Mixing Mogas with 100ll

Hi Michael

The 100/130 was for full leaded fuels, lean then rich condition.  The 130 was to describe how the fuel reacted to detonation at full rich for max power when going into high performance climb or with fighter aircraft for power in extreme conditions such as a dogfight.  The introduction of 100 LL, low lead, was to clean up heavy deposits in engines that would foul spark plugs and cylinder heads along with other things.  At that time they dropped the double ratings for any normal flying.  

As far as Rotax goes it is certainly, even in the carb versions, lean at the normal cruising power settings.  This would be the lean burn area around 65 to 80% power settings.  This is simply to have a cleaner burn and save fuel.  When it comes to the injected versions we see a 20 to 25% fuel saving at these same settings.  The takeaway here it is indeed lean and not running rich at all.  

There is no doubt that engines that run lean of peak are more susceptible to detonation.  For this reason the MON fuel rating of 100 in Avgas is a very desirable number to fight detonation.  Swift 94 UL (UL stands in this case for unleaded) has been used for years by some operators in Rotax and seems to work fine.  The other bonus for us is the Reid Vapor Pressure, RVP, in Avgas is much better to reduce any vapor locking conditions.  

Cheers

I fill the aircraft with 91 UL Chevron, plus 30 gallons in cans in the last month summer fuel is available. In my region of California we get summer fuel until late October.  But once on winter fuel, I always mix 50/50 with avgas.  Like Octane, RVP is linear. Most winter gas has an RVP of about 12, and avgas is always about 7, mixing 50/50 will get you to a reliable RVP of under 10.  Honestly, I’m more concerned about the RVP of winter fuel than I would be about running fuel with AKI of 90.  We can get a hot day in the winter and I don’t want to be using a fuel with an RVP of 12-13.  

I did a little digging on the 100LL -vs-100/130 question and found a concise article at General Aviation News that explains it well.

https://generalaviationnews.com/2007/10/19/the-difference-between-100130-and-100ll/