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Some people say it's at the end of the compression stroke, some people say beginning of the power stroke..

What do you guys think ?

5 Answers 5

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The power stroke (the bang in the series: Suck; Squeeze; Bang; Blow) begins after top dead center (TDC) past the compression (Squeeze) stroke. The spark does ignite the air/fuel mixture prior to the piston getting to TDC. This is called advanced timing or before TDC (BTDC). Timing is set this way for two reasons:

  • First, firing it BTDC creates more power. It has to do with the the combustion fighting against the piston makes it a little more rowdy.
  • Secondly, if it fired after TDC, the air/fuel mixture would not have enough time to burn, especially during higher RPMs.

Why then are we still in the compression stroke after combustion has started? There are two reasons:

  • The strokes are measured by the piston position, not what is happening within the combustion chamber.
  • Even though the ignition process has started, the contents within the cylinder are still being compressed during the final stages of the stroke until after the piston swings over TDC into the power stroke.
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That's a great question! Four cycle engines have four steps to operate(cycles):

  1. Intake(Suck)
  2. Compression(Squeeze)
  3. Ignition-Power(Bang)
  4. Exhaust(Blow)

As seen here

Technically, the spark ignites the air-fuel mix compressed at the end of the compression stroke, beginning the Ignition/Power stroke. Technically. In reality, the question can become a little more complicated. Modern machines have computers, called Engine or Powertrain control modules, that adjust when the spark plugs ignite within the cycle. This reduces emissions and increases power/efficiency. Advance spark timing means that ignition actually happens at the end of the compression stroke, before the piston is at Top Dead Center(TDC) and retarded timing means that it occurs after TDC on the compression stroke, which would be the power stroke.

That's why your question is a tad bit philosophical, as @LoztInSpace points out. It all depends on the make/model of you car and what, exactly, your car is doing at the time that you're asking. On older model cars, spark timing is set by the distributor and can adjust some depending on the make and model of the car(See the comment below). Sometimes in performance uses, the timing is static. For an example on what this sounds like, you can check out some racing videos. While the cars are idling, they'll sound horrible. The reason why is that the timing is actually set to provide the very best performance at very high RPMs.

I hope that this answers your question. Regardless, please let us know!

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    Good answer, but on older cars, timing was set manually but not permanently by the distributor. In other words, timing was not fixed and static. From at least the '50s till the electronic era, timing was automatically adjusted for driving conditions. A set of rotating weights in the distributor advanced the timing a few degrees at higher rpm through a linkage, and a vacuum diaphragm on the distributor retarded the timing through a linkage during high vacuum (low power) conditions. If either mechanism were faulty, the car wouldn't run right.
    – MTA
    Dec 18, 2023 at 17:25
  • Thanks @MTA. I didn't know that! And it makes perfect sense that such a thing was not only possible, but in use. I edited my post to make it accurate.
    – Geary
    Dec 18, 2023 at 17:54
  • @MTA I believe the vacuum diaphragm advanced the timing under high vacuum conditions, not retarding it as in your comment. motortrend.com/how-to/…
    – HandyHowie
    Dec 18, 2023 at 18:35
  • You have some problems with your answer which makes it very inaccurate. Whether a car is old or new, they've always (always, meaning since they figured it out) have had some type of ignition advance. In older cars it was done three ways: pre-advance; vacuum advance; mechanical advance. Now it is all done electronically by the ECU, but the advance is still there and happens throughout the RPM range. As for performance, the noise has basically nothing to do with timing, but rather with where the engine is designed to run at in the RPM range. Dec 18, 2023 at 18:59
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    @HandyHowie - You are exactly right. So many people get it wrong with carbureted engines where they'll put the vacuum feed below the throttle plates, which does exactly opposite of what you want it to do. Putting it above the throttle plates advances the timing when you open the throttle, which is when you need it. Dec 18, 2023 at 21:35
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Maybe a philosophical question. The spark defines the transition between the two.

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It takes time for the flame front to progress through the compressed mixture after the spark fires. It is desirable for the mixture to burn close to TDC (Top Dead Centre) so it is ignited by the spark a few degrees before TDC to account for the delay. As the delay varies with operating conditions (pressure, temperature) the ignition advance has to change. At higher RPM there is less time so the advance is greater.

Today the ignition timing is done by a computer whereas in the past fly out weights and a vacuum diaphragm were used and, before that, a manual control often mounted on the steering wheel.

So, to be clear, ignition takes place towards the end of the compression stroke.

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As a stroke, the Power stroke is down, from TDC. However the spark can fire quite a few degrees BTDC. If you are running E85 especially. So during the dying moments of the compression stroke.

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