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Discussion Starter · #1 ·
:) Hello !

Today I was reading a couple of old SnowTech magazines.
One thing that got my intrest was that they stated in the magazine that the 800 HO engine comes in 2 different versions !
Hey whats this I thought.

The 800 HO engine was introduced in 2003 in the Summit 800 X ( ZX ) and the 2003,5 Summit 800 HO ( REV )
In 2004 it found its way into the rest of the REV fleet and was a couple of years later superceeded by the 800 PT engine.
But back to my Questions...

SnowTech states that the 800 HO comes in two different versions

Quote : SnowTech ( March 2003 ) Whats New for 2004

"The 800 HO was optimized for maximum power at higher elevations.
That means they ( ROTAX ) had so much compression it was questionable safe at lower elevations unless the fuel was fresh premium.
Enough flatlanders wanted the 800 HO package, so Rotax had to figure out how to make a version safe for sea level running.
The solution is a slight reduction in compression from 13,5:1 ( Summit ) to 12,5:1 ( Mxz )
This should provide safer operation yet a significant power increase over the standard Rotax 800 twin. "

Quote : SnowTech ( Feb 2004 ) Dear ralph

"The Summit 800 HO has a higher 13,5:1 compression, where the MXZ 800 HO version has 12,5:1 compression ratio.
Both specify 91 Octane is required on the gas cap, but the Summit has a timing compensation feature that the MXZ does NOT have, were the timing is advanced as you climb in elevation , and retards the timing as you come down in elevation.
This does give us a bit of fudge factor to work with.
This is what it boils down to - if you left the stock head on the Summit, you would have to be pretty faithful in using fresh 91 Octane fuel.
Does this mean you'd damage it if you took on a tank of 89 ? Probably not.
If you ran multiple tanks of 89 ? Then it might depending on if you're holding it open on a lake for a long pull, or if you're working it hard in wet heavy snow.
Ski-Doo says that the Summit DOES require at least 91 Octane fuel.
Install the MXZ head on a Summit engine and you´ll be 89 safe for sure, and very likely 87 safe with the timing retard factor.
According to Ski-Doo . it should be 87 safe, but I'd like a bit of cushion here.
How about installing the higher compression head from the Summit on a MXZ ?
I'm sure a few will try this but without the variable ignition timing feature of the Summits, it will be that much more critical to have good fuel.
It has been suggested that in this case 93-94 Octane may be required for long hard pulls for long term reliability.
We'll see how this plays out in the field as the season progresses."

I then go and check my sleds ( 2005 ) Owners Manual and it clearly states : 800 HO LA

Does LA stand for Low Altitude ?
I even check my identy tag on the engine but there it was nothing about the LA version, ?

What does this compression thing consist of ?
Other head and other base gasket, or ?

I have never heard of that the Summit 800 HO also had a different ignition !
Can somebody confirm or explain some more ?

:unsure: HELP ! :crazy:

4,869 Posts
you can run higher compression in the mountains with hotter timing

same engine just diferent timing and gasket thickness
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