# Poor man's compression test?



## handyrandyrc (Dec 9, 2009)

Just talked to a two-stroke guru here at work. Now, he mostly works on snowmobiles and dirt bikes, but he said the principle is the same. At this altitude, he says he likes to have 120 psi for 'good' compression on a 2-stroke.

He told me the easiest way for him to do a quick test is to pull the spark plug and turn the crank over. If he can hold his thumb over the plug hole, he knows compression is dead. If air is pushed out, he knows compression is good. Now he said he's used to 200-400 cc 'lungs' so there is more volume of air. However, our thinking was 'pressure is pressure', right?

120 psi is 120 psi whether it's 50cc air volume or 400cc volume, right? Or am I missing something else here?


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## mtngun (Dec 9, 2009)

Pressure is pressure.

But ..... what's so hard about using a compression gage ?


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## bcorradi (Dec 9, 2009)

120# of compression may be good on a snowmobile, but not on a saw.


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## StihlyinEly (Dec 9, 2009)

mtngun said:


> But ..... what's so hard about using a compression gage ?



Nothing. If you have one. Given the title of the thread, my guess is he doesn't have access to one and, even if he does, it's useful information for those of us who don't.


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## handyrandyrc (Dec 9, 2009)

Well, that answers my question on the compression on this Poulan 4200 I am working on.

I can hold my thumb over the hole and rotate the flywheel by hand. I feel pressure build under my thumb, but I can definitely hold it over the plug hole without leaking any.

I'm gonna have to pull the head off and see if piston/rings can be done.


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## 93Dakman! (Dec 9, 2009)

And that depends on how fast you turn the crank too. I know my 6401BB has more compression than a Diesel volvo. Its still hard to pull even with the decomp pushed in. But I can turn it slowly with my thumb over the hole and no air pushes past.


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## mowoodchopper (Dec 9, 2009)

Compression tester World !

It should have its own forum and I already named it! LOL


I know alot of guys on here love their compression testers and their minimum
compression reading for a saw to be a runner.
IMO its pretty much pointless , I you cant tell from pulling over a saw and the way it runs if it has good compression, you should get an electric saw and then you dont have to worry. Also all of these saws that are torn down or junked because they blow under 150 what a waste . And the myth a saw wont run with under 120 is just that a myth. Its very easy to start a saw and see if it is strong or not. 

Next I'm gonna invent a chain sharpness gauge, it will be computer operated and it will scan your chain cutters and if it says no good you just throw it away. It will save all that silly stuff like feeling your chain or trying it in wood that's all a thing of the past. If you dont have a sharpness tester before long you will be a rookie!


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## Slamm (Dec 9, 2009)

mowoodchopper said:


> Compression tester World !
> 
> It should have its own forum and I already named it! LOL
> 
> ...



Yeah !!!!! What he said ........... Who needs compression anyways?? I think its over rated at best.

Instead of making pop-up pistons and increasing compression, I'm going to dish out my pistons to lower the compression. I'm sure they will run a lot strong after that.

A lot of times when I compression test a saw and its running over 150 psi. I'll dishout the piston to bring the static compression down to something more mangeable like 120 psi. It makes for an easier starting saw, while simultaneously increasing the amount of time I get to spend running my beloved chainsaws, because it takes longer to get anything cut. Perfect concept.

Thats what I do,

Sam


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## mowoodchopper (Dec 9, 2009)

Slamm said:


> Yeah !!!!! What he said ........... Who needs compression anyways?? I think its over rated at best.
> 
> Instead of making pop-up pistons and increasing compression, I'm going to dish out my pistons to lower the compression. I'm sure they will run a lot strong after that.
> 
> ...




 Thanks for the great idea, I'm gonna lower all my saws compression.
I think Ill just drill a small hole in the top of the cyl, this compression stuff sucks!


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## blsnelling (Dec 9, 2009)

Pressure may be pressure, but the chainsaw will move *much *less volume, making it easier to hold back.


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## Slamm (Dec 9, 2009)

mowoodchopper said:


> Thanks for the great idea, I'm gonna lower all my saws compression.
> I think Ill just drill a small hole in the top of the cyl, this compression stuff sucks!



You're too late I already have the cordless drill with an 1/8" drillbit into the top of my 084 ........ whoa, wait ................ There done............................................. Now lets try it, <Pull <Pull <Pull .... Hey this is great, it pulls over a lot easier now.

