fuel octane, ignition timing, and performance

[bug1200]

I tried searching google but i cant really find good answers... so im hoping someone here could answer my question. so here goes:

Does higher octane fuel really burn slower than lower octane fuel?

If so, then say for example that a normal type 1 engine only requires 91RON fuel (which it does). and for some reason, they discontinue 91RON fuel with 97RON fuel being the only choice. assuming that everything else in the engine is the same, wont the performance decrease? since the higher octane fuel burns slower than the lower octane fuel, the time it takes to fully ignite the mixture is much longer isnt it? therefore, the ignition timing would actually "retard" in a way... right?

should i advance the ignition timing to compensate for the "slower" fuel? by how much should i advance the timing? how many degrees advanced per octane number up?

[ps2375]

in a stock motor, you won't really see/feel much difference either way. In a motor built to take advantage of the higher octane fuel, the difference will be noticeable. Kinda like the people over here tuning their turbo motors for e-85 and then having to dial it back for less than e-85 or for regular pump gas.

[atticus finch]

[mark tucker]

to test fuel&rate it they put it in a old single cylinder engine and see where it knocks.pretty scientific aint it. I only run preamium in my bug, it like it. my wifes honda element loves it.better power&mpg a must when towing our heavy popup camper(it's loaded even a slide out).
you may find better mpg&power by tuneing your car to the fuel, not relieing on tuneup specs for fuel that hasent been avaliable in 30+ years.just a thought do as you wish.

[timvw7476]

Higher octane fuel is more chemically stable,
you capitalize on that stability with higher compression, lower octane fuel
would "auto ignite" from latent heat(heat generated from previous combustion
cycles) if fed to such an engine, the higher octane is resistant to that latent
heat in the combustion chamber, leading to trustworthy ignition from the
sparkplug, not some hot carbon lingering in there.
An engine builder builds an engine for higher octane ratings, you don't
advance the ignition event to capture that advantage. All that will do is make
your otherwise standard octane burning engine seem more "peppy" for
a brief window of time, leaving too much heat in the heads, taking
operational hours of use out of them.

[atticus finch]

[bug1200]

[bug1200]

actually, i just asked this question because i won a free tank of "97 octane premium" in an event here

[mark tucker]

fill up the biggest tank you can find!!!!! cr isant all the issue with fuel requirements. a wide piston to head clearance can detonate sooner than a tight deck.and the tight will usualy have better tq and mpg and be a happyer running engine. so you can raise the cr and get rid of the detonation and have cleaner chambers,better mpg,power,tq. there are many things that alter many things that alter more.

[modok]

Yes using a higher octane than needed will decrease performance.

Hard to pass up a free tank of gas.......
Advance timing 3 degrees, and let us know if you can feel the loss. I can, but it's not much, maybe you won't notice.

[mark tucker]

I dont think so, never seen it decrese power/performance in anything. I have seen race fuel do it but not pump gas. somany thiongs are diferent and somany factors, try it&see. although if you are not using E fuel now than there may a decrease if it's on the leen end to start with. I have no issues with running Egas. our pumpos say 10% but when tested some come out at around 20%.like everything else nothing is what it is supposed to be.

[bug1200]

im gonna empty the tank first before i fill it up with the "97 octane" gas so i could really take advantage of the free gas. its already on the R sign right now and maybe about half a liter left inside...

then im gonna do some testing.

btw what does the R mean? refill? reserve?

[modok]

It means "reserve"
same word in English and German

[bug1200]

[bug1200]

I did some more research about octane level and burn rate and found conflicting results. other sites said that higher octane burns slower, others faster, and others no difference.

but! i read from the articles of other sites, vintage car sites, that: the formulation of todays fuel is different from the fuel that our engines were designed for. and again, i found conflicting results. some say it burns slower, some say faster... ill just conclude that todays fuel is "different" from yesteryear, and i need to make necessary adjustments to cope with the change.

so, i filled the tank with the "97 octane" fuel, and drove around for awhile. there was no difference... before i made some adjustments.

then i did some testing...
my initial setup: solex 30 pict 3 carb, 112.5 main, 65 idle, 125 air. dizzy is a bosch 113 905 205t vacuum distributor with stock vacuum can timed at 0tdc

first i made sure that the carb was good, the distributor had the stock vacuum can and so on...
note: the engine usually runs on when turned off after driving it hard (i dont have an idle cut off valve).

after filling the tank, i drove it for awhile to make sure that the fuel being fed to the engine is the 97 octane. there wasnt any difference, it felt the same.

so what i did next was advance the initial timing to 10btdc. the total timing will therefore be (based on the data from oldvolkshome) 43-45 degrees! thats high! the results... well the idle went up, very high, i had to lower it by 5 turns. and the idle was smoother. acceleration "felt" lighter, oil temps dropped about 5 degrees (based on my vdo gauge) and no pinging/detonation. not to mention that the engine did not run on after turning off...

i didnt stop there, i was afraid that the engine was pinging and i just couldnt hear it. so i changed the vacuum can to the one for the bosch 113 905 205k and set the timing to 10btdc as my baseline. then i followed the guide here: http://classicmechanic.blogspot.com/2011/05/ignition-timing-with-modern-fuels.html?m=1

i ended up with an initial timing of 13btdc. which, based on the data from oldvolkshome will give me a total advance of 37-41 degrees! i didnt hear any pinging/detonation, so i think its ok... i did notice that my engine is very happy at this setting. it runs cooler, acceleration feels lighter, smoother idle, no more engine run on after turning off, what more can i say?

here: does higher octane fuel burn faster/slower than lower octane fuel? i doubt it. octane rating is only a measurement of how resistant the fuel is from being ignited by anything other than the spark plug (eg. carbon buildup, higher cr, etc). so maybe even if the fuel i put in my car is (just an example) lets say 200 octane, and i over advance the timing to the point that it fires while the piston is still going up, am i still going to have pinging/detonation/knock?

and also according to my research, higher octane fuel needs more spark energy to ignite. the ignition systems of our volkswagens are weaker than the ignition systems of todays cars right? if so, wont the chances of a misfire go up? and if so, higher octane fuels that fuel manufacturers market as performance fuels might actually decrease performance if the engine and the ignition isnt built for it due to the higher probability of the spark not being able to ignite the fuel.

this leads me to another question, if i switch back to regular fuel, does that mean i have to retard the timing again? i dont think so, because (just reread the part where i said does higher octane fuel burn faster/slower than lower octane fuel).

and finally, does the formulation of todays fuel make it burn faster/slower than the fuel of yesteryear thus "advancing/retarding the timing in a way"? I dunno... maybe? hopefully someone more experienced could answer this question?

please correct me if im wrong in any of these statements, what i wrote here are only from my research, and i still have my doubts...

the tests i have done were all only on the same day, i dont have time to test each setup for very long. but the route was the same, so is the traffic condition, and weather/outside temp.