Another fuel octane thread...
 

The recent post on "what octane fuel should i use" got me thinking. What if I want to go to the drag strip or autocross with the most power available?

Is high octane fuel actually bad for rotaries just because it is not needed? The rotary engine doesn't need a high octane fuel, because it doesn't even burn all of it as it is. So it is often recommended not to waste your money on the 97 at the gas station.

However, with more octane at its disposal, wouldn't this be a good thing either way in terms of power? Or would it simply make no difference?

If I wanted to, I could get some 100 octane (avgas) from the airport. I want some facts backing up your answers too, not just what you've heard other people say. Thank you much.

I read somewhere that a few rotary race cars actually had a special LOW octane gas brought in for race, but thats just what I heard and I have no proof to back that up.

Just as an educated guess, I would say that no, it wouldnt help just because octance is a resistance to detonation, so the higher the octane is, the higher the compression can build before ignition, but since our engines run best (or better?) with lower compression, I dont think you would want a higher octane. I could be wrong, and once again, have no proof. Im standing down now ;).

~T.J.

CLose TJ. At least you do have some idea about it. :) Im not a scientist, and I didnt stay at a Holiday Inn Express last night but I'll give you a brief explanation as I see it.
Yes Octane number is related resistance to detonation. Soooooooooo, lets talk about that first. What is detonation? Detonation is an uncontrolled burning in the chamber. Instead of a nice flame front, it literally burns all at once (like an explosion). Lower octane fuels have a quicker burning characteristic. Higher octane fuels do the opposite. In an N/A rotary we have a long narrow chamber unlike a boinger. We need that flame to travel a greater distance than in a boinger. And with a limited amount of time to burn it, the air fuel charge needs all the help it can get. Well we have 2 ignition systems, which helps get things going, but a quicker burning fuel will ensure that its all consumed before we reach the exhaust phase. That means less unburnt fuel going out the exhaust, and more power made per rev.

carl is right, now for turboed engines. lets say that you have 1 atmosphere of preasure b4 it's compressed. lets say you run 8:1 jsut to be easy(this is not realy realistic, what i'm about to say, but it's easy to help explain that turbo engines will have more preasure in the chamber than n/a engines). so after is compressed you have 80 atmosphers of preasure. then you run 10 psi boost. all the sudden you have 800 atmosphere of preasure. thats a shit ton. the higher octane resists deonation because the molecule is larger or has an additive like ethnol. that means it takes much more preasure to build up the heat to cause detonation. but if you have the preasuer built up enough, there will eb enough energy to burn all of the gas. now if we talk in terms of n/a engines you won't have as much preasure built up in the chamber which realy means that you won't be able to produce the heat/energy to burn all of the high octane fuel. also go ask the second(turboed) or 3rd gen guys waht they run for gas, i bet it's 91 at the most.

We went through this in the previous thread. You can't positively say that a higher octane gas burns more slowly than a lower octane. There is no direct relationship between octane an burn rate. Look at this table:

http://www.me.mtu.edu/~slpost/CLASS/hcprop.html

The hfg value is known as the heating value, this is the amount of heat energy in the fuel. Notice on this chart that although hfg is decreasing, octane value varies up and down.

The same can be said for burn rate. It's dependent on the method used to raise the octane of the gasoline.

A fuel can have the molecular structure that takes alot of energy to break up, but once it does, it does it fast. On the other hand, a molecule can be very easy to break apart, but it can take a long time to fully break up.

You can take two gasolines of the same octane rating from two different sources and get two different burn rates, total heat release, and engine power levels.

As for the flame front not moving fast enough, forget it. Flame propigation rates are so high, that any fuel that will burn, has burned during the combustion stroke. Incomplete combustion is mostly due to the geometry of the combustion chamber and the turbulence of the airflow within the chamber.

I understand the concept of what you're all saying, except I wasn't aware that octane slows down the speed of burn.

T.J.: Stand down, soldier!!! Ayh said, stand down soldier! :p

Thankyehmuch for the answers.

That's what I'm saying isn't necessarily the case. There are different methods to changeing the octane of a fuel. Some decrease heating value and some don't. Some decrease burn rate and some don't.

So to go back to the original question. There is no DIRECT relationship to the octane of the fuel and the amount of energy you are going to get out of it. So there is no way to know if there will be a loss in power when using higher octane fuel.

The best thing to do is research fuels. I'm sure someone out there (maybe check the SAE website) has done testing to find the heating value of different gasoline mixtures (suppliers). The gasoline with the most stored energy is the one you want. Again, it won't be DIRECTLY related to octane rating.

noone said that ther was a realation. i simply said the burn lasts too long and will still be burning outside teh chamber, so you won't get the full amount of engery pushing against the rotor b/c the burn last to long b/c it wasn't heated up enough by the compression

Guys, it's really this simple. Lb for lb of fuel, high octane and low octane have roughly the same energy content. The octane is a rating that deals with the anti-knock capabilities of the fuel--not the energy content.

Yes, it is true on some cars if you use higher octane fuel you get better mileage. Not because the fuel has more energy, but because that particular engine runs better and delivers more output for a given flow rate of fuel (mpg).

Ok... So what you've all basically said, is that a higher octane gas most likely won't improve the efficiency or rate of burn enough to justify the extra money and/or increased compression. I'll have to research it a bit more like purple82 said.

Danke.

This is like god damn ping pong. First higher octane in a rotary is unproductive... tehn it doesn't matter... then it is unproductive again... then it doesn't matter... tehn it matters but it won't be unproductive and by this time i have no hair.
Are there any concrete answers? I just want to know what i should fill my tank up with... that's all.

Zachstylez

Easy answer: The Cheap Stuff.

