If octane is good, is more better?

[VTRDark]

We have all heard it before "shall I use higher octane fuel in my VTR?" Well here is a really interesting article I just read and thought I would share.

Have you ever taken your baby to the gas station and decided to treat her? (I'm talking about the bike, of course.) So you pull up to the pumps and go for the highest octane you can get. After all, nothing's too good for your bike, or is it?

What's special about higher octane fuel? More power? Better gas mileage? Longer engine life? Most of us know that you need a higher octane fuel if you get "knocking" (detonation) or "pinking" (pre-ignition) in your engine; but is there any benefit in using a higher octane fuel than your engine needs? Conventional wisdom held that the higher the compression ratio, the higher the octane you needed to avoid detonation. So how do modern fours with 12:1 compression get away with regular gas while my '70 Bonneville at 9:1 needs 94 octane?

Early motorcycles weren't designed with efficient combustion in mind: fuel quality was poor, and compression ratios were as low as 4:1 to avoid detonation. In England, Harry Ricardo carried out the first scientific examination of combustion efficiency. By incorporating an inspection "porthole," Ricardo was able to observe the flame path inside the combustion chamber. His experiments led him to recognize the importance of combustion chamber turbulence in promoting efficient combustion and preventing detonation. An engine so designed could run with higher compression (and therefore more efficiently) on the same fuel. The earliest motorcycle incorporating this concept is the 4-valve Triumph Ricardo of 1921.

Along the way, Ricardo also discovered that the more iso-octane in the fuel, the more resistant it was to detonation, and developed a scale to measure detonation resistance: the octane rating. Gasoline made up of 100 percent iso-octane was given an octane rating of 100.

But producing a fuel with 100 percent iso-octane was impractical and expensive. Thomas Midgley Jr, working for DuPont in the US, discovered that different chemicals added to gasoline would prevent detonation, therefore increasing the "octane" rating. Of the compounds he tested, tetra-ethyl lead, TEL, was the most cost effective to produce. Alkyl halides also added to the fuel avoided lead buildup in the engine by combining to form volatile lead halides, which went out the exhaust.

TEL had other advantages. The lead acted as a metal lubricant, preventing exhaust valves from "welding" themselves to the soft iron valve seats then in use, and so preventing valve seat erosion. But it had one big disadvantage: Lead is extremely toxic, especially to children, and the high levels of lead in the air around major highways became associated with a number of illnesses. As a result, lead has been banned as a fuel additive in most jurisdictions. It's been replaced with a swath of other octane boosters, like toluene, xylene, ethanol, methylcyclo- pentadienyl manganese tricarbonyl (MMT), methyltertiarybutyl ether (MTBE) and more.

Meanwhile, developments in cylinder head design were improving combustion efficiency also, allowing higher compression ratios and therefore more horsepower. In the UK, Harry Weslake discovered that creating "swirl" in the combustion chamber improved mixing and combustion efficiency increased, while also reducing detonation. The best-known example of Weslake's principle is the "bathtub" combustion chamber used in the first-generation Minis.

But there was another important feature of Weslake's design. Although the combustion chamber was heart-shaped, the piston crown was, of course, round. This meant gases were "squished" back into the combustion chamber. This forced unburned mixture back toward the flame front, improving combustion efficiency further by "scavenging" these gases. Squish became the holy grail of engine builders and led to some innovative designs, such as the wedge combustion chamber (Erling Poppe's Sunbeam S7 for BSA), and the in-piston combustion chamber (the Moto Morini "Heron" cylinder head).

Until the 1970s, most motorcycle engine designers stuck with the hemispherical combustion chamber, which offered high volumetric efficiency (good gas flow) and therefore, high performance. But it had two major drawbacks. First, for increased performance, larger valve sizes, valve lift, overlap and higher compression were all needed - but the geometry of the design limited these. Second, combustion efficiency was low, also limiting compression ratio and generating higher emission levels.

Formula 1 has often been the proving ground for new ideas, and so with combustion chamber design. Around 1966, Frank Duckworth (the "worth" in Cosworth) designed a new four-valve cylinder head with a shallow combustion chamber and narrow included valve angles for the Ford-Cosworth F1 car. The Cosworth DFV V-8 became the "winningest" F1 engine ever with 167 Grand Prix victories. It's still the basic blueprint for all modern high-performance four-stroke engine design.

All of these developments allowed the use of higher compression ratios without detonation by improving combustion efficiency, so that now a modern high-performance sportbike with 12:1 compression can run comfortably on 87-octane gas.

So back to the original question. Given the advances in engine design, is there any benefit to using higher-octane fuel? First, always use as a minimum the octane rating recommended by the motorcycle manufacturer. And also bear in mind that octane is measured differently in different countries. Triumph, for example, recommends 95 RON octane for its bikes, but in most of the US, the highest octane you'll find is 92. Turns out that's quite OK. In the UK, octane is measured by the "Research" method (RON); while in the US, we measure the average of "Research" and "Motor" octane ratings (R/2 + M/2). US ratings are typically five points lower on the scale, so UK 95 RON is roughly equivalent to 90 octane US.

The higher the octane rating of gasoline, the more detonation inhibitor compounds it contains. Most of these compounds have lower heat content - combustion energy - than the alkanes in the fuel they displace. So, in most cases, the higher the octane, the lower the combustion energy. Not only are you wasting money buying higher octane fuel, in most cases you'll be getting marginally inferior performance too.

