Common Questions: Long winded answers - KawiForums - Kawasaki Motorcycle Forums
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post #1 of 8 (permalink) Old 06-17-2011, 03:13 AM Thread Starter
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Talking Common Questions: Long winded answers

There are certain questions or "mods" that everyone does or inquires about. Explaining the reasoning behind these things grows tedious so I'm going to slowly create a thread with common mods/questions and try to answer them. It'll be a good resource IMO for the umpteenth incantation (Wow umpteenth is really a word? It didn't red-underline it) of things like, "Do I need to remove my Kleen-air?" or "Do I need to re-jet for a slip-on", etc. I'll keep the first two posts for links to the individual posts.

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Cam Chain Tensioner (CCT)

This comes up: A lot. People always inquire about the "ticking noise" that is common to many Kawi's. Simply saying, "It's your cam-chain tensioner" is akin to telling someone the reason to something is, "Just because". I've explained it before to people and hate to have to repeat it over... and over... and over again. So the first "segment" in this details what a CCT is, what it does, and why you might want to switch to a manual one or plain replace your aging one for a new one.

What IS a CCT?

Quite literally THAT. This isn't my picture, but it shows a CCT and gives a sense of scale.

So what does this small device do?

This was a prime example I found while scouring Images.Google.

This engine is very similar in geometry to a majority of the inline 2 and 4 cylinder engines found on most motorcycles. I circled the CCT in the picture. The CCT extends putting pressure on a guide (often backed by a hard plastic) which in turn keeps the cam chain taught. The timing chain is the long chain that is driven by the crankshaft (very bottom point of the chain in this picture) and turns the camshafts (The two large sprockets at the top of the picture).

The camshafts have to rotate precisely with the crankshaft as they control the valve openings. This poses 2 problems:

1) The camshafts fluctuate between being under load while opening a valve to providing a force to the chain when valves are closing. This causes the chain to tighten and loosen. This will affect timing if the sprockets are allowed to move on their own if there is too much chain slack.

2) The length of the chain will cause the long section of the chain to move (perhaps violently) when a camshaft alternates between being loaded and unloading.

For this reason a cam chain tensioner is used to push on one side of the long section of the cam chain to minimize the amount of slop in the chain (to reduce timing variances) and to prevent the chain from slapping back and forth.

In the above picture there is a guide the chain slides along on the far side of the engine and a similar one of the CCT side. This way the tension from the CCT pulls the chain tight and it glides along the guides.

So what's the Kawi tick?
For simplicity sake most motorcycles use a spring-loaded CCT with ratcheting teeth. Chains STRETCH in time so the CCT needs to be able to self adjust.

The CCT shaft has teeth machined in it so once it extends to a certain distance it can't retract. The more the chain stretches the more the CCT self adjusts by clicking to the next teeth. These little teeth DO break though. This allows for the cam chain guide to be able to move more and the end result is a ticking noise of the cam chain slapping on the guide. Also the distance the teeth are apart from one another dictates how much ticking noise their is. If the CCT teeth are close together the engines are typically quieter. If they are far apart and a CCT is close to clicking out a tooth then there is going to be more noise as the chain can slap back and forth more.

The fix?
A common fix for this is to help the CCT along. A lot of people remove the cap from the back of the CCT and you can manually lengthen the CCT until it clicks to the next tooth. This works, but you have to be careful not to lengthen it too much as then unnecessary force is going to be applied to the chain. This can also be bad if the CCT has broken teeth as the next "click" might be further down because of missing teeth.

Here is a picture of a CCT with the back screw out and a screwdriver on the CCT manual adjustment screw. This is actually me retracting a CCT to install it, but that's neither here nor there.

If you have broken CCT teeth then replacing the CCT is in order...

Another popular aftermarket fix is to just change out the CCT for a manual one:

APE CCT's are super common. As you can see they are nothing more than a bolt in a machined mounting block. You adjust the bolt to tension the cam chain guide until clicking is eliminated, and tighten a set-nut to "lock in" the setting. This fixes the issue and the only drawback to them is you periodically need to adjust them as the cam chain stretches.

