Auto Fuel

Subj: Fuel
From: (Dennis Friedrich)
Recently bought a Swift with a C-125. Was wondering about the use of auto fuel in it. Have used it successfully for years in a Cherokee. Any ideas or advice? Thanks.

As I recall, you now have N3773K. You'll never get me to recommend car gas. Now, I know it has been used successfully by many, and it has an STC from Peterson. On a well - worn 125 it might be more successful than on a fresh engine, due to the fact the engine is well broken in, and you won't have any excess heat from the break in process to contend with. A local Swift was using it, and the only problem he ever had was after one flight, it was very hard to restart. (indicating vapor lock) I am more afraid of alcohol and other solvents attacking the rubber components in the fuel system. I guess, if you're careful, it can be done.  --  Jim

Subj: Airplane gas
From: Steve Whittemberger < >
While fueling up today a guy was talking about a lot of stuff. He had a 540 that he uses Amoco 93 car gas in. Says it runs great and he only did it based on a study that said it was okay in aircraft engines. I asked him if it was just in a pinch - but he said no he uses it all the time. Strongly recommended that I give it a try. I'm pretty satisfied with the 100 LL although our lowest prices in the area are now up to ~1.90 a gal. (Still cheaper than the bandit at the local FBO who is now up to 2.60. Amazing.- very few buy there gas here because of the price. Most fly 5-15 min away to get gas - even the guys with big aircraft and deep pockets). What do you think of this 93 octane approach. It would not be convenient to lug car gas in 5 gal cans to the hangar - but would save 30 cents a gal. Not worth it I believe. I'm more interested in the viability of using 93. He says only the Amoco has the right ingredients ---

I'm not much in favor of car gas in airplanes. Besides the safety possibilities, I can't see pouring gas into an airplane from 5 gallon containers to save the difference from $1.50/gal to $2.00/gal. -- Jim

(Editor's note: There is an STC for running car gas in the Swift. If any of you Swifters are doing that we'd like to hear about your experiences via email please.)

Swifter Larry Owen was kind enough to send an email detailing his experiences running auto fuel in his Swift via the STC paperwork...

From: Larry Owen <>
Subject: April #2 GTS Internet Update -Reply
I have a STC for the Autogas for my Swift with a C-145. (At least that's what the paper in back of my logbook says). I have never had any sort of problem using 91 to 93 Octane autogas. It is a pain in the butt to haul around but the field I've been at, the FBO is a sometimes sort of thing, not enough real business from what I can tell. Saving $10 a fill up is the only bonus and I agree that it's not that big of a bonus. Per STC I use both fuel pumps for takeoff and landing and have never had any problem. The only difference I (might) have noticed is the plugs stay cleaner with Autogas. Aviation 80/88 and 100LL seems to load them with lead faster. Flying in the El Paso area, we get plenty of heat and density alt problems but I have never had a vapor lock problem. I've seen the fuel pressure gauge wobble a bit after a hot takeoff but it doesn't drop below 4-5 lbs but for a fraction of a sec, then stabilizes a 6+.

If I understand correctly (?) both the real Octane requirement and the effective compression of the engine is related to the density alt. The higher the density alt, the lower the octane number needed to stop "knock" and the lower the effective compression of the engine (less air - less compression). Both put a larger octane "pad" between what the engine needs and the Autogas can deliver. On the other hand, I can see where flying at sea level on at warm / hot day could push your octane requirement pass what Autogas could handle. All bets are off if you have "tweaked" your compression above 7 to 1. (That's why the WWII Warbird's "hot" engines use 120 / 140 Octane) From what I have read, mixing gas is in proportional with the mixed octane. (Half 80 and half 100 is roughly 90 octane). It's just real tough to know / remember what the octane of the fuel in your tank is after a few mixed fill ups!

