Rotax Max compression

Hi guys, has anyone on here ever measured the compression on a rotax senior pre-evo engine ? If so what were your results ? Does anyone know what normal is?


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Good question. I don’t have an answer as far as pumping pressure is concerned I’m afraid. Web search seemed to turn up figures around 160psi which sounds plausible, but I wouldn’t stick my neck out for it.

Couple of anecdotes for two stroke compression testing.

Cylinder lube can make a big difference in the reading, so try to maintain a constant amount between tests.

Gauge readings vary, so comparing your psi/kpa with another’s can be tricky. For sure, you’ll want to at least get a baseline with your own gauge (note air density if you can) to compare.

Hey James, yep I am aware of all the points you mentioned.

I bought a mityvac digital gauge off Amazon to track mine and it’s been around 150psi constant with throttle wide open and engine at operating temp…sea level pressure.

Was just wondering if anyone else ever checked.

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Yip do it all the time to gauge ring wear.
They range from 145-155psi for good senior engines.
Engine cold for consistency. Never done them hot.
We run squish 1.0-1.15mm here.

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Hi Marin, and this is on the pre-evo seniors? Or does it not matter?

No difference between the 2. :grinning:

Good deal.
I’m always cognizant of folks that are merely reading the topic as well, so I wanted to add that context.

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Marin, does the amount of compression a particular engine makes translate to which would perform better or worse? For instance, in the other post we were discussing the barrels…would one of those cnc barrelled new cylinders put out more comp?

New CNC barrels are no different as the squish is the same. And the port timing small differences are non existent at cranking speeds.
Static compression is a nothing value on all performance engines (4 stroke and 2 stroke) as there are so many variables that contribute to it. but generally in these motors, running low squish and high compression does help bottom end. whether this is due to change in port timing or compression or a little of both is very subjective. Don’t get to hung up on it as its something you can do much about with the control specs of the rotax. But that being said I have checked to compression on a couple of engines that I knew were fast and their compression was at the higher end. I have know idea what squish they were running though. High exhaust port and a carefully prepped reed block is the critical areas of these engines.

Ok understood on what you’ve said. I have checked the compression on one of the fast engines here and it was 157psi vs mine at 150psi. I am considering switching to one of the new cnc barrels and see if I get any performance gain as everyone else is still on the older barrels.

I think it’s worth comparing other factors too if you have access to the higher PSI one. Perhaps do a leakdown on both to see if it’s related to ring seal. Measure squish. Verify\compare head volume. Ring end gap. Anything you can think of really.

I agree. Reed block set makes a difference as the curve of the petals varies and changes with use.
Also oil type. Most run Maxum 927 here just for the ring seal action of castor oil.
Chris, where are you looking for a performance gain (rpm wise)?

I run Bel-Ray H1R which is 100% synthetic ester based. I’ve heard the castor builds up carbon in the engine that’s why I never used it.

In my opinion my engine is not stronger anywhere so its an overall performance boost I think.

Yeah, I’m not convinced in power gain…its just the mantra. You know the old story, someone says something and everyone goes but…what if it does…better run it. never used 100% castor based oils, Maxum is meant to be a blend I think.