Rotax 377 Manual needed

I am looking for a copy of the 1986 377-447-503 owners manual

I am tryng to determine the correct compression ratio for a 377UL motor

I have the 1990 owners manual, and also the 92 and 94 revisions of it.
The 377 is listed as having compression ratio of 11.6 to 1

A well known (but will remain unnamed) site says something significantly lower

I am trying to determine if there are really two ratios - or if the site is in error. Two ratios would mean my rebuild project might have me looking for different heads

Tried e-mailing website "contact-us" link - no reply

There is an earlier -1986 - version of the manual available online. But it is only a partial copy - the technical data pages are not there.

Any old timers out there have a copy of the 1986 manual ? - or any other official Rotax document showing a compression ratio other than 11.6?

Thanks

Looking up the engine online says a compression ratio of 9.3 to 1. I had engine training as A F mechanic as well as a bit of hot rod and sports car and motorcycle experience, but unless 2 strokes are radically different, a compression ratio of 11.5 to 1 would be too high and would detonate on normal gasoline. I tiink an normally aspirated OFFY engiine may be 11 to 1 but running on methanol. I dont have much 2 stroke knowledge or experience.
We built a Starlite with a Rotax 377, back then but I dont have it or the manual any more, you might try to see if anyone has that type of plane. It sounded loose but ran good once under power. A tiny and fun little airplane. Unlike so many basic and cheap homebuiits,, this one wasnt a powered hang glider or putt putt,this thing was streamined, light weight and would move, seems to me it would top 130 mph. Test flying it was an experience may a bit dicey when I found cockpit was too small for a regular parachute.

Not sure if this will help or not. But you may find your answer or info you seek here.

https://www.cps-parts.com/catalog/rtxpages/articles.php

Tony

Go to rotaxowners.com and you should be able to pull up the specs on your engine. There are two types of compression listings for a 2 stroke. I can't remember the specific connotations but one is for when you crank it over by hand and the other is when it is running. I had an old Ducati 1965-66 maybe, but it had a 12.5 to 1 ratio and ran on ethyl pump gas ( mid to upper 90's octane) it was finicky on tuning but it ran like stink. 4-stroke engine.

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Originally Posted by cliffo

Go to rotaxowners.com and you should be able to pull up the specs on your engine.

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Yes already tried there - Their database of engine manuals does not go back far enough to cover the early engines - like the 377. In fact, it does not cover any of the points ignition based engines 377, 447, 503.

It only has data for the engines that went to CDI ignition (447,503). The 377 was never offered with CDI

That's why I am looking for other "old" data from Rotax

Check one of your search engines for, manuals library .com , there's 3 different manuals listed for the 377, this should get you what you need, operators manual, repair manual, and specs.

Ultralight news has all the specs. for the 377 compression ratios included, it will tell the 2 ratios I was referring too.

The 11.6:1 is probably a calculated or theoretical compression ratio. The actual or effective compression ratio is probably in the neighborhood of 7:1.

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Originally Posted by cliffo

Ultralight news has all the specs. for the 377 compression ratios included, it will tell the 2 ratios I was referring too.

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That is the website I was refering to. It has a number of mistakes - easy one to spot is the sparkplug is listed as 12mm when they are 14mm.

Another one is that they list is the 447 as having CDI ignition - then give the contact points gap - CDI doesn't have points.

That is why I am trying to find two Rotax sources saying 11.6. from different time periods, just in case they made changes over the years. Manuals I have cover the last years of the 377 (90's) I need the early 80's stuff to see what was going on then.

If I can find another official Rotax manual that says 11.6 I will know the site is wrong.

If another ROTAX manual says 9.6 theoretical then there must be two different style heads - regular and High compression

If there are two Theoretical ratios - then I will need to widen my parts search for different heads

447"s have been fitted with both CDI and points, I have an 80's model with points, don't know when they became available with
CDI. As far as the heads go I know the older (70's) Cuyunas had a wedge type combustion chamber on the 430 that was a higher compression, but I'm not privy to any thing like that with the 377. You might try referencing old sled motors. Good luck on your quest

Two cycle compression ratios can be "calculated" as in a four stroke using bore and stroke or "effective" based on the exhaust port closing position. This may be the difference in compression ratios you are seeing. Also the start versus run compression ratio accounts for the compression release if installed.

My comment only. The facts may differ and others may have conflicting opinions.