I guess my short answer is, I use it, it works, I don't have any problems with it, but it's not that big of a deal. You can save a lot more $ by not getting the four color-moving map-voice activated-latest and greatest tool / toy and fly more and push buttons less. I guess that is about 2 cents worth.... Larry Owen, N78287

I read on the Swift e-news letter about the use of auto gas and had to put my two cents in. I had a Cherokee with an auto gas stc. It was a pain to fill her with 5 gal jugs but I was saving money. Then one warm Feb day in Western PA (it does not get all that warm) I was taking some friends for a ride, slow flight, circles around their homes and that kind of stuff. We returned to the airport, unloaded passengers, did a run up then off we went. Just as we broke ground the engine bogged flat on its face. PGH-Boquet is short and has a cliff at each end, we got on the ground safely, looked things over, did a good run up and tried again. This time we flew, but the engine fuel pump would not maintain pressure until about 15 minutes into the trip home after things had cooled down. We kept the boost pump on until then. This was a Lycoming 0320. Anyway, that was the last auto gas I ran and the last time I ever had the vapor lock problem. Bottom line is my life is worth more than the money I was saving using autogas. Many others I have talked to have had no problems, but for myself, NEVER AGAIN ! -- Don Cumpston <> via the "Yahoo! Globe Temco Swift Club"

The Reid Vapor Pressure (RVP) for automotive gasoline, on a standard day, is about 14 psia with 15 psia being the cutoff. RVP for avgas is about 7 psia on a standard day. At 100 degrees (F), the RVP for automotive gasoline is 35 pisa, while avgas is just approaching 15 psia. That is the reason automotive gasoline has the potential for vapor lock Vs avgas. This just might be one of those rare cases in life where... YOU DO GET WHAT YOU PAY FOR! -- Steve Wilson <> via the "Yahoo! Globe Temco Swift Club"

(Editor's note: You may or may not like to use auto fuel, but the STC itself may be a good thing for operators of Swifts with low compression engines. I have read recently that if and when the new 82octane unleaded avgas becomes available it will be permitted only in aircraft with STC'd autogas approvals. If anyone has additional info on THAT subject, I'd love more input...)

Subject: Re: Auto Gas
From: Ed Lloyd <>
To: Steve Whittenberger <>
Steve, another thought on the auto gas. I don't think I would have tried the auto gas if I had not had the STC for auto gas. Don't know what is done to the fuel system / carburetor to permit the use of auto gas of at least 88 octane. I'll hit Jim Montague and see what he says. I've been told it has something to do with the alcohol content in auto gas, but I'm not sure. I will send this to Monty and see what his input is. Jim, you see the discussion, question is, when the auto gas STC is performed, what is done to the fuel system / carburetor to permit it's use? Thanks.....Ed

Nothing! The STC just cautions you, "don't use any gas with alcohol in it". Amoco and Union and perhaps several other name brands claim not to ever use alcohol, but I'm not convinced. I overhauled a C-85 a few years ago, the owner used car gas in it and tried to get me to warrantee it after a couple of exhaust valves started leaking. I told him to take the cylinders to the shop that had overhauled the cylinders for me. They looked at them and told him the cylinders had been run on car gas and told him to hit the road -- in other words, no warrantee. He readily admitted using car gas. Avgas is cheaper than cylinders! I had another friend whose engine quit on takeoff after the car gas ate up the rubber tip on the needle in his carburetor. Save a buck, lose thousands of bucks! I don't think anyone can guarantee no alcohol ever in their car gas, in this cold country they seem to put it in at the refinery to prevent freezing of the various valves before it even gets to the gas station. -- Jim

From: Wesley & Susan Knettle <>
Subject: Re: April #3 GTS Internet Update
The information I've read on the new Avgas is it is an approved replacement for Red 80/87 Avgas which means it is approved in all engines rated for 80/87 or less octane. this may include the auto-gas STC holders but I believe the EAA & Petersen folks will have to issue supplements or amendments to their STC's before it can be used. They'll probably have to establish compatibility between auto-gas and the new 82 octane avgas. I don't believe the FEDs have done this study for them. the FEDs have established compatibility between the new 82 and the old 80/87 and 100LL. As for the pro's and con's of using the auto-gas STC, I can only see a couple of real benefits,. One-In an emergency you can legally use auto-gas to get you home. Two-if it does cause a power loss then your insurance will settle with your next of kin since you were using a legal fuel. I don't approve of full time/long term use. I have used it when nothing else is available, within the constraints of the STC, current operating conditions and the limitations of myself and the aircraft. If the conditions are good for vapor lock then I will not fly it. Just my 2 cents. -- Wes Knettle

From: James Coats <>
Hey Jim,  Have a friend that recently bought an AA-1A Grumman Yankee. I'm going to go ahead and tell you first that they are using auto gas. Two times after landing on rollout with the throttle at idle the engine has quit. The first time it restarted, but the second (today) it never would. They had it towed to the shop but their A&P hasn't looked at it yet. They also experienced a major power loss at 3500ft last Friday night and that shook them up a little. The engine just lost power like the mixture was pulled full lean. After going through the emergency checklist and pulling the carb heat they managed to get it to restart (Thank God!) This sounds like carb ice to me, but could something else be causing a problem, or perhaps something vaporlock related with the auto gas? I'd appreciate your thoughts! -- James Coats

