Compression In Different Engines?

[Vintage Engine Repairs]

The real simple answer is half the load. I can run 29° full advance on 18 or 25hp old Rude in stock trim. Add bigger boat, more wheel or loaded and it will ping on 29°. No big deal. Change the fuel or back off a bit on full advance.

My milling saw is at 32° with 18 off the base no band cut. Now with 63 off the base and a band cut, not flat either, it hates 32° starting with no decomp. Tested it again last night with a 42 404 7p. It needs to come back but now it's on pump 93 and ran fine on 89 before. The game has changed enough it might only want 25 now with higher compression. My guess is near 200psi. I'm already on the ragged edge and managed to rework the intake system now running 193°D. She'll eat but how will it handle 100° temps. Last summer it was parked. The years before it ate 40" oak on hot laps in after noon sun mis summer no sweat. If it runs hot I can open the chamber some or go to 94 rec gas with the ignition backed down more. Have mind set on a new chamber shape anyway.

Need a link to that forum please. I have my old Rudes and a few inline Mercs still resting in trailer boxes. A few V4s and V6s of each. All carb engines. The old fun ones that suckup gas with crossflow setups.

That’s very Interesting, 32 degrees is standard advance at full throttle especially those that have variable advance. If you’re getting problems with that, I wonder if it’s likely your fuel, tuning or something you’ve done to the saw when porting?

200 psi isn’t that much higher than new stock - my 034 super Is 187psi bone stock and not seated rings. 25 degrees is way ******** even on 1960’s long stroke muscle saws which are generally around 28. A lot of ****** and increased compression and you’ll be losing performance rather than gaining it. Sounds like some weird stuff going on.

What saw is it?

 

[bwalker]

And they would still send you over the handlebars when it kicked back.....

I've heard people say that, but honestly I never found them all that difficult to start. Just don't try kicking one over without a boot on.
Now days alotnof guys add decompression valves to them.

 

[N8Dawg]

Need a link to that forum please. I have my old Rudes and a few inline Mercs still resting in trailer boxes. A few V4s and V6s of each. All carb engines. The old fun ones

Man, I'm having no problems what so ever mucking up my posts, trying to operate this new to me forum. So if that's the case, you know why.
https://www.screamandfly.com/showthread.php?99345-Octane-vs-compression

 

[sean donato]

I've heard people say that, but honestly I never found them all that difficult to start. Just don't try kicking one over without a boot on.
Now days alotnof guys add decompression valves to them.

it was a 96, no compression release. Hardly went 150lbs back then, it was my older brothers bike. Sent me flying a few times.

 

[EchovsStihl63]

It’s generally accpected 2 stroke engines need at least 120 psi compression minimum to run. However, I saw a video on an old Johnson
outboard twin cylinder engines has 70 psi and runs great and the tester was accurate, he tested his chainsaw that made 150 psi. What determines the amount of compression that is required for a 2 stroke to run? How can one engine only need 70 psi where another needs 120 better ideally 150?

I worked at a Marina as a teenager, we had an old Johnson 75 2 stroke on a pontoon fuel barge for another rental location that we ran back and forth to the main location to fill up every few weeks.

One day I was riding it and it just started sounding terrible and quit. So I sat for a bit and hit it and it started back up but I called back to the mechanic and told him what happened and that I thought the oil injection was going out cause it hadn't used that much oil. He said something along the lines of if its running it must still be getting oil and to keep driving, lol one of those "yes sir" and ask no more questions deals.

5 minutes later she seized for good that time so I had to wait on him to come out and tow it back with 300 gallons on board.

Anyway it cooled off and un seized by the time we got back and to our surprise actually started but basically was struggling just to idle. Compression gauge showed all 4 cylinder between 55-60psi, and you guessed it, the oil injection had stopped working, but 18 year old me couldn't possibly have any clue about what was going on haha.

 

[Vintage Engine Repairs]

I worked at a Marina as a teenager, we had an old Johnson 75 2 stroke on a pontoon fuel barge for another rental location that we ran back and forth to the main location to fill up every few weeks.