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Lightbulb 520 Conversion Kits

520 Conversion Kits

I see this in people's signatures all the time and it drives me a little crazy. There is legitimacy in this "kit", but people kind of get mislead. It'd be like advertising putting, "Metric threaded spark plugs" on their bike... Allow me to explain a bit:

You know what type of chain comes FACTORY on ZX6r's (Even 636's)? A 520 series chain. The 7r's and 10r's use a 525 series chain as they produce more torque and need the stronger chain. In THOSE cases people will put a 520 CONVERSION kit on their bike to reduce rotational mass (for racing). For anyone with a 6r or 6rr, 250r, 650r, 500r, ANY of the cruisers just about, etc. you have a 520 series chain from the factory and aren't converting anything.

Why does this drive me nuts? Because it tells nothing about the modification! You might have changed your sprocketing to move your power-band around either to give you more torque in lower gears, or to lower your RPM in higher gears. THAT'S the modification.

Wait a tic. So what's the "520" even mean?
Good question! The wider the rollers on a chain, the more a force is distributed. If you have a very narrow roller on a chain then MORE force will be localized and possibly wear it out prematurely. If a chain is really wide then the force is spread out (distributed) so the chain will last much longer. The trade-off here is rotational mass. The bigger the chain is, the more mass your engine has to move. That's why LARGER displacement motorcycles might "convert" to a more narrow chain. You trade some of the reliability and longevity of the chain in for some horsepower.

Reading a chain: The first number (ie. the '5' in 520) refers to the pitch of the chain. This literally is a measurement between the centers of two rollers.

This number translates to one EIGHTS of an inch. So a 5-series chain is 5/8" from roller center-to roller center.

The last 2 digits refer to the width of the rollers in 1/80". Basically the same thing as the pitch, but there's two digits so it's 1/80" instead of 1/8".
Example: A 20 series chain is 20/80" or 1/4". A 525 chain is 25/80" or .3125" wide (5/16").


I thought I'd also take the time out here since the topic is chains/sprocketting for racing. Another common theme I see all the time (and I see it on the streets a lot) are people getting their sprocket/chain kits with aluminum rear sprockets. There's a reason why these kits are much cheaper than others...

An aluminum sprocket of mine which lasted less than 5,000 miles next to a brand new Stealth Sprocket.

Aluminum is a very soft metal (when compared to steel) and is really bad to use as a high-contact surface. The exception to this is if the surface is hard-anodized with something such as Nikasil. Cylinder for instance in a vast majority of modern motorcycles are aluminum with a coating of Nikasil. These cylinders will endure millions of cycles of piston ring movement. Without a hard anodizing the same cylinders will literally fail almost immediately.

So while YES you are shaving weight from a rotating assembly and consequently are increasing power to the wheels, that same aluminum sprocket will not last longer than 4,000-6,000 miles. Basically you can look forward to changing your rear sprocket and chain every oil change!

Aluminum sprockets are fully justified in racing vehicles as they aren't concerned with the longevity of the sprocket past a race or perhaps 1 season. For a street ride it is completely not justified.

Long story short: Unless you spring for a hard anodized coated sprocket (which creeps upward in price) don't consider any sprocket/chain kit with aluminum rear sprockets. If you pay a little bit more (compared to a steel sprocket) for an aluminum hub/steel ring sprocket (such as Stealth Sprockets):

From thread:

Not only is the sprocket very light (compared to stock), but it will last the LIFE of the motorcycle. Believe it. Note the picture I used as the title to this post as well. A large majority of "520 conversion" kits come standard with aluminum sprockets so as a warning make sure you LOOK at what the kit comes with!

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Lightbulb Kleen Air systems

Kleen Air

I attribute a lot of misinformation and logic with engines to the '70's. A lot of things changes in the world of engines and a lot of ideas popped up back then and have been regurgitated so much that even if they are wrong, they somehow become "truths".

Kleen Air systems are an emissions tool that motorcycles AND cars use to help clean up exhaust systems. My whole opening tangent should hint to where this is going. ANY TIME you use the word "Emissions" people automatically equate that to "Power robbing". Rewind a bit: Emissions equipment DID affect power significantly in the early 1970's. This is a fact. Emissions equipment now either has little or no affect on power. The problem is so many hotrodders of yesteryear were gung-ho about ripping out emissions "crap" and gutting catalytic converters that people just got used to doing it and not questioning whether or not it should be done.

