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 Post Posted: Sun Jun 06, 2010 11:42 pm 
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My question is how do we determine the best gas to use for a given engine? With so many variables between brands short of trial and error, what should we be looking at to pick a fuel? As an example, the picture below shows 3 different octane rated fuels from Sunoco. The odd thing is that the Maximal has the highest octane rating yet looking at the distillation curve is completely evaporated at a lower temp than the 110. I understand that the components used in the mix will have a hand in octane rating independent of their evaporation temps, but doesn't the evaporation temps have some hand in this? And wouldn't a fuel with a narrower distillation curve be easier to tune for and with the lower peak evaporation temp burn more completely, requiring less fuel and make more power?

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 Post Posted: Mon Jun 07, 2010 2:49 am 
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HI Mark, people concentrate on Octane because its the one test that's related to detonation resistance. The octane test engine is low RPM and that assures sufficient time to vaporize the fuels on test but how that relates to high Rpm's and race engines with porting problems and fuel separation etc and other associated problems is unknown.

The distillation temps are a vague area because they only list the temperature not the specific heat required to reach that temperature. Although there is a general trend amongst HC;s that the more complex the molecule is the higher its gas state energy level is, it would be useful to know exactly what you are buying with different fuels. Just because a fuel blend of 500 chemicals has a 90% distillation temp stated doesn't mean it will be 90% gas in the time period available prior to ignition in the engine. The data presented is only a rough guide. For instance look at the Reid vapor pressure, a higher number indicates a more volatile fuel. See the Reid for maximal is higher than standard leaded that means it is more volatile, it evaporates quicker, but maximal has a higher distillation 10% point than standard leaded, which generally means it should have a lower Reid index.
Fuels with lower 100% distillation temps do reach a greater gas concentration earlier in the cycle, but that point may be after ignition. Street unleaded is terrible requiring 430 degrees for 100% generally. That's why you have to control the heat input sources of a street engine differently to a race engine. Street fuel is blended for EFI cars to pass emissions running at temperatures generated by stoichiometric combustion. So it has a high 100%.
Race fuels are designed for richer than stoich burns where the peak temps are lower. However a race engine with high cylinder pressures making 700hp is going to have a high temp peak simply because the pressure is great. So you have to have high octane chemicals to avoid erratic combustion across TDC. Cylinder pressures are loosly related to HP per cubic inch. A big engine can use street fuel to make 700 hp and it can be a bit sloppy about how it does it but a small one cant, it has to be exact with a small margin of error in design and tuning etc.

The optimum fuel for any combo has to be found by trial and preferably with measuring tools like Dyno's and gas benches and ignition scopes. Ignition scope traces reveal information about the vaporization prior to ignition and can be used to correlate to why a plug looks like it does with colors etc. Gas benches reveal if the chamber has areas of insufficient burn and the efficiency of the utilization of oxygen. EGT's reveal wasted energy. Dynos show the power.
When confronted with an engine that makes maximum power at AFR's like 12.2 etc you have to use a gas bench to see why. If the fuel is not being vaporized because of a distillation thing then you can see what happens when you change fuels. Just answering your question with " if it makes max power at 12:1 then the fuel is wrong is not a good answer. Many times people change fuel and see a result but if they cant qualify what changed then they dont know if it could be further improved. I constantly have battles over octane, often the octane rating is there to mask a problem of a lean area. So how can one say you need this octane for that compression. I simply cant do it.


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 Post Posted: Mon Jun 07, 2010 6:33 am 
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To add to what Shrinker said…
The completeness of the combustion of reactants is something of importance to think about. This means that all fuel molecules will have been fully oxidised and that all oxygen has been consumed by the reactants of the oxidised fuel molecules. Reaching this outcome of complete combustion has nothing to do with the type of fuel that is used. Any type of fuel can be completely reacted and all the oxygen consumed if the fuel and oxygen relationship is correct.
The closer you get to having the fuel and oxygen relationship correct on a molecule by molecule level, the greater the efficiency of energy (that your engine has), into the fuel load is applied.