One day I was riding it and it just started sounding terrible and quit. So I sat for a bit and hit it and it started back up but I called back to the mechanic and told him what happened and that I thought the oil injection was going out cause it hadn't used that much oil. He said something along the lines of if its running it must still be getting oil and to keep driving, lol one of those "yes sir" and ask no more questions deals.

5 minutes later she seized for good that time so I had to wait on him to come out and tow it back with 300 gallons on board.

Anyway it cooled off and un seized by the time we got back and to our surprise actually started but basically was struggling just to idle. Compression gauge showed all 4 cylinder between 55-60psi, and you guessed it, the oil injection had stopped working, but 18 year old me couldn't possibly have any clue about what was going on haha.

Haha thanks for sharing. Fun to look back at memories like that. Better to just bypass the oil pump and premix lol

 

[huskihl]

Have a collection of older v6 Mercury outboards myself. The early 3 litre had 90lbs-110lbs per the manual. Mine has 85-90. The 2.0, 2.4, and 2.5 liter have 110 - 130lbs, per the manual, Some have the exhaust notch that give a lower reading at cranking speed as air escapes easier than at higher engine speed.
I frequent a performance boat forum, the good engine builders, and ole time hot rodders recommend no more than 150lbs of compression and run on pump gas. As it causes detonation, and the pistons will melt with these engines. It is also general consensus at this boat forum, that higher compression increases low end power, but not so much in the higher rpm range, it can actually decrease power if detonating. These engines run about 25 degrees of timing at about 6000rpm.

I'm curious how some of the guys cutting squish, and base for 200+lbs of compression on a chainsaw, are getting away with pump gas? Seems that would melt the piston with sustained use, or the big end bearing, and crank wouldn't live long. If I had to venture a guess, it might have something to do with the rpm, and timing these engines run compared to a marine application.

Whether or not an engine will detonate is dependent on compression ratio, not compression. And the bore/stroke ratio comes to play as well. A ported saw with 225 PSI only runs about 8.5:1 compression ratio. Most of that is because the piston is so large diameter compared to the stroke.

 

[N8Dawg]

If I did my math correctly, (probably should have stayed in school longer, instead of going to the woods). A 2.5 litre merc would have about a 6.6 dynamic cr ratio, and blow about 110-115 on the gauge. If the heads were cut for 145-150 psi, the dynamic cr would be about 7.5....
I so much want to divide 14.7 into a compression gauge, but that's not legit I hear.

I think what Lightning said about load is very relevant also. Boat engines have to be made for a wide range of application. The manufacturer has to over compensate for the fact that these things could be put on a barge. Idle relief slots, and lower compression likely saved a few warranty claims.

 

[bwalker]

Whether or not an engine will detonate is dependent on compression ratio, not compression. And the bore/stroke ratio comes to play as well. A ported saw with 225 PSI only runs about 8.5:1 compression ratio. Most of that is because the piston is so large diameter compared to the stroke.

Very true and while the bore is large in relation to the stroke, the bores are still small and thus resistant to detonation. The reason for this is detonation takes time and a larger bore requires more time to combust completely and thus detonate easier. Which is also why high octane fuels do not burn slower....

 

[bwalker]

If I did my math correctly, (probably should have stayed in school longer, instead of going to the woods). A 2.5 litre merc would have about a 6.6 dynamic cr ratio, and blow about 110-115 on the gauge. If the heads were cut for 145-150 psi, the dynamic cr would be about 7.5....
I so much want to divide 14.7 into a compression gauge, but that's not legit I hear.

I think what Lightning said about load is very relevant also. Boat engines have to be made for a wide range of application. The manufacturer has to over compensate for the fact that these things could be put on a barge. Idle relief slots, and lower compression likely saved a few warranty claims.

Turning a screw is inherently high load.

 

[N8Dawg]

I

Turning a screw is inherently high load.

I try to mitigate that as much as possible by aeration, and surface drive.