Ok so enough of the rant. Here's what the Kleen air system does on a motorcycle and a little insight into if removing it should be done in your case:

First off Whitey did a great thread showing the innards of a Kleen Air system. This should really cement HOW the system functions and to clear up some misnomers about what is inside.

So I will do a little more "theoretical" functioning of the system and the if you should remove it bit.

First off the big common question/incorrect assumption. YES the Kleen air port(s) are on the valve cover. NO this is not a breather. By removing and plugging or bypassing these ports you will not affect crankcase pressure one iota (well unless you connect the actual breather port TO the Kleen air port then you create crankcase vacuum).

From the thread:

This is the connection for the Kleen air system on a 4-cylinder Kawi. For parallel twins there is only one port instead of two. Again this is NOT a breather in any sense! If you go to Whitey's thread he shows what is underneath those ports and it in no way leads to crankcase.

What happens in this system is fresh air (from the airbox because it is filtered) is drawn through this and pulled into the exhaust. The added air in the exhaust allows for hot exhaust gas to continue to burn with the introduction of more oxygen. This in no way affects power negatively (if anything it allows for additional scavenging and that's a GOOD thing). This is a prime example of emissions equipment not hurting anything! The plumbing weighs all of a pound so there isn't much savings in weight by removing it.

So WHY remove it!?
Ah that's the REAL question! Kleen air systems potentially do have some problems on modified bikes.

1) Bikes with aftermarket exhausts don't contain catalytic converters in the header and allow for freer flow. Additional air in the exhaust tends to reignite and "pop": especially on decel. If you have an aftermarket exhaust and are getting a lot of after-firing either restricting the Kleen air to not pull in as much air, or more often removing it (or a third option which I'll discuss later) is in order.

2) Bikes using an aftermarket O2 (lambda) sensor or being sniffed on a dyno. Additional air in the exhaust can cause false O2 readings. A bike running the correct stoichiometry might read leaner than it really is due to additional O2 being pulled into the exhaust. For dyno runs especially simply clamping the Kleen-Air supply hose to prevent air flow should be done. For aftermarket O2 sensors THEN the Kleen-Air system might need to be removed.

So how do I remove it? If you really need to remove the system then there are multiple avenues.

1) Plugging the system.

From the thread:

From the thread:

With 4-cylinder bikes you can simply tie the two Kleen-Air ports together which essentially keeps either of the reed valves from opening (and thus no air is pulled into the exhausts). This is what is done on the first picture. With parallel twins such as Ninja 250r's, EX500's, Ninja 650r's, etc. you plug the single Kleen-Air port. The picture on the bottom is an example from a Ninja 250r. For temporary scenarios such as dyno tuning you can just clamp the hose shut instead of removing it. This is also a good method to determine if eliminating the Kleen Air system is a wise move. Simply clamp the Kleen-Air hose with vice grips and then see if it fixes the popping your your exhaust. If not, remove the vice-grips and nothing changes.

2) Block-Off Plates.

From the thread:
Hoser also does one of a 6r here:

Several companies make (and it's easy enough to make your OWN even) block off plates. This does the same thing as plugging the port.

3) Creating a Crankcase Evacuation system

Sounds pretty technical, but it's really not. In piston-engines the constant movement of the piston(s) in cylinder(s) and blow-by causes air to constantly not only circulate, but if it were sealed pressure would build until your seals blew out and the engine would seize. For that reason engines utilize a "breather" to allow air to enter and a PCV (not to be confused with a Power Commander V) to allow air to escape. In this case there is one all encompassing port on the transmission connected to the air-box. If air needs to leave the crankcase it will be pulled into the engine and burned... It is easy to locate as usually at the bottom of an airbox there will be a hose 3/8"ID - 1/2"ID leading to the transmission.

So one strategy to improve piston-ring seal in race cars is to hook the PCV valve up to a vacuum source (and plug the breather). The seals are safe as there isn't above atmospheric pressure in the crankcase and the vacuum acts to PULL the piston rings outward on the pistons so they maintain a better seal (and hence higher cylinder pressures lead to more POWAH!). This tactic is called "Crankcase Evacuation" and is a proven and standard practice in most all of motorsports.

