Octane issues?

[robwbright]

I have 2 questions:

1: If you are running below 10:1 compression, can you really burn anything over 100 octane? I've been told that you need a high compression motor to burn high octane fuel.

2: What is the difference between leaded and unleaded race fuel as far as combustion? Does the unleaded stuff burn better and therefore justify its higher octane in a lower compression motor?

Thanks

Rob

 

[Rich Rohrich]

I have 2 questions:

1: If you are running below 10:1 compression, can you really burn anything over 100 octane? I've been told that you need a high compression motor to burn high octane fuel.

The fuel will "burn" whether the compression ratio is 5:1 or 12:1. If the compression ratio is low enough a 100 octane fuel might not be "needed" but it will burn regardless.

2: What is the difference between leaded and unleaded race fuel as far as combustion? Does the unleaded stuff burn better and therefore justify its higher octane in a lower compression motor?

Under normal running conditions leaded and unleaded "burn" the same, they differ when things start going into extreme conditions.

Try these links for some additional information.

http://www.ericgorr.com/techarticles/Fuel_Basics.htm

http://www.ericgorr.com/techarticles/fuel_terminology.htm

 

[robwbright]

Rich:

I essentially knew NOTHING about this topic until 6 months ago. I still know/understand very little. Nevertheless, I had already read your article on Fuel. I will review the terminology page.

I guess i should be more descriptive. I know the high octane stuff will burn, but will it burn completely?

This all stems from a conversation with my father-in-law (racing carts and/or modifeds for 40 years).

He disagrees with you on a couple points. He maintains that an engine of less that 10:1 compression WILL NOT burn anything over 100 octane efficiently/completely. Essentially that using 110 octane in a 9:1 motor has the same effect as taking 10,000 mg of vitamin C - it won't hurt you but you'll just "piss it out" in the form of splooge. He also cites repeated cases of cart and modified drivers who increased their lap times simply by changing from 108+ race fuel to either pump or AVGAS.

He is no chemist by any means, but he does have 40 years of day to day experience and testing, building motors, etc. . .

I notice that the info on Eric's site has some/all his built motors specs running under 10:1 compression.

I tend to think you and Eric are right on point, however, my father-in-law knows more about motors and transmissions than anyone I personally know . . . perhaps he's just been misled.

At any rate, until I have Eric put in the 144, any recommendation for race fuel for a 2002 RM 125, Bill's Pipe and Silencer? I may be interested in switching from the AVGAS.

Thanks for the input and any other thoughts you might have.

Rob

 

[Rich Rohrich]

Rob - I'm pressed for time at the moment so I'm going to cheat a bit and do a cut & paste of a previous post that even though a bit off the exact topic does go into the basics of why one fuel can make more power than another. It isn't really a matter of octane it's a matter of matching the distillation curve to the application. If this post doesn't clear things up let me know and I'll try and get back to it later tonight.


The cut and paste part :

The differences in terms of fuel requirements bewtween the two engine types can be broken down into three basic requirements :

- throttle response
- octane requirement
- piston crown cooling

Regardless of the engine type we want the best throttle response we can get, so we lower the fornt end temperatures of the fuel's distillation curve to sharpen the response. If you get too aggressive with this you can make the fuel prone to pre-ignition (firing the mix before the spark) . Normally this isn't a big deal on a four-stroke, but on a two-stroke with it's hot piston crown, deposits in the chamber from oil that glow red hot, and high specific output this is a very real concern. So on the two-stroke fuel we want to balance response with pre-ignition control.

High output two-strokes will tend to have higher octane requirements than a comparable four-stroke, partly for the reasons listed above. As the output and peak rpm of four-strokes increases the intake valve tends to be closed later thereby limiting the trapped (or dynamic) compression ratio. This is part of the reason that some 14:1 mechanical compression ratio engines can run without knock even on pump fuels. Four-stroke specific MX fuels or fuels designed for high speed 4 valve engines will tend to be lower octane in most cases.

As alluded to above piston cooling isn't an issue for four-strokes but it is a major concern for high output two-strokes. The closer you get to the thermal limits of a particular engine design the more useful fuels with high end points (90-100% on the curve) become. The end point distillation temperature of these types of fuels is high enough that some of the fuel can make its way into the combustion chamber in liquid form. While this would normally prove to be a liability in most engines it could be useful when you are tuning to the ragged edge. The liquid fuel droplets entering the combustion chamber will leech some heat and help to cool the piston crown. In an engine that is pushing the thermal limits this can be the difference between a win and a holed piston. Some specialty fuels like Philips P45 are designed with a large jump at the end of the distillation curve to specifically provide this cooling effect. It’s not uncommon for a fuel with higher octane (like Phillips B37 118 octane) but lower end point temperatures to experience preignition or knock problems long before fuels with high end points. It's the tuning equivalent of making lemonade out of lemons. For engine combinations that aren't near their thermal limits like MX applications this high end temperature just ends up as oil spooge dripping out of the silencer even when the jetting is lean.

On a high speed four-stroke like a YZF250 a fuel like this will just limit power in the upper rpm range. The intake tracts are so short and there is so little time available to vaporize the fuel that high end point fuels just flatten them out and dump raw fuel into the exhaust. **** This is a key point

An interesting side note here is the end point distillation temperature of 101 LL Avgas is high enough that some of the fuel can make its way into the combustion chamber in liquid form. As you start flirting with the thermal limits of a two-stroke engine Avgas can become much more useful.

There's a lot more to this story but my conference call is about over so it's time to get back to work

One last thing, for our purposes the differences in laminar flame speeds of the various fuels are of no consequence, and can be set to the side.

 

[robwbright]

Ok Rich I think I'm getting it . . . Thanks!

1: So part of what you're telling me is that a 2 stroke requires more octane than a 4 stroke at a given compression . . . etc?

I know the following question will vary given the particular bike and application, but all things being equal:

Take a basically stock RM125.

2: Do we consider a basically stock bike a "high output" application?

3: What would the difference in octane requirements and other fuel factors be given a change in compression from 8:1 to 10:1 to 12:1? Is there ANY direct correlation between compression and octane in a given application or is it only the distillation curve?

Finally, take a mid-top ported polished RM 144.

4: What would be the compression requirement of such a bike given a usage of 100 LL fuel vs. 118 race fuel or whatever race fuel you like? Is it higher or lower with the race fuel?

Thanks a lot Rich you've been a great help. I could read technical manuals for months and still not learn as much as you and Eric teach us on here in a matter of hours.

Rob