What a great idea,

Sam


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## mowoodchopper (Dec 9, 2009)

Slamm said:


> You're too late I already have the cordless drill with an 1/8" drillbit into the top of my 084 ........ whoa, wait ................ There done............................................. Now lets try it, <Pull <Pull <Pull .... Hey this is great, it pulls over a lot easier now.
> 
> What a great idea,
> 
> Sam



Dang I was gonna patent that!!


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## stipes (Dec 9, 2009)

*On a 80cc saw...*



handyrandyrc said:


> Just talked to a two-stroke guru here at work. Now, he mostly works on snowmobiles and dirt bikes, but he said the principle is the same. At this altitude, he says he likes to have 120 psi for 'good' compression on a 2-stroke.
> 
> He told me the easiest way for him to do a quick test is to pull the spark plug and turn the crank over. If he can hold his thumb over the plug hole, he knows compression is dead. If air is pushed out, he knows compression is good. Now he said he's used to 200-400 cc 'lungs' so there is more volume of air. However, our thinking was 'pressure is pressure', right?
> 
> 120 psi is 120 psi whether it's 50cc air volume or 400cc volume, right? Or am I missing something else here?



At the GTG I went to and a friend tryed to drop start my sp80 without the decomp on and before I yelled stop,,,,it about flew outta his hands....Thats compression...


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## stipes (Dec 9, 2009)

*Be like me...*

Drop starting Brad's 090....End up with a broken arm,,or black eye......LOL!!!


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## blsnelling (Dec 9, 2009)

stipes said:


> Drop starting Brad's 090....End up with a broken arm,,or black eye......LOL!!!



I do it You just have to "mean it"!


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## mtngun (Dec 9, 2009)

bcorradi said:


> 120# of compression may be good on a snowmobile, but not on a saw.


The OP has the misfortune of being in Rexburg, Idaho, at 4800 feet elevation. The rule of thumb is you lose 3% compression/power per 1000 feet, so a saw that blows 120 psi in Rexburg might blow 140 psi at sea level.

Force on thumb = pressure times area of spark plug hole. As long as the spark plug hole size is the same, force on thumb at a given pressure will be the same regardless of engine displacement.

But, I prefer to use a compression gage.


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## Rookie1 (Dec 9, 2009)

handyrandyrc said:


> Well, that answers my question on the compression on this Poulan 4200 I am working on.
> 
> I can hold my thumb over the hole and rotate the flywheel by hand. I feel pressure build under my thumb, but I can definitely hold it over the plug hole without leaking any.
> 
> I'm gonna have to pull the head off and see if piston/rings can be done.



Heres a backyard test Randy. I assume you poulan wont start because of low compression right. Take out the plug and squirt some oil into cylinder. The put plug back and try. Oil will temporarily seal rings and give good compression.


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## StihlyinEly (Dec 9, 2009)

mtngun said:


> The OP has the misfortune of being in Rexburg, Idaho, at 4800 feet elevation. The rule of thumb is you lose 3% compression/power per 1000 feet, so a saw that blows 120 psi in Rexburg might blow 140 psi at sea level.



Thanks for that info, gun. I cut for about a year at 9-10,000 feet in Colorado and could never remember the elevation compression tradeoff figures.


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## gee_dubya75 (Dec 12, 2009)

*where should I get a compression tester?*

Is this one at Baileys decent?

http://www.baileysonline.com/itemdetail.asp?item=G+320HD&catID=

I'm thinking about buying a used saw this next week and would like to check the compression before I buy it. What kind of compression should a Makita DCS 601 have? 

Where are some places that one could by a compression tester locally? Does Autozone carry something like this?


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## handyrandyrc (Dec 13, 2009)

I picked up this one from Amazon for 11 dollars. Tried it out today and have 130-ish PSI on the old Poulan. It has a rubber nipple that will fit in any spark plug hole, and a valve that holds the pressure reading.

Link to Amazon


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## artie__bc (Dec 14, 2009)

For saws, just pick them up by the starter handle. If it has enough compression to hold itself up without the rope pulling out it's either seized or has great compression. Usually the saw will slowly 'chug' its way down the rope. If it drops quickly, put it back on the shelf and keep your wallet in your pocket. Make sure the decomp is closed, though!


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## gee_dubya75 (Dec 14, 2009)

How do I edit a post? There is no 'edit' icon visible to me in my post above even though I'm logged in.

I had a question about the compression of a Makita DCS 6401 but I left off the 4 accidentally.


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## Rookie1 (Dec 14, 2009)

gee_dubya75 said:


> How do I edit a post? There is no 'edit' icon visible to me in my post above even though I'm logged in.
> 
> I had a question about the compression of a Makita DCS 6401 but I left off the 4 accidentally.



