Leaded 110 Octane

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Leaded 110 Octane

Postby jOe » Thu Jul 18, 2019 6:18 pm

There’s a Sinclair station that sells 110 octane leaded gas. I bought a couple of gallons and put it in both of my XLCHs- both have stock compression . No problems, both ran fine. Now I’m wondering how far I can advance the timing before running into any problems.
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Re: Leaded 110 Octane

Postby LDB » Thu Jul 18, 2019 7:25 pm

As long as the combustion heat inside the head and cylinder remains within the octane level's resistance to detonation or knock as it's commonly called, the distance of the timing advance would have no effect unless the timing is so advanced that too much of the burn is occurring to close to or before TDC, in which case the heat would get high enough to cause detonation with any octane and the engine would not run properly anyway. The best part of the burn should occur a little ATDC (actual crankshaft position not the timing setting which would be BTDC in accordance with the speed and load). As long as the octane level is sufficient enough to prevent knocks with the timing setting in that position then a higher octane would have no effect.
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Re: Leaded 110 Octane

Postby Ferrous_Head » Sun Jul 21, 2019 4:27 pm

My concern would be that for whatever reason you weren't able to get the leaded fuel at some point and HAD too use a tank of rubbish fuel.
On stock compression you should be running 45 (I presume it's a 900) BTDC. The thing is, that really is an |proximate" number. Every engine is different and some actually like a little more and some less.
Also the quality of the fuel WILL vary and that effects it as well.
The air density WILL affect i as well.
The real advantage of having access to higher octanes is being able to raise the compression ratio. Which I wouldn't suggest you do on a toad bike.
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Re: Leaded 110 Octane

Postby jOe » Sat Jul 27, 2019 4:29 pm

All good points. Why then is this level of octane used for aviation?
I wouldn’t think airplanes have abnormally high compression (no idea ). I suspect it might have something to do with the oxygen content at altitude or ambient operating temperature.
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Re: Leaded 110 Octane

Postby LDB » Sat Jul 27, 2019 6:33 pm

Not all piston engine aircraft require 100 octane. But the more advanced piston engine aircraft with higher compression ratios or turbo charging need higher octane.
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Re: Leaded 110 Octane

Postby Ferrous_Head » Sat Jul 27, 2019 10:38 pm

Nor being an airplane man I have no real experience.
But, higher altitudes = richer mixtures and also ambient temperatures will be lower, Both of which would make life a little easier for the engine.
But they are going to need all the torque they can get. So I suspect that in fact they do run relatively high compression ratios.
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Re: Leaded 110 Octane

Postby panic » Tue Jul 30, 2019 8:41 am

Higher altitudes makes existing mixture setting too rich.
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Re: Leaded 110 Octane

Postby wz507 » Tue Jul 30, 2019 10:03 am

jOe wrote:All good points. Why then is this level of octane used for aviation?
I wouldn’t think airplanes have abnormally high compression (no idea ). I suspect it might have something to do with the oxygen content at altitude or ambient operating temperature.

Like others posting here, I too am not an aviator. That said, with respect to the reason for running high octane Av gas, I would think that like everything associated with aviation it is about “safety factor”. As others have noted many of the engines do not require high octane fuel and in general are relatively low compression engines, e.g., consider the Continental engines used in the ubiquitous small Cessna airplanes.

https://www.continentalmotors.aero/engines/400.aspx

However, there is one thing an aviation engine should never do and that is to experience detonation, as detonation would be a one way street heading down, and that’s a bad direction to be heading unintentionally in an aircraft. Although high octane fuel may not be required to operate an aviation engine, it provides a significant safety margin against detonation. Air cooled aviation engines operate in a very wide range of temperatures (arctic to desert) and perhaps when you are stuck in line on the tarmack of a desert runway waiting for clearance to take off, and then experience delay after delay, you’d be glad you had a significant safety margin against detonation when you finally throttle up.
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Re: Leaded 110 Octane

Postby JohnF » Tue Jul 30, 2019 10:54 am

Aviation gas/fuel is also very dry and contains additives that remove moisture and prevent vapor locking. Higher octane, dry fuel and anti vapor lock additives are all things required for aviation gasoline. Quality octane improvers typically contain all the required additives to meet aviation GASOLINE requirements. On scooters I’ve found faster starting, improved throttle response, better fuel economy and smoother running conditions when using aviation and other higher octane gasoline’s.
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Re: Leaded 110 Octane

Postby Lisa » Tue Jul 30, 2019 11:18 am

You can advance your timing to the point where it kicks back and rips your ankle knee or leg apart. LOL
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