Timing is Everything! : Engine Timing

Engine timing : When it comes to the operational health of your internal combustion engine, it’s true what they say…

“Timing is everything.” This statement is often true throughout life whether we are talking real estate, comedy, or you name it. When it comes to engines there’s a double meaning (at least). Cam timing controls the valve train, ignition timing the spark: two systems that must work very near perfectly together in a gasoline engine. Fuel and intake also need to be carefully tuned to work with these systems but they’re slightly more flexible. The exhaust system must also be compatible for best “breathing” effect.

Engine timing can be, and is, “mapped out” in degrees: with two-stroke engines there are 360, a four-stroke 720. Yes, a circle has 360 degrees. If we start at Top Dead Centre, 360 degrees later we arrive at TDC again. Yes, that’s why a two-stroke sounds so busy—it has only half as much time to perform the necessary functions that make an engine work.

A two-stroke must do all the same work but in one complete revolution not two as in the four-cycle (cycle and stroke are interchangeable terms here).

A four-stroke has a bit more time relatively speaking, though it’s still happening faster than your eyes could follow once the revs climb much above idle. In older non-computer bikes a strobe light can be used to freeze the timing marks and make them visible.

To understand engine timing, first we need to know how the system works. Let’s look at a four-stroke single to make it easy. Stating at TDC (we could start anywhere) gives us the location of the piston. It also means the crankpin or journal is at the top (if you could see the crankshaft).

So with the piston moving down, air and fuel are drawn into the cylinder. This is the first or “intake” stroke. At sea level there is 14.7 psi of air pressure pushing the fresh air through your filter against a less than perfect vacuum created by the piston moving down.

At a higher elevation that pressure goes down about one psi every 2,000 feet. This is why your bike makes less power in the mountains (less air means less fuel can be completely burned).

There is very little time for this air-fuel mix to get in, with the valves opening and closing at a tremendous rate of speed. A hotrod engine will often use a cam that holds the valve open longer (duration) and higher (lift), which helps a bit, though usually at some cost to durability and emissions.

At some point near Bottom Dead Centre the intake valve closes (this depends on cam timing). The piston now moves up compressing the mixture on the second, or “compression,” stroke while the ignition system and timing cause the spark to occur before TDC.

The swing analogy: Finding the “sweet spot” of an engine’s timing is both art and science. “Imagine pushing a kid on a swing,” says Rich Burgess. “If you push at exactly the right time everything goes smooth and far. Push too soon and you are doing extra work. Push too late and not much thrust is delivered.

How much before or “in advance” this needs to happen depends heavily on cam timing. The greater the number of degrees before TDC that the sparkplug fires the more “timing advance” is present; getting closer to TDC the timing is retarded.

The high-octane (premium) fuel necessary for high compression engines actually burns a little slower than regular and needs a bit more time (microseconds) to get going. It is also important to get the maximum push down on the piston. If you find the timing sweet spot you are there.

Too much cam and timing could cause a “backfire.” I once saw a cammed-up Shovelhead blow off its carb! Early bikes actually had two twist grips: one for throttle and one for manual timing. The operator would retard timing for easy starts and as the revs climbed he would advance timing to give the fuel time to start burning.

Now on the way down again we are on the third or “power stroke” in which the controlled burn of the fuel drives the piston down and the bike forward. Before we hit Bottom Dead Centre the exhaust valve opens to give the spent “charge” a head start out of the engine.

Now the piston heads back up pushing the hot used gases out on the “exhaust” or fourth stroke. Just before TDC the intake valve will probably open for a “scavenging” effect.

The process keeps repeating as long as the engine gets what it needs—air-fuel-spark—at the right time and only if the engine has enough compression. My old buddy Mike Legg, who trained in the Canadian military, put it this way: “Suck-Squeeze-Bang-Blow.” Good summary. One I like less but hear on occasion is “An engine is just an air pump.” Well, some pumping is happening, but there is a lot more going on.

The idea for this article occurred to me as I was installing my warmed-over Evo back into my Dyna. I decided to use the Ultima nosecone single-fire ignition that had been in my 120 when it suffered an extreme overheating situation after the oil pump jammed. Sometimes the nosecone style systems can fail if subjected to very high temps. I wanted to see if it was still good (it was!).

These are one of the least expensive ignitions on the market and work very well. One thing though: when “timing” mine by the book, it says with the engine at TDC on cylinder #1 “turn the plate clockwise till the light just goes out.” That is what I did. The bike would start on choke but ran badly, acting like maybe the carburetor low-speed jet was plugged. The carb had been sitting a long time but I knew it was clean so I started playing with the timing. Sure enough, I turned it another five degrees or so and the bike ran great! I thought, “Hey there is an example of how the systems have to work together.”

Be careful here if the timing is too far off because you could badly overheat your engine. I once had the chance to tune an Evo using a dyno, and it was interesting to see how a very small change could mean a horsepower or two. I would recommend getting it done this way if you can. You will wind up with an efficient and “safe” setting.

As you understand how the engine has so much going on, it’s easy to see how things can go wrong. Understanding how the two- and four-stroke systems work is the basis of all engine trouble shooting. What I have learned so far, from advice, books and experience about engines has been a long and fascinating road with no end in sight. Knowing how things are supposed to work is a real good starting point.

By Rich Burgess Canadian Biker Issue #321

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