timing-race gas vs pump gas

[plovett]

Anybody know what a gasoline burn curve looks like?   That would be burn rate on the left or vertical axis and time (or maybe crank degrees) on the horizontal or bottom axis.

My guess is that rate is not constant.  It will start off slow, increasing as the flame spreads, then reach a fairly constant speed until most of the fuel is expended, then fall off again.  It might be something like a bell curve with a flat portion on the top?

If that is true, the initial part of the burn would affect how long it is before the mixture reaches its "constant speed".  I think this would affect the optimal start of ignition.

Maybe the small difference in starting conditions is too small to make a measurable difference in ignition timing.  That's kind of where I was coming from in terms of relating octane and timing. 

Also, I once dyno-ed an engine with race gas and the power kept increasing with more timing, up to 45 degrees before I chickened out.  I can't remember what octane it was now.  Something between 104 and 110.   I now have another engine with the exact same heads and it likes 37 degrees total timing with 92 octane gas.   There are other differences between the two engines which affect timing, as well.  Still the exact same heads, though.  I think the timing was accurate both times.   Same timing gun and even the same balancer on the engine.  So I don't think you can say a certain set of heads always wants the same ignition timing regardless of other factors.

Maybe directly relating octane to timing is not warranted.   But surely burn rate, or the average burn rate, varies between different gasolines?

I'm just throwing that out there for discussion.

Flame away!  Get it?   

[Tobbemek]

One thing for sure is, early ignition fights against power.If you have to advance your timing because fuel with higher than necessary octan for a certen engine combo to get maximum power you could lose power. There is so many things  with vaporizing, easily burning carbon molecules and how many of them that play a roll how fast a fuels burn rate is besides just octan witch i believe we all understand easily. All to getter its to complicated for me to discuss.

[Barry_R]

Anybody know what a gasoline burn curve looks like?   That would be burn rate on the left or vertical axis and time (or maybe crank degrees) on the horizontal or bottom axis.

My guess is that rate is not constant.  It will start off slow, increasing as the flame spreads, then reach a fairly constant speed until most of the fuel is expended, then fall off again.  It might be something like a bell curve with a flat portion on the top?

If that is true, the initial part of the burn would affect how long it is before the mixture reaches its "constant speed".  I think this would affect the optimal start of ignition.

Maybe the small difference in starting conditions is too small to make a measurable difference in ignition timing.  That's kind of where I was coming from in terms of relating octane and timing. 

Also, I once dyno-ed an engine with race gas and the power kept increasing with more timing, up to 45 degrees before I chickened out.  I can't remember what octane it was now.  Something between 104 and 110.   I now have another engine with the exact same heads and it likes 37 degrees total timing with 92 octane gas.   There are other differences between the two engines which affect timing, as well.  Still the exact same heads, though.  I think the timing was accurate both times.   Same timing gun and even the same balancer on the engine.  So I don't think you can say a certain set of heads always wants the same ignition timing regardless of other factors.

Maybe directly relating octane to timing is not warranted.   But surely burn rate, or the average burn rate, varies between different gasolines?

I'm just throwing that out there for discussion.

Flame away!  Get it?   

All of those are great questions.  And all of those are actually fuel chemistry variables.  I had a friend from a pretty good sized gasoline manufacturer try to explain some of this to me a long time ago. 

Burn rates, evaporation rates, octane values, and energy content are all somewhat dependent upon each other, but no blanket statement can be made or interpreted from one to the other.  As an example, race gas will normal have a high octane to resist self ignition under high cylinder pressures.  But it may also have a faster burn rate once lit - - since we have less available time in the combustion cycle to convert the fuel to energy at high RPM.  Exactly the opposite of the oft cited urban statement that high octane race fuel burns slower.

[fryedaddy]

thanks to everyone who replied.it looks like i will leave it alone.most of that chemistry stuff is over my head.i thought it was a simple question. lol no real reason im going to run the race gas but i dont go over 10 miles on a trip and the price of the better gas is no big deal no more than i drive.last spring i had the gas boiling in the carb problem because of the winter blend pump gas with ethanol .im going to run the 110 whether i need it or not.i was just wondering if i needed to make a timing change or not.

[plovett]

All of those are great questions.  And all of those are actually fuel chemistry variables.  I had a friend from a pretty good sized gasoline manufacturer try to explain some of this to me a long time ago. 

Burn rates, evaporation rates, octane values, and energy content are all somewhat dependent upon each other, but no blanket statement can be made or interpreted from one to the other.  As an example, race gas will normal have a high octane to resist self ignition under high cylinder pressures.  But it may also have a faster burn rate once lit - - since we have less available time in the combustion cycle to convert the fuel to energy at high RPM.  Exactly the opposite of the oft cited urban statement that high octane race fuel burns slower.

Thanks, Super-Bear! 

I try to throw myself out there as a sacrificial fuel source to promote the combustion of thought.  I don't care if I'm wrong as long as me being wrong stimulates someone with more knowledge than me to propagate glorious hp.

I'm going away now.  Far away..........   

[Barry_R]

I really and truly do not have the answers to fuel chemistry questions.

I was simply fortunate enough to meet somebody who was willing to let me "know that I did not know"....

[Katz427]

I'll add a couple comments from my experience. Doing experiments on gasoline in the last 1960's , "hi test" burned slower and one could pick up a bit by advancing timing. Our test engine was an old 283 Chevy from the mid 60's. Then along came unleaded and the Clean air act. Gasoline makeup changed significantly. The lighter aromatics were taken out. The burn rates were adjusted by improved refining and changes to the chemical composition. The need to reduce peak temperatures that produced nitrogen oxides that were the major components of smog. As we all know the chemical makeup of gasoline has changed​ over the years. Today I would imagine the burn rates are tailored for the majority of fuel injected combustion engines on the road and as direct injection is adopted will change again.

[Tobbemek]

    "know that I did not know"... For me that is one of the more important phrases been working as a technician in  nuclear power plants   to know when you dont know avoiding mistakes by learning from people that do know and educate your self and then take it from there. Bad enough blowing up a good FE engine but a Atomic station well well. Remember the " dueling" 390 ? when Blair mixed pump with race gas playing it safe with the knew BBM heads and iron CJ heads he sead " He know pretty well wat the CJ heads needs beating on that iron for years " and  ended up was it 32 ore 33 ¤ with the BBM.s and 40¤ something with the CJ.s same cam same engine.
   Conclusion to your question, if you ask me nobody dont exactly know in your particular case one has to test what your combo likes best .
    Like in Blairs case with the 390.s  with mixed pump race gas fore me it indicates that you dont have to advanc your timing, so many             different dyno shows with modern FE heads that coms in around 32¤ total.
Som one ells my have an another  opinion thats fine with me because i just dont exactly know in your case, but you will not hurt your engin without advancing your timing and will not get a core melt down.

[fryedaddy]

i am going to leave it in the 32-33 range,i was just wondering because im not going to run winter blend ethanol pump gas in my car any more after the boiling problem last spring. thanks to all