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100LL aviation fuel in a Mercury 300x

Old 10-07-2015, 09:17 PM
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Default 100LL aviation fuel in a Mercury 300x

I'm just wondering if you can run 100LL in a 300x outboard? Any sensors that might be effected by the extra lead in the fuel? Do they run a catalytic converter? Thanks...
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Old 10-08-2015, 08:06 AM
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100LL I believe stands for 100 octane Low Lead-- there isn't extra lead in the fuel-- it most likely is lead free.

What grade of fuel does the owners manual require- I would not do anything more than what is required by the manufacturer -- heck some engines can run on fruit fly urine - but that doesn't mean you will get any more performance out of it. Just as a side bar - I have a 7.4 MPI (310 hp). When I first got it I put in 92 octane - when I pulled the boat out I noticed a little soot on the transom - cleaned this up and the next fill up when to the required 87 octane - Not a hint of soot!! Because of the lower flash point of the higher octane fuel some of it wasn't burning ergo soot. In fact there was NO difference in speed/performance with the higher octane.

I'm a firm believer that the engineers that developed these engine systems (in most cases) were a heck of a lot smarter than me!!

just my 2cents

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Old 10-08-2015, 08:28 AM
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Originally Posted by 3pointstar
100LL I believe stands for 100 octane Low Lead-- there isn't extra lead in the fuel-- it most likely is lead free. It does have lead in it that is where some of the additonal octane comes from. Av-Gas is great stuff have run the stuff in all my 2 stroke turbo snowmobiles and they love it. Have to replace an O2 sensor maybe every 1000 miles, thats the only sensor I can see being affected if there is one.
What grade of fuel does the owners manual require- I would not do anything more than what is required by the manufacturer -- heck some engines can run on fruit fly urine - but that doesn't mean you will get any more performance out of it. Just as a side bar - I have a 7.4 MPI (310 hp). When I first got it I put in 92 octane - when I pulled the boat out I noticed a little soot on the transom - cleaned this up and the next fill up when to the required 87 octane - Not a hint of soot!! Because of the lower flash point of the higher octane fuel some of it wasn't burning ergo soot. In fact there was NO difference in speed/performance with the higher octane.
This is a very good point, run the least amount of octane you need, higher the octane the more heat your engine/cooling system will need to get rid of.

I'm a firm believer that the engineers that developed these engine systems (in most cases) were a heck of a lot smarter than me!!

Dam straight, I cant tell you the amount of hrs we spend on the dyno trying to get as much hp out of an engine with the least amount of octane, if we say run 87 run 87 if we say run 91 well you better dam well run 91 (usally motor have knock sensors in them at that point for the idiot that runs 87).

just my 2cents

3pointstar
If your motor is bone stock then run what merc says, if you have different timing/fuel maps and you need to run more octane then 100ll is a good choice.
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Old 10-08-2015, 03:01 PM
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I'm looking for a non ethanol based fuel. Being a pilot, it's my best option. Does anyone know if a 300x, being fuel injected, will have a problem running 100LL?
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Old 10-08-2015, 03:49 PM
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100LL Still has plenty of lead in it. I ran it in my carbed 540's with no ill effects, however I don't think I would run it in a modern day EFI engine.
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Old 10-08-2015, 04:17 PM
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Originally Posted by jamontes
100LL Still has plenty of lead in it. I ran it in my carbed 540's with no ill effects, however I don't think I would run it in a modern day EFI engine.
I have run it in my fuel injected engines it is good to go. The best bang for the buck for leaded fuel you can get and it wont go bad in a few months.
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Old 10-08-2015, 05:09 PM
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Originally Posted by turbom700
If your motor is bone stock then run what merc says, if you have different timing/fuel maps and you need to run more octane then 100ll is a good choice.

I don't know who posted the paragraph starting with Dam straight ......That WAS NOT me 3pointstar. I agree with his/her comment but it didn't come from me

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Old 10-08-2015, 06:19 PM
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Originally Posted by 3pointstar
I posted the paragraph starting with Dam straight ......That WAS me 3pointstar. I agree with his/her comment but it didn't come from me

3pointstar
I had to do that only for fun. If I reply to your post with quotes I can change stuff.
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Old 10-08-2015, 06:38 PM
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Originally Posted by turbom700

This is a very good point, run the least amount of octane you need, higher the octane the more heat your engine/cooling system will need to get rid of.
Not true.

I have a significant history running highly modified 2 strokes in the desert. The higher the octane the slower the burn and more resistant to detonation. your fuel charge actually burns it does not explode. Exploding is detonation. Higher octane also has less thermal energy. Higher octane is used because you can run higher compression, more ignition timing etc. Things that can possibly make more power, but also make the engine more susceptible to detonation.

Higher octane alone will actually make less power. We used 112 octane fuel in the desert because it actually makes the engine run slightly cooler.
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Old 10-08-2015, 08:25 PM
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Wow boating in the desert! That's a new one
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