Home
Basic Dirt Bike How-To's - Video
Dirt Bike How-To's - Video
Living The Moto Life - Video
Bike Tests | Shoot-Outs - Video
Forums
What's new
Latest activity
Log-In
Join
What's new
Menu
Log-In
Join
Navigation
Install the app
Install
More options
Close Menu
Forums
MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Octane issues?
Reply to thread
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Message
[QUOTE="Rich Rohrich, post: 974445, member: 16241"] 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. [b]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.[/b] [i]**** This is a key point[/i] 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. [/QUOTE]
Verification
Which ocean is California closest to?
Post reply
Forums
MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Octane issues?
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
Accept
Learn more…
Top
Bottom