A very nice looking exhaust driven crankcase evacuation system on a turbocharged race car. For forced induction vehicles you almost need a crankcase evacuation system of some sort.

On motorcycles it is very easy to get a slight crankcase evacuation effect by reconnecting your Kleen Air hose from the airbox to the breather port on the transmission. The reed valves:

atop the valve cover ensure that air can only travel one way. So whenever there is vacuum in an exhaust port (such as after the exhaust valve closes) anything connected to the Kleen air port will experience vacuum. This entails there always being a vacuum present in the crankcase so you get free crankcase evacuation. And as a bonus since the air inside the crankcase isn't very oxygen rich you fix afterfiring in the exhaust which is likely why you wanted to remove the Kleen-Air in the first place.

An example of the Kleen-air mod done on a 636:

The path from the Kleen-Air ports on the valve cover to crankcase vent on the transmission:

Well that's about it. People always put the Kleen-air removal "mod" at the top of their "to-do" list along with anything emissions related and I think a little education goes a long way. Just because people are all jumping the bandwagon to do something doesn't automagically make it a good idea...


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post #6 of 8 (permalink) Old 06-17-2011, 03:16 AM Thread Starter
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Lightbulb Rejetting/Remapping for a Slip-on?

Rejetting/Remapping for a slip-on?

Preface: Before someone asks, with carburetors you have to rejet (a broad term really which is kind of misleading) to alter how much fuel enrichment the carburetor(s) add. If you have a fuel injected bike you have to remap (usually with something like a Power Commander or Bazazz) which alters how long your fuel injector(s) are held open. When I mention rejetting/remapping I don't mean you have to do both to your bike, I mean whichever suits your engine's fuel enrichment process needs to or doesn't need to be performed.

It really is a running joke come Spring time every year when a new crop of riders get a hold of shiny new bikes and want to soup them up about rejetting/mapping for slip-ons issue. This question has come up so many times and not only do the answers vary, but so does the logic behind those answers. It makes me feel like I'm taking crazy pills sometimes!

So to tackle this issue let me say first and foremost the answer is... No.

Now here's the logic. Modern sportbike exhausts really aren't that restrictive. Changing mufflers out doesn't do much if anything for freeing up backpressure in the exhaust. Most slip-ons don't yield ANY power gains as they don't do anything for performance. Slip-ons might sound nicer (which is subjective really as every time I hear a Yoshi can it just pisses me off), and the big help is the weight savings! You really shave a good 10 lbs off of some bikes by swapping out the factory can. That's pretty significant.

But xx manufacturer has dyno sheets! It's proven with a Power Commander or jet kit it netted more power!
People always forget the whole bit that retailers are trying to SELL you their expensive products. If Leo Vince started saying, "Buy our slip-on! It sounds sweet, but doesn't do dick for performance" and then Micron says, "No, buy OUR slip-on! It also sounds sweet and DOES improve performance!" people would stop buying Leo Vince slip-ons in favor of the Micron. So all slip-on manufacturers tote the power gains their product outputs (with dyno sheets). How do they do this if it's not true?

Well what if I told you I wanted to sell you the chair I'm sitting on for $180,000? You'd laugh at me. If I said, "I'll throw in the house it's in as well!" then you'd think it was a good deal? No you'd realize it's all a ploy for me to sell my house, not the chair.

That's exactly what slip-on manufacturers do and they pull one past a lot of consumers hook, line and sinker. To improve fuel economy all bike manufacturers run their bikes a little leaner (especially in the part throttle zone). What happens when you put a Power Commander or rejet a bike? You usually lose a little bit of fuel economy, but gain power in your mid-range AND a little on top as well. Slip-on manufacturers always suggest you to rejet or remap to get the most from your slip-on, but in reality you are getting ALL of your performance from the remap or rejetting and none of it from the muffler itself.

Yes that is a long winded explanation, but there's a reason. The moral of the story is NO you don't need to rejet or remap for a slip-on. It's not doing anything to lean out your A/F ratio like a full exhaust will so you don't need to worry about "damaging" anything. If you want to rejet or remap I wholeheartedly thing you should, but not because of the slip-on, but because your engine WILL benefit from it.