We all *know* N/A rotaries run fine on the cheap stuff. You don't seem to be running a turbo, and that setup seems to be the only debate here. For turbo you may want a higher octane to prevent detonation caused by the high compression. Any other time, why spend the extra money? 65c/L or 87c/L , which would you rather pay if you don't have to?

Jon

so octane rating just tells you how much resistance it has to detonation but not the flame rate. do you have anymore info about the different fuels/additives that affect the flame rate?

Ok, so thinking of the octane as a fuse before a firecracker. Lower octane is an easy to light fuse, higher octane means you have to hold the lighter to the fuse longer before it catches, but when both the firecrackers go off, its still the same thing.

So, with that in mind, couldnt I say that if the spark plugs in an engine are the lighter, then with a high octane fuel its going to take longer to light the fuse (ignite the air/fuel mixture), but a lower octane fuse would light easier.

Now, with that in mind, couldnt I say that if you were trying to make the most power and burn most of the air/fuel efficiently, that the longer it has to burn in the combustion chamber, the more will be burnt? And if thats true, isnt it true that something easier to light to begin with would have a longer time to burn because it didnt waste all the time trying to be lit to begin with?

Is anyone even with me here? Are we on the same page, or are you reading a different book?

~T.J.

Ok everybody Very simple... Why don't we just run 89 octane, which burns quickly enough, but also burns long enough ? You get good ignition + it then burns fully meaning u get the best ignition to burn time ratio ! :D

This is totally un-scientific, but nevertheless, shuts up most people who dn't have a clue what to put it :P

jk people, but I always run either 87 or 89 and no stupid civic beat me yet so ! .. works for me !

Without going into the technical side of fuel, for about three months I conducted a practical test on standard versus high octane fuel.

The test vehicle was my 1985 RX-7 with the 12A engine.

The test track was part of my twice a month 1200km trip to and from work. The actual test run was 410km from the outskirts of a town on flat slightly curvy road with minimum traffic, mostly 100km speed restriction and about 150km of 110km restriction, and only two small settlements when speed drops to 60kmph for about a minute. I keep within 5% of the speed limit as traffic fines are really steep. Its a lovelly feeling to have a Civic roar past and catch up with them 30km down the road stopped by flashing lights and a $400 fine!

In total I collected 10 sets of data with no real difference in weather conditions. The average consumption was 8.82 litres per 100km on high octane,
8.85 litres per 100km on standard. All results were within 3% of each other except one into a strong wind.

My conclusion was that there was no statistical difference between the fuels, and the extra cost of the high octane was not justified. The limitations to the test was the engine cruised along at 3000-3300rpm. Thus I have no knowledge of what happens at 5000+rpm.

Its a boring 4 hours of my life thus the test runs added a bit of interest. One way i used shell fuel the other caltex but again no statistical difference between the results.

TJ, I like your fuse and firecracker analogy. However, instead of thinking that it takes longer to light one fuse then the other, think of it taking more heat. The easy fuse need a match, the hard fuse needs a candle. The spark plug is a blow torch. The spark plug is so far beyond the ignition temperature of either that there is no difference in ignition time.

I've also read a reseach paper available from MIT that states that the difference in burn rates between regular and premium gasoline isn't enough to be significant inside an engines combustion chamber. Measurable but not meaningful in the real world.

And finially, I have actually tested regular vs. premium on a stock 12a on a dyno. Premium made 2hp more then regular. I believe that variations in a particular batch of fuel and the margin of error from one run to the next on the dyno could account for this. However, it didn't make LESS hp on premium like lots of people profess.

So, in my SCCA Spec-7 RX7, which runs a stock motor, I run premium. If this was a street car, there's no way I'd spend $.20 a gallon for 1-2 hp. On a race car, absolutely.

82 octane fuel!?
 

Hey guyz while this post is up here, and sense weve established that lower octane runs rotaries better what would happen if I ran 82 octane fuel in it with mmo because its avaliable around where I live..... would I get better power curves or more power or what?

Re: Another fuel octane thread...
 

all other things being equal, 87 pump fuel will (probably) yield more power than 93 or 94 (or higher) pump fuel. Higher octane burns slower, I use 86 in my FB race car (non-turbo). Turbo is a whole nother story.

Higher octane burns slower also

Yeah if you were running 10 bar (aka 147 psi)

10 psi = .68072 bar = .68072 atmosphere

Re: Re: Another fuel octane thread...
 

Blanket statements make for incorrect statements.

So what's the next two posts say? Now that it been established that regular is better then premium...

Doesn't anyone actually read what's put in front of them? Or is it a case of, any information that doesn't agree with someones preconceived ideas is ignored?

Let me try again. On a dyno, a stock 12a makes MORE hp on premium then regular.

Forget theories, if you haven't shown it on a dyno, its all just hot air. Please prove me wrong, show me YOUR dyno sheets. But don't tell me anymore of your theories.

So your dyno runs were made back to back?
If so, that tells us that they are basically equal. Now lets steer this towards another tangent. If octane is un related to flame front travel, then what is? Or more importantly, is there a theoretical way to extract more power by manipluating the fuel?

Carl,

I'd refer you back to this to answer some of the questions. Flame front travel speed is dependent on many things including in-cylinder turbulence, fuel diffusion rate into the air (and AFR), the molecular makeup of the fuel, the shape of the combustion chamber, the location of the spark, and lot of other things.

But like I said above, unburned fuel in a combustion chamber isn't due to burn rate, its due mostly to the shape of the combustion chamber, cool spots and whatnot. So a faster burning fuel won't really help you.

However, a fuel with a higher heating value WILL yield more power, basically, there's more energy stored up in the fuel and so you get more energy out of its release.

I'm not an expert in fuel formulization, so I couldn't point you to the right fuel to use but the table I've linked to above should give you the idea.