Optimum compression readings for '86 140 Evinrude?

[andys15]

What's the optimum (like new) readings for this engine. Seems to be a wide range of acceptable readings on here. My engine is around 100 psi on all cylinders, yet I'm hearing others on here around 120 psi and even a 150 psi on someone's 85hp evinrude. I though that would have had lower compression being that it uses the same piston I think.

 

[Cricket Too]

Re: Optimum compression readings for '86 140 Evinrude?

100 psi on all 4 is good. Actually very good, for a 20 year old engine. Some motors run higher some run lower, what you should really be concerned with is that they are all equal or within 10% of each other, this matters more than the actual number. You should decarb the engine every 50 hours or at least once a season to keep the compression strong.

 

[Paul Moir]

Re: Optimum compression readings for '86 140 Evinrude?

On a 2-stroke, static compression relies heavily on the port timing. Since port timing is a function of engine design and performance, you cannot compare one hp engine to another. For example, later big V-6 J/E engines are well known to normally produce around 90 psi static compression while my brother's 4hp looper produces around 140psi.<br />So you really cannot compare across the board, or even between a few years as port height is a common 'tweak'. Static compression doesn't have a strong relation to performance on 2-stroke engines.<br /><br />Then there's all the other variables. Gauges and testing practices seldom agree. If you're at a high altitude even, you can expect lower readings. If the engine is warmed up you could get higher or sometimes lower readings. Battery state will even affect it.

 

[andys15]

Re: Optimum compression readings for '86 140 Evinrude?

Originally posted by Paul Moir:<br /> On a 2-stroke, static compression relies heavily on the port timing. Since port timing is a function of engine design and performance, you cannot compare one hp engine to another. For example, later big V-6 J/E engines are well known to normally produce around 90 psi static compression while my brother's 4hp looper produces around 140psi.<br />So you really cannot compare across the board, or even between a few years as port height is a common 'tweak'. Static compression doesn't have a strong relation to performance on 2-stroke engines.<br /><br />Then there's all the other variables. Gauges and testing practices seldom agree. If you're at a high altitude even, you can expect lower readings. If the engine is warmed up you could get higher or sometimes lower readings. Battery state will even affect it.

Interesting, I thought those v-4 engines used the same block and same bore pistons. I kind of assumed the power difference in them was due to stroke and compression (spacers on the heads or something).

 

[Dhadley]

Re: Optimum compression readings for '86 140 Evinrude?

Theres a world of difference between the 140 looper and a 140 crossflow. 100 is about right for a small bore V4 looper. Or even the big bore V4 looper.