Compression In Different Engines?

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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 did a fuel system replace on an old gray Echo string trimmer with 89 psi a few yrs ago.... ran perfect with power even. Still running to this day!

And yeah.... I told the guy it was due for new parts but he got it for $25.00. So if it blows up, not a big loss.... his words.


A lot of outboards got impressive pistons....


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I did a fuel system replace on an old gray Echo string trimmer with 89 psi a few yrs ago.... ran perfect with power even. Still running to this day!

And yeah.... I told the guy it was due for new parts but he got it for $25.00. So if it blows up, not a big loss.... his words.


A lot of outboards got impressive pistons....


View attachment 1151809
They look like the piston on the old Sears gear drive saw. Tecumseh AH 47.
 

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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?
Most 4-stroke have auto compression release on the camshaft for easier starting, so you get a false low reading
 
Compression ratios are a sign of the times and the material available. Engineers understood high compression long ago. They had to because they understood steam engines. More pressure more torque real simple.

The popup pistons being shown are not for high compression but crossflow engines, not loopers. Loopers have flat top pistons most times. Crossflow engines are also detonation prone with long traveling flame fronts and lean spots within the chamber area. Rarely were these 120 psi in old outboard engines. They won't tolerate much beating @130 psi without being polished and rounding over in the hotspot areas. Beyond that they fail from being cast type a lot of times in old stuff. Good pistons from forged slugs ups the game a bunch to 150 psi. No worries at higher RPM if you balance the bottom end proper.

Hand starters become an issue with higher static compression ratios. Old cars come to mind first. Smaller stuff follows suit with no decompression system during cranking.
 
As a kid, early 1960s, BS REEL MOWER, wrap the rope around notched pulley to start. It wud start with a half-wrap and easy pull... wish I still had that mower; Dad bought for $10 used, had been traded in at the local ACE.
 
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.
 
Johnson/Evinrude; I once joined a group and I got a load of abuse, called a liar etc. when I told them my Yamaha 55hp compression pressure was 150 psi even. this is the pressure in the factory manual. I don't understand the OMC guys... anyway...
 
As a kid, early 1960s, BS REEL MOWER, wrap the rope around notched pulley to start. It wud start with a half-wrap and easy pull... wish I still had that mower; Dad bought for $10 used, had been traded in at the local ACE.
I did that with my Johnson outboard 35hp 2 cylinder 2 stroke. (150 psi in each cylinder for reference in this post). I didn’t get away without a number of rope lashes across my chest.
 
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.
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.
 
I did that with my Johnson outboard 35hp 2 cylinder 2 stroke. (150 psi in each cylinder for reference in this post). I didn’t get away without a number of rope lashes across my chest.
Have a pair of those. One is on my 17ft Starcraft boat. The other should be next to it. 1977 engine for one reason, electronic ignition. They are probably the best grunt outboard of it's time in 35hp. It will spin a four blade comp prop stock. Not many under 40hp will from back then. 55hp was the two cylinder beast. Jimmy had one on the crick remote setup 17ft Lonestar. Mine has been everywhere including offshore out front north of AC and out of Capemay Harbor but not in the Delware Bay. It turns fast there and isn't shallow like the upper Chesapeak is. Good times.
 

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