Rotax ultralight manuals for the 503 show CHTs of 350-430F as the normal range with max 480F.
The old 377 was about the same range - so your 380 Formula looks to be "right"

350F is about where I would want to be with any trail sled. We used to run 400 on the race sleds.

 

Rotax ultralight manuals for the 503 show CHTs of 350-430F as the normal range with max 480F.
The old 377 was about the same range - so your 380 Formula looks to be "right"

350F is about where I would want to be with any trail sled. We used to run 400 on the race sleds.

Right on thanks very much....most of the info I was finding was all for aviation. Guess I'll have to lean out the skandic tomorrow and see how it runs.

 

An additional note from an aircraft perspective, the 480 is the maximum CHT because it is approaching the plastic deformation limit of the aluminum heads and pistons. Plastic deformation is where the shape changes and will not change back as it cools.

Also, further instrumentation to keep an eye on would be your exhaust gas temperature, EGT. EGTs give a better idea of what your air/fuel ratio is doing and could help prevent a melt down due to a lean running condition from a plugged up carburetor.

Another thing to keep in mind is to not trim the instrumentation leads when installing passive thermocouple instruments like CHT and EGT as wire length can dramatically affect your readings. Likewise ambient temperature of the connection at the back of the gauge can influence readings far more than simply adding or subtracting the temperature ie: EGTs tend to read 100 F higher in cold temperatures.

Hope this information is of use.

Doug M

 

An additional note from an aircraft perspective, the 480 is the maximum CHT because it is approaching the plastic deformation limit of the aluminum heads and pistons. Plastic deformation is where the shape changes and will not change back as it cools.

Also, further instrumentation to keep an eye on would be your exhaust gas temperature, EGT. EGTs give a better idea of what your air/fuel ratio is doing and could help prevent a melt down due to a lean running condition from a plugged up carburetor.

Another thing to keep in mind is to not trim the instrumentation leads when installing passive thermocouple instruments like CHT and EGT as wire length can dramatically affect your readings. Likewise ambient temperature of the connection at the back of the gauge can influence readings far more than simply adding or subtracting the temperature ie: EGTs tend to read 100 F higher in cold temperatures.

Hope this information is of use.

Doug M

Great info, thanks very much. Is the higher EGT temp in cold weather caused by the denser air leaning out the mixture?

 

I have never been able to get a reliable answer to that question. Some believe the higher temperature is due to denser air making the engine effectively leaner and thus a higher EGTs. The other train of thought says that ambient temperature effects the mechanical junction of the wires causes the high readings. Me personally, I always erred on the side of caution on my two stroke ultralight and ran one notch richer on the needle once the temperatures cooled off.

Great info, thanks very much. Is the higher EGT temp in cold weather caused by the denser air leaning out the mixture?

 

Denser Air does the oppisite, it enrichens the Air/Fuel mixture. That is why you need Bigger Jets at Higher elevations or Colder Temps. Leaner Burn equals Higher Temps.

 

Denser Air does the oppisite, it enrichens the Air/Fuel mixture. That is why you need Bigger Jets at Higher elevations or Colder Temps. Leaner Burn equals Higher Temps.

I respectfully disagree with a few things. Denser air does not enrichen the fuel mixture, it leans it out. Denser air increases the air drawn per stroke, once that air reaches engine temp. This leans out the fuel mixture and this is why you increase jetting at lower temps. The additional fuel compensates for the additional air. Also, at higher elevations, you require smaller jetting to compensate for the thin air, not larger.

 

Yes My Bad, Thinner Air, requires Leaner Mixture.

Reminds me of a Radar Run.

1/2 mile, -29F, I put down a 78 mph Pass early, I tried a few runs after and never beat that number.

My nemises a 340 Twister, (I was running a 7500 Blizzard) he made many runs but only made 77 mph.

Then late he placed a 81 mph run. I was shocked. I went and asked "What did you have to do?" He dropped 5 sizes on the Main Jets.

A Rotax would have been brought home in a Box, 5 sizes!

 

Out of curiosity which gauge did you go with?

 

I am also interested in which gauges you used?

 

I used one made by trailtech that I bought at Royal. Internal battery is still working at the point.

Here's an amazon link so you can check it out. There are newer models available that are powered off the sled, have backlit displays, gps speedometers, etc. if you want to spend a bit more.

Amazon.ca

 

I used one made by trailtech that I bought at Royal. Internal battery is still working at the point.

Here's an amazon link so you can check it out. There are newer models available that are powered off the sled, have backlit displays, gps speedometers, etc. if you want to spend a bit more.

Amazon.ca

Thanks man!