Detonation

Motorcycle.com Staff
by Motorcycle.com Staff
I recently bought myself a brandy new 2004 CBR1000RR. I absolutely love the bike, but for one thing, it has a pretty bad detonation problem. Every time I roll back on the gas it sounds like the change in my pockets is jingling. If I'm at a steady cruise it's fine, but loading the engine even slightly brings the pinging right back. This is the only problem I'm having with the bike. There is no missing, no starting problems or anything else to speak of. I've heard many opinions from the mechanically disinclined among my friends. So let me eliminate those here:



a. It's not a loose chain

b. It's not gravel hitting the exhaust

c. My bike was NOT designed for 87 octane

d. It's not lifters, piston ring break-in or any other mechanical noise

I also have a 2002 ZRX1200 that runs happily on 93 octane all day long under all conditions. I know it's only 10.5 to 1 compared to the Honda's 11.5, but I wouldn't think a single point would cause such a dramatic difference in detonation. I've even tried a tank of CAM2 100 octane. There was a slight difference, but not enough of one to merit always running the bike on gas that costs $4.00 a gallon. Shouldn't modern sportbikes with compressions going over 12 to 1 have some kind of knock sensor? I know detonation is bad on a motor and the delicate throttle hand required to avoid it is annoying me. I didn't put down over 10 large on this bike to ride in the granny lane.

Thanks

There's detonation (spontaneous combustion after spark plug ignition) and then there's pre-ignition (ignition prior to the spark plug firing), and regardless of which one you have, both are fatal to an engine. Basically, you've got dual-ignition (at different times) going on here, and when the flame fronts meet in the cylinder, you hear the "knocking" or "pinging," an audible sound indirectly resultant from the pressure spike -- similar to the resonance and vibration you'd note if you whacked a block of metal with a hammer. In addition to the shock of these conditions, pressure spikes in general are a bad thing. PV=NRT, so when the pressure spikes in a constant volume (relatively here due to the short time span we're dealing with), so does the heat. Ever inflate a tire with a Co2 cartridge? The cartridge gets really cold, really fast from the internal pressure drop. Engines are the opposite: Combine a lot of heat with rapid pressure spikes and you can surmise why it's so deadly. Again, this is bad and the first thing you need to do is take it to your dealer! Write up a written report to document the problem for your warranty needs.

Still speaking generally, these two different types of events are usually either too much compression (yes, one point is a huge change) or too much ignition advance. Check the cranking pressure in all cylinders to make sure they're the same -- if one is off, most likely something's mechanically wrong. If not, it could be an ignition control/blackbox problem.

There's other complications: fuel starvation could be one. A cooling problem another. Glowing carbon deposits at the piston edges can cause detonation. The point here is that this is a really really bad thing with many systems that could be failing and causing the problem, so get it fixed! Technically, yes, some gasoline engines can live with detonation for a long time, but it's likely that a high-revving high-compression motorcycle won't be one of them.

For people more interested, entire books have been written about locating peek pressure, burn rates, gas expansion and the timing needed to maximize power output. It's pretty interesting stuff. To get started with a good general reading on engines, try Edward F. Obert's 1973 text "Internal Combustion Engines and Air Pollution."

--MO

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Motorcycle.com Staff
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