Now if you do have a full exhaust you must rejet or remap. NOT BECAUSE OF THE MUFFLER in the system though! The header design on a full exhaust changes things like port velocity, scavenging characteristics, etc and it affects how "free" your engine breaths. Your A/F ratios will be all over the place if you run stock maps/jetting after doing a full exhaust and you really can run dangerously lean and melt a piston. Again this is something that the muffler doesn't affect as the muffler doesn't change your header design one bit.

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post #7 of 8 (permalink) Old 06-17-2011, 03:17 AM Thread Starter
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Lightbulb Rejetting/Remapping?

Do I need to rejet/remap?

Similar to the "Do I need to rejet/remap for a slip-on" topic, this one comes up a lot for a different reason. There is a very fundamental concept that is completely ignored by a huge majority of people when they first start learning how an engine works and that's "Stoichiometry".

Don't worry if you haven't heard that word, you still might know what I'm talking about. Back to the original concept at hand there are certain correlations which people make and sometimes they are totally incorrect. One such pairing people make is that an engine consumes fuel and makes power. This is true. The problem is then they go one step further and assume if the engine takes in MORE fuel it will make MORE power. If it were that easy than high volume fuel pumps would be the first modification everyone performed.

Fuel consumption does NOT correlate to power!

If anything the BSFC (Brake Specific Fuel Consumption) which ties the amount of fuel an engine consumes versus the amount of power output is better the LOWER that number goes. Making an engine do more with less fuel is icing on a cake to a well performing engine. Now back to stoichiometry.

Stoichiometry is the measure of how complete a chemical process is by parts of a chemical equations. For gasoline for instance stoichiometry equates to 14.7 parts of AIR to 1 part of fuel. That is the "magic" number for gasoline. Most fuel injected engines operate as closely to 14.7-15.0:1 air fuel as they can. Running leaner than that causes the combustion process to finish early after all the fuel is consumed (loss of power) and worse it causes the engine to run HOTTER. Under high cylinder pressures gasoline acts as a sort of inter-cooler transferring heat. By leaning out too much things can go south in a hurry and you can literally melt pistons.

So fuel is very important for cooling an engine. The "ideal" range for gasoline is 12.5:1 - 13:1 air/fuel for high cylinder pressures. This isn't the most efficient burn as you are running excessive fuel, but this is the ideal ratio to run for POWER and safety. Running leaner doesn't normally help with power and will cause the engine itself to heat up or possibly melt. Running richer than 12.5:1 (12.0:1 is actually best for super high cylinder pressures such as with forced induction applications as they need MORE fuel to cool things down) finds the engine losing power as you start getting too far away from a stoichiometric correct mixture so the air in the mixture is consumed quickly and then you're left with unburned fuel. On the extreme end fuel will actually start to puddle and liquid fuel can vapor lock an engine (which can literally blow it up) or at the least start washing oil off of the cylinder walls increasing friction. Bad news bears.

The combustion process is easy, but doing it correctly is difficult. This is what makes fuel injection preferred for doing this all best. When mapping an engine (preferably on a DYNO) part throttle conditions (low load) you shoot for stoichiometry for your fuel (again that's 14.7:1 for gasoline) and high load conditions REQUIRE more fuel: usually around .85 Lambda (12.5:1 - 13:1 for gasoline). Running your bike on a dyno and getting a baseline of your A/F ratio will tell you if your need to rejet or remap. Don't let anyone tell you that you need a bunch of dyno time if your A/F ratios are steady and within range. I never tune engines by looking at torque numbers. I always watch the air/fuel ratios. Anytime you can get them steady and in the correct range the power will literally follow suit AND responsiveness will be had.

Side tangent: Carburetor "jetting"
Ok I got all the crap about stoichiometry out of the way. An additional note is how to 'jet' a CV style carburetor which is what is found on nearly every carb'd engine on sport bikes over the past 30 years. Again stoichiometry is key to a good tune. Simply changing the "jets" in a carburetor won't do this for you though. Actually the jets require the least amount of work IN jetting a carburetor. That's why "rejetting" is kind of a misnomer.

The main jets in a carburetor meter fuel pulled into the carburetor at MAXIMUM load and at full throttle. What happens when you are cruising at 1/2 throttle and start going up a hill? The main jets aren't fully uncovered and you don't have the throttle all of the way open so another system is in effect.

