I stopped at Delle, UT for fuel yesterday, taxied to the pumps for mogas and a gal came out and said that they were out of premium. Only had low grade which is 85 oct. Minimum octane for mogas stc is 87. Hypothetically, if I had gotten gas can you figure amount of gas still in tanks assuming 100LL and average the octane rating and be legal? How about octane boost? I will fill up at Spanish Fork before heading home.

Tim

It is 7 to 1 compression ratio. 100LL is bad for the engine.

Tim

Tim

The regular octane sold will sometimes vary with the altitude of the area. I think at higher altitudes the octane is lower. I also think that avgas octane and auto fuel octane are calculated differently. You might e-mail Petersen and ask him. I have been to Delle many times. That was when they still had the restaurant. Auto fuel is good. All service stations in Northern Idaho have gone over to the dark side with ethanol.

flyer

Tim,

I don't understand your question. You stated that the only fuel that was available was below that specified in the STC, right? Does the stc specify that you can average, or does it say that the gas you pour into those tanks has to meet certain specs? I'd guess the latter.

In any case, the risk with a lower octane fuel than is recommended with aircraft engines is the potential for detonation. Detonation, by definition, is very destructive in nature, and is not something I would flirt with, even experimentally.

But, who knows??

Actually, that's the point, isn't it?

MTV

Last time I was at Cal Black, Cal told me that when the current tank of Mogas was gone, they would be going to 85 octane mogas.

JrCB
The avgas vs Mogas debate is almost like talking politics, but on the low compression engines, the mogas seems to be fine, if not better because of the reduced lead. Then you factor in saving $1.50-$2.50/gal, X 12 gal/hr.(avg. burn for O470) saves you about $25/hr. That becomes pretty significant. 100 hrs/yr.=$2500 savings. You can see the temptation.
Gary

I had a conversation with an engine guy about required octane. I have an IO-520 in my Bonanza that requires 100LL. The guys at GAMI have instrumented this engine, among many others. At sea level, standard day the 520 is on the ragged edge of detonation and their graphs show this. The internal cylinder pressure is about 800 psi. The absolute worst thing you can do in this instance is to reduce RPM and this is why you are taught to reduce manifold pressure first after takeoff. The more rpm the better as each cylinder stays at the high pressure peaks for a shorter amount of time. Now I don't live at sea level, I live at 3650. At 4000 msl, standard day the 520 only makes about 400 psi. Since the internal cylinder pressure almost solely determines the octane requirement the 520 no longer needs 100 octane where I live and certainly not at any cruise altitude, 91 octane car gas is more than enough at takeoff here. Temperature has little effect on octane requirement. At typical cruise altitudes around here, 7500-11,500, upper 70's to low 80's is more than enough octane. If you are concerned about having too low an octane simply reduce manifold pressure, you fixed pitch guys reduce rpm. Going extra rich does not make up for a lack of octane, you need to make less power, either by reducing manifold pressure or by climbing. All of this info is available from GAMI and the APS engine guys. They have the sceince and will show you exactly what your engine is doing. Curiously neither Continental or Lycoming can show you this.

BM,

Interesting comments.

The Continental IO 520 as installed in the C-185 and 206/207 and 210 all turn 2850 rpm for takeoff, and recommended procedure (from the AFM) on those engines is to reduce rpm to 2700 rpm and full throttle for METO power as soon as safe after takeoff. That equates to 285 hp, which is, I believe, the max power rated for most Bonanzas with those engines.

So, if GAMI claims the Bonanza installation of this engine, turning at only 2700 rpm, is on the ragged edge of detonation, how come the Cessna engines haven't been blowing up regularly?

Something doesn't make sense in that argument, I fear.

MTV

AS I am very new here, (although I have been a lurker for a while), I will suggest the following with utmost respect to those who know more than I in these matters.

For very short term usage, ie. emergency puposes, even very low octane can be used reletivly safely. I've run 78 octane in a go480 to get me home, I was however extremely careful to reduce power as quick as possible after t/o, in fact it was a lower powered t/o as well, just enough to get me off the water.

The real danger is the vapour point of the mogas.

