Commandos and fuel octane rating

A while back there was a discussion about Norton engine development. Somebody mentioned Joe Craig running Manx Norton engines very lean. A lot of guys believe raising compression ratios improves performance. However compression ratio, ignition advance and air/fuel ratio are in balance when you have maximum power output. Raising compression ratio has the same effect as increasing ignition advance or increasing the air/fuel ratio.
I always wondered why Manx Nortons had sodium cooled exhaust valves. Apparently the heat in the exhaust valve is a major source of detonation.
In my own situation with the 850 engine, I use methanol fuel which has unlimited antiknock and I run it as lean as possible with a lot of ignition advance with a 2 into 1 exhaust system. When people use petrol as fuel, they might run into more problems than I do.
The following video is interesting :

Running a cam with a high degree of valve duration plus a lot of carburation, results in unburned fuel passing out the cylinders. Increased ignition advance tends to reduce the unburned fuel, but nevertheless, fuel burning as it passes the valves causes the valves to run hotter.
If the exhaust contains unburned fuel it tends to combust in the exhaust port and exhaust headers giving the crackling and popping sound characteristic of race engines.
If we are talking motorcycles with chrome header pipes, blue pipes also result from the fuel burning in the exhaust stream.

Slick

Ignition timing, carb settings, valve timing and exhaust design (eg megaphones) are a rich area for experimentation on a race bike.

The title of the thread is fuel octane rating. Another variable but in my experience once you have a sufficiently high octane for your motor a higher octane does not give more power. Plus no hard and fast rules for what's necessary because different countries have different fuel mixes and even octane ratings. RON in NZ but I think the US is different?

Years ago I discovered Gordon Jennings articles on carb tuning and ignition timing.

I have used his methods for both petrol and methanol. He likes projected plugs if they have piston clearance. He does not like the "racing plugs" with the spark hidden down a hole in the plug.

He believes any tiny pepper spots on the white of the plug central porcelain are caused by very very early detonation causing oil to get past the rings. I have used this to set timing for petrol on my Dommie at 28 deg full advance and 31.5 degree for methanol. Once this is correct I established carb jetting setting using fuel analyser on a dyno.

The bike went well and measured over 50 bhp rear wheel on a dynojet dyno on petrol. At the time it was the second fastest 500 cc Clubmans bike in NZ. The fastest was Peter Lodges clubmans class ES2. Peter is a good tuner. I'm never going to catch him.

Good post - I'll watch the whole thing when I have time!
At risk of repeating what's in the video... I read somewhere that the Germans analysed a captured P-47 P&W R-2800 engine, and dismissed the sodium content in the valves as impurities!
I also read that (as mentioned in the video) the P-47 was rated to use 150 Octane aromatic fuel, and it also used water injection, while we ran 100LL in the Battle of Britain, and the Germans ran 87 Octane initially, with Methanol-water injection and 96 Octane fuel only coming in towards the end of the war.
Our little 110cc H***a derived race engine seems to work OK with very modest tuning mods with Shell V-Power (99 RON), plus a dash of, er, 'upper cylinder lubricant' to add some tetra-ethyl lead to keep the knock at bay.
It was dyno'd at a whole 11.5 bhp, but since that's over 100bhp/litre from a 2 valve commuter bike engine - which cost £155 - I thought that was a decent result.
The dyno testing highlighted that it was slightly lean, so I went up a size on the main jet, which did seem to take the edge off it slightly.
It previously ran in an endurance race for 7 hours without any detriment in its lean condition, so I'll revert to the original jetting for this year.
If we were allowed to run a bigger carb I would expect another 3-4bhp, but since the regs mandate a standard H***a C90 carb it does choke it rather a lot, but conversely it stops the motor from being overstressed

Where I live in Australia, there is a species of bird which has one wing shorter than the other. However they have all seemed to have disappeared lately.

johnm said:

Ignition timing, carb settings, valve timing and exhaust design (eg megaphones) are a rich area for experimentation on a race bike.

