total timing too high?

[plovett]

I do not have enough time to get too deep into this discussion - but need to point out a couple things. 

Fuel burn rate and octane are two different characteristics of race (or street) gasoline.  Octane is the resistance to self ignition and does not tie directly into the rate of burn once that ignition occurs.  They are obviously inter-related as all chemistries are- but race gas does not burn slower than pump gas as any sort of rule. 

Think about it for a moment.  Both fuels are designed and optimized for a particular need.  A street engine will need to run its best at idle and lower RPM.  A high performance application will need to run its best at very high RPM.  At high RPM you have much less time available to use for each combustion event.  In order to get the combustion event completed and get the most power from the engine you will want the race gas to burn faster...and it does....

I was confusing, maybe contradicting myself, with time of combustion and rate of combustion. 

One might think that combustion starts with the spark and that is that.  Then you have the rate of combustion, and that is that.  I'm thinking there's a lot more to it which I'm sure you know.

I'm guessing the octane affects the early combustion rate.  So after the spark, the octane affects the early part of the curve?  This in turn affects the required timing, which makes us think the combustion is starting later, but it really isn't.  It's just the early part of the process is slower?  So you have to start it earlier to get to the same peak pressure at the same time?

paulie

[plovett]

Not to confuse the issue, but I think compression ratio affects this, too.  Higher compression imparts more energy to the air and fuel molecules which makes them easier to ignite.  It also moves the molecules closer together for more and more frequent collisions, increasing the speed of combustion.  I imagine this is obvious for the professional engine builders, but I'm still thinking it through.   

paulie

[chris401]

I do not have enough time to get too deep into this discussion - but need to point out a couple things. 

Fuel burn rate and octane are two different characteristics of race (or street) gasoline.  Octane is the resistance to self ignition and does not tie directly into the rate of burn once that ignition occurs.  They are obviously inter-related as all chemistries are- but race gas does not burn slower than pump gas as any sort of rule. 

Think about it for a moment.  Both fuels are designed and optimized for a particular need.  A street engine will need to run its best at idle and lower RPM.  A high performance application will need to run its best at very high RPM.  At high RPM you have much less time available to use for each combustion event.  In order to get the combustion event completed and get the most power from the engine you will want the race gas to burn faster...and it does....

Thanks for pointing that out. Another testimony to stable fuel is a hot soaked carb. Although my truck will run 87 octane and only sacrifice .5 mpg it does boil the fuel and flood. It takes about 30 minutes sitting off with the current 1" aluminium adapter. No problems running 93 octain or stacked with a phenolic spacer under the adapter.

[fryedaddy]

i dont know if any of this would come into play,but my comet weighs 2950 with me in it ,high stall auto ,low geared rear

[Dot Heton]

As long as we're all here talking about timing...

I set the timing in my truck by putting it at 32 degrees at 3000 rpm or whatever rpm it stops advancing. The initial timing ends up at 14 or so depending on how fast the idle is. BUT THEN I hook up the vacuum and it will advance even more than my 32 degrees when over 3000 rpm.  How much farther is hard to tell, am I doing this right?

[jayb]

That sounds right, its not unusual to have advance up to 45 or even 50 degrees when the engine has vacuum.  The engine can use a lot of advance in a high vacuum cruise situation.  Sounds like you are doing everything right to me...

[Drew Pojedinec]

Paulie, if you ever get interested in studying fuel movement, timing effects, etc this is a great search engine of papers that were written when this was THE cutting edge field.

https://ntrs.nasa.gov/search.jsp?N=17

[plovett]

Paulie, if you ever get interested in studying fuel movement, timing effects, etc this is a great search engine of papers that were written when this was THE cutting edge field.

https://ntrs.nasa.gov/search.jsp?N=17

Thanks Drew! I will check it out when I get a chance.  Probably 3 am.