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Av & Mo Gas

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Well-Known Member
Apr 27, 2010
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The decision whether to use Av or Mo gas should be an informed one.

They are not identical & my goal is to expose some possible pitfalls.

It seems it might be easiest be rewrite my "POS deleted" comments in several sections here.

Some comments may be "controversial" (?) because of your own experience with these fuels.

These comments WILL differ because there are many forms of Mo-gas.

It can vary by Region, Season , Type of Crude oil available , Product demand & batch produced.

Alcohol has been discussed by many already so I will leave that out.

Reid Vapor Pressure ( RVP) is the tendency for a fuel to evaporate.

Av-gas is produced with the same RVP all year.

Mo-gas is blended on a Regional/Seasonal basis.

High RVP is desireable in COLD times to enable easy starting.

In the HOT time of year Low RVP is blended in to reduce Vapor Lock & for environmental concerns.

In some states this changeover is MANDATED by law.

High RVP may have the following difficulties ;

1.Greater tendency to Vapor Lock @ High altitudes.

The Brits put an Altitude Restriction on Mo-gas when they began to use it.

2. Carb Icing- The FIRST (?) US STC for Mo-gas was issued to EAA for use in a Cessna 150.

A condition of the STC requires increasing the Idle RPM to 700.

A POL buddy " Crude Dude" & I tested samples of Av & Mo gas .

They were placed in a Bell Jar that was evacuated to increase "Altitude".

They were both stable up till about 10K .

After that the Mo-gas increased the rate of "Boil-Off".

At some point which I forget the Mo-gas was really churning.

In this regard the use of Northern Winter fuel being used on a trip to Arizona in August could produce difficulties.

That is not a concern with Av-gas.

There will be a couple more sections on this topic.

Disagree with me if you want.

Just don't say they are identical; PLEASE.

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