Does higher octane fuel affect timing?

I had been using low-grade Shell bronze 87 fuel in my '73 850, with Boyer, and just for a change I refilled my tank with high grade fuel Shell V-power 91
high-grade fuel. After a 1/2 hour run at city and highway speeds, I notice that my header pipes are a significantly darker blue shade than before the switch, suggesting more fuel igniting outside the cyclinders. Does this mean that the higher grade fuel is igniting slightly outside the ignition timing tolerance, as was initial adjusted when using low-grade fuel?

The way I understant it, the higher the octane rating, the slower the burn. And if the spark isn't timed advanced enough, some fuel will still be burning when the exhaust valve is opened.?

But I will read Ludwig's link, there is a lot of information there. Thanks Ludwig.

Graeme

I forgot to mention, in Australia Shell has, or had, optimax, which seemed to be a very dense fuel and was terrible with carb. bikes. It fouled plegs very quickly.
In my and lots of others experience,

Timing is fixed and will not be changed by different octane fuels, I believe, but may be wrong, higher octane fuels when ignited burn hotter, but most old bikes will run happily enough on most commercially available roadside petrol. I'm currently rebuilding an XLR 250 that ran on aviation spirit - for a while untill the piston seized and shattered. Generally bikes that are designed to run on very high octane fuel are heavily modified, cams, compression ratios valves etc and along with these changes Ignition timing changes are to suit the new cams and components but not the fuel.

Generally, the higher the octane, the slower the burn, and the less likely you'll get compression ignition (dieseling).

One of the side-effects of removing tetra-ethyl lead (TEL) (back in the '70s in the US) was the reduction in Octane rating and increased knocking. This was countered by retarding timing and lowering compression ratio. With the advent of fuel injection, variable valve timing, and electronic engine management systems, many modern vehicles are back up to fairly high compression ratios on pump gas [My Triumph is 12:1 and runs on 89 (R + M /2) octane].

A good rule of thumb is to use the lowest octane your motor will run well on - you don't gain anything by running a higher octane than needed.

running higher octane lets you bump the advance up. in old muscle cars you can grind the profile of the advance plates and change springs to tune the motor. i had an old pontiac 455 that liked 40 degrees of advance. i'm new to the motorcycle game and just replaced the points on my '72 w/ boyer micro mark 3. i run avgas 100 to keep from eating up my fiberglass tank. i am also going to fiddle around w/ advancing the timing a degree or two. my question is if i bump the advance to 33 degrees what does that do to initial w/ a boyer fitted. avgas here in s indiana is 3.40/gal. not much more than 93.

Try Hirsch tank liner in your glass tank, then you can use what ever is available.
But I wish avgas was the same price here as what you're paying

The octane rating is a measure of the resistance of gasoline and other fuels to detonation (engine knocking) in spark-ignition internal combustion engines. The higher the octane rating, the slower the fuel burns. High-performance engines typically have higher compression ratios (and hence longer piston strokes) which requires higher octane (slow burning) fuel to allow the piston power stroke to complete before the fuel is completely burned. If the fuel burns faster than the downward movement of the piston allows, then the pre-detonating fuel results in a loud "knocking" sound and vibration. A lower-performance engine will not generally perform better with high-octane fuel, since the lower compression ratio is fixed by the engine design.

The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of iso-octane and heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of iso-octane in that mixture is the octane number of the fuel. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.[1] This does not mean that the gasoline contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties. Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.

Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner.

It is possible for a fuel to have a Research Octane Number (RON) greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, AvGas, liquefied petroleum gas (LPG), and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher — ethanol's RON is 129 (102 MON, 116 AKI). Typical "octane booster" gasoline additives include MTBE, ETBE, isooctane and toluene. Lead in the form of tetra-ethyl lead was once a common additive, but since the 1970s, its use in the United States, and most of the industrialised world has been restricted, and its use is currently limited mostly to aviation gasoline.

ludwig said:

recklessridr said:

.. The higher the octane rating, the slower the fuel burns. ..

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This is a myth !
Burning speed and octane nr are unrelated .
And even IF this was true , it would only be one of the many factors that determine the speed at wich the compressed mixure burns .
Did you ever wonder why a Commando engine needs only 28 - 32 deg full advance , while comparable engines need 35 - 40 deg ?

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I've read that it's related to the shape of the combustion chamber at TDC. If you took an impression of an engine like a 650 triumph it would look like a folded pancake. Long flame travel with a narrow path to cover it. The last thing they need is slower flame travel. Higher octane's utility is in it's ability to resist detonation, which isn't the same thing as burning. It's an explosion where what you want is expansion due to heat.

1st of all, LUDWIG, i dont appreciate you calling me a liar. you might take the oppurtunity to research what it is that you don't know about I.C.E., to make an informed comment for all. as a professional locomotive mechanic, it is my livlihood to know what octane is, and is not!
2nd. for the rest of you who made comment to my posting, do not confuse " octane " & "cetane" ratings. cetane, which gereraly refers to diesel engines & fuel, in laymans terms, is a rating of the quality of ignition. combustion chamber shape , compression ratio & type of fuel used, will directly affect timing.
additionaly, timing is a product of many factors. compression ratio, combustion chamber shape, fuel type & octane rating, turbulence caused by " squish " , inlet velocity, scavanging effect, etc. the differences in timing between a 750 NORTON & a 750 TRIUMPH for example ( for discussion purposes, assume identicle compression & displacement ), can be explained by the fact that a NORTON is of an " UNDERSQUARE " design. ( i.e. stroke > bore ) the TRIUMPH is of an " OVERSQUARE " design ( i.e. stroke < bore ) using this as an example, it will explain the different timing requirements for the norton, and a comparable engine.
octane, (resistance to detonation) & cetane, ( quality of ignition ) would both directly affect timing requirements.

recklessridr said:

additionaly, timing is a product of many factors. compression ratio, combustion chamber shape, fuel type & octane rating, turbulence caused by " squish " , inlet velocity, scavanging effect, etc. the differences in timing between a 750 NORTON & a 750 TRIUMPH for example ( for discussion purposes, assume identicle compression & displacement ), can be explained by the fact that a NORTON is of an " UNDERSQUARE " design. ( i.e. stroke > bore ) the TRIUMPH is of an " OVERSQUARE " design ( i.e. stroke < bore ) using this as an example, it will explain the different timing requirements for the norton, and a comparable engine.
octane, (resistance to detonation) & cetane, ( quality of ignition ) would both directly affect timing requirements.

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your argument on Norton V Trimpuh needs correction. the turnip has a lousy combustion chamber and piston dome design. it is a deep hemi shape with a big lump on the piston shoved in to the chamber to interfere with flame travel whereas the norton has a very shallow hemi chamber with NO piston crown to interfere. and I wont get into the rest ie squish, turbulance or velocity.

i think you,ll find both triumph and nortons are under square

Hey Singring The Madnorton answered your question. Timming is a mechanical thing only changed by mechanical adjustments in some fashion.

lud the wiki insert was put here specificaly for you . i don't clam to know all there is to know about the subject, but i do hold a masters degree in industrial diesel applications, & an associates in petroleum fuels applications.
the point is , when discussing octane & detonation, the burn rate is directly related in the same context.
guys, u r rite! i should have said honda