[Antipodean Andrew]

I have never been sure as to which fuel I should use in my C5, regular or premium fuel. When I first got the bike I used premium because that's what I have always put in motorcycles due to the higher compression ratio of motorcycle engines. Of course the Enfield isn't very high compression at 8.5:1.

The manual only states I should use the correct fuel without telling what that is! The tank had a sticker on it saying I could use E5 or E10, but that only relates to ethanol and common New Zealand fuels are ethanol free.

After noticing some discoloration at the first bend of the exhaust pipe I did a bit of reading about slower burning higher octane fuel and how the flame front could continue past the exhaust valve, so decided to try regular fuel. Anecdotally, the engine seemed to run a bit cooler and the discoloration hasn't got worse. The bike certainly seems to run fine on regular fuel. When I took my spark plug out at 3000km (1900mi) it was in a clean condition with a very fine grey coating. Looked better than the one in the Haynes Manual.

Our common fuels are 91 RON (US equivalent 85 (R+M)/2) and 95 RON (US equivalent 90 (R+M)/2. From what I've read, India has been using fuel with a lower octane than our regular for years and is only now changing to 91 RON.

Searching old posts about this topic reveals everyone has their favorite octane number. Does anyone know the science of this and the compression ratio at which one should step up to a higher octane fuel? Cheers.

[AzCal Retred]

Pragmatically, if "regular" is running fine, you're done. Octane is a measure of resistance to detonation/preignition. The fuel/air mix is supposed to burn across the combustion chamber in a uniform manner and create a pressure pulse at the correct time. As compression increases, depending on combustion chamber temperature and shape, amongst many other variables, the fuel/air charge becomes more liable to either burn too rapidly from adiabatic heating and mimic an explosion (detonation) or the radiant heat from the burning fuel front pre-ignites the rest of the charge ahead of time, creating a pressure spike at the wrong time. Proper combustion chamber shape puts the ignition point in a pocket in the center of the cylinder so it can burn radially outwards, which tends to minimize the possibility of either detonation/preignition. Modern 4 valve combustion chambers routinely operate at 11/1 to 12.5/1 compression ratios on regular.

Myself, I always run Premium, as my seat of the pants dyno seems to tell me that my engines run better in mid-range on it, and on hot days it gives you some added slack against either detonation or preignition happening. Besides, these are near 80 MPG engines, so how much are you really saving? If the air temp is cool, under 60F (15C) Premium likely doesn't matter. Air cooled motors will benefit more from Premium as they tend to run hotter. The 2 valve motors tend to have a less than optimal combustion chamber shape, but at CR's of under 8.5/1 it's likely not a big issue. So - Regular in cool weather, Premium for hot weather.

In the 1960's when Honda went GP racing, they learned that detonation/preignition became a non-issue at engine speeds above 20,000 RPM...good to know!    In their 50cc twin they had to run Regular, as Premium burned too slowly to keep up with the pistons. For our Royal Enfield 2 valve pushrod motors, I can adjust my timing on my Pre-Units to burn about anything at their 6.5/1 CR, and as you pointed out, the UCE was built to survive on Indian gasoline.

[Antipodean Andrew]

Thanks for the great answer. We are coming into summer here, so I might go back to premium for the hotter days. When I take my bike to the drag strip and exceed 20,000 RPM I will be sure to use regular. 

[Ove]

"20,000 rpm"... and be sure to take a shovel and a wheelbarrow with you 

[Richard230]

I have always used 87 octane (regular) E10 fuel for my Bullet and it runs fine on that stuff.  My 2002 Yamaha FZ1, with an 11.4 compression ration also specifies regular fuel and uses that without a hint of detonation. My 2009 BMW F650GS requires 91 ROZ/RON regular unleaded gasoline. It has a compression ratio of 12.1 and also runs without issues on E10, 87 octane fuel - as well as getting between 60 and 70 miles per gallon from its 4.4 gallon fuel tank. 

[Carl Fenn]

Well it’s all E5 or E10 in uk now contains ethanol which is hydroscopic with means absorbs water ethanol also has harmful effects on rubber parts and fuel systems so it pays to check these periodically for any signs of deterioration or adverse effects, it is known to be bad for older cars and bikes.

[johno]

Having already suffered a 'melted' fuel level float, I am sticking to the higher octane E5 in the UK

[Richard230]

Having already suffered a 'melted' fuel level float, I am sticking to the higher octane E5 in the UK

The original float on my 2011 B5 managed to last for five years while soaking in E10 before dissolving into into its original petrochemical components. I replaced it with a new float and so far it has managed to last another 6 years, so I guess I am about due for another new float soon.    Fortunately, I have Hitchcocks on speed-dial. 

[Morgan65]

You can’t go wrong with premium. That been said in my REs I run regular about 75% of the time in my REs and haven’t had any problems with that.  I usually get 10% better mpg with premium. If pump prices are within 10% of regular then I buy premium.

