compression numbers question

[bob johnson]

many people say that the actual number isnt as important as if they are with in 10% of each other, and thus the motor is runnign smoothly...but

you hear numbers like 90 psi

and then the next guy has 120 psi across the board , and still another guy has 150 psi across the board...

i have always thought, heck same motor same year same modle the motor with the higher compression motor should seemingly have more HP!!

well i went and bought a used set of heads for my 1995 60 degree looper 115 hp, because i wanted to try and look over the possible causes of one of my heads running hotter than the other..

they came in nice, and cleaned them up and removed the seal inserts for the T-stats...then i went and took a head off my spare moroe which is also a 60 degree looper but it is a 90 HP isntead of the 115, but the parts all interchange. I t has been said that the difference was all in the carbs...well after i cleaned up the 90 hp head, i measured them ..

sure enough the combustion chamber is about .080" deeper on the 90 hp than the 115 HP.

since the bore and stroke are identical... the compression MUST be higher in the 115HP!! RIGHT???

this all leads me to think that compression is VERY important not only in comparison to the other cylinders, but also to the amount!!!

the higher compression a motor is , the more it probably will produce the HP sticker on the COVER!!!!

Funny on the site i look up parts, they say the 90 hp head will fit the 115 and 90 hp models..

sure they FIT, but they are not the same!!!!

bob

 

[JB]

Re: compression numbers question

I agree that a higher compression RATIO generally makes more power, Bob, but static compression psi is not a measure of compression ratio.

Here are some reasons why. The psi you would measure in a compression test of a 2 stroke outboard can be changed by:

Engine temperature.
Amount of lubrication present on cylinder walls.
Quality of the seal in the compression guage.
Accuracy of the compression guage.
Condition of the battery, wiring and starter motor.
Skill of the person taking the measurements.

None of those factors has anything to do with the compression ratio or the engine power, but each will affect all cylinders equally. Factors that are in one cylinder only (Broken ring, failed head gasket, carbonized rings) will cause that one cylinder to measure very different psi than the others.

Therefore we call a test that produces nearly equal readings on all cylinders an indicator that the engine is healthy and don't pay much attention to the psi readings beyond that.

There are some exceptions. If a head gasket fails between cylinders they will both read low. In a two cylinder engine that requires visual inspection if both cylinders read very low. . . .like 50psi or less.

In 4 stroke engines the rules are very different, mostly because of valves that can leak and are often imperfect in all cylinders of an auto engine, for example.

 

[bob johnson]

Re: compression numbers question

thanks for the reply....my interest peaks when I had figured that the heads on the 60 degree looper eagle motors were the same since they INTERCHANGE between 90 hp and 115 hp.

now if i was to buy a used motor of each and i got 130 psi i each cylinder...in both motors (one 90HP and one 115HP).. i would think i did good...those are numbers that have been thrown around alot...120 PSI and up is USUALLY the sign of "GOOD COMPRESSION"

now imagine my suprice when I now know that the bore and stroke of each is the same...but the Cubic centimeters of volume in each head was much different!!

id think..whoa...the 115 should have MUCH higher compression numbers

and to think that the carb throats are much larger on the 115 hp as well

but they are cramming that into a smaller volume head..

SEEMS strange...the psi numbers would be similar!!

but then again, i am learning 2 stroke mentality after years of only knowing 4 stroke engines and teminology..


this all fell into my lap because i happen to have a motor of each HP of the same series....

I went and looked up the 9.9 and 15 hp of earlier years, because they shared the same kind of components, only to have a few things different that made for the HP difference..

and the heads were ecaxtly the same part number on the 9.9 as the 15...so same volume....so the increase was all carb and porting..

BOB