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Discussion Starter #1
Sorry for what appears to be a really silly basic question, but when engine timing retard or engine timing advancement are discussed does it mean spark timing?

I don't see how it could be anything else on an engine without variable valve timing.

Lastly, if I'm correct so far, does the timing have two parts?
1. When the spark occurs.
2. How long the spark lasts. (would this matter at all, beyond the basic "long enough to get the fuel/air mixture ignited"?)
 

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FF's Night Security
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Timing could mean many things on a motor. It could mean valve event timing, spark timing, and so on. Typically when you hear advance or retarded timing it does mean the spark timing but you need to know more about the what particular timing event someone is talking about on the motor.
 

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Can be injector timing as well.

How long the spark lasts is not timed per se, rather built into the equipment although conditions in the cylinder can affect it minutely.
 

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Yup, when not specified timing usually refers to spark timing though as Sleepy mentions there are other "timed" events happening as well.

The duration of the spark isn't a large consideration beyond adequate, unlike duration when referring to valve opening.

FWIW