 

[Okie]

I've seen several small chainsaws (30-40cc) start run and cut good on 70-80 psi. (that should have 135# or more)
I've got about 3 very accurate small engine compression testers and I get more info9 by looking through the muffler port at the piston first nd if I see damage I don't care too much about the compression. I've seen high compression on them with severely scored piston and I then know it's on borrowed time. If the piston is not scored and it has low compression I know then that it might can be saved.

On multi cylinder engines I pay attention to cylinder balance. If one is weak and others are normal it's a heads up time.

 

[ZeroJunk]

FWIW, Stihl 4Mix motors have a little tab sticking out of the cam that centrifugal force pulls back in at RPM.

So, compression gauge not much use.

 

[Okie]

FWIW, Stihl 4Mix motors have a little tab sticking out of the cam that centrifugal force pulls back in at RPM.

So, compression gauge not much use.

So, compression gauge not much use.

I'm assuming this is a question.
Yes it has good use but not the first thing I go to when checking a chainsaw, but for small cc engines it needs to be a accurate reliable tool.
That is why I have two gauges for when I see a low reading on a small cc engine I verify with my other gauge. (I've seen them compression gauges read wrong and will cause a guy to get on the wrong troubleshooting page fast)
I've seen few times (more than once) where a good auto mechanic would bring me a chainsaw that would not run and they had overhauled the saw with new piston and jug and it still had reported low compression. They were using their big high dollar fancy Mac, snap-on AUTOMOTIVE compression tester that was lying to them when used on a small cc engine. When I told them such their jaw just dropped usually saying it reads ok when tested on my air compressor tank and the gauge even reads lower AFTER I installed the new parts in the saw..
Their saws usually had a bad carb and would not run.

 

[ZeroJunk]

So, compression gauge not much use.

I'm assuming this is a question.
Yes it has good use but not the first thing I go to when checking a chainsaw, but for small cc engines it needs to be a accurate reliable tool.
That is why I have two gauges for when I see a low reading on a small cc engine I verify with my other gauge. (I've seen them compression gauges read wrong and will cause a guy to get on the wrong troubleshooting page fast)
I've seen few times (more than once) where a good auto mechanic would bring me a chainsaw that would not run and they had overhauled the saw with new piston and jug and it still had reported low compression. They were using their big high dollar fancy Mac, snap-on AUTOMOTIVE compression tester that was lying to them when used on a small cc engine. When I told them such their jaw just dropped usually saying it reads ok when tested on my air compressor tank and the gauge even reads lower AFTER I installed the new parts in the saw..
Their saws usually had a bad carb and would not run.

No. It was a statement. The cam has a metal tab that sticks out and keeps the valve from completely closing until the RPM comes up.
I know this is the chainsaw forum, but this pertains to 4Mix Stihl string trimmers and backpack blowers.

Just in case somebody pulls on one and says this thing doesn't have any compression.

 

[sean donato]

No. It was a statement. The cam has a metal tab that sticks out and keeps the valve from completely closing until the RPM comes up.
I know this is the chainsaw forum, but this pertains to 4Mix Stihl string trimmers and backpack blowers.

Just in case somebody pulls on one and says this thing doesn't have any compression.

Use a leak down tester for little 4 strokes anyway. Been the std for kohler, Briggs etc for years.

 

[ZeroJunk]

Use a leak down tester for little 4 strokes anyway. Been the std for kohler, Briggs etc for years.

Hell, to work on four strokes I would have to double my IQ .

 

[sean donato]

Hell, to work on four strokes I would have to double my IQ .

I doubt it.

 

[Okie]

Hell, to work on four strokes I would have to double my IQ .

I think that 2 stroke engines are more of a pain upstairs as compared to the 4 strokes.

But whichever o the two you don't like will be the more painful down low and up stairs.

 

[NorthernLonewolf]

I was about to post a new thread about this, but I found this one, so I was wondering if anybody knows what the average compression is for the Homelite 7-29? Mine gives 125 PSI.