If you are not running a pure gravity fuel system, or you don't have a fuel pump mounted in the lowest point of your fuel tank, you could be seconds away from an engine failure at the altitudes that most pilots on this site fly at.

Be very careful with mogas, there will be NO warning and No inflight cure for a vapour induced engine failure.

mtv wrote:BM,

Interesting comments.

The Continental IO 520 as installed in the C-185 and 206/207 and 210 all turn 2850 rpm for takeoff, and recommended procedure (from the AFM) on those engines is to reduce rpm to 2700 rpm and full throttle for METO power as soon as safe after takeoff. That equates to 285 hp, which is, I believe, the max power rated for most Bonanzas with those engines.

So, if GAMI claims the Bonanza installation of this engine, turning at only 2700 rpm, is on the ragged edge of detonation, how come the Cessna engines haven't been blowing up regularly?

Something doesn't make sense in that argument, I fear.

MTV

That actually goes along with what I said. Adding rpm does not make the situation worse. But if you added an inch, or two or three, of manifold pressure you would make the engine detonate. For any given manifold pressure setting more rpm subjects each cylinder to the peak of pressure for less time.

I am not an expert here but I would not hesitate to mix 100LL and regular gas. I am sure that neither you or your engine would tell any difference. I have had the discussion about different octane fuel several times. High octane is made for high compression. What people don't usually relize it that the higher octane fuels are actually harder to ignite, thus they can be used in the higher octane engines without preigniting the fuel from heat and compression.

In 99, my brother and I bought two BMW F650's. They are designed to run on regular. My brother always insisted on running premium. We were at a station that had run out of premium and he was forced to use regular. His bike got 4 mpg better and ran better too. He now uses regular.

I have an STC and I will run 87 octane. That is what my engine was designed to run on and I really can't tell any difference in performance. I let my flight teachers son use my plane to practice for his check ride and he was telling me he was burning 6 GPH. He uses Avgas and I have never burned over 5 GPH using mogas. I will stick to Mogas when I can but you need to do whatever makes you comfortable. Just my thoughts.

Tim, if I understood it correctly, you question was can you mix 85 octane mogas with 100LL avgas and achieve a resultant octane that is sufficiently high enough to both prevent detonation and meet the minimum specified in the STC?

I called my father and posed the question to him - he was a research chemist for Exxon for 37 years. His answer was that yes, the octane of the mixed fuels would be higher but it would be tough to estimate exactly how much higher. If you mixed them in equal proportion (50/50) you could be reasonably assured that you would be above 87 octane, but the exact rating wouldn't simply be an average of the two ingredients.

Jaerl wrote: In 99, my brother and I bought two BMW F650's. They are designed to run on regular. My brother always insisted on running premium. We were at a station that had run out of premium and he was forced to use regular. His bike got 4 mpg better and ran better too. He now uses regular.

Wow, that's a new take. Everybody I've ever talked to who used premium in an engine calling for regular says they get better mileage on premium as justification for the switch.

Vick wrote: If you mixed them in equal proportion (50/50) you could be reasonably assured that you would be above 87 octane, but the exact rating wouldn't simply be an average of the two ingredients.

The main reason for that is you're dealing with two different scales. The mid grade gas at your gas station is made simply by mixing regular and premium.

Here in Utah, the Mavrick stations sell four grades of gas not three. They don't have four tanks, the high and low octane fuel is mixed at the pumps. As for the Premium Vs Regular debate, most peope fall into the illusion that "if it cost more it must be better". For some of you that comute the same route every day, can you throw in a tank of premium and see what happens to your mileage?

On the other hand I had a 61 Mini Cooper in High School that had been bored too 1293cc's with 12:1 compression. If I didn't mix premium and at lease half 100 Octane Avgas the thing would rattle like crazy any time you got on it. Always looked funny that tiny Mini being fueled by the fuel truck. Plus it only had a six gallon tank but they were always willing to fill me up. I have always got the best results using the octane the engine was designed for.

When GAMI...

....starts manufacturing/certifying and providing warrenty for GAMI engines, after a few 100,000 hours of operation....I'll accept their "science."

