What Would Be “Acceptable” in Practice.... compression test.

[Mikal]

New to Laverdas, (1975 3C) new to this forum (thanks for having me). I am sure this has been asked but I can't find it so reluctantly I turned to ChatGPT for an answer on what compression numbers are good. Coming from the Ducati world they seem low:

Condition	Approx psi	Notes
“Perfect / new‑ish / ideal cylinder”	~ 125–135 psi	Around spec (i.e. 130 psi)
Slightly worn / “good”	~ 110–125 psi	Still usable, though less margin
Borderline / suspect	~ 90–110 psi	Might show loss of power, suspect wear
Too low / needs attention	< ~ 90 psi	Likely need top end work, valves, rings, head gasket

 

[Vince]

Typical Jota with 10 to 1 pistons are 150psi, mine was 205 at one point, but it's a long way from std.

 

[Mike_P]

My ‘79 Jota 180 is 175 psi each pot if that helps

 

[Paul Marx]

Unless one is catastrophically low, it imports that they're more or less equal across cylinders. Compression will very much depend on general wera of the engine.
An old engine can have high compression due to coke in the combustion chambers.

Paul

 

[Rob]

Standard 3C won't be 10:1, so I'd expect to see 150 - 170 tops. What matters as much is that all 3 readings are similar

 

[piranha-bro2]

Different cams also create different comp results. Even running slightly advanced (for more midrange) could affect the reading. It's only turning at starter motor speed.

 

[iis_iis]

FWIW, do a wet and dry comp test, first the dry on all three, no more than 5-10% difference with wide open throttle, ignition rendered inoperable. Then the wet.... then the smoke screen post testing, a smile here.

As the lads indicate, matter of many factors, if a standard style 3c motor, then 150 psi would be a good test, perhaps a little less down to 120, lower than that i would be alerted. Wet test may rise another 40-60 above dry test. HTH j.

 

[EncinoBoy]

Mikal: Stay away from the Chatty universe and all of the rest of the fourth hand mis-information
on-line channels. Ask us as the experience and knowledge level is "The Source".

 

[hooksey]

Let sleeping dogs lie…….. you will only be disappointed

 

[Mikal]

Thank you all for your replies. As I mentioned I come from the world of Ducatis where 150 psi for the machines I work on would be the norm. The 3C I have in my shop is 135-135-140 psi. Borescope showed cylinder walls in good shape so I figure I'd leave well enough alone. I was just curious as to what a "normal" 3C would yield. The PO stuck 4C cams in it but didn't swap pistons. Cheers.

 

[CLEMTOG]

a leak doen test is a much better way of determining the condition of your motor.
CLEM
who once did 40,000 KM with a deliberatley very low comp ratio of about 7:1

 

[Alzero]

a leak doen test is a much better way of determining the condition of your motor.
CLEM
who once did 40,000 KM with a deliberatley very low comp ratio of about 7:1

I didn’t want to go there, but I’m glad you did!