Well I think with this reply members will figure it out,if thay havent already. 
Edit, I kooked this up on FAQ.
If you have registered, you will be able to edit and delete your posts. Note that the administrator can disable this ability as he desires. Your ability to edit your posts may also be time-limited, depending on how the administrator has set up the forum.


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## lone wolf (Dec 14, 2009)

*tech question on test?*

any input on doing test hot or cold or how ambient temp changes readings?


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## stihlboy (Dec 14, 2009)

room temp is best, if cold you lose some compression because the rings arent expanded fully


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## handyrandyrc (Dec 14, 2009)

I picked up a cheap 11-dollar compression gauge from Amazon.com and tested. Just a hair over 130 PSI! I think my thumb was a fairly good indicator of where things were at.


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## spacemule (Dec 14, 2009)

mowoodchopper said:


> Compression tester World !
> 
> It should have its own forum and I already named it! LOL
> 
> ...


That's what I've been saying for years. I wonder how many of the saw geeks on here wear pocket protectors.


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## andym (Dec 21, 2009)

I've been fighting with a Poulan Pro 260 and just tested the compression. I guess 40psi is too low ehh? This saw is like brand new too.


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## Arrowhead (Dec 21, 2009)

gee_dubya75 said:


> Is this one at Baileys decent?
> 
> http://www.baileysonline.com/itemdetail.asp?item=G+320HD&catID=
> 
> ...



I have that one from Baileys, I like it.


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## Nandy (Dec 21, 2009)

Well, If you want to know if you have good compression the only way to do it is with a compression gauge, nothing less...


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## Anthony Pillois (Dec 14, 2017)

mowoodchopper said:


> Compression tester World !
> 
> It should have its own forum and I already named it! LOL
> 
> ...


I agree with you on the low compression I did a video on utube with a comparison between a Mac 1010 that I knew had 150 lbs of compression and a Poulan pro 5020av that I got over two years ago that then only had 30 lbs of compression at the end of the video I started up the Poulan its under Anthony Pillois low compression check it out


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## JohnsonD (Dec 14, 2017)

And now for the poor man's ignition tester. Simply put your pinky in the spark plug boot and pull the cord.


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## lone wolf (Dec 14, 2017)

JohnsonD said:


> And now for the poor man's ignition tester. Simply put your pinky in the spark plug boot and pull the cord.


That works pretty good actually.


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## Anthony Pillois (Dec 14, 2017)

JohnsonD said:


> And now for the poor man's ignition tester. Simply put your pinky in the spark plug boot and pull the cord.


that's one way of doing it with shocking results lol another is just lay the plug on the head and pull the rope


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## Little Al (Dec 15, 2017)

bcorradi said:


> 120# of compression may be good on a snowmobile, but not on a saw.


Got to agree anything below 150psi is a bit on the weak side for a saw also a "redneck check for compression" is hold the recoil handle & let the wight of the saw hang free if it pulls the start cord out in a series of jerks the the saw saw has compression how much is a different ball game I have 2 ALPINA saws the prof 55 has 225 psi compressiom & the 070S has just north of 200psi


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## lone wolf (Dec 15, 2017)

Slamm said:


> Yeah !!!!! What he said ........... Who needs compression anyways?? I think its over rated at best.
> 
> Instead of making pop-up pistons and increasing compression, I'm going to dish out my pistons to lower the compression. I'm sure they will run a lot strong after that.
> 
> ...


But can you do all that precisely without a gauge?


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## lone wolf (Dec 15, 2017)

I rebuild MS 200T saws and sell them would a buyer be satisfied if I told him it passed a drop test? I bet he would not! I have 2 gauges one Snap On the other US Gauge and I calibrate them together with the gauge on my shop compressor and they all read the same so I say they are pretty accurate! Also I have a Snap On adapter for the smaller plugs and it took a while to dial that in I had trouble with adapters before this one then I had trouble with the wrong* Schrader* valve finally I got it calibrated.


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## Riles32807 (Dec 15, 2017)

I worked as a bicycle mechanic for a while. I cringe when someone pinches a tire and tells me there's enough air in the tire. I think I have a reasonably strong grip and I can't tell the difference over about 15 psi. Most mountain bike tires take 35 psi or more, road bikes usually take 65+.

If your on the spot looking at a saw to buy and fix, the thumb test, or the drop test might be useful to get an idea of whether it's worth your time and money. But its not useful as a diagnostic tool.


