What is compression?

[minnnt]

It's probably a simple question, and i have done a search but it just brings 40 pages of results with all the threads containing the word compression.

Can someone please explain compression to me and if i was to buy a saw with bad compression what would i need to do to make it have good compression again?

Thanks, David.

 

[TreePointer]

You should get good replies in this thread. Take a look at these two animations to start:

Two Stroke Engine

How Two-stroke Engines Work

 

[Geno44]

David, Compression ratio is a comparison of the volume of space above the piston at the start of compression to the volume of space above the piston at the top of its stroke. This in turn, bumps atmospheric pressure up about the same ratio. If there are 7 cubic inches above the piston at the start of compression and 1 above it when it is at the top, the compression ratio is 7 to 1. If atmospheric pressure is 15 psi then 7 to 1 should yield about 105 pounds of compression pressure. In a four stroke cycle engine, compression ratio is figured with the piston at the bottom and at the top. In a two stroke cycle engine, the piston has to get above the ports to start compressing. An engine with poor compression is not getting a good seal in one or more places which could be gaskets, piston rings, piston/cylinder condition, cracked parts or bad valves if a four stroke. I hope this helps you.

 

[timberwolf]

If there are 7 cubic inches above the piston at the start of compression and 1 above it when it is at the top, the compression ratio is 7 to 1. If atmospheric pressure is 15 psi then 7 to 1 should yield about 105 pounds of compression pressure.

Unfortunatly it's just not that simple. For example a stihl 660 has 10cc of volume above the piston at TDC and makes about 150-160 psi of compression, so would need to compress a little more than 10 volumes of air to get the 150 psi of compression. In that case 10 times 10 cc = 100cc of total trapped volume which is greater than the total swept volume of the saw. Given the exhaust durration is close to 180 deg the trapped volume would be about 65 cc so by the formula would have a 6.5:1 compression ratio. But 6.5 x 14.7 psi = 95 psi and is far short of the actuall 150-160 psi that should be expected.

What needs to be looked at to relate compression ratio in a two stroke to cranking pressure is trapping efficiency and to some extent dynamic compression. Those topics are pretty well covered too in the two stroke engine writings.

 

[sloch24]

Simple answer fora simple quesiton

Can someone please explain compression to me and if i was to buy a saw with bad compression what would i need to do to make it have good compression again?

Thanks, David.

Here's the simple answer to your question. The fuel/air mixture needs to be pressurized in order to burn correctly. If you have a scored piston and or cylinder, or bad bad rings, or a faulty decompression valve, the pressure will not build to what it needs to be to ignite the fuel/air mix and cause combustion. I'm sure that there are other variables that I'm not touching on (such as blow-by), but I'm going for a "simple" answer.

Generally when I buy a used saw, I'll pull the plug and pull it over cold until the needle stops on my compression valve. I'm not sure of the minimum allowable (do a search and you'll find it), but a new saw should be 150+ and a used saw (from my experience) should be between 135 and up. Some engines will seem to run fine at lower compression, some will not.

I'm not an expert, so hopefully someone will come along here and give the numbers.

 

[timberwolf]

I don't think you can just know what to do to improve compression on a used saw. If bore in cylinder is good and compression low new rings or new rings and piston will bring it back up. But if the bore is dammaged then you almost need to plan on it needing a ne cylinder and piston. Sometimes a bore can be cleaned up with sanding or very light honeing, but most times results are marginal.

Just about anything below 140 psi in a modern saw is showing wear. anything down 120 range is toast.

some of the newer saws have stock compression up in the 170-180 psi range stock, stihl 361, 460, and newer solo's and dolmars come to mind these saws I would think would be running poorly long before they were down to 120-140 psi on the compression gauge.

 

[litefoot]

It's probably a simple question, and i have done a search but it just brings 40 pages of results with all the threads containing the word compression.

Can someone please explain compression to me and if i was to buy a saw with bad compression what would i need to do to make it have good compression again?

Thanks, David.

Wow, some really technical answers here. I won't try add to the analytical description (although there's some good stuff I learned here), ,but maybe what you're asking is how much does it costs to restore compression.

The two things that I am most concerned with in a used saw are compression and spark. If compression is bad, then the piston, rings and/or cylinder may be bad. If you fixed it yourself, you could be in for $200-300 in parts alone. If there is no spark, then the ignition module could be bad. A new module can run over $100.

Of course, those are worse-case numbers. The repair may be as simple as an air leak at the cylinder base.

 

[Geno44]

David, Timberwolf is certainly correct in his more technical explanation. Another factor might be turbocharging or supercharging. I didn't know where you were starting from in your understanding and thats why I said about 105 but there could be a wide variance here.

 

[minnnt]

Blimey! Haha, that has totally blown me away, i'm all in a tizz now. I am trying to understand saws more and how they work. Or at least enough to get me by without it having to go to a mechanic/dealer to be sorted etc.

I have seen a 038AV Super on ebay which says it has poor compression and on pulling it only moves about half way and could posibly have a seized engine. What could this be and how expensive would it be to fix? I know it is not easy to say without seeing it or inspecting it but what would it need if it was seized?

Thanks alot, David.

 

[sloch24]

Blimey! Haha, that has totally blown me away, i'm all in a tizz now. I am trying to understand saws more and how they work. Or at least enough to get me by without it having to go to a mechanic/dealer to be sorted etc.

I have seen a 038AV Super on ebay which says it has poor compression and on pulling it only moves about half way and could posibly have a seized engine. What could this be and how expensive would it be to fix? I know it is not easy to say without seeing it or inspecting it but what would it need if it was seized?

Thanks alot, David.

If it's seized, you can't get a compression reading. As for price, at minimum you'd need a new piston and cylinder.. depending on what damage was done during seizure you could need a new crank, bearings and seals as well...

I don't know what you mean about moving only "half way". Are you talking about the reading on the compression tester? If that's so, it's not seized and just has poor compression (depending on what the "half way" reading is).

Like you said... not easy to say without seeing it or having all the information.

 

[minnnt]

I'm not sure either, thats how it is wrote in the advert. He could mean that the cord is only pulling out half way.

 

[litefoot]

I wouldn't pay more than $50 USD for that saw...whatever that translates into for you.

 

[PatrickIreland]

I'm no chainsaw expert, but I've been fiddling with cars for 15 years. As a rule of thumb for any engine you want to see 16-18 times the compression ratio in PSI for a good engine.

I'm not sure either, thats how it is wrote in the advert. He could mean that the cord is only pulling out half way.

 

[seagull369]

If the plug's removed and I put my thumb over the hole while pulling the cord, could that be a 'backyard' way of testing for low compression? If I can still keep my thumb sealed up against it, I'm thinking that it's not what it should be... I don't have a compression gauge, hence the question.

 

[PatrickIreland]

That's a backyard way of testing for ANY compression. A tester is only about $15, and will tell you if the engine is still worth a go or due for the tip.

 

[drmiller100]

Blimey! Haha, that has totally blown me away, i'm all in a tizz now. I am trying to understand saws more and how they work. Or at least enough to get me by without it having to go to a mechanic/dealer to be sorted etc.

I have seen a 038AV Super on ebay which says it has poor compression and on pulling it only moves about half way and could posibly have a seized engine. What could this be and how expensive would it be to fix? I know it is not easy to say without seeing it or inspecting it but what would it need if it was seized?

Thanks alot, David.

it needs a new piston, rings, and cylinder.

if you want to learn about chainsaws, it is a fun project if you are somewhat mechanically inclined. It is pretty easy to do, and a LOT of fun when you get it running!!!!

As far as difficulty, similar to changing a water pump on a mid 80's automobile.