Detonation


Burned and melted pistons, shattered rings, blown bearings - uncontrolled detonation is an engine killer. Anyone who operates a motorcycle should know how to recognize and avoid it. Since the Blast is being particularly aimed at new riders, here's a brief on the basics.

Detonation, "knocking", and "pinging" are different terms for fuel combustion gone awry. In normal internal combustion, the fuel/air charge burns and expands; in detonation, it literally explodes within the confines of the combustion chamber. The detonated charge does no work, and all its heat is waste heat, which only tends to worsen the problem by robbing power and making the engine work harder and hotter. The force of the explosion breaks through the fuel/air "boundary layer" that normally protects internal engine parts from direct flame impingement and heat transfer, and this can cause overheating, which also makes the problem worse. The explosion violently impacts the piston and rings, and puts heavy shock loads on bearings.

On a motorcycle with a big single piston like the Blast, you'll hear it - it's a hollow metallic sound like someone is knocking or pinging on the engine with a hammer. It's not like any normal engine noise, and it's loud enough to hear over the wind. I think you'll know it right away. But here's something to keep in mind, from page 76 of the owner's manual:

"The ignition control unit on Blast Models use a two-stage curve. In certain transient load conditions, as the throttle is opened, the timing changes from normal to fully advanced. At this point, the operator can sometimes hear a noise that is similar to pre-ignition detonation. This noise should not be confused with detonation, which can be stopped by the use of higher grade fuel.  It is caused by the instant pressure rise in the combustion chamber as the spark advances rapidly.  This noise doesn't affect engine performance."
On the Blast, I sometimes heard a transient light clicking that may have been from the ignition advance; but detonation sounded like a half-inch ball-bearing knocking around in the combustion chamber, and when it did occur, it was constant at steady load and throttle. Another tip-off is decreased fuel mileage; detonated fuel is wasted, and I got only 44 mpg during the persistent episode in Death Valley.

Sportster engines have a reputation for pinging; some tuners blame it on the stock ignition timing advance curve. But the Blast isn't exactly a Sportster, and there are other possible causes of this engine disorder: lean carburetion, overheating, and low-grade fuel are some of the usual suspects. In fact it may be not one but a combination of things that brings it on.

I think it was a combination in the Blast because the onset of detonation was inconsistent under the same load, temperature, and throttle conditions. In 3,000 miles I had three instances, and in two of them I got rid of it immediately. The first was in Vegas at 2,000 feet, about 80 degrees, climbing a grade at 75 mph and about 85% throttle. When I heard knocks I opened the throttle wide, and the knocks were gone. The second was in similar conditions at 600 feet; knocking continued at full throttle, but disappeared when I downshifted and maintained speed in 4th. It was only below sea level that I had to drop to 65 to stay out of the ping zone, and with more altitude I was able to use full throttle again. I think separate things were "stacking up", and they all had to add up to cause the knocks.

The likely combination? I could easily get lost here, but I'll take a guess: timing slightly off, fuel slightly off, and in the Valley, unusual high pressure leaning the fuel mixture enough to add to the problem.

Carbs meter air by volume, so denser air at low elevations means a greater mass of air is mixed with a fixed amount of fuel, and the engine runs leaner. I suspect we rode through the wind into a high-pressure area, and that, together with the increased pressure below sea level, might partly account for the knocking, and also explain why it got worse in the cooler, and denser, evening air. Maybe.

I'm told the Blast was extensively tested in the heat of Australia as well as Arizona, and was intended to be "detonation-proof". It already uses the cooler 10R12 spark plug that's being recommended for ping relief in the Twins. They say the bike puts out only 30% of emissions allowed under the California regs, and richer jetting was possible, but not needed. I can say the bike ran perfectly after a few minutes warm-up, and there were no other signs of leanness.

Spark timing slightly off is a likely suspect. I rode right past the first two scheduled service checks, which included timing checks. This would be the second thing I'd look at, after fuel.

I think fuel quality had a part because, other conditions being equal, the onset of detonation was inconsistent from tank to tank. On the road, I used, and mixed, every brand available. I used premium 92 when I could get it, maybe 98% of the time. The Blast owner's manual recommends a minimum 87 pump octane, with this little note: "You may find that some gasoline blends adversely affect the starting, drivability, or fuel efficiency of your bike. If you experience one or more of these problems, we recommend you try a different brand of gasoline or higher octane rating". Web scuttlebutt says single refinery lots are being sold under different "brands" these days. Quality may vary from lot to lot, but I couldn't find one brand consistently better or worse than another.

Backroad touring is one of the Single's strong points, and out on the road fuel quality is taken on faith. Based on experience with a variety of fuels, I don't anticipate any problem as long as the rider is savvy enough to recognize pinging if it occurs and ride around it. Ease up and reduce the workload on the engine, and if that doesn't work, stop! and let it cool down thoroughly.

If detonation occurs more than once in a blue moon on "premium" fuel, it's time to rule out things like timing and carburetion. We'll be getting valuable feedback on this as more riders start racking up miles on the new Single.

Note: If you want more, I got some of this from an excellent little article at Motorcycle Online, which also has illustrations.

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joe@gazette9.com


 

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