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I can’t find info on aceptable octane rating for mogas. My aerovee is 8:1 and currently using 100LL as specified in manual. The local gas station has ethanol free 93. Is this ok for 8:1? I was thinking about using a blend of 100ll and mogas to cut down on any potential lead buildup. Or using TCP additive for lead scavenging.
Thoughts?

Darick -

You can always find the latest AeroVee assembly manual online at the Aeroconversions website. Now of course they change parts over time and don't keep the old versions handy, so the assembly instructions themselves may not be 100% correct for your particular engine; but AFAIK their recommendations on things like Oil quantity and Fuel type have been consistent over time...

So check out Page 6 of the manual, where they talk about fuel and compression ratio (under the "Fuel System" section):
http://www.aeroconversions.com/support/ ... Manual.pdf

Enjoy,

--Noel

I wish I knew more about what determines the octane needed. The Aerovee manual says you need 100LL to run 8.0 and can only run 7.0 on AKI 90 mogas. I think engines certified to run on 100LL commonly use 8.5 and what I've heard is that they have low detonation margin at full throttle. Yet my motorcycle has 11.3 compression and runs fine on AKI 89. What determines the difference? Does the Aerovee just have a really conservative knock margin?

I'm sure the fact that cylinder temps in air-cooled engines are ~100C higher than in water cooled ones plays a role. Volumetric efficiency also plays a role, since higher VE means you start with a higher cylinder pressure before you start compressing it. But I don't think my 16V DOHC bike has lower VE than the Aerovee.

I've been using TCP for the life of my AeroVee engine (3.5 years) with excellent results. A can lasts for quite a long time.

lutorm wrote:I wish I knew more about what determines the octane needed. The Aerovee manual says you need 100LL to run 8.0 and can only run 7.0 on AKI 90 mogas. I think engines certified to run on 100LL commonly use 8.5 and what I've heard is that they have low detonation margin at full throttle. Yet my motorcycle has 11.3 compression and runs fine on AKI 89. What determines the difference? Does the Aerovee just have a really conservative knock margin?

Modern engines have things like fuel injection and variable valve timing to ensure better conditions in the cylinders. The AeroVee (and indeed your carbureted Lycomings and Continentals) have none of this. Also, motorcycle and automobile engines are only run at full power for very short bursts (i.e. when you accelerate up to speed) whereas aircraft engines are running continuously at max power - or close to it. Its just not an apples-to-apples comparison.

--Noel
P.S. The short answer to your question of "what determines octane needed" is a combination of: (a) How much do you need lead or another additive to aid in cylinder cooling, (b) how susceptible is the engine to knock/ping/detonation [and note that this is not a fixed value - its dependent upon the temperature of the cylinder head and the air-fuel mixture], and (c) what effects do the different types of fuel have on the overall stress and energy (i.e. pressure and temperature) on the moving components of the engine.

Darick wrote:The local gas station has ethanol free 93. Is this ok for 8:1?

Based on my experience, yes.

My engine was also 8:1 CR and I used 91 octane ethanol free gas with no problems. You are more likely to have vapor lock problems with mogas so you need to insulate the entire fuel system inside the cowl.

Here's a picture I've taken of a page from my Great Plains Type 1 VW aircraft conversion manual by Steve Bennett. It goes into a bit more detail than the AeroVee manual on this subject:

Michael, the page is cut off hiding some good info. Can you take another picture of the right side?

Darick wrote:Michael, the page is cut off hiding some good info. Can you take another picture of the right side?

Hover your mouse over the photo & right click. Now select "Open image in new tab" and then you will see the entire photo in the image displayed in the new tab that is opened.

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
158.6 hours / Status - Flying

I also download the image on my phone to see the whole thing.