Commandos and Higher Octane Gas

I think this question answered for stock bikes is yes. Having said that Ethanol and unleaded fuel would be of greater concern unless the valve seats have been replaced or compression ratio increased maybe?

I'm not sure I subscribe to the ideas mentioned that you can run a high compression high performance engine off 87 octane and expect the same results. If that's the case explain to me why racing fuel and aviation fuel would range from 93 to 110+?? I know the aviation industry has stricter regulations on additives required for lifespan (most prop planes sit) like preservatives and lead for valve seats etc but the octane in these fuels and racing Motorsport fuels is there for a reason. I think the answer lies in the compression ratio.

Ok so is the octane effecting the thermodynamic properties and unlocking more hp? My Porsche 911 runs much better on AV gas and my brothers 930 was rebuilt to run 96-100+. It will run 93 but it doesn't produce the same results. I get better results running AV gas or race fuel in my Ducati at the track, the bike just runs smoother, maybe no more power but ain't pulls harder out of turn 10 and will wheelie easier heeled over with good gas and :lol: cold air.

I'm happy being wrong with my assumptions and lack of attention or knowledge retention from physics and chemistry in college but hey the truth and fact overrules assumption right? Let's talk about oil next lol :lol:

Rusty bucket said:

I think the answer lies in the compression ratio.

Click to expand...

There is a lot more involved than just the compression ratio. The amount of swirl and squish in the chamber influences octane requirements greatly -as does the head material and temperature.
A water cooled aluminum head normally requires the least octane and an air cooled cast iron head needs more octane -if everything else is the same.

It also depends on the cam and porting. They determine how much cylinder pressure will be created and at what RPM. Very high rpm engines often get by with much lower octane just because there is not as much time for the end charge to get hot. High rpm engines usually run lower peak cylinder pressures also.

If high octane is not needed to resist detonation -then there is no advantage to additional octane, but there is often an advantage to race fuel because it is blended with tighter controls and contains less of the light end products that are in today's pump gas. Good race fuels are usually more dense and contain more BTUs of energy per gallon. Jim

There's more to race fuel:

http://www.racegas.com/fuel/1

I ran 13.5/1 in a SBC on the street using Sunoco 260 back in the day. The way I got away with that was the 4.88 or 5.38 gears. Any lugging would lead to severe detonation; the engine always had to revved. Trying to minimize shifting whilst keeping one arm around a girl sometimes led to the spark plug center electrode insulator cracking and falling down over the gap. That only happened with Champion plugs, never AC commercial. Squish clearance was about .035 +/- and I think the resulting turbulence really helped to tame detonation.

comnoz said:

Some race fuels contain lead which will leave a deposit in the chamber and on the plug. The color of this deposit was a good indicator of the air fuel ratio.

Click to expand...

What can the average guy wrenching in his garage without access to special sensors or a dyno look at to determine if the air fuel ratio is close when running unleaded E10?

htown16 said:

comnoz said:

Some race fuels contain lead which will leave a deposit in the chamber and on the plug. The color of this deposit was a good indicator of the air fuel ratio.

Click to expand...

What can the average guy wrenching in his garage without access to special sensors or a dyno look at to determine if the air fuel ratio is close when running unleaded E10?

Click to expand...

You can look for signs of melting [rounding] of the edges of the center electrode. They will eventually wear round but a low mileage plug should hold the sharp edges for quite a while. Timing that is too fast will round them quickly also.
This is not reliable when using a platinum or iridium center electrode. By the time they melt the piston will be long gone.

You can look at the side electrode. If the mixture is correct and you have the correct heat range of plug then there will be a change in the color about halfway down the side electrode.

You can look at the color around the bottom of the threads. After some miles the carbon around the bottom of the threads should be gray with a little black just starting on one side.

Pipe color helps too. No blue on chrome or yellowing on stainless means the mixture is probably rich.
Medium bluing at the first bend would be normal.
Heavy bluing means either the mixture is lean or the timing is too slow. Jim

Much appreciated Jim, I'm going to print this out for reference.

I've always gone by the philosophy that I've never had a bike show any issues by having 'too high' octane. But we all know that too low octane can, basically, destroy an engine (holed pistons, seizure, etc).

Add to that many old bikes sit around for prolonged periods AND that modern fuel goes 'off' (loses its octane) quickly...

So, I've always seen it as a simple, sensible, precaution to use high octane fuel. The fact it costs a bit more is utterly insignificant given the potential downsides of low octane (above) and the low annual mileage most of us do.

I do admit though that this has led to me building my bikes accordingly! My 850 Commando runs 10.5:1 CR with no pinging etc at all on pump 'super unleaded'.

I always use 85 or 87 octane (the cheapest they got) in my norton and it does just fine.
My bike has low compression and I do all of my riding in between 6000 and 100000 ft. altitude.

Mark said:

I always use 85 or 87 octane (the cheapest they got) in my norton and it does just fine.
My bike has low compression and I do all of my riding in between 6000 and 100000 ft. altitude.

Click to expand...

How does the bike run at 100,000 ft? Must be a pretty awesome view at that altitude.

Mark and comnoz are both from Colorado.

I see a pattern here. :lol:

Dances with Shrapnel said:

Mark and comnoz are both from Colorado.

I see a pattern here. :lol:

Click to expand...

Hey, my injection compensates for altitude. But I have to wonder where Mark got the 0.0002 main jets.....

^Altitude

It runs about like you would expect, weak but still capable.
Unlike a lot of the guys here, I don't treat my MK3 like a hotrod and try
to make it perform like a modern sportbike.
I try to ride it for what it is..... an old bike.

right now it has about 33,000 mile on a stock engine rebuild. Single 36mm mikuni,
old Boyer, NGK bp7es plugs. It's tired, and up on the mountain passes, (10,000 ft.),
I would realistically guess it is only making 20 or 25 horsepower.

It does the job, usually hauling my Ol' lady and saddlebags full of gear.
This pic is about 25 miles up the road from my house:



Normally two up, Sunday afternoons and such:

Mark said:

I always use 85 or 87 octane (the cheapest they got) in my norton and it does just fine.
My bike has low compression and I do all of my riding in between 6000 and 100000 ft. altitude.

Click to expand...

10,000 ft is easy.

My other bike has 6 carburetors and it is definitely wheezing and wanting to ping when I get up high.
(runs like a rocketship at sea level though).

Mark said:

My other bike has 6 carburetors and it is definitely wheezing and wanting to ping when I get up high.
(runs like a rocketship at sea level though).

Click to expand...

Mark, where dya get the 6 into 2 inlet manifold from...?

Fast Eddie said:

Mark said:

My other bike has 6 carburetors and it is definitely wheezing and wanting to ping when I get up high.
(runs like a rocketship at sea level though).

Click to expand...

Mark, where dya get the 6 into 2 inlet manifold from...?

Click to expand...

Must have a strong grip on his right hand to twist the throttle :lol: and think of all those cables to set up :cry:

Same spot, different bike.
6 cylinders - 6 carbs.
This motorcycle does like the more expensive fuel.