blah13

I know when you replace air filter/exhaust with more free flowing components its most time recommended to re-jet the carbs to prevent detonation/lean running. If the bike runs stock on 87 octane why not go with slightly higher octane? I know the increased airflow will need "more" fuel, not just higher octane but would this yield decent results while still reducing detonation?

rdblckSV650S

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by blah13 &raquo;</TD></TR><TR><TD CLASS="quote">I know when you replace air filter/exhaust with more free flowing components its most time recommended to re-jet the carbs to prevent detonation/lean running. If the bike runs stock on 87 octane why not go with slightly higher octane? I know the increased airflow will need "more" fuel, not just higher octane but would this yield decent results while still reducing detonation?</TD></TR></TABLE>

You shouldn't really experience detonation by doing filter/exhaust. The detrimential result of those parts is the increase in running lean. Most bikes run lean from the factory for emissions reasons. By adding parts that "increase airflow" you allow more air to get into the combustion chamber. This ups the ratio of air to fuel. Unlike cars, most bikes do not have O2 sensors to see this adjustment in airflow and allow the ECU to make slight changes. Thus the re-jetting or PC.

Higher octane fuel will precent detonation yes, but fuel is fuel, and the bike will always give the same amount of fuel unless you alter it, so by adding "increased airflow" mods, you will lean out the bike.

Luke

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by blah13 &raquo;</TD></TR><TR><TD CLASS="quote">I know when you replace air filter/exhaust with more free flowing components its most time recommended to re-jet the carbs to prevent detonation/lean running. If the bike runs stock on 87 octane why not go with slightly higher octane? I know the increased airflow will need "more" fuel, not just higher octane but would this yield decent results while still reducing detonation?</TD></TR></TABLE>

A higher octane will not prevent detonation it will give you more. More octane = higher burn. When you fit farty exhausts the airflow is greater therefore you need more fuel to richen the combustion mix. A higher octane with the stock weaker mix will make things a little worse.

Oh and dont listen to people who say you sonw have any issues. The main problem if you run it nice and lean will morelikely be a burnt valve than detonation.

JDMTurboCivic11

Quick question though, since i get mixed answers on this.... are you actually supposed to rejet the carbs?

Luke

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDMTurboCivic11 &raquo;</TD></TR><TR><TD CLASS="quote">Quick question though, since i get mixed answers on this.... are you actually supposed to rejet the carbs?</TD></TR></TABLE>

It depends on what bike. Some you have a jet, some needles, others timing and or injector mods.

You need more fuel whichever way you choose to go.

rdblckSV650S

Luke, i'm pretty sure higher octane PREVENTS detonation, not increases it. That's why there's fuels like C16 and what not. That's why when my friend's integra started knocking he had to put higher octane in it. That's why when i advanced my ignition timing on my car i had to put higher octane in it.

http://www.team-integra.net/fo...Topic

Apocalypse

Yeah I'm gonna have to go against you on this one Luke... higher octane is just an increased resistence to detonation experienced with more compression. It's true however that lean is lean is lean and the only way to "fix" it is to increase fuel flow.

Luke

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by slvrblckf4i &raquo;</TD></TR><TR><TD CLASS="quote">Luke, i'm pretty sure higher octane PREVENTS detonation, not increases it. That's why there's fuels like C16 and what not. That's why when my friend's integra started knocking he had to put higher octane in it. That's why when i advanced my ignition timing on my car i had to put higher octane in it.

http://www.team-integra.net/fo...Topic</TD></TR></TABLE>

Higher lead content reduces the onset of detonation. Higher octane in a weak mixture will detonate as bad as a weak mixture with lower octane fuel. Octane is not used to 'jet' engines, air/fuel ratio is.

Apocalypse

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Luke &raquo;</TD></TR><TR><TD CLASS="quote">Higher octane in a weak mixture will detonate as bad as a weak mixture with lower octane fuel. Octane is not used to 'jet' engines, air/fuel ratio is.</TD></TR></TABLE>

I don't think anyone was denying that... just disputing that higher octane would cause more detonation.

Sly_Samba

Higher octane results in a slower burn thus, REDUCING detonation

Just a reminder detonation is when the gas ignites due to being compressed not the spark plug. So, common sense would tell you that the higher the octane the less chance of detonation.

wheatyaaron

quick answer:

high octane gas doesn't have more fuel in it to make up for the additional air taken in by free flowing exhaust and air filter. they are not related. higher octane fuel is a little less picky about the air/fuel ratio (rich or lean conditions) that occur through changes in altitude/air temperature/etc. and will put up with much higher compression ratios without detonation.

if you want your engine to last, run the expensive gas. if you ride a typical cruiser, it's likely fine to run the cheap stuff. the race gas smells cool, but isn't necessary on the street unless you're on an exotic bike. it won't solve a lean condition caused by mods.

free flowing exhaust and high-flow air filters can lean a bike out quite a bit and require idle enrichment (idle mixture screw out 1/2 to 1 turn) at least and often a jet kit with richer main jets, richer needles, different springs and sometimes different slow jets. or an adjustable programmable fuel system like the yoshimura EMS or vance & hines system on fuel injection bikes.

detonation is a knock typically caused by too low of an octane, aka too volatile, fuel for an engine's compression ratio.

pre-ignition is causes even rougher running, and is a result of glowing hot metal in the combustion chamber igniting the fuel/air mixture too early....and is typically caused by a lean condition...because you put a race exhaust and k&n air filter and hogged out your intake ports and put big nasty cams in and bored it out and stroked it and...and means you will even sooner be buying engine parts.

motorcycle engines typically run high compression because they're not designed for longevity. they're designed for high power/weight ratios to toss you off the back...and require higher octane fuels with more stable vapors. you want your 14.7 air/fuel mix to not go boom when it hits the 600 degree exhaust valve as soon as the compression begins. that's bad.

get a mechanic to tune your fuel system to keep up with your intake/exhaust system, or start stockpiling engines to swap. you'll need them.

Luke

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Apocalypse &raquo;</TD></TR><TR><TD CLASS="quote">

I don't think anyone was denying that... just disputing that higher octane would cause more detonation.</TD></TR></TABLE>

The only time I have used higher octanes is with race fuels, and with the switch from leaded AVGAS to Elf I can tell you that unleaded higher octane race fuel is much less forgiving than leaded. Pistons, cranks, plugs everything has a harder life.

Anyway to the original guy the answer is rejet