Bottom line - stick to the manufacturer's recommendation!

http://www.roadrunner.travel/magazine/r ... 6/page/82/

(:-})

[lumpyv]

that is interesting.

I have always convinced myself that my bike does a few more mpg when I stick the good stuff in ?

probably my imagination?

off on a tangent (as we do) I have noticed that my bike used to do a steady 70mph at 4000 rpm. it now does about 74 at 4k?? (if it keeps this up it should be good for about 175 flat out in 5 years )

the only thing that has changed recently engine wise is an oil change. heine gereke to Gulf and a new oil filter?

whats going on there then ??

[Watty]

Dodgy speedo

[lumpyv]

Dodgy speedo

ha ha , could be ?

[VTRDark]

It could be something as simple as atmospheric pressure/climate conditions/temperature which is altering the fulling in your carbs. It could even be the new oil and filter, but I doubt it. Maybe a different brand of fuel or the fuel recipe has been altered by Mr BP or whoever.

Any one of those things or you have lost weight My guess is climatic conditions.

(:-})

[Saintsman27]

I do not think that high octane petrol .. super unleaded makes any difference ..
in fact it makes my VTR run hotter with noticeable pinking sounds sometimes at
tickover... it by far better with regular unleaded ... with MURCO or ASDA coming out
tops .. strange but true for me ... look at the gas in the "USA"...the octane rating is just about
90 even lower than the UK.... all petrol powered vehicles run on lower octane and they get
by on it just fine ... have a butchers at the " Superhawk" forum sometimes it is mentioned.... higher
octane gas is available but the norm octane for stateside gasoline is low.. maybe we should
have a low octane petrol here in the Uk... but I believe that nowadays here our fuel is doctored with alcohols
etc on a European directive.. and maybe it is low anyway..

[VTRDark]

I agree. Mine is the same with higher octane.

maybe we should
have a low octane petrol here in the Uk...

Gone are the the days of 2 star, 3 star and 4 star.

(:-})

[tony.mon]

off on a tangent (as we do) I have noticed that my bike used to do a steady 70mph at 4000 rpm. it now does about 74 at 4k?? (if it keeps this up it should be good for about 175 flat out in 5 years )

the only thing that has changed recently engine wise is an oil change. heine gereke to Gulf and a new oil filter?

whats going on there then ??

Tyre wear.
As the tyre tread wears away, you get a smaller circumference, which means that for a given speed the tyre revolves quicker.
Simples.

[VTRDark]

Genius

(:-})

[lloydie]

I've used Shell v power and it runs a little smother , I put it down to old fuel !!

How many people use the "good stuff" compared to the normal stuff ?
My thinking is as most people use the normal fuel as it's cheaper it's refreshed in the petrol stations tanks a few times a week where as the "good stuff" only gets refreshed every few months so the "good stuff" is old fuel and is less affective than the normal petrol .
I've worked in a petrol station so I know the good stuff tank don't get used or filled as often .

What I'd like to try is race fuel or aviation fuel :-)

[Virt]

I run shell v power on my first ride after payday and then normal asda petrol..

I was told v power has more additives and can help flush the pipes/carbs etc? It does make the bike feel smoother too (though probably a placebo affect)

Sent from my GT-I9505 using Tapatalk

[VTRDark]

What I'd like to try is race fuel or aviation fuel :-)

Have you drank all the Meths then

Race fuel/avgas is going to only benefit an engine with extremely high compression. Stick it in your bike as it stands and it will run like a pig. There's bound to be a lot of wastage too. Higher octane burns slower as to not detonate an engine. Stick high comps pistons in and some really extreme high lift, long duration cams in, it will go like a rocket top end but will soon go bang. You will also have crap bottom end/mid as this is the sacrifice for going too extreme at the top.

Cam and compression ratio compatibility

Cranking Compression Formula

(:-})

[StuartWags]

"smart" engines have sufficient sensors and control systems to enable them to adjust the combustion characteristics to get the best out of the fancy fuel. Simple engine do not so any change is negligible i.e. same amount of energy going in, ignition point the same etc

More likely to get / notice better performance on a cool damp day. i.e. air is more dense, giving higher mass air flow, dragging in more fuel. Moister / water in the air also acts as a antiknock agent.

I used to do this sort of stuff once when i was in research but i think those brains cells have long been depleted

[lumpyv]

off on a tangent (as we do) I have noticed that my bike used to do a steady 70mph at 4000 rpm. it now does about 74 at 4k?? (if it keeps this up it should be good for about 175 flat out in 5 years )

the only thing that has changed recently engine wise is an oil change. heine gereke to Gulf and a new oil filter?

whats going on there then ??

Tyre wear.
As the tyre tread wears away, you get a smaller circumference, which means that for a given speed the tyre revolves quicker.
Simples.

hmmm hadn't thought of that one? am I going quicker then or is it just "indicated". the tyres still have a fair bit left in them, especially the front.

[fabiostar]

regardless what road engine i have. i always put super in every 3rd or 4th tank. if nothing else its gives the valves and piston a wee wash and lube.. my storm seems to favour the super when the air temp drops and gets hungry for revs.