. It is designed to easily let the air flow one way and then close tight.

When it is open, right at the last split instant before it closes to trap the compressed air, what is the total volume being measured?

 

[MechanicMatt]

Yes, less volume will give you higher compression. The volume of the Gauge, tube and cylinder never changes when you are testing regardless of the position of the valve.

No i didn't get it running. I have no need for that saw was just having fun.

:bang:

 

[dboyd351]

Bottom line is, my saw needs parts that are worth more than a running saw. :yoyo:

Your saw might be salvageable without buying any parts. There is a method of using muriatic acid to remove metal transfer from the walls of the cylinder and restore compression. If you have 110 psi right now you could well be a good candidate for it. The saw might run with 110 psi, as is. I've always heard 100 psi was about the minimum for a saw to run.

 

[WV Mountaineer]

Your saw might be salvageable without buying any parts. There is a method of using muriatic acid to remove metal transfer from the walls of the cylinder and restore compression. If you have 110 psi right now you could well be a good candidate for it. The saw might run with 110 psi, as is. I've always heard 100 psi was about the minimum for a saw to run.

Whoa, that sounds super cool regardless if it works or not. I'll try to find something online to teach me how to do it unless you or someone else are willing.

 

[WV Mountaineer]

I won't argue compression testers with anyone. I just want to learn what it is that I don't understand.
I think I have a grip on it but maybe not.
P=F/A seems pretty straightforward

It's more of a riddle than anything Stephen. Once you get the fact that the valve is open at the instant max pressure is registered on your gauge, everything else you said will be accurate.

I wish I could buy you a beer.

 

[tomscott]

WV Mountaineer, I mean no disrespect, but you are wrong.

Stephen C and others that have tried to explain that the volume of the hose adds to the total volume of the combustion chamber are absolutely correct.

Liquids and gases both have some similar tendencies but they are treated differently with different engineering formulas because by definition fluids are incompressible. We are dealing with gas here which is compressible.

The heart of the combustion engine is compression and is arrived at a design value by the compression ratio. The compression ratio is simply how much volume is above the piston at the bottom of the stroke versus how much volume remains at the top of its stroke.

Adding the hose adds to the volume above the piston. If you add volume above the piston you change the compression ratio.

That is why on the first pull the compression reads so low. The check valve traps this pressure in the hose, and each successive pull reduces the error a little more, until you can't make the gauge read any higher because the pressure in the hose has been fully compressed to the maximum pressure that the engine can produce. Stephen C described it very well.

If the check valve is upstream the pressure is released from the hose between each pull and the hose will always be part of the "new" compression ratio that you have artificially created with the additional volume of hose.

I truly mean no offense, but I wish you hadn't of played the engineer card, it makes the rest of us engineers look bad.