fuel octanes

[ljrichey]

just a question for the more educated---what kind of heat difference will heads and block run with different octane fuels? for instance 100 aircraft vs 87 pump reg--also do you have any other engine advantages with higher octane fuels--or are there disadvantages?

 

[tx1961whaler]

Re: fuel octanes

What kind of motor? There is generally no advantage whatsoever in deviating from the manufacturer's recommendation for octane ratings for an unmodified motor. This includes inboards, outboards, and cars.

 

[ljrichey]

Re: fuel octanes

i myself am running a 78 140 evin & an 89 175 johnson ---just didnt know if a motor could get pre ignition with higher octane fuel &or heat build up---i'm sure some of the older 50s & 60s motors the ones that could have run ethyl & with a lead base could have been set up a little different

 

[kenmyfam]

Re: fuel octanes

Stick with the recommended octane. Lead was added as a lubricant for valve gear. 2 strokes having no valves have no use for the lead anyway.
Just my 2 cents though.

 

[Silvertip]

Re: fuel octanes

Higher octane fuels are required to avoid detonation. Your engine is not in that category and is designed to run on 87. You are simply wasting money on the more expensive fuel. Cars on the other hand have an ECU and knock sensors that can advance the timing (within certain limits). Depending on the fuel being used, timing can be advanced by the ECU to take advantage of the increased protection from detonation. My flex fuel car for example can run E-85 which is 104 octane. It performs better on E-85 but it uses more of it.

 

[F_R]

Re: fuel octanes

By itself, higher octane does not mean higher head temperatures. There is only one reason for higher octane---to prevent pre-ignition and detonation. Pre-ignition and detonation will cause abnormal temperatures, even burn pistons. So you see, higher octane prevents those high temperatures, not causes them.

So, bottom line is if an engine is designed to require high octane fuel, you must use it or you will get pre-ignition and detonation. On the other hand, if an engine is designed to use lower octane there is no danger of pre-ignition and detonation, so there is no need of preventing it. There are a few outboards that do need high octane, mostly older ones. Read your owner's manual to see what you are supposed to be using. And don't be messing with the designed set-up (timing, carb jetting, and compression ratio, etc)

What I just said applies to non-computerized engines. Modern automobiles are a different story. The modern car has a computer controlled engine that actually de-tunes the engine if necessary to prevent damage from pre-ignition. In that case, low octane causes automatic de-tuning and lousy gas mileage.

Much of the misunderstanding comes from the old wives tale that high octane means higher energy content. That is a whole different topic.

 

[5150abf]

Re: fuel octanes

High octane is linked to performance because high performance engines use it but by itself it doesn't give you any more power, as others have said it is solely to eliminate preignition so it is actually harder to ignite than 87 octane so your engine could actually run worse.

I see kids all the time putting $5 a gallon race gas in moms Oldsmobile thinking it is going to make it faster which of coarse it doesn't, it just gives Johnny a lot less money to play with.

Now, you don't say what engine you have but if it is a 2 stroke they are all low compression engines and don't need the high octane, 87 is fine.

 

[ljrichey]

Re: fuel octanes

thanks guys---answered some misconceptions that i had on octanes in outboarding---i also hear a lot of bad talk on ethanol blend fuels in boat motors ---just had these questions on my mind -Thanks for the responses