Bob

Bonanza Man wrote:

Jaerl wrote: In 99, my brother and I bought two BMW F650's. They are designed to run on regular. My brother always insisted on running premium. We were at a station that had run out of premium and he was forced to use regular. His bike got 4 mpg better and ran better too. He now uses regular.

Wow, that's a new take. Everybody I've ever talked to who used premium in an engine calling for regular says they get better mileage on premium as justification for the switch.

Premium fuel is a common mistake that the rotary engine crowd makes. They think that the premium will help it run better, when in fact they want the lowest octane possible, it requires a quick burning fuel to make use of the long combustion chamber. This is why they have 2 sets of plugs firing at different time as well. A high octane fuel burns too slow, and will still be burning when the exhaust port opens.

Jaerl wrote:
On the other hand I had a 61 Mini Cooper in High School that had been bored too 1293cc's with 12:1 compression. .

Was it a Mini Cooper S. If so I am sure that you miss it. I was a Bug Eye Sprite guy in 64.

45 gal of 85 car gas and 10 gal 100LL averages to just a bit over 87. Now are the two octanes computed the same, do not think so but my question was hypothetical of course.

Great responce to my post. Thanks.

Tim

Hi Tim

My other Brother had a 60 Bugeye, fun cars. My Mini was an 850 with a bored out 1275 S engine. Dry suspension. I was pushing 130Hp with a 1,000+ lb car. Back then 8"wide Goodyear trailer tire were as closest to racing tires as I could afford. I put on all the S stuff except the dual gas tanks. I do miss it sometimes but it was so obnoxious to drive around with everyone staring and making the same stupid coments. #1 Stupid coment - Did you get that out of a Cracker Jack Box? #2 Stupid comment: Is that the car that they use in the circus? Guess I outgrew that a few years back. I can tell you that more than one muscle car around here felt the wrath of that little Mini. (usually more than once because no one could belive that the little toy car just stomped their ass:)

As for the Premium vs Regular debate, read this. It is one of hundered articles that all say the same thing. http://www.straightdope.com/columns/rea ... egular-gas

I would never put low octane regular Mogas in a high compression aircraft engine and run it like it should be run. You will tear it apart. You can run higher octane in a low compression engine and the only damage done is to your wallet. I have a Contential O-200 and a O-300, both were designed to run on 87 octane so I am comfortable with Mid Grade Auto fuel.

There are other factors to consider also. Lead vs no lead and I have heard that the carb is more susceptible to icing with Mogas, and there are additives in Auto Fuel.

Auto fuel has additives in it that do not evaporate. If you have ever cleaned out a gas tank of a car that has sat 5+ years you know what that nasty stuff looks like. Looks like tar, not what you want in your planes fuel system. If I ever let my planes sit for more than a month or two will I fill them with Avgas.

As for lead. I try to put some Avgas in once in a while to keep some lead in the system. It lubricates the valves and keeps them from sticking. If your engine has been built or rebuilt since the 70's it probably has hardened valve guides and shouldn't be a problem.

If you have to use Auto fuel, I wouldn't worry at all about mixing octanes since that is how it is made in the first place. Just try to get it close to or a higher octane than you need. Better safe than sorry considering what these engines cost to rebuild. If your engine is pinging or ratteling back off and get some higher octane in there as soon as possible. I wouldn't add any octane boosters because I think most of them are just alcohol anyway.

z3skybolt wrote:When GAMI...

....starts manufacturing/certifying and providing warrenty for GAMI engines, after a few 100,000 hours of operation....I'll accept their "science."

Bob

Well, Lycoming "science" has always been that Auto-gas will destroy your engine and they tried to put the fear of you-know-who into everyone for 50 years. Now (starting late 2008) out of fear of becoming irrelavant, they suddenly have "No Technical Objection" to running it in my 0-360 because they see the suppliers of the world moving away from avgas. I have no STC to do so, and won't, but to sit back and accept the status quo kills innovation and makes one feel like a cold-war Soviet.

GAMI has done great service to the community and have shown may how to extend the life of big-bore engines. We're all welcome to take it or leave it. But if you're waiting for a blessing from Textron/Teledyne.........

I like to keep one eye on the innovators and one on the manufacturers. In aviation, sometimes, they seem to despise each other and thats not such a bad thing.