[Richard230]

You can’t go wrong with premium. That been said in my REs I run regular about 75% of the time in my REs and haven’t had any problems with that.  I usually get 10% better mpg with premium. If pump prices are within 10% of regular then I buy premium.

In California at my local stations, up until this year, premium fuel used to be 10 cents more than mid-grade and mid-grade was 10 cents more than regular gas. So premium fuel was only 20 cents a gallon more than regular. However, starting this year, many gas stations changed their prices so that mid-grade became 20 cents a gallon more than regular and premium was 20 cents a gallon more than mid-grade, making premium fuel 40 cents a gallon more than regular. That additional cost for premium pretty much eats up any savings that you might get in increased fuel mileage by using 91 (it used to be 92 up until a few years ago) octane instead of 87 octane "regular" gas.

I have always heard that regular fuel should be fine in most engines as long as you don't experience any "pinging". That would certainly be the case with Euro 3 and earlier Royal Enfields. However, many new engines incorporate very high compressions and a "knock" sensor that can adjust ignition timing to advance timing when using premium fuel is being used. That will (or can) allow the engine to make more power and efficiency.

I have no idea if the latest Royal Enfield Euro 5/BS6 engines incorporate a knock sensor.

[Carl Fenn]

Yea l think E5 is the best bet, well the only bet now the change was so quick, mind you this fuel will see a lot of the older cars off the road unless you know about mechanics and fuel systems, they will need yearly checks and examination of components.

[Bilgemaster]

Seeing as how the vast majority of Enfield Bullets of any type have been sold and happily operated decade after decade in lands where getting a roadside fill up from some random guy or gal with "mystery fuel du jour" dispensed from old cooking oil jugs or soda bottles may be the norm, I think we may be overthinking this.

I seem to recall my 2005 Iron Barrel's Owner's Manual  mentioning a minimum octane of 86 or 87, though I do not recall if any octane measurement standard was mentioned. But let performance be your guide: If she pings or knocks, especially under load, such as when climbing hills, then maybe step up a grade (or check your timing and carburetion).

More important than octane in most lands I'd say, is to eliminate or minimize the amount of ethanol in one's fuel, which can cause all sorts of issues, especially with older vehicles. Personally, my Bullet is on a steady diet of ethanol-free 89 octane (U.S. style "(R+M)/2") with a dash of Marvel Mystery Oil (maybe an ounce or two per tankful) to keep those jets tidy and serve as a cooling top end oil.

As already mentioned in a previous thread, if you want to check for the presence of corn spew in your fuel you don't need a fancy test kit. Just raid the pantry for some cheap food coloring, put a sample bit of fuel in a small jar or other container, add a drop of food coloring and try to mix. The food coloring will not mix with gasoline, but only with water or ethanol. If the droplet just sinks to the bottom, your fuel is pure. If the color changes throughout with a shake or swirl, you've got spew in the brew.

[Morgan65]

Our premium is a over kill on our RE singles compared to east India quality grade fuel so I’ve heard. Some  of my bikes in the winter time only gets ridden once a month, so I use a fuel stabilizer and fill the tanks to the top with premium as it has more additives in it that helps keep the fuel injectors clean. I feel this way the fuel injectors are less app to give me issues.

In California at my local stations, up until this year, premium fuel used to be 10 cents more than mid-grade and mid-grade was 10 cents more than regular gas. So premium fuel was only 20 cents a gallon more than regular. However, starting this year, many gas stations changed their prices so that mid-grade became 20 cents a gallon more than regular and premium was 20 cents a gallon more than mid-grade, making premium fuel 40 cents a gallon more than regular. That additional cost for premium pretty much eats up any savings that you might get in increased fuel mileage by using 91 (it used to be 92 up until a few years ago) octane instead of 87 octane "regular" gas.

I have always heard that regular fuel should be fine in most engines as long as you don't experience any "pinging". That would certainly be the case with Euro 3 and earlier Royal Enfields. However, many new engines incorporate very high compressions and a "knock" sensor that can adjust ignition timing to advance timing when using premium fuel is being used. That will (or can) allow the engine to make more power and efficiency.

I have no idea if the latest Royal Enfield Euro 5/BS6 engines incorporate a knock sensor.

[Richard230]

Our premium is a over kill on our RE singles compared to east India quality grade fuel so I’ve heard. Some  of my bikes in the winter time only gets ridden once a month, so I use a fuel stabilizer and fill the tanks to the top with premium as it has more additives in it that helps keep the fuel injectors clean. I feel this way the fuel injectors are less app to give me issues.

I don't have your riding problem. I typically ride each of my motorcycles, including my Bullet, once a week. 87 AKI octane E10 gas works fine in my bike and I have never heard any pinging from the engine under load. But then the ambient temperature around here rarely gets over 80 degrees F.