Compression Specs and 2-Stroke Test Procedures published by Rotax?

[Riverguy42]

While scouring the web looking for compression specs and test procedures actually published by Bombadier or Rotax, the interesting thing is that the ONLY place I can find these is in the maintainence manuals for the 2-stroke Rotax AIRCRAFT engines. This might be because the FAA forces aircraft engine manufacturers to do this, but why would Rotax keep this 'confidential' for Jet Boats and PWC?

My factory Sea Doo shop manual (1996 Sportster/Speedster/Challenger) does not specify either the psi numbers, nor the correct procedure for testing.

From the Rotax aircraft manuals I'm seeing minimum acceptable numbers of 80-87psi. The highest 'minimum' number I can find is 100psi from an authorized Rotax Aircraft Engines dealer in California, who also says "below 80 psi is time for top-end rebuild". The same dealer says the top of the 'good' range is 120psi while cranking a warmed-up engine at a minimum of 300rpm.

I can't find any reference to any compression spec above 120PSI.

Here and on the various PWC websites I'm reading things like "150/150 is perfect and 120 it’s toast". Where are these higher numbers coming from, and what is the procedure for getting them? Also, several posts have raised the question as to why Sea Doo doesn't specify what the compression numbers should be?

I was seeing 123/125 on my new-to-me Rotax 717 (1996 Sportster). I used a precision pressure gauge to verify the calibration of my compression tester and battery was strong and cranking very fast, throttle wide open, engine was warm.

After reading "...and 120 it's toast" I was getting ready to do a top-end teardown, but now I am reading that these numbers are 'perfect'. Likewise, the nearby Sea Doo dealer says 120 psi is about right for these engines when new. With a new impeller and wear ring it's running perfectly, topping out at ~43mph.

Can anyone say where these 150 psi numbers are coming from, and what is the detailed proceedure for getting them?

 

[mikidymac]

Where?
From 30 years experience working on these, even when they were brand new in the 90's, testing brand new engines from the factory and knowing what the compression is when they won't run in the water anymore.

A perfect new topend from the factory or quality rebuild will be at 150 psi on the 580-787 engines and 130 on the 951 carbed engines, Di is a little higher. At 120 psi for the 580-787 and 110 on the 951 the engines are worn out and typically will not start in the water.

Correct procedure is a good calibrated gauge with the correct schrader valve location in the tip of the hose. Fully charged battery, sparkplug out with the wire grounded and crank with the throttle wide open and crank until the gauge needle stops rising. That is your cranking compression.

 

[Poormanracing]

To many variables to do a compression test a rich running ski will give you higher readings just do a leak down on the top end

 

[slatere17]

@mikidymac Can you elaborate on the reason for holding the throttle wide open? I have an educated guess for a carb ski, but I've personally measured the difference on an RFI ski (~6psi) which I don't have a good explanation for.

Also, any idea how altitude affects the compression readings? I'm in CO so our air density is around 86% of sea level (5000'+) which would in theory put our "perfect" 150 psi at only 129. I'm sure it's more complex than this but i'm sure it has a measurable impact.

Also FWIW, I have a 97 (carb) and 98 (fuel injected) and they are both well above the 120 mark with plenty of hours on them.

 

[mikidymac]

The throttle open allow more air in to be compressed.
On the RFI ski it also kills the fuel injectors.

Yes, your compression should be lower at altitude but I am not sure what it equates to.

 

[JC-SUPERJET]

Just the Air Density Ratio. Air Density at Altitude versus Sea Level Air Density.

Assuming the Stock New OEM Compression Readings are taken at Sea Level, then multiply by:

Altitude Factor
1000' .9711
2000' .9428
3000' .9151
4000' .8881
5000' .8617
6000' .8359
7000' .8106
8000' .7860

"When performing engine cylinder compression tests, your readings will get lower the higher in altitude you go. Here is a chart to show the correction factor."

Temperature also affects the Air Density so that Chart is an approximation. Close enough.