Timing advance for avgas

[SlowMoe]

From what i read avgas is a slow burning fuel so i was thinking +7 but I would like to have a second opinion before I buy a timing plate (I already have a modded stocker...)

 

Thanks

[SlowMoe]

This will be on a drag engine w/ vitos pipes, 160lb rider, approx.180 lb. compression, sea level

[MatHolland]

I'd like to know this too. I'm going to run half avgas and have 93 because that's the only thing I can get where I live. I got a cut and milled head with 180 psi in both cylinders 500ft above sea level. My timing is set at +4 now.

[SonOfSand]

I was told by Kevin @HJR (who designed my engine) not to up my timing past +4 with 180 psi on a fresh build and running straight 100LL. @5000 feet above.

Edited March 15, 2011 by SonOfSand

[MatHolland]

I'm 400 ft above sea level. You think I'm ok to run straight avgas?

[SonOfSand]

I'm 400 ft above sea level. You think I'm ok to run straight avgas?

 

I think you will probably be fine, I have the same compression and timing have run avagas with great success.

[SlowMoe]

I was reassured by a very popular site sponsor that avgas 100LL is 100 octane in the air and 115 octane at sea level.

[okbeast]

I was reassured by a very popular site sponsor that avgas 100LL is 100 octane in the air and 115 octane at sea level.

 

Do explain more, if possible?

[SlowMoe]

Do explain more, if possible?

Well I had read that avgas 100ll is actually 115 octane, so when i saw a site sponsor selling a used head rechambered for race gas only i asked him if avgas would work, and he said it would. I also told him I heard it was really 115 at sea level and he said that was true.

So I told him it was kinda silly to pay 10 bucks a gallon for race fuel when avgas is around 5 bucks. He agreed. That's all that was said.

Apparently when you elevate fuel it reduces the octane rating.

[SlowMoe]

not sure how altitude changes octane rating of gas since octane rating has zero to do with altitude.

altitude will affect the octane rating the engine will require, the higher the elevation, the lower the octane rating you need.

This is a statement I pulled off an aviation forum:

 

95 Octane sounds only a little lower, but remember, fuels have two octane ratings. A rich octane and a lean octange. 100LL is known as 100/130. Lean octane is 100 and rich is 130. 95 is estimated to be 95/115. So at a lean octange you are only 5 octane lower, and at a rich mixture you are 15 octane (actually performance numbers) lower. So the power setting the aircraft can operate at cruise will be only slightly lower, but the power settings on takeoff might be substantially lower.

Edited March 23, 2011 by SlowMoe

[SonOfSand]

I did a lot of reading on this along time ago. 100LL does not have a higher octane rating than race gas, you need to research "motor octane number" (MON), i believe 100LL was 98 or 100, and race was 104. The "MON" number is the best way to compare them.

[SlowMoe]

Avgas grades are defined primarily by their octane rating. Two ratings are applied to aviation gasolines (the lean mixture rating and the rich mixture rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this case the lean mixture performance rating is 100 and the rich mixture rating is 130).

 

In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing demand these have been rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to attempt to eliminate errors in handling aviation gasoline, it is common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100). More recently, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this grade is called Avgas 100LL, the LL standing for 'low lead'.

 

Road fuels tend to be measured on a RON scale, for which unleaded fuels tend to be 95 - 98 RON but are only 85 - 87 MON. Avgas is measured on Lean Mixture (similar to MON) but also has a Rich Mixture Octane rating.

 

The Lean Mixture rating is 100 octane (15 octane higher than the comparable 85 MON for unleaded Mogas) but Avgas also has a Rich Mixture rating of 130 which allows higher supercharger boost pressures to be used without detonation occurring. This is particularly a problem when using high power settings at low altitude, for example during take off.

 

Gasoline used for aviation fuel generally has two numbers associated with its octane rating. Examples of this include the (now almost completely unavailable) 80/87 avgas, and the 100/130 avgas. The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the Motor Octane Number (MON) rating given to automotive gasoline. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure.

 

 

 

It is obious to me that the mon is 100 or so, which, as said earlier is 4 less than typical race fuel, but it is the 130 rich octane number that catches my attention.

I might be way off here, but on a two stroke with an expansion chamber doesn't the engine operate at a simulated supercharged condition which would lean toward the 130 octane rating?

[SlowMoe]

Delete

Edited March 23, 2011 by SlowMoe

[SonOfSand]

I don't think so. MON is what you need to look at. forget 130