To give yourself a head start prior to all the testing that’s required to achieve the best end result, (Just like you are trying to do here),you need to sum up,or discect or diagnose if you will, the type of engine that you have. You need to look at how much energy input the whole combination is capable of.
Make yourself a list of questions to answer before you choose your fuel or before you build an engine.
Do I have enough compression for the cam that I have..in other words. Now that I have a cam that keeps the valves open for a longer period of time, thus reducing the compression stroke,do I have enough static compression to makeup for this loss.
Where does the intake valve close…is it past 90 degrees or is it before.
If I want to choose a higher octane fuel,why is it that I want this? What is the reason behind my thinking? Is it because I might get detonation? Why do I get detonation? Can I stop detonation from happening and then can I use a fuel that vaporises a bit easier?
What size carb shall I use? What determines the size I choose? Is it based on getting the most air into the cylinders at the compromise of fuel quality entering the cylinders that might help reduce the detonation possibility? Can I start with a smaller carby and see the positive effects on the amount of gassed fuel in the cylinder at ignition time?

And so on and so on…
Come up with a combination based on fuel burn efficiency and gassing the fuel rather than basing it on reaching some numbers ,like HP numbers or AFR numbers or EGT numbers…etc..
Base the combination on something different,something that you can see and expect to see the effects of when you make changes to see whether you can burn that fuel load better.

Talk to people about engine combinations and what that means? What do people base engine combinations on?
I think your question is a good one and I think that Shrinkers answer is a good one.Its so involved,isnt it..Injection has stuffed it all up…injection has stuffed the fuels that we buy and try and run carbys on,cause the injection system is based on delivering HC’s to the cats for them to run…so the distillation temps and reid vapour pressures and so forth have this in their thinking when they make the fuels..


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 Post Posted: Mon Jun 07, 2010 7:06 am 
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Good reading but Some of this I don't really understand so go easy on me =; ...My first engine had a giant dome with a 114cc head to get the compression to 13 to 1...Now a days with the newer bigger CI motors I see the same head cc or larger and a much smaller dome and a compression ratio of 14,15 or even 16 to 1.
Now back them with the larger dome there was always a concern with "flame travel" and the possibility of detonation. With the smaller dome I would think that flame travel would be a lot better and the possibility of detonation would be less............ #-o ...

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 Post Posted: Mon Jun 07, 2010 5:23 pm 
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When using large domes the 'Flame travel' issue is actually flame extinguishing. When the large dome is at TDC or even 10 deg before, it is in close proximity to the developing flame kernel, once you place a surface close to the fire the fire is affected by the boundary layer of the surface, the fire looses energy. That's not the optimum thing to do to something that your trying to get started. Those old days of big domes created erratic slow burning due to disruption of the flame kernel. Now day we have small chambers and mirror dished pistons etc so that the flame path is just in an open space relatively. Sparkplug positions have also changed over the years, nowadays the plug is on the exhaust side in the hotter more vaporized zone. In the old days it was on the intake side or in the middle, lots of cold air, no vaporization, slow to start flame kernels that needed different fuels to burn. Ahh the old days of innocence and all that mattered was it lumped over a lot.


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 Post Posted: Mon Jun 07, 2010 7:40 pm 
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How detonation is linked to flame kernel development.
The flame kernel growth rate determines the pressure development of the entire chamber, that's fairly obvious if you think about it, but what that also means is that if the pressure developed is limited the heat is also limited. Heat and pressure are linked, heat goes up the pressure goes up. So how does that affect detonation? Well a cylinder that doesn't develop sufficient pressure has a higher chance of leaving behind partly combusted HC's.
The concept to think about is this (it doesn't happen this simple)-- Combustion is a sequence, a molecule can be oxidized like a fuse cord in an old cannon. In the chemical world if you remove a carbon or Hydrogen from say one end of a molecule then that molecule will reform into a different shape and that's a different chemical. This process is combustion but that's only a partial combustion , because we didn't oxidize all the molecule. Combustion ceases once there is no oxygen to do it with or the energy level (pressure or heat) drops too low. Once a molecule has been partly combusted its a lottery as to what it might be chemically. But one thing is for certain its going to be hotter than a fresh new molecule coming on the intake charge. So when these partly combusted hot molecules get compressed the SECOND TIME AROUND they get too hot and detonate especially if there not chemicals of high octane. So basically detonation can occur because you failed to complete combustion. If the partly combusted molecule is a chemical that's not high octane your stuffed. All of a sudden you have detonation in the areas of the chamber that trap gases or dont scavenge residuals.