I would guess his problem is something similar to carburetor ice -- induction ice caused by water in the fuel. I would guess they picked up a tankful of fuel with water in it. The tanks should be sumped to determine if there is a presence of water. The proper thing then would be to defuel the airplane and start over, preferably with aviation gas. The quick-and-dirty way to proceed would be dump in a can or two of "Heet" or some other automotive deicer and see if that homogenizes the water into the gasoline. There may be an approved aircraft deicer additive - Prist comes to mind. The reason most deicers are not FAA approved is they will continue to draw moisture from the atmosphere until you do definitely have a bunch of water in the fuel. So use deicers with caution. -- Jim

"SEASONAL" AUTO FUEL... (060500)
From: Allen E. Andersen <>
Subject: June #4 GTS Internet Update
Concerning the Yankee with the "auto fuel" problem. I have witnessed this several times particularly this time of the year going from winter-spring to Summer flying. The problem seems to be limited to low wing airplanes which "suck " the fuel up to the engine.

Here's what probably happens. The fuel manufacture' s formulation between summer and winter fuel changes during the year, with the winter fuel being made with a higher vapor pressure which allows the fuel to vaporize at a lower temperature for easier starts in cold weather. This also allows for easier vapor lock, since the fuel will become vaporized easier especially in warmer weather. If you have a tank full of "winter blended" auto gas, and you fly on an 80 degree day in the late spring or early summer, especially with a low wing airplane with no head pressure on the fuel (like a high wing plane has), you run an excellent chance of getting vapor lock, bubbles in the fuel and other bad things happening to you.

A friend of mine had the same thing happen in an old Bonanza and was fortunate to get it down OK. The solution is not to quit using auto fuel, but to make sure that you burn the auto fuel from the season of the year which you fly. (i.e. fresh gas). The solution to the Yankee problem is to drain out about half the auto gas and fill with 100LL. The mixture will lower the vapor pressure of the auto fuel and you will have no more problems. It is also a good idea to occasionally fill up with the expensive aviation fuel to help keep the vapor pressure under control, by diluting the auto gas. I have seen several identical situations to what you describe, but it has always been using "Pure " car gas, and never a mixture, and always using "winter formulated gas" in the warm summer weather. -- AL

From: Anonymous
Subject: auto gas
The cost of 100LL has just been increased to $2.40/gal. while the local gas station's per-gallon cost have been in a dive bombing run and some are around $1.15 - which got some of us into a hangar discussion of Amoco 94 and its STC for AvGas. The local AI seemed to think EAA held an STC for autofuel but that it applied to some engines and not to others, particularly those with higher compression ratios. Question: Do you know if there are any STCs for a Lyc. O-360 180hp, and if so, are there more than 1. Several of our pilots come to the airport with the trunk full of gas cans. Please do not publish my name.

The 180 Lyc is a 91 octane engine. It is approved for car gas under STC SE 2563CE held by Peterson Aviation, Inc 984 K Road Minden, NE 68959 The Swift airframe is approved for car gas on another STC with the 145 Continental. You would have to talk it over with your FAA inspector on how to make out a 337 form for the O-360 Lyc in the Swift airplane, if you want to do that. The EAA might have an STC also. The STC specifically says using a combination of 100 LL and car gas is OK. I would use say, 10 gallons of car gas with 10 gallons of 100 LL. I try to cc the answers to questions like this to Denis so he can include them in his update. I will cut and paste this one to keep you anonymous. -- Jim

Subj: Questions concerning the Swift.
From: (JM)
What is your opinion on using Mogas? The plane has the STC for it. I used it for quite a number of years in the 1948 170 (O-300 engine) I owned prior to the Swift without problem. My Swift has a C-145-2. I know that every once in a while you have to run 100LL (w/TCP). Thank you for your time a comments. Jack Meyer

 I don't like car gas, but some guys have used it with success, the engine should be well broken in and use a little 100 LL occasionally. Remember the Swift is a low wing airplane and will be more sensitive to vapor lock. The money you save on gas can be used to by new valve guides! (or a little Marvel Mystery Oil?) -- Jim