Unleaded auto fuel

Papa · #1 01-05-2009, 06:09 PM

Well, I reached the point in my phase I flight testing where I wanted to see how the engine would run on unleaded auto fuel. I have a Superior IO-360 with one mag and a single Lightspeed Plasma II+ EI. I had flown down to about 5 or 6 gal in each wing tank, so I filled the right tank up with 91 octane no ethanol auto fuel ($1.70/gal) and the left tank with 100LL ($4.00/gal). I did my takeoff and climb with the 100LL, and when I reached my cruise altitude (7.5K) I leaned the mixture to 30 degrees LOP. After letting things stabilize, I switched tanks to the auto gas and waited several minutes to see if I could see any change. The winds were very strong, and the ride was bumpy, but as near as I could tell there was no change in performance. I would have flown more tests including climb performance, etc., but given the winds and ride I didn't think I would get much valuable info. I returned to the pattern and landed uneventfully burning the auto gas, but when I taxied in I noticed that the engine didn't feel as smooth at idle as I was used to. I played with the mixture some, but no help. Back at the hanger, I tried one ignition at a time. What I found was that when I was idling with only the mag, the engine was idling very poorly. When I used the EI, it was better, but not as smooth as I'm used to. I then switched tanks back to the 100LL, and within a few minutes the engine was idling as smoothly as it aways does. I guess from here I'll wait to a better day to fly more testing, but I'd classify today as mixed results. I would sure have liked to have seen no difference between the two, but that wasn't the case, on the other hand at cruise power the auto fuel seemed great. If anyone else has any experience along these lines I'd love to hear about it. Perhaps I should just adjust my idle up to 900 from the 650 I have now and say "good enough." I really like saving $2.30 a gal.

Mark

David-aviator · #2 01-05-2009, 07:32 PM

Mark,

The vapor pressure of the mogas is not as good as 100LL, you could have had some bubbles in the fuel from the fuel controller up to the divider. Winter blends of the stuff test out worse than summer blends.

CESSNADON · #3 01-05-2009, 08:00 PM

The STC's for mogas for certificated airplanes require increasing the idle RPM, not sure how much, but I don't think it was a high as 900rpm. You might also need to enrichen the idle mixture a bit.

dtw_rv6 · #4 01-05-2009, 08:09 PM

Superior recommends a change to the timing of your engine (can't remember if it is advance or retard) if you intend to use auto fuel for extended periods. I regularly burn auto gas in my Superior engine, but I have two magnetos. The idle does seem a little lower for me too, but that hasn't stopped me from running mogas.

Don

JordanGrant · #5 01-05-2009, 09:23 PM

Mark, keep the data coming as you try that out. I plan on doing the same thing this spring/summer. I have the O-360 with one Lightspeed and Airflow Performance fuel injection. As soon as I can acquire a tank to fill with MoGas, I'll probably give it a try.

airguy · #6 01-05-2009, 09:23 PM

Whose injector setup were you running?

Greg Arehart · #7 01-05-2009, 10:45 PM

Mark,

Yes, please keep posting your results. I am currently at 20 hours and considering going to 91 on my Superior IO-360, so am very interested in whatever you find.

greg

Papa · #8 01-06-2009, 07:39 AM

I'm running the Precision Airmotive Silver Hawk fuel injection system. The Superior manual does address the timing issue in their manual. They recommend the the timing be set to 22 BTDC instead of the normal 25 BTDC for extensive use of auto fuel. The logic is that auto fuel burns faster than 100LL so less advance is needed. The problem is that I see myself using both fuels, so I'm not planing on adjusting the advance at this time. It will be several days before I have the time to fly again, but when I do I will try to record and post more information.

Mark

F1Boss · #9 01-06-2009, 08:04 AM

Hi fellas:

SAFETY. It's kinda important....

I have used MOGAS in more than a few Rockets -- they will all run on the stuff, with a few caveats. I have never purchased premium or mid-grade -- only regular -- and this is OK with 9.5:1 compression and cold air induction w/electronic ignition.

The rough idle is genuine -- you should be running on 100LL during that phase, and others.

My personal process:
Keep a min 50% mix of 100LL/MOGAS in one tank (maybe 75% in the winter), and whatever you end up with in the other tank (call this second tank your MOGAS tank). Do not switch sides for this procedure, once you start it. We don't need any extra confusion -- we have plenty to start with.

Use 100LL on the ground, taxi, takeoff, climb, descent, pattern, and landing.

On your long X-CYs, run the mogas tank dry, so you have plenty of 100LL left for the landing phase.

The ship with carbs handle the MOGAS easier than the FI equipped ships -- especially if you have a vapor return set up at the carb inlet. I don't recommend such a setup with FI.

Remember: MOGAS has a higher vapor pressure in the winter (more susceptible to vapor lock); this allows your car to start easier. You can use this to your advantage when starting and warming up your Lycosaur, but remember to switch to 100LL before taxi.

I have experimented with ethanol mixtures up to 30% (in summer), and found no difference in performance. Do not take this as a recommendation to ignore warnings against using MOGAS with ethanol in it -- the problem is corrosion associated.

Your spark plugs will last longer with this procedure, as the engine doesn't run on a constant diet of leaded fuel.

My 'toy' airplane burns 40GPH in cruise; 2.5x that on takeoff -- you can bet the farm I use MOGAS whenever possible, but I keep the mains with a min of 25% 100LL (to manage the vapor pressure), and follow the procedure lined out above. With 6:1 compression (supercharged to 37"MP on takeoff), she doesn't really care about octane -- the engines only require 78 octane.

Carry on!
Mark

Papa · #10 01-06-2009, 08:25 AM

Hey Mark,
Thanks for the thoughts on the mogas. Depending on the outcome of the rest of my testing, I'm kind of leaning toward the separate tanks concept too. I'll stay with 91 octane mogas with no ethanol IAW the superior manual and keep that in the right tank, with the left tank being 100LL. I will experiment around with the mogas idle some more, and may increase my idle speed up to closer to 750-800 RPM. If I'm not happy with the smoothness there, I'll probably just stick to 100LL for ground ops, takeoff, and landing as you recommend.

Mark