Octane/Detonation

[Racey Rider]

So whats the story with Octane?
Am I right that I should run the lowest octane fuel I can without causing Detonation?
That- if my bike runs well on 91, that theres no advantage in running Race fuel (98 Octane), or Av Gas (100 + Octane) just cause others are doing so!
They do so (Presumably), because their motor is so tweaked that it detonates with out it?
The lower Octane fuel burns quicker, giving out more Hp than a higher Octane/ slower
burning flame front??
That the days air temp can change which Octane I need on that day?
The altitude of the track can cause a necessary change to higher Octane?
My radiators inability to cool the motor well may cause a necessary change to higher Octane?

And whats the story with Detonation?
I understand (Somewhat), what happens in the motor, but when is it most likely to happen? At full RPM? only under load? while idling? all of the above?
Is the sound of detonation easily recognised?
And lastly,,, If I get good enough to be beating you on the track,,,, would you still tell me
if you hear my bike detonating!!?? :confused2

Please (Those that Know)(That's probably means your Dad KK) correct me where I'm wrong!

[wkid_one]

PMPL 

 Octane

Octane is a rating of a fuels resistance to ignite. The higher the octane, the harder it is to get to ignite. A higher octane may be necessary to prevent pre-ignition and detonation in a high performance engine. Higher octane fuel will generally burn slightly slower than a lower octane fuel which could require a change in ignition timing. Using more octane than you need will not help power, the slower burn rate will actually cause you to lose some power.



Detonation



Normal combustion will take place at a pretty steady rate (for a given rpm and load), when a large amount of the charge burns extremely fast and uncontrollably, it is know as detonation. Detonation can destroy an engine in a matter of seconds. It is common knowledge that heat causes detonation, but heat it not the only cause. Pressure plays a big role in it also. You can get detonation by running with too much ignition advance, if heat was the only cause, signs of detonation would around the exhaust valve, but this is not the case. Usually when detonation takes place from too much advance, the detonation occurs on the intake side of the chamber, which is the coolest side of the chamber. This happens because detonation did not occur until the pressure got excessive, which was after the spark. By the time that happened, the charge near the exhaust valve has already been burned. Most of the time, detonation will occur after normal combustion has started. Apart for destroying pistons and spark plugs, light detonation can cause all sorts of other problems, like fatiguing cranks and rods quickly and pounding bearings to death, so avoid detonation at all costs.



Pre-Ignition



When the charge lights off before the spark, it's called pre-ignition. This can happen with or without detonation, but usually will cause detonation in a high performance engine. Hot spots in the combustion chamber are the usual cause of pre-ignition. A spark plug with a high heat range can cause a hot spot. When pre-ignition occurs, the charge begins to burn, then when the spark plug sparks there are two flame fronts. This is very unpredictable and can lead to detonation, because it will act just like too much ignition timing, but it wont be controllable.



Motor Octane Rating



The motor octane rating, referred to as MON (motor octane number), is the best rating to use when selecting fuel for your race or high compression engine. When testing MON, the fuel is heated to 300° F and the intake air is heated to 100° F. The test engine is a single cylinder 4 cycle engine that is run at 900 rpm. Ignition timing is varied with compression ratio. Engine load is varied during test.



Research Octane Rating



Known as RON (research octane rating). Tested at 600 rpm with a fixed timing of 13° BTDC. The fuel temp is not controlled at all and the intake air temp is varied with barometric pressure. This is done to covert everything to a SAE standard day, which is 60° F, 0% Humidity, and 29.92 inches barometric pressure. The RON should not be used when selecting fuel for a race or high performance engine. The RON will always be higher than the MON.



(R+M)/2 Rating



This is what you get at the gas pumps. It is average of the RON and MON. It is ok to use this for lower compression street motors, but when you get much over 10:1, you should really pay attention to the MON. The closer the RON is to the MON, the more stable the fuel is. This can be very critical when running 7000+ rpm.



Specific Gravity



This is the weight of the fuel compared to water. If a race fuel has a .75 Specific Gravity (or SG) it is 3/4 of the weight of the same amount of water at the same temperature. Fuel with lower specific gravity will have a higher BTU content when burned and be more stable at high rpm's. A lower specific gravity also makes jetting more critical because the engine will run leaner (less dense fuel). A fuel with a higher SG will run cooler (if no jetting changes have been made to compensate), because a denser fuel will make a richer mixture. A fuel air mixture is not measured in volume, it's measured in weight. For a 12:1 mixture, you have 12 lbs. of air to 1 lb. of fuel. See Race Fuel Specifications for a list of common racing fuel SG's. 

