Octane Boost Question For J Smith

[Prof. O/B]

Hi Guys, Thanks for the hellos. Your assumption is pretty much on Raceman. I'll assure you that it you mix a 50:50 mix you will always get a minimum octane between the two numbers you mentioned. I believe that it was 92 + 102 = 194/2 = 97 octane.
In reality, that number will always be higher than that when blending with a leaded fuel. The reason being, say for instance, you add 2 cc per gallon of lead to gasoline and let's hypothetically say it boosted the fuel 4 octane points; then we increased the the cc's to 4 per gallon we should expect it to increase it four (4) more numbers or 8 octane numbers. However, this is not the case. You will find that almost without exception that by adding any particular booster or distillate works on a deminishing return. In this case the addition of 4 more cc's only yields us an increase of only 6 octane numbers. This does inversely, makes the lead more effective when mixing a leaded fuel with an unleaded one. The real octane one might expect would be more like 98 octane. However, your method is most certainly proves a reasonably close, safe approximation.
I was most interested in the post concerning the gasoline representative not understanding why the jet ski lost, I believe it was said - 3 mph. I was not surprised at those results and neither should the rep have been. Being a rep doesn't necessary equate to being a "rocket scientist" as someone had noted in a previous post. He's been told, and rightfully so, that his gasoline has more energy than does pump premium. therefore it was only natural for him to conclude that the jet ski would go faster with a higher energy fuel. That energy is usually expressed in BTU'S per Lb. for anyone who really cares. The problem of course was the engine was over-octaned with the race gas for it to be successfully used in this application. Most everyone knows that too much octane degrades engine performance, and it is also true that most don't really know why. And a greater portion don't really care. That's OK, but to be a practicing "artisian" in extracting HP from an engine, which we are most all interested in, it seems odd to me that the very 1st. priority would appear to be to have a conceptual understanding of the combustion process. I could not conceive that an intimate understanding of that knowledge would be anything less than a boon to someone who wanted to win.
Having said that, the problem with the race gas primarily was the addition of lead in the fuel. If one would imagine a "dry" grass field and we were to start a small fire (in ignition terminology a flame "kernel") in the middle of that field, the fire (or flame front) would spread across that field in exponential fashion, becoming faster and faster until the field is consumed.
That's a great scenario, know let's throw a kink in the process by going to various spots in the fuel and washing down certain spots with water. Now if you are hunting the ultimate octane booster this it, it does burn at all. One would quickly see that the addition of the watered-downed spots would momentarily "impinge" upon the combustion process as it the flame came to the damp spots. This is the way lead works, it momentarily interrupts the flame front in the same way. After a while the flame, in this case would drive the moisture out of the grass and it to would be set afire, however, this slowing down of flame front propagation is exceptable with one minor exception. If the exhaust port opens before the process is complete the unburnt fuel (power) goes out the exhaust port. So, while the the race gas has more energy than pump premium, it is of little importance if it cannot be utilized.
The same can be said of any additive or distillate that slows or interrupts the combustion process. Whether it be octane booster, toluene or whatever. The aromatic hydrocarbons used to "drown" the knock out of pump premium and work much in the same way as lead, as they are harder to burn. So with this in mind, when we discuss octane maybe we should express conceptually in another manner..........."A resistance to burning",
Prof. O/B

[Mark75H]

Doesn't this assume an unproven that fuels that are more knock resisitant are always slower burning?

[DSwartz]

Prof. O/B
Thanks for your reply.
I,m using 2 gal. 100LL with 3 gal 91 pump. I'm figuring that comes out to about 95. Close?

[O/B Dude]

Hey DS,

100+100+91+91+91=473 divided by 5 total gallons = 94.6 octane. 91+91+100=282 divided by 3 gallons = 94 octane. 94 is enough to run well above 150 psi compression on a Merc. I don't know much about them JohnnyRudes.

Sean

[O/B Dude]

Mark75H, yes. The whole idea on higher octane is to burn slower. You can actually make your motor run worse with too much octane and not enough compression and/or timing to take advantage of the slower burn. It burns too slowly and produces less power because the piston is already too far past top dead center by the time the mixture finishes its burn. On the other hand, if it burns too fast for the spark timing, compression ratio, and combustion chamber design, the burn will complete too far before the piston reaches top dead center and you'll get "knock". I have heard many people confuse octane rating with BTU rating and think premium fuel always make more power because its more "explosive", for lack of a better term. Just because a fuel has higher or lower octane rating doesnt necessarily mean the fuel has higher or lower heat producing capabilities, and total heat output is what makes power. In example... Alcohol has a higher octane rating than pump gasoline but has a lower BTU (British Thermal Unit - heat measurement) and makes less power with all things being equal. Alcohol requires a richer mixture, more timing, and more compression to make good power. 91 octane premium and 87 octane regular unleaded have the same BTU rating, its just the knock resistant octane rating that allows one to make more power than the other.

[Cp]

Thanks for the time! Your grass fire analogy is stunning.

Hang around if you can.

Cp

[Markus]

Actually, Mark75h knows a bit about gasoline, and his question is not as innocent as it seems. Octane rating is defined from the resistance to uncontrolled burning of the fuel. It generally correlates with flame front propagation, but it is not the same thing.

For example, supposedly, avgas burns slower than race gasoline with the same octane rating. Or at least, that is what some vendors of race gasoline say.

[Scream And Fly]

I don't want to hijack this thread, so I'll be quick:

Welcome back, Prof. O/B! A lot of people sure do miss you here (including myself).

Greg

[Jay Smith]

Glad to see ya back Uncle Chauncy........

Jay & Cat

[O/B Dude]

Marcus, I was trying not to get too technical and just address what we performance boaters deal with in our engines. There are a lot of other considerations with regard to fuel quality, knock, ping, pre-ignition and detonation. Octane is actually a chemical product of the refinement of crude oil into gasoline. Octane rating is basically a comparison of gasoline's performance to that of pure octane. If anyone wants to know more about gasoline, octane, etc...I just did a search and found a website that explains it pretty well.
http://auto.howstuffworks.com/gasoline3.htm

[CDave]

I seem to remember reading or hearing that race fuel, say Sonoco purple, has a different specific gravity than pump gas and flows more through the same size jet.

Is that true? If it is, I'm figuring not only are you getting a higher octane=slower burning fuel but you are richining the mixture at the same time which I'm guessing slows the flame front down also.

[Jay Smith]

You are OH so correct !

[O/B Dude]

Specific gravity doesn't effect the volume that flows thru a given jet or orifice but it can effect the float level in a carb. Specific density effects the volume at which the fuel can flow thru a jet. There are minor variances between race fuel brands and between specific octane fuels made by the same fuel producer. Its the non-gasoline content of race fuel that tends to make the difference in gravity/density between various grades of fuel. In example, mixing alcohol with gasoline. Alcohol has a spec gravity of .79 and gasoline is around .69. (Varies with blend) A 1:1 mixture would provide a gravity of .74. Specific density of gasoline is around 42lbs per cubic foot. Of course mixing in additives of a different specific density will change that figure. Hope this helps.

[O/B Dude]

Here are a few websites with definitions, information and a mixing chart for pump fuel and race fuel.

http://www.76.com/products/76racing.asp#100OCT

http://www.aliveweb.com/race_fuels.html

http://www.aliveweb.com/fuel_facts.html

[Scream And Fly]

Interesting debate here... I won't say much about the fuel/octane issue since I prefer to leave it to the pros.

I do know, however, that some of the people posting in this thread have built/modified some of the fastest outboard drag engines in the world, and therefore have a very clear understanding of this subject matter.

Greg