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SB 3-13-01 Ethanol

HDDAHL

Active Member
It is clearly preferable to use non ethanol gas in our RV -12's, as spelled out in this service bulletin. However I cannot get higher than 89 octane fuel without ethanol in southeast SC and northeast GA.

With that in mind I thought a 20-25% mix of 100LL with 89 octane ethanol free would work. Lockwood aviation in strong language advised: never upgrade 89 octane by mixing with 100LL (Mixing 100LL with 91 or higher octane is permitted). Pressed they could not give any technical reason why.

I cannot see the logic in this. With this service bulletin, I am thinking about disregarding Lockwood's "prohibition". Anyone have any thoughts or advice on this?

Doug Dahl
RV-12, RV-9A
 
I've posted this before, but going through Repairman school with Rainbow and Instructor Jim Scott - who has worked on thousands of Rotax engines - he stated the cleanest engines he has ever encountered were running a Mogas/100LL mix.

In December, he came and ran my engine for the first time and set all the parameters. I queried him again about his recommendation. He reiterated it. At the school, he stated a 50/50 mix was what he saw as being the best; in December, he said about 25% 100LL would be excellent for regular use.

That's what I'm loading. The 100LL is 115 octane, and will make up for possible low octane filling station gas, if that is the problem, and will boost the total octane without causing a lot of lead-in-the-fuel problems with regard to maintenance etc.

I believe him and trust him and that's what I'm doing.

Bob Bogash
N737G
Loaded 20 gals of fuel today - 24% 100LL
 
What? 100LL is 100 motor octane.
Car gas octane is the average of motor and research octane, with a typical spread of about 8 points. e.g., 89 car gas is about 85 motor octane.
 
I'm not going to start into a petroleum grade discussion - this has been covered in great depth eelsewhere on this forum - and there are far more knowledgeable individuals that I am or ever will be. But.....

100LL is somewhat of a misnomer - in that it probably is not 100 and also not Low Lead, although it is lower in Lead than the fuel it largely replaced. It used to be - as I understand it - what we used to call in the good old days 100/130.

100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions.

Any which way, 100LL has a substantially higher octane rating than MoGas and a mix will boost the overall octane of of the fuel load, at the very least making up for any shortcomings in the MoGas being used. Avgas also has certain other benefits - like a longer shelf life, and much tighter quality control, that will benefit the mix.

With that - I bow out of this discussion and leave the why's and wherefor's of the various octane ratings etc to the petroleum engineers.

Bob Bogash
N737G
 
Copied from www.rotex-owner.com

Avgas and Autogas mixing


For the Rotax 912/914 series engines Avgas and Autogas can be mixed together in any ratio; this is done primarily to increase the octane rating of Autogas.

Method:
-100LL has a minimum motor octane of 104
-Regular automotive gasoline (87 AKI) has a minimum MON (motor octane
number) of 82.5

You can get a fairly close guess of the octane number by multiplying the MON by the percentage of that fuel in the mixture.

EXAMPLE 1:
50% 100LL and 50% Regular
(104 x .5) + (82.5 x .5) = 93.25 MON (rough approximation)

EXAMPLE 2:
75% Regular with 25% 100LL
(104 x .25) + (82.5 x .75) = 87.8
The fuel retailer will be able to supply the MON number of the fuel you are
purchasing.
 
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