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Fuel Octane Rating

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RFazio

Well Known Member
Can someone explain to me what it means when the name plate on my Lycoming says the Octane rating is 91/96? Recently a few gas stations in my area are selling high test fuel 93 octane ethanol free. It's $3.05 a gallon, lots less than the $5.00 for aviation fuel. Was thinking of mixing half and half. Good idea? or bad?

Thanks in advance

Richard
 
That refers to the lean and rich octane ratings. Different than auto rating which is RON and MON.
 
RFazio said:
Can someone explain to me what it means when the name plate on my Lycoming says the Octane rating is 91/96? Recently a few gas stations in my area are selling high test fuel 93 octane ethanol free. It's $3.05 a gallon, lots less than the $5.00 for aviation fuel. Was thinking of mixing half and half. Good idea? or bad?

Thanks in advance

Richard
Click to expand...

It means the minimum octane requirement is 91 MOTOR octane. Car gas is rated as the average of Motor and Research Octane - typically 93 'octane' car gas is about 89 motor and 97 research octane. So running all mogas 93 is just a bit shy (at 89) of the requirement. How much margin is there in the requirement? Hard to say. Certainly a 50-50 mix with 100LL/mogas 93 would easily be over 91. In fact since the first bit of lead raises octane more than the last bit of lead, I'd say 25% 100LL + 75% 93 mogas should be just fine.
 
i have been saving $$$ like this for about ten years now. 50/50 mix will be fine. :)

let me know if anything good is going on over your way this summer.
 
Just be watchful for vapour lock issues.

I know folk with O-360's and they run OK on it in the cruise but if they land the heat soak gives awesome vapour locking to a scary level :eek: Even with the pump on.

Be careful with spark timing, all the EI's advance, some more than others, so the higher CHT with Mogas combined with the advance means you run higher peak pressures(CHT) and not more HP. If done to outer limits you will get less.
 
Auto Gas

I have an IO-360 with dual PMAG's (running on the standard curve on a low compression engine). I've been running 100% Hi-test (93 oct) auto gas for the last two years. The ONLY time I've had vapor lock issues is in the spring time when the Winter blend fuel is still available on a warmer day. In every case, turning ON the AUX pump mitigates the condition. The higher pressure injected fuels system also helps mitigate vapor issues.

Hot starts are another issue, especially on a hot summer day. I have to intentionally flood the engine to push out any vapor conditions, and keep the AUX pump ON.

In order to use gas with some Ethanol in it, the whole fuel system MUST be Ethanol compliant. This means none of the old 303 or 606 hose types in the fuel system. You must use PTFE (Teflon) gas hose where flexible lines are required.

One other issue: change the "O" rings in any of the quick drains that don't already have Ethanol compliant materials....

Read more: http://www.hotrod.com/how-to/engine/hrdp-1101-performance-fuel-hoses/#ixzz3essi5P35
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RFazio said:
Can someone explain to me what it means when the name plate on my Lycoming says the Octane rating is 91/96? Recently a few gas stations in my area are selling high test fuel 93 octane ethanol free. It's $3.05 a gallon, lots less than the $5.00 for aviation fuel. Was thinking of mixing half and half. Good idea? or bad?

Thanks in advance

Richard
Click to expand...
 
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