The Cessna 120-140 Association Forum

As I understand it the two players are Petersen and EAA. Both are the same price.

What’s the difference between the two? I’ve read both websites and can’t really tell. I found some old forum posts that talk about fuel specification that appears to obsolete.

With either, is there any sort of modification that would be necessary to the Marvel-Shebler or Stromberg carbs we use?

Hello Daniel,

I have had the EAA auto fuel STC for some time and, have used it for years with Shell Premium MOGAS. Very satisfied. No fouled plugs of course.
No high Pb contents in the annual metallography oil checks. If you use 100LL Pb in oil will go to the stratosfere.
I have Marvel-Schebler.

No modification at all in the plane, not in engine neither in the airframe.
Was for me just paperwork, plus signatures, plus adding the STC Documents to the flight manual.

But... Two decal placards,provided with the STC, are stick near your fuel caps warning you about the gas normalisation.
It is clearly stated that you must use only MOGAS (C + H molecules) and no oxygenated molecules. So Alcohol is prohibited.

One very important difference, in my opinion, is this:
If you use 100LL the supplier is entirely responsible for the quality of gas.
If you use MOGAS, the pilot has that very big responsibility.

Now,
Is a low alcohol fraction really a problem in a high wing plane, flying at relatively low altitudes, with rubber components that are alcohol resistant ?
Or the norm is very conservative ?
In Argentina premium MOGAS has 7.5 % alcohol, tested and verified.
For this matter I have the question but not the answer. More knowledgeable people in this forum will give us the answer.

Kind regards,
Horacio

I believe you are asking "what is the difference between the two STC's?" I wish I could help, I've only see the EAA STC.

V529 wrote: ↑Mon Mar 30, 2020 11:54 am I believe you are asking "what is the difference between the two STC's?" I wish I could help, I've only see the EAA STC.

Yes, this is my primary question. Here’s a link to the Petersen STC from the FAA database:

https://rgl.faa.gov/Regulatory_and_Guid ... enDocument

They're functionally equivalent. Buy from whomever you want to support.

I forget which one I have, but man is it nice paying $2.08 a gallon at a local Murphy for Rec fuel. I've tested each batch I've got for ethanol and nothing yet. Flying for $10.00 how can ya not go flying??? I'm still going to give her a shot of 100ll too just not full strength...

FWIW: I'm testing for ethanol using water based food color... put gas in a pop bottle, put some food color in, shake it like crazy... Is the food color still in droplets or blended with the gas? No blending, no ethanol....

I don't know if I'd trust that test. When I used to test, I found that the alcohol comes out of solution with the gas and combines with the water, so I wouldn't expect the food coloring to combine with the gas. IIRC, we put about 10% of water with 90% of gas. Mark the level of water in the bottle. Shake it up and then let it separate. If you end up with more "water" than you started with then you have alcohol in the gas. No increase means no alcohol.

I agree with the test method John describes in his last post. That is the way to test.
You are very lucky to get alcohol free gas !

Kind regards.

We have quite a few gas stations in Michigan that sell Alcohol free premium. (it's 90 octane) Handy to have around. It has always run about $1.00 cheaper than Av Fuel.

I have the Petersen Aviation STC (1984) and other than the paperwork and a few placards there is nothing else to do. The STC clearly says to avoid fuels containing alcohol and gives the procedure described by John to test for alcohol.

One additional point of caution in the STC is regarding “Vapor Lock”. Due to the higher volatility of auto fuel compared to avgas, auto fuel has more potential for vapor lock. The most conducive situation to the formation of vapor lock is using “winter blend fuels” during hot weather. So far my C120 with C85-12F hasn’t had any problems.

Maverick Gas Stations where I live, currently sell alcohol free gas (88 Oct.) for$2.59 Gal. compared to $5.24 Gal. at the airport. A $2.65 per gallon difference!