I'll take a quick step back and go through the list of how to PROPERLY tune a CV carburetor:
1) Adjust main jet sizing looking for PEAK power (ideally 12.5:1 - 13:1 again for gasoline).
2) Adjust the needle height so under light load and half throttle you are at 14.7:1 (if you don't have a dyno usually cruising at perhaps half throttle and then suddenly opening up to full throttle and finding the needle height that offers the best response).
3) Adjust idle fuel mixture screw. If out of range, step up to larger pilot jets.

Make sure to sync the carburetors beforehand and check each carburetor's float height to ensure all the carburetors are functioning equally.

The needle height is that transition element between the main jet and light throttle/no load. Once the engine is tuned for maximum power (main jet region) tuning for part throttle plays off of the main jetting, and then the idle mixture smooths the transition between starting and part throttle.

Here's a handy chart showing the various tuning elements of a CV style carburetor versus their effectiveness. Notice the main jet doesn't do much until you're almost at 3/4 throttle? Meanwhile the clip position (needle height) and needle taper cover a huge area of your tuning.

So the next time someone asks if they need to "rejet" know that jetting is only a small part of tuning a carburetor. Technically if you only adjust your main jets you aren't going to be helping yourself out very much. Also when you hear someone boast that they have a stage "whatever" jet kit on their bike they likely don't know their ass from a hole in the ground.

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Lightbulb Octane requirements


This is a real touchy subject and has been covered so many times and there is so much mis-information out there it's disgusting. The main thread I refer people to is Nevada's thread where he literally links to Kawasaki's website: with regards to octane ratings. There has literally been 350+ comments of nothing but people arguing the point into the ground. People just won't accept being told differently about something they learned incorrectly. Which goes towards my first point...

General rule: Whatever the manufacturer of your particular engine suggests to use, USE IT! Using 93 octane over a manufacturer's suggested 87 octane rating is pointless. I don't care what your uncle Frank, your "buddy" who's been racing cars since the 60's, your friend who works at Jiffy Lube, etc. says. The engineers that designed the engine and expect thousands if not millions of them to wind up on the road probably are pretty knowledgeable about how the engine should run.

A quick side note: All octane systems are not created equal. I've constructed a little chart below.

System used in:.........USA, Canada...........Europe, Asia, pretty much everywhere else
Equivalent octane...........87.......................................91-92
In the U.S. the (RON+MON)/2 measurement is usually just simplified to "AKI" or Anti-Knock Index.

What's the point of this? Kawasaki's are made usually in Japan or other Asian countries. The system used over there for rating octane is not the same used in places like the USA. So no, they don't just have higher octane fuel over there they just rate it differently. So YES most all Kawasaki sport bikes require 91 RON octane fuel... In the USA that's 87 AKI octane NOT 91.

Octane rating only is a measurement of detonation resistance. Nothing more! You can have higher BTU content fuels of the same octane rating. Most fuel distributors will specify that if you look in the right places. So there literally are 87 octane fuels with higher BTU content than 93 octane gasolines and vice-versa. Octane rating tells the end user absolutely nothing about this. Nothing. Nada.

In fact there are oxygenated race fuels with octane ratings in the low 90's AKI with significantly more BTU content than 109 octane leaded race fuels. Believe it. Hell pure ethanol is usually in the vicinity of 110 OCTANE! AKI. You know what the BTU content of ethanol is? Usually 2/3 that of gasoline! So there is far less heat output per equivalent volume of ethanol. The trick here is you have to BURN 25%-33% more of it per volume of air to maintain stoichiometry So it's a wash!

Even with high BTU content fuels you probably aren't going to get your moneys worth or any sort of power benefit from it. Why? Because higher BTU content fuels don't automatically completely burn just because it has a higher concentration. Super high BTU content fuels usually don't finish burning by the time the piston reaches BDC and continue burning while the piston is trying to travel up the cylinder. Yes this COSTS your power!

The best fuel you can use is one that burns the most complete burn it can by the time the piston reaches BDC. It's not terribly complicated a concept, but people try to take concepts which might prove their side of an argument even if the concepts don't really apply or if there is a ton more to the concept disproving them.

