Compression test psi

Thundersporter

New Member
Has anyone carried out a compression test on a 390. I was wondering what a good psi reading is. Did mine yesterday with the decompressor disabled and got 200 psi, but don't know is that's good or bad.
 

Fasteddy

Member
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I would go with good, IIRC there was a thread that calculated it to be around 130'ish psi, 9.5 bar, 137 psi is the spec on my LC8 KTM.
What symptom are you trouble shooting?
 

musti

Member
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I'm a bit too late to the party, but still would like to share my findings as of today.

As per the user manual, the compression ratio for KTM 390 is 12.88:1

To convert this to PSI (pounds per square inch), the compression ratio has to be multiplied with atmospheric pressure. The atmospheric pressure at sea level is 14.69 PSI

Therefore, 12.88 * 14.69 = 189 PSI

Now, KTM 390 engines come with auto decompressor pin on the left valve exhaust cam. Therefore there will be a lower pressure reading.

Hence, the compression range should be between 90 - 120 PSI [around 8 kg/cmsquare] - wide open throttle - dry

If you add oil (to check for piston ring leak) then the compression will increase a bit - which is a good sign.

I have not tested it on my end, but if you OR anyone tests it out, I would be grateful to learn about it. I am thinking of purchasing a compression tester/ guage myself. May be I would be able to share.

Hope this helps.
 
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1JZSupra

Active Member
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Did you try a leak down? When tested mine it was high on compression like that as well.... even with a blown head gasket lol.
 
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