The title of the thread is fuel octane rating. Another variable but in my experience once you have a sufficiently high octane for your motor a higher octane does not give more power. Plus no hard and fast rules for what's necessary because different countries have different fuel mixes and even octane ratings. RON in NZ but I think the US is different?

Years ago I discovered Gordon Jennings articles on carb tuning and ignition timing.

I have used his methods for both petrol and methanol. He likes projected plugs if they have piston clearance. He does not like the "racing plugs" with the spark hidden down a hole in the plug.

He believes any tiny pepper spots on the white of the plug central porcelain are caused by very very early detonation causing oil to get past the rings. I have used this to set timing for petrol on my Dommie at 28 deg full advance and 31.5 degree for methanol. Once this is correct I established carb jetting setting using fuel analyser on a dyno.

The bike went well and measured over 50 bhp rear wheel on a dynojet dyno on petrol. At the time it was the second fastest 500 cc Clubmans bike in NZ. The fastest was Peter Lodges clubmans class ES2. Peter is a good tuner. I'm never going to catch him.

Click to expand...

The first decision which must be made is which fuel you intend to use to stop detonation. My thought about sodium-cooled valves is they were drilled down the stem, then filled with sodium metal. In the old days, a sodium cooled valve for a Manx cost 15 pounds, and a piston cost 30 pounds. In those days, my wagers were aboiut 10 pounds per week. So in 1973 when I was offered a 30M for $15000, I did not buy it. By the time it returned to New Zealand in about 1990, having been restored, it's price was $60,000. We got decimal currency in the mid 60s.
In Australia, all the Manx Nortons in A grade ran methanol, so sodium-cooled valves were not necessary. But I never knew about the detonation problem with petrol. I have never owned a Manx, but I rode the one I was offered. It was slower in a straight line than my 500cc Triton, but much quicker in the corners, and not as dangerous.

When I built my Seeley 850, I never believed in it, so it sat unraced until I moved to Benalla. I now believe the 850 motor is an excellent design. Once the crank balance factor has been rectified, it can be made to go fast enough to win races. But it is different from other motors, it needs to be used differently. When you can accelerate from the beginning of a corner, around it and right up the next straight - it is a very convincing argument, even if the acceleration rate is relatively slow. The gearing on my bike is stupidly high and I use close ratio gears. With torque, you never know how much you have until you raise the gearing. A Commando based bike can feel as if it is going as fast as it can, but just rev and go nowhere fast. When you lower the gearing, the bike usually does not accelerate faster. If you lower the gearing on a bike which has a light crank, it will usually come out of corners like a rocket. With the heavy crank, nothing changes when you lower the gearing. It accelerates at the same rate. - With close ratio gears, you lose less revs on up-changes, so acceleration is quicker. The heavy crank makes the bike safer in corners. My 500cc Triton was always trying to kill me.
When I was a kid, I always knew Norton were smarter than most. A Manx was a money-pit. So I raced what I could afford. My Triton 500 was good enough, however I am lucky to be alive. When I think about it these days, I am horrified. There was one crash where I survived by sheer luck. I have a dislocated collar bone, but by the time I reached the ripple in thev road, I was on my side, not the top of my head.

johnm said:

Ignition timing, carb settings, valve timing and exhaust design (eg megaphones) are a rich area for experimentation on a race bike.

The title of the thread is fuel octane rating. Another variable but in my experience once you have a sufficiently high octane for your motor a higher octane does not give more power. Plus no hard and fast rules for what's necessary because different countries have different fuel mixes and even octane ratings. RON in NZ but I think the US is different?

Years ago I discovered Gordon Jennings articles on carb tuning and ignition timing.

I have used his methods for both petrol and methanol. He likes projected plugs if they have piston clearance. He does not like the "racing plugs" with the spark hidden down a hole in the plug.

He believes any tiny pepper spots on the white of the plug central porcelain are caused by very very early detonation causing oil to get past the rings. I have used this to set timing for petrol on my Dommie at 28 deg full advance and 31.5 degree for methanol. Once this is correct I established carb jetting setting using fuel analyser on a dyno.