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 Post Posted: Mon Jun 07, 2010 8:01 pm 
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shrinker wrote:
Combustion ceases once there is no oxygen to do it with or the energy level (pressure or heat) drops too low.

Important point that, for understanding Widebands and gas benches.
If the combustion continues in the exhaust it of no use as far as pressure generation in the cylinder goes, but the WB and the gas bench will read the more completely combusted situation as it exists in the exhaust. We common people don't have tools to read whats happening in the chamber. AHHH but yes we DOOOOO. Its called the spark plug. Anytime you have a white plug and the AFR is supposedly Richer than stoich then you have a lack of vaporization in the flame kernel(its lean at ignition time). Principally, the only way you can get zero carbon deposits on a rich mixture is if its not rich in the area you are looking at. (fuel washing not included, that's a different thing to a combustion discussion. ) That statement applies equally to unleaded fuels as leaded. It matters not that the fuel is unleaded. Unleaded fuel does not magically create zero carbon, there's plenty of it in your exhaust pipe, just look at stuff in a different way and you can learn new things.


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 Post Posted: Mon Jun 07, 2010 9:33 pm 
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I'm sure my engine is outside the "norm" for bracket engines, Mine is a 461 CI small block with flat tops and SB2.2 heads, just over 15 to 1. The combustion chamber in mine should be one of the best designs, coupled with the flat tops I should have little issue there. The cam is big, 312/331 at .020, 284/300 at .050, 207/219 at .200, .4544"/.4559" lobe lift, .863"/.866" gross valve lift, .843"/.838" net valve lift. LSA is 114 LSA and ICL ended up at 108 which gave it the best peak torque and HP on the dyno. 29˚ timing.

I guess my point was there anything in the info given by the fuel companies to help make a choice to start with, something to look for. You made a comment on a prior thread that a chemist could solve the fuel issues for most engines, I wanted to look the other way. I guess there are too many variables and unknown info from the fuel companies. Back to trial and error.



shrinker wrote:
Ignition scope traces reveal information about the vaporization prior to ignition and can be used to correlate to why a plug looks like it does with colors etc.


It's been a long time since I had access to an engine scope, early 80's. I remember working on an import once finding an intake leak on one cylinder, the spike for it was different than the other three. I do have a regular 150mhz oscilloscope collecting dust, never got around to figuring how to set it up on a car. How will the trace look different with different vaporization prior to ignition? Will it show it requiring less voltage peak to fire the mixture? As far as plug color, we have some fuels that sometimes color the plugs due to the dyes that are in the fuel to contend with.


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 Post Posted: Mon Jun 07, 2010 10:41 pm 
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The voltage to ionize a gap varies with the AFR. the leaner it is the more voltage required. The important part of scope traces is the duration time and voltage during the duration. There are many books or internet sites with information on the subject but to me all are lacking for racing. Thats what your interested in.
If someone wants to get into scopes etc then go buy a Picoscope It is a small box that attaches to your laptop and converts it into a data logging scope. There terrific, so fast and detailed it will blow off any auto scope in a workshop.

Anyway I look at the arc line on a race engine in a different way, I correlate what I see in conjunction with what I have done and what other things I know about the engine. You have to do it that way because whatever you read about scope traces is usually orientated to service work on normal engines etc. Once you gain skills over time of what gases are produced and AFR's and sparkplug reads and ignition box designs you can see how every tool helps the next.
The arc duration line of a Crane Hi-6 is a wonderful thing. Provided the ignition leads etc are up to scratch the Crane is able to display flame propagation start point, changes of vaporization during the duration and even be representative in some way of the way the engine generally runs, like how it vibrates or 'feels' to me when driving on the dyno. Really good engines have smooth flat durations, rough ones have jagged lines.


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