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## Dennbb (Jan 21, 2021)

Slamm said:


> Yeah !!!!! What he said ........... Who needs compression anyways?? I think its over rated at best.
> 
> Instead of making pop-up pistons and increasing compression, I'm going to dish out my pistons to lower the compression. I'm sure they will run a lot strong after that.
> 
> ...


Good way to go. Compression ratio is more critical that high compression. Using a 162 husqvarna as an example. The squish at .031 calculates to 13.7 : 1. With the squish at .021 it's 16.2:1. Anything over 14:1 is deisel territory and gas will ignite from the compression. Which is why gas in a deisel engine will grenade the engine. Most saws stock are 11.5 - 12.5 for a reason. They don't want to reach the deisel threshold. I was taught 45 years ago to never assume that the engineers don't know what they're doing. I'd be willing to bet that 70-90% of ported saws on the internet have less power than stock if they were all dynoed. There's a placebo effect in loud saws.


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## Cantdog (Jan 21, 2021)

Dennbb said:


> Good way to go. Compression ratio is more critical that high compression. Using a 162 husqvarna as an example. The squish at .031 calculates to 13.7 : 1. With the squish at .021 it's 16.2:1. Anything over 14:1 is deisel territory and gas will ignite from the compression. Which is why gas in a deisel engine will grenade the engine. Most saws stock are 11.5 - 12.5 for a reason. They don't want to reach the deisel threshold. I was taught 45 years ago to never assume that the engineers don't know what they're doing. I'd be willing to bet that 70-90% of ported saws on the internet have less power than stock if they were all dynoed. There's a placebo effect in loud saws.


Hmmmm.....well good luck with that.......we'll see what the respondents say....LOL!


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## sundance (Jan 22, 2021)

Dennbb said:


> Good way to go. Compression ratio is more critical that high compression. Using a 162 husqvarna as an example. The squish at .031 calculates to 13.7 : 1. With the squish at .021 it's 16.2:1. Anything over 14:1 is deisel territory and gas will ignite from the compression. Which is why gas in a deisel engine will grenade the engine. Most saws stock are 11.5 - 12.5 for a reason. They don't want to reach the deisel threshold. I was taught 45 years ago to never assume that the engineers don't know what they're doing. I'd be willing to bet that 70-90% of ported saws on the internet have less power than stock if they were all dynoed. There's a placebo effect in loud saws.


I'd be curious to see the compression ratio math to support your example. Doesn't seem plasible.


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## Dennbb (Jan 22, 2021)

sundance said:


> I'd be curious to see the compression ratio math to support your example. Doesn't seem plasible.


Of course it doesn't seem plausible because it goes against what youtube followers are led to believe. The math is pretty simple. It's not my math. It's the universal excepted math for calculating . compression ratio. Add the swept volume and trapped volume. Divide by the trapped volume. The swept volume on the afore mentioned saw is 43.8 cc and the trapped volume is 3.4 cc which adds up to 47.2 cc then divide 47.2 by 3.4. The math for finding the trapped volume is more complicated and I don't have the time or inclination.


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## Bob Hedgecutter (Jan 22, 2021)

He told me the easiest way for him to do a quick test is to pull the spark plug and turn the crank over. If he can hold his thumb over the plug hole, he knows compression is dead. If air is pushed out, he knows compression is good.

Does that same Guru have the ability to jam his todger up a dirtbike exhaust and be able to tell if it is running rich?


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## Ted Jenkins (Jan 23, 2021)

What is wrong with just pulling the starter rope. After fifty years of pulling ropes I think one would kind of get the idea when compression was weak. Thanks


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## kevin j (Jan 23, 2021)

Dennbb said:


> Of course it doesn't seem plausible because it goes against what youtube followers are led to believe. The math is pretty simple. It's not my math. It's the universal excepted math for calculating . compression ratio. Add the swept volume and trapped volume. Divide by the trapped volume. The swept volume on the afore mentioned saw is 43.8 cc and the trapped volume is 3.4 cc which adds up to 47.2 cc then divide 47.2 by 3.4. The math for finding the trapped volume is more complicated and I don't have the time or inclination.


Using the total cylinder volume is a nominal compression ratio, like 4 strokes are calculated. But the actual compression ratio for a two stroke is the volume when the exhaust port closes. Clearly the actual CR is not 15-18:1. More like 6 or 8 to 1.


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## Dennbb (Jan 23, 2021)

kevin j said:


> Using the total cylinder volume is a nominal compression ratio, like 4 strokes are calculated. But the actual compression ratio for a two stroke is the volume when the exhaust port closes. Clearly the actual CR is not 15-18:1. More like 6 or 8 to 1.


You need to study more.


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