The bike went well and measured over 50 bhp rear wheel on a dynojet dyno on petrol. At the time it was the second fastest 500 cc Clubmans bike in NZ. The fastest was Peter Lodges clubmans class ES2. Peter is a good tuner. I'm never going to catch him.

Click to expand...

I always thought tiny white specks in the plugs were specks of aluminium from the piston. When you get that, you are jetted far too lean. If you hear a miss when you are riding the bike - stop and fix it.

N

acotrel said:

I always thought tiny white specks in the plugs were specks of aluminium from the piston. When you get that, you are jetted far too lean. If you hear a miss when you are riding the bike - stop and fix it.

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No. Black pepper. Burnt oil spots.

If you have tiny aluminium beads you are in potential disaster territory. And in 90 % of cases I would hold they are due to too much ignition advance and not lean tuning.

My experience is that many people run way too much advance and then flood the bike with over rich mains to attempt to prevent detonation.

Again Jennings would say get your ignition timing right first. Then tune the carb.

Seriously that guy knew a lot about motorcycles.

On Manx compression ratio. Back then, when push starting was the norm, they had to set the compression not for max power but slightly lower to get the blasted thing running.
A good day, my Manx is a 4 steps starter. With 11:1 CR. Uses double plug washers, otherwise the piston closes plug gap. Uses 102RON/90MON fuel.
On air/fuel ratio, Most people think that around 12.7 gives best power. Which is a rich mixture. Also the engine runs cooler.
On octane rating needed. Back in the sixties and seventies, we had 98RON and 100RON leaded gasolene. Guess that our brit bikes was built for that.
What octane rating do you use nowadays?

Mike40M said:

What octane rating do you use nowadays?

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In my area it's all E10 so octane is not even a consideration - pump gas "premium" does the job. I generally use Shell or Sunoco, but in truth, most stations sell the same stuff (out of same trucks) with additives added when they deliver.

According to ExxonMobile, in the US:

• Regular (the lowest octane fuel–generally 87)
• Midgrade (the middle range octane fuel–generally 89–90)
• Premium (the highest octane fuel–generally 91–94)

BTW-1, even in the 60s when Sonoco had "260" (102 octane leaded if I remember right) and about 6 grades, they were mixing regular and premium at the pump to get those different grades. I suspect that is the same for all today to mix the two or three grades they sell.

BTW-2, people who remove the ethanol from US gasoline are running really low octane gas! For E10 to be tested at 91-94 with ethanol, it is WAY lower without the ethanol!

The US Dept of Energy says this:

"The large number on the yellow gasoline pump octane label is the minimum octane rating. The (R+M)/2 Method you see on the label refers to the average of the Research Octane Number (RON) and the Motor Octane Number (MON) ratings. To determine the RON, the fuel is tested under engine idle conditions with a low air temperature and slow engine speed. To determine the MON the fuel is tested under the more stressful conditions of higher air temperature and engine speed."

This explains some of this in more detail: https://www.energy.gov/energysaver/tracing-gasoline-well-your-tank

Mike40M said:

On Manx compression ratio. Back then, when push starting was the norm, they had to set the compression not for max power but slightly lower to get the blasted thing running.
A good day, my Manx is a 4 steps starter. With 11:1 CR. Uses double plug washers, otherwise the piston closes plug gap. Uses 102RON/90MON fuel.
On air/fuel ratio, Most people think that around 12.7 gives best power. Which is a rich mixture. Also the engine runs cooler.
On octane rating needed. Back in the sixties and seventies, we had 98RON and 100RON leaded gasolene. Guess that our brit bikes was built for that.
What octane rating do you use nowadays?

Click to expand...

I'm only talking about a 500 Dommie.

10.25 :1 CR, PW3 cam. I used New Zealand Mobil 98 RON pump gas. I have used Shell leaded No 1 race gas which I think is about 105 RON but found no advantage on the dyno or on the track. Anyone a bit younger than me could bump start it pretty easily. Especially if it's warm.