So why do high performance vehicles require higher octane fuels?!
Excellent question! By adding iso-octane, MTBE (Now illegal in the USA), ethanol (ethanol is 110 octane mind you!) to a fuel you raise the octane rating. Recall octane means NOTHING for power, but is a measurement of detonation resistance. HIGH-performance turbocharged, supercharged, or high compression-ratio engines are all tempting the same demon: detonation. The higher the cylinder pressures you get, the more unstable the charge in the cylinder becomes before it spontaneously ignites. Octane simply inhibits the fuel from suddenly detonating. It's not necessarily a slower burning fuel, but one which is more stable at higher cylinder pressures. That's IT!

To demonstrate what octane has an affect on I've concocted spark hook curves of a theoretical engine designed to run on 91 octane fuel. By advancing the timing you come to a point where you find maximum power at a certain ignition timing. You actually start to LOSE power if you advance timing past that point. THIS is how manufacturers determine timing for engines. They vary the load and RPM of an engine and measure power, advance the timing, measure power, repeat. Once they find maximum power they set the timing for that load/RPM to that point. Normally manufacturers will actually err to the left of that point so detonation isn't an issue. This is why most sport bikes are meant to run on 87 octane. They COULD make more power on higher octane fuels, but you would need an ignition advancer to find that power (simply putting in higher octane fuel won't do anything).

In each spark hook I have put red cross-hairs where the engine will experience detonation. 93 octane fuel gives a good safety buffer, but the engine will still find maximum power with 91 octane and not risk detonation. The safety buffer comes from the fact that fuel loses octane if it is allowed to evaporate... Then I put an example of 87 octane fuel being used. The engine will detonate before the ideal timing point so you would have to sacrifice a bit of power by retarding the timing. THIS example is similar to sport bikes. Sport bike manufacturers will err on the timing and lose a couple of horsepower so bikes will work on 87 octane. To get a couple more horsepower you can use 91 octane, but you need to advance the timing!

Lastly I put a spark-hook test for an engine with 110 octane fuel. You can advance the timing really far, but the engine will literally LOSE power if you do run the timing that far. You can run 110 octane fuel and run 41-42 degrees of ignition timing and not gain 1 horsepower over 91 octane fuel. You could also run 45 degrees of ignition timing to realize the potential of your fuel's detonation resistance and lose 5 ft-lbs of torque in the process. It sounds stupid, but misinformed people WILL do that.

Yeah but my sport bike has 13:1 compression ratio! That's way more than perhaps an 11:1 C/R Corvette and THOSE take 91 octane!
This is an interesting topic. The volume of one cylinder in a 6.0 liter V8 is 750cc's. Your 599cc ZX6r has an average cylinder volume of 150cc's. Each individual cylinder in that Corvette engine is FIVE TIMES the volume. That's a lot of squeezing a charge into one cylinder! Compounding that is that larger engine sees a much lower red-line. That's a large surface area for fuel to disperse across and transfer heat from things like the piston and combustion chamber. So when its charge is most critical at perhaps 6,000 RPM the fuel inside the cylinder is able to absorb MORE heat and then detonate. The 6r engine though doesn't see these cylinder pressures at 6,000 RPM as that's barely into cruising territory. When the cylinder pressures really start to climb past 10,000 RPM the charge in the cylinders is ignited so quickly it doesn't have much of a chance to heat up so the income charges are able to keep things much cooler. Another angle to imagine this would be some hot coals. Imagine a huge batch of hot coals. Now spray those coals with a squirt bottle filled with water and then wait perhaps 30 seconds. Then spray them again and repeat. Your efforts to cool the coals is going to be for not. You're not going to affect them one bit as the large surface area and time between spraying won't be able to keep up with the rate at which the coals are burning. Now get a smaller coal pit and this times spray it with water constantly. That constant bombardment on a smaller area is going to cause the coals to cool down...

I hope this has been somewhat informative. I realize no matter how you explain concepts such as this one, or how much evidence or logic you use people will not accept it. So I say take what I've said as a logical and educated explanation of a topic or completely dismiss it as the ramblings of some Internet ass-hole. I'd rather have that then someone picking apart things to suit their own ideology with half-truths and hearsay.

Last edited by VeX; 08-18-2011 at 04:35 PM.
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