fuel question

[jimbo]

So I am needing a minimum fuel grade placard for the RV and have a question about fuel grades that a quick research hasn't answered. The engine manual for my Lycoming clone experimental D2 engine with 8.5:1 compression says:
fuel grade 91/96 or 100/100LL minimum octane aviation fuel

Is 91/96 or 100 avgas even made anymore? Is not I guess the minimum would be 100 LL avgas and should be placarded as such.

The 91/96 spread is confusing and not sure I understand it.

According to this article:
http://www.eaa.org/autofuel/autogas/...vs%20Avgas.pdf
mogas octane is about 5 octanes lower when tested on an airplane engine. In other words, a mogas octane of 96 would be needed to have it perform to 91 octane in an aircraft engine. Still, if I am understanding this right, even with comparable octanes and considering ethanol free mogas, there are other differences between mogas and avgas. So I guess mogas at any octane is out?

Thanks for any input.
Jim

[Sam Buchanan]

Originally Posted by jimbo

So I am needing a minimum fuel grade placard for the RV and have a question about fuel grades that a quick research hasn't answered. The engine manual for my Lycoming clone experimental D2 engine with 8.5:1 compression says:
fuel grade 91/96 or 100/100LL minimum octane aviation fuel

Is 91/96 or 100 avgas even made anymore? Is not I guess the minimum would be 100 LL avgas and should be placarded as such.

The 91/96 spread is confusing and not sure I understand it.

According to this article:
http://www.eaa.org/autofuel/autogas/...vs%20Avgas.pdf
mogas octane is about 5 octanes lower when tested on an airplane engine. In other words, a mogas octane of 96 would be needed to have it perform to 91 octane in an aircraft engine. Still, if I am understanding this right, even with comparable octanes and considering ethanol free mogas, there are other differences between mogas and avgas. So I guess mogas at any octane is out?

Thanks for any input.
Jim

These questions and many more about mogas are currently being discussed in this thread:

http://www.vansairforce.com/communit...ad.php?t=68032

[N1593Y]

Originally Posted by jimbo

> ...
Is 91/96 or 100 avgas even made anymore?

No, 91/96 disappeared before 80/87 about two decades ago and 100 became 100LL with a reduction in maximum lead (TEL) content, otherwise they are the same.

> ...

The 91/96 spread is confusing and not sure I understand it.

All avgas is designated by a MON octane spread. In this case 91 is the lean MON octane, 96 is the rich MON octane. The rich MON octane has been dropped as of late when listing avgas octane, so 100LL is 100 MON octane lean. Actually the full spread of either 100, or 100 LL avgas is 100/115. The only difference is the amount of TEL added to maintain the octane rating. In the case of 100 LL, the amount of TEL cannot exceed 2 g / gal, which coincidentally is the old spec for 91/96. The amount of TEL allowed in 100 avgas was 4 or 6 g / gal, don't remember which. These are maximum TEL levels, the minimum is "some" TEL. Avgas made to ASTM D-910 must have some TEL in it. It cannot be unleaded.