[What?]

Awesomely informative and thorough answer Wkid.

[i]
(R+M)/2 Rating

This is what you get at the gas pumps. It is average of the RON and MON.[/B]

Bugger - I didn't know that! I was always led to believe that the pump claim was RON. S'pose even a know-all like moi can learn something new every day...

One point about RON versus MON, as taught to me by a certain fuel company, is that a low MON fuel can cause high-speed knocking in an engine which will destroy pistons and things, where the same fuel has a high-enough RON to prevent pinging at lower revs. This is common in engines such as Holden sixes, where our 96 octane fuel is OK according to the handbook, but the MON of our local gas is too low, so the old Holdens and Falcons tend to blow up running on gas which is seemingly OK for them. I learned this the hard way...

[wkid_one]

Sometimes I even surprise myself - the internet is a powerful tool when used properly...

website I got it from was..

http://www.grapeaperacing.com/GrapeA...fueloctane.cfm

[Racey Rider]

Choice!
Thanks for that.

So your saying I should just chuck some fuel in it and GO FOR IT!

[wkid_one]

Basically

[NZred]

WOW Wkid_one..

Did that cover Av Gas too ?
I kinda sorts recall that when you play with Av Gas.. you need to jet up quite a bit,.. or it runs too lean and burns stuff.
(Dunno if Av Gas would be needed in road engines.. over kill)

[What?]

Originally posted by wkid_one
Sometimes I even surprise myself - the internet is a powerful tool when used properly...

website I got it from was..

http://www.grapeaperacing.com/GrapeA...fueloctane.cfm

'On ya, Wkid. very informative site...But, umm, I think it is American??

According to Mobil, the pump octane figure here is RON, not the averaged figure.

NZRed, avgas is for aeroplanes. Yes, it has a high RON, but it is designed for low-revving engines (I am guessing it has quite a low MON, but none of the NZ fuel companies seem too keen to state what it is). The fuel companies all say that automotive fuels should not be used in aviation and vice-versa. If you want extra-high octane fuel for your bike, buy race gas.

[wkid_one]

As an aside - Moibl has installed Race Gas pumps in a lot of its stations....apparently great for cleaning injectors...in case the police pull you over...as you can't run it on the road.....give away is the nice light grey pipes and sweet smell.

 

 

[wkid_one]

Right you are What - but was more interested in the detonation.....and yes it is in RON

[What?]

Originally posted by wkid_one
As an aside - Moibl has installed Race Gas pumps in a lot of its stations....
 

Sounds good to me - and to my CB1100...

[wkid_one]

Seems to be mainly focused around areas which have a large recreational and competitive ski boat fraternity....but it aids us bikes.

[Kwaka-Kid]

i say good stuff wkid! but yes during my rental car job i was talking to some random fuel know-it-all and he explained to me some random reason (im guessing that average thingy) as to why over in the states there highest octane fuel or somthing is rated @ 89octane, but thats because they have a different way of calculating, please dont take my exact figures as fact, point im getting across is he said that there numbers are lower, yet equals out to be same fuel often.

Last thing: so out of all this, when my RVF400 is taken out to the track, what should i run it on?

And wkid, i checked with police and as far as they know race gas is legal? and that you might be confused with somthing else.  But then again thats the police, but hey, who cares, if they dont know, they cant ticket you -talked to mate and he said go for it, hes never given ticket for it.  Why would this be? are the road tax's not put into the price? (thats my only guess)

[Dave]

That was all nice racey,but in the real world-put in 100 octane fuel,it will allow you to lean off your engine and make more power-also the fuel is very consistant unlike pump unleadeds.
You will not hear detonation in a two stroke, the first thing you'll know is either pitting on the top outside edge of the piston ( if yyour checking frequently) or a hole in the piston.
If you do a plug chop and jet your main jet according to plug color,that should take care of any detonation at full throttle-then start to look at the needle adjustment-remember i said to watch out for your bike surging and bucking on light throttle in long sweepers-that is the first sign.raise the needle or go to a thinner needle until this stops.
Now the only thing that will make it detonate is if it overheats-so watch your temperature.
Come and see me at manfield in august and i'll give you a hand.
(also remember that your two cylinders will probably need different jets.