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1990 Johnson 25 compression high elevation

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mrivera77524

New member
Amigos,
First question is what is good compression for this motor at sea level? AI/google giving me two different answers, the first being 80-90 and the other being 90-105. So, what IS the right answer?
Second, at higher elevation should compression read lower? I've spoken to a racecar mechanic in my town that says "yes, you'll lose 15% compression due to our elevation" and the internet says multiply the regular compression by 86% and that'll give you your compression at this high, which is pretty much 15% less. Other mechanics have told me elevation doesnt matter.

The motor im looking at reads 75psi on both cylinders and I DID NOT have the throttle open. Didnt learn about that until this morning after I tested it. Thank you in advance for the help.

Marcos
 
Throttle does NOT need to be open on that 2 stroke!----Elevation does matter.----What elevation are you testing at ?----Do you have a known good gauge ?
 
Elevation is grade 10 physics. At sea level a standard atmosphere is 14.7psi. If your motor has 7:1 compression… 7x14.7=103
As you increase elevation, the column of air is less, a standard atmosphere at 5200 feet is about 12.1 psi. Therefore, in the same motor., 7x12.1=84.7
On a really hot day, the air expands,has less density, and again you have less initial pressure to compress.
Same thing with a low pressure area, taken to an extreme, if you’re in the very centre of a strong hurricane you develop less power
That is the reason behind Turbo, charging and supercharging, to increase the initial pressure of the air charge going into the cylinder
 
Edchris177 said:
Elevation is grade 10 physics. At sea level a standard atmosphere is 14.7psi. If your motor has 7:1 compression… 7x14.7=103
As you increase elevation, the column of air is less, a standard atmosphere at 5200 feet is about 12.1 psi. Therefore, in the same motor., 7x12.1=84.7
On a really hot day, the air expands,has less density, and again you have less initial pressure to compress.
That is the reason behind Turbo, charging and supercharging, to increase the initial pressure of the air charge going into the cylinder
Click to expand...
Understood.
Now I just need to know what “good” or “acceptable” compression is at sea level for my motor. Once I know that, I’ll know what “good” or “acceptable” compression is for that motor at 5,200 feet.
 
Intake and exhaust ports are WIDE OPEN as piston moves from BDC .----Compression does not start until after exhaust ports close.-----Many folks just are not aware of the simple fact !!----Repeat---Throttle position does not matter on your 2 stroke.
 
mrivera77524 said:
I’m at 5,200 feet.
Why does throttle not being open NOT matter? Other forums and Google say it does.
Click to expand...
Perhaps your Google searches have come up with four stroke motors, as you know, a two-stroke works differently.
Plug this phrase into Google search, and amazingly the AI explanation is pretty good.
“why doesn't throttle position matter when measuring compression on a two stroke”
 
Edchris177 said:
Perhaps your Google searches have come up with four stroke motors, as you know, a two-stroke works differently.
Plug this phrase into Google search, and amazingly the AI explanation is pretty good.
“why doesn't throttle position matter when measuring compression on a two stroke”
Click to expand...
Understood. ✔️
Now, the question is: is 75 psi on a 1990 Johnson 25 2 stroke “good” “bad” or “really bad” at 5,200 feet of elevation. That’s the real question here.
 
Last edited:
Ok so I found this out. I bought an aluminum boat that came w a 1982 Johnson 25. Didn’t know anything about it. I was told I couldn’t get parts for it. So I sold it for $100. I asked the guy about compression and he said it runs and has 95psi in both cylinders. So. That sucks. Shouldn’t have sold it. And that tells me the 1990 w 75psi has low compression. Aghhh I’m a little frustrated. But live and learn.
 
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