On Manx sodium cooled valves on production bikes. Was introduced in -51 and discarded for -59 season.
My guess is that obtainable fuel quality had improved.

johnm said:

N

No. Black pepper. Burnt oil spots.

If you have tiny aluminium beads you are in potential disaster territory. And in 90 % of cases I would hold they are due to too much ignition advance and not lean tuning.

My experience is that many people run way too much advance and then flood the bike with over rich mains to attempt to prevent detonation.

Again Jennings would say get your ignition timing right first. Then tune the carb.

Seriously that guy knew a lot about motorcycles.

Gordon Jennings | Cycle World | FEBRUARY 2001

magazine.cycleworld.com

Click to expand...

Thanks for that link.

I have mastered the hacksaw, and feel a kindred spirit in Gordon.

I run Premium 98 non A10 fuel in all my bikes, runs cleaner and revs better than 95 and 91 just makes my Norton ping, I use to run valve supplement with my fuel at the right mix but found the motor ran to rich so only used it in every 3rd or 4th tank full, but these days I don't use it at all.
Leaded fuels were so much better for running than unleaded fuel in our old bikes but years ago I was given an old Honda dirt bike that still had leaded fuel in the tank, my mate and I was on the piss one afternoon and decided to do a burn off test in the garden a small cup of 91 unleaded fuel lit up and the flame only raised knee high, then we did the 98 unleaded premium and it went about hip high, then we did to old leaded fuel, man did that flamed up double the 98 unleaded premium, it's been in the tank since 1985, I sold the Honda to a mate son and they got it running on that old tank of leaded fuel and rode it around using that tank of leaded fuel.
The unleaded fuel of today have so much additives added to it, it's not funny.
We had a 94 Toyota Corolla but we ran 91 unleaded in it, ran ok but every so often we put a tank of 98 unleaded premium in it and after a few days it ran a lot better, in fact it loved it and also got a bit more out of the tank mileage wise, just seems to run cleaner and better with the higher octane.

Ashley

I once talked with a guy working with cattle feed blends. He constantly monitored the ingredients market and adjusted mix for lowest price.
Guess it is the same with fuel mixes. Not a consistent product. Reason I use race fuel.

Consistency is probably the reason for Elf racing fuel in MotoGP. The maximum usable compression ratio probably gives no more power than any other when the ignition timing and fuel/air ratio are optimised. If your engine is jetted rich, raising the comp. ratio will give you more power, but so would increasing the ignition advance or leaning-off the jetting.
If the optimum combustion conditions improve with pressure, higher octane rating fuel might give more power. Then what we are probably talking about is chemical composition and mixing.

A non-scientific motorcycle rider's comment:
I always thought it was easy to tell the difference in performance between pump premium fuel and high octane race fuel without changing the tune. Back when race fuel was somewhat affordable, I used to buy it in 60 gallon drums from a speed shop to run in 2-stroke and 4-stroke off road bikes. I wasn't racing. I liked the way it worked. Plus it smells better than pump gas to me. At $20+ per gallon today it isn't something I'll be entertaining again. Now I use poor man's race fuel in my street Norton by mixing pump gas with off the shelf octane booster. The difference in performance from pump gas with off the shelf octane booster is not even close to what race gas was like, but I think octane booster along with lower compression helps with possible detonation on extremely hot days. I carry a bottle of octane booster for additional fill ups on longer rides as well.

Rockithound said:

I have spouted off on this topic before: I run my Commandos on aviation fuel purchased at a small local airport: 100 octane LEADED gasoline. Not only do the bikes run like it's 1973 again, I still use the original fiberglass petrol tank on a 1970 Commando S. Last time I purchased (August '23) price was a little over $4 / gal. Other then the fact that the fuel is dyed blue in honor of "no road taxes collected" (but who's checking?) - zero issues.

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That's fine if you never go very far. A lot of my riding requires fill ups on the road.