Avgas Update

[Jim Hudson]

Lurker Carl
That's what I wanted to say but I don't claim to know anything. On a Briggs engine I usually find a flywheel key almost sheared from hitting rocks if its on a lawn mower.

[Buzzard Wing]

Guess I am glad my Gravely has electric start.... and it will get a lot less use now that I have a Cub.

I also thought AVGAS was very high octane.

[George Willer]

Guess I am glad my Gravely has electric start.... and it will get a lot less use now that I have a Cub.

I also thought AVGAS was very high octane.

The lowest grade of avgas is 100 octane, as far as I know... much higher than necessary or helpful. It does have one small advantage in an engine in good condition... it deteriorates less in long term storage. The other advantages are mostly imaginary... or apply to an engine that is not set up as intended. Most of us who maintain our engines properly have no problems at all that can be traced to pump gas. Some of my tractors are run so little that they haven't had any fuel added for at least 3 years... even the junk fuel, and they start and run just fine. I choose not to "reinvent the wheel" or become a fuel chemist when there isn't really a problem.

[Lurker Carl]

Most of us who maintain our engines properly have no problems at all that can be traced to pump gas.

My sentiments exactly.

[Harold R]

The other advantages are mostly imaginary... or apply to an engine that is not set up as intended

The best the Brits could do with the Rolls Royce Merlin was about 1500 hp. Even after super-charging. We gave them some 100 octane, and the rest is history. Yeah, about a 500 hp jump, just from fuel.

I share your opinion though. A normally aspirated engine, at a standard atmosphere, +/- a little, pump gas should be just fine. Provided it's not some type of racing engine with the timing advanced, or super high compression. I also agree pump gas ain't what it used to be.

[WKPoor]

I just got back on so I'll try and defend myself a little here. As for the gravely- I have tuned the mags carefully, even retarding the implulse way past TDC and still they will want to fire backwards now and then. Only fix I've found there is electric starter. As for lead content in the Avgas, I'm of the understanding that its actually higher than the old pump gas used to be. LL means its less than older Avags. No, I don't run it in my car, Not in anything that says EFI on it. I've had no trouble on machines that say EFI unleaded only. Jet fuel is basically just Kerosene and will not improve diesel performance and doesn't have lubricant in it for the injector pump. Some add it in if they use it. Can someone tell me why my Mercruiser is hard to start hot and my brother, who has the exact same engine in his boat only it fuel injected, has no hot start trouble?
Now George, I've been accused of re-inventing the wheel my whole life-seems to me the wheel gets re-invented everyday.

[George Willer]

Now George, I've been accused of re-inventing the wheel my whole life-seems to me the wheel gets re-invented everyday.

Maybe that's right... I've also done my share of inventing but I think the round ones are still best, so I don't try to invent another shape. I was merely taking a cue from all the troubles others don't seem to have using the same junk you are complaining about. Maybe we're just lucky.

We can discuss this better at CubFest while we have something cold and cylindrical to keep our hands cool!

[WKPoor]

I was going to suggest just that George on my next post. This will be a great topic for Cubfest. I'll look forward to meeting you and I'm sure we'll have some interesting conversation.

[Carm]

In order to actually compare the octane rating of Avgas with auto fuel, you have to use the same method of rating. Auto fuel uses the R=M/2 method and Avgas uses something different (the method escapes me). 80 octane used to be available until recently, we ran it in the Stearman and ehe Cessna 140, but they stopped making it. So now we run the 100LL. In comparing the 80 octane Avgas with 87 octane auto gas, I don't think it is an octaane reatin so much as it is a fuel quality issue. The Avgas is made for long term strorage, has virtually no capacity to carry water, no varnishes etc. So it seems to me that 80 octane and especially 100 octane Avgas would naturally vaporize and perform better than even premium pump gas because of the quality of the fuel. If I can find some 80 avgas, I may try and see how it runs in the Cub versus 100 ans pump gas of different grades.

[Jim Becker]

Can someone tell me why my Mercruiser is hard to start hot and my brother, who has the exact same engine in his boat only it fuel injected, has no hot start trouble?

I know absolutely nothing about that specific engine. Is it an inboard or I/O with the engine enclosed in a box? In general, if one engine has a carburetor and the other fuel injection (of virtually any type) and all else is the same, the injected engine will have less hot start problems. Fuel in an injected system is mostly under pressure while the carburetor depends a lot on vacuum. Lower pressure results in more opportunity for vapor bubbles in the system. That is why the old vapor lock problems are virtually non-existant in the newer cars with a fuel pump in the gas tank. This is true even though newer engines run hotter on "junk" gas.