Why Things are the way they are: ENGINE TIMING

“Yep, that engine is out of time.”

It sounds good, doesn’t it? When someone mentions that an engine’s out of time, you can’t help but think he knows a lot about what’s happening inside a motor. But what is timing? It’s funny how often I ask this and receive an answer that’s just way off base. For our purposes here, let’s assume we’re working with a fairly basic and stock engine — nothing exotic — and explain exactly what timing actually is.

Two types of time
No, not a.m. and p.m. There are actually two different types of timing that need to be correct in every four-stroke engine, and the two aren’t really related. Both are important and contribute to engine performance, but in vastly different ways. Over the years, people and companies have worked out a number of creative ways to manipulate both types of timing to increase efficiency and power. Because they are often confused (and confusing!), we’re going to examine both. You probably weren't expecting a two-for-one deal on Common Tread today, were you?

Valve timing
The first is valve timing. In order for a four-cycle engine to work correctly, the valves must open and close at fairly precise times.

In a “freewheeling” engine or a side-valve engine, which are both typically fairly low-performance units, if the valve timing is wrong, the engine will not run. In an "interference engine," the valves and pistons occupy some of the same space at different points in time, so correct timing is even more important. Most modern motorcycles are of this design. If the valve timing is incorrect, not only will the engine not run, but the piston could smash into the valves, causing catastrophic damage. Usually, the result is bent valves and damaged pistons.

If you hear someone talk about their timing chain or timing belt, this is the timing that those pieces control. High-performance engine builders will sometimes change valve timing for heavily modified mills, but for the most part, valve timing should be set to manufacturer’s specs. Due to the fact that the penalty for fouling up valve timing — like during a timing belt replacement — is fairly high (engine rebuild), many riders opt to leave related jobs to a pro.

Ignition Timing
OK, push all that stuff aside for now. We're moving onto the other type of timing. Ignition timing has to do with when the spark occurs relative to the travel of the crankshaft and piston. Theoretically, the engine can make its biggest “bang” when the air-fuel mixture is squeezed into the smallest space, which occurs at the piston’s top dead center (TDC). In practice, that doesn’t work, because the air-fuel mixture doesn’t light off instantaneously; it needs some time to burn. Because of this, most engines will specify that the spark occur a certain number of degrees BTDC, or before top dead center. That bit of time between the spark plug lighting off and TDC, expressed in degrees of crank angle, is known as advance.

Ideally, after the mixture then begins to burn, the maximum pressure in the cylinder will occur shortly after TDC, when the explosion can move the piston with the most oomph. (As a point of reference, anywhere from 10 degrees to 40 degrees of advance is common for a conventional engine design.) Advance helps the engine burn the air-fuel mixture completely. Proper ignition timing helps your engine make the most power throughout its rpm range, and also eliminates knocking and misfiring, which helps not just performance, but also engine longevity.

Now here's the complicating factor. The time the air-fuel mixture takes to burn is relatively constant. However, the time it takes the engine to cover 36 degrees of crankshaft rotation is going to vary based on the engine speed. What this means is that the amount of ignition advance — how many degrees the spark needs to occur before TDC — should vary, depending on engine speed, to make the maximum power. (Other factors come into play in the real world, and we'll touch on them briefly, but for now we’re keeping things simple.) Generally, as engine speed picks up, advance needs to increase because the piston is moving faster but the time the fuel takes to burn is still relatively constant.

Have you ever heard of an “advance curve?” This is so named for the plotted graph that shows the ignition advance relative to engine rpm. If, say, a theoretical engine needs only 12 degrees of initial advance (at idle) but perhaps 36 degrees at redline, there needs to be a way to advance the spark timing smoothly up and down the rev range. How is this achieved? Well, the need to advance the ignition has been dealt with in a lot of creative ways since the early days of motorcycling.

In modern engines, the spark timing is controlled by the engine’s computer. In most engines, a Hall effect sensor (a sensor that responds to changes in a magnetic field) is generally attached to the crankshaft. The sensor allows the computer to “see” where the crank is relative to TDC, and the computer will trigger the ignition coil to fire based on how it’s been programmed. Note that some systems are very simple, using only the crank’s angle, and others are quite complex, taking into account load, throttle angle, rpm, and other factors. Interestingly, due to the high potential for mishap (and legal and environmental issues), most OEM electronic ignitions leave no room for adjustability. Replacing the ignition with an aftermarket unit or reprogramming the ECM is often the only way to gain control of the ignition events on a modern motorcycle.

Prior to electronic ignition, advance was controlled mechanically. Normally, a set of steel weights would be attached to the camshaft, and they would be flung outwards as the cam spun faster. The points breaker would also be attached to the weights mechanism, so the ignition would advance or retard automatically in lockstep with the speed of the engine. Because the weights moved in an arc, the advance curve was continuous.

Even before this, way back when, the ignition timer used to be connected to a cable. That cable ran to the grip that was not controlling the throttle. (So that’s the left grip on a Harley and the right one on an Indian. Other early bikes had tank-mounted levers.) When the rider wanted to start the bike or come to an idle, both grips would be rotated forwards to close the throttle and retard the spark. (This was before the days of throttle return springs, so you had to manually move them forward, not just let go of the grips.) The rider then twisted both grips backwards to raise the engine speed while simultaneously advancing the ignition manually. Nifty, eh?

Manipulating timing
Like most mechanical concepts in motorcycling, once the principles are understood, creative ways are found to exploit them. Both valve timing and ignition timing are altered in a number of ways in an effort to produce power.

The need to increase spark advance relative to engine speed was understood fairly early on in the motorcycle’s development, as evidenced by the early spark control mentioned previously. The progression to advance weights provided a much more consistent application of advance. It was soon realized that the “curve” of the advance could be modified with lighter or heavier weights and springs of different strengths.

Similarly, as mentioned, electronic spark control developed from simply monitoring crank angle to taking into account a number of inputs and incorporating sophisticated algorithms to time the combustion event to occur in order to make the most power. A good example of this is Harley’s new Milwaukee-Eight engine. The engines feature individual knock sensors on each cylinder to feed information to the computer about detonation within the cylinder, allowing the computer to adjust spark timing for each cylinder independently, taking into account changes in temperature, fuel quality, and other parameters.

Of course, power can also be made by monkeying with valve timing, as well. This is normally accomplished through a variable valve timing system. (Variable valve timing is a bit different from something like Honda’s VTEC system, which incorporates changes in lift to the VVT system, in case you’re wondering why we’re not covering that here.) The most common form of variable valve timing is known as cam phasing. Instead of the cam being connected to the crank directly via the cam chain, the timing chain solidly drives the outer portion of a cam phaser, which in turn spins the inner portion of the cam phaser, which in turn is solidly mounted to the cam. The “slop” or takeup between the inner and outer sections of the cam phaser allows for the timing of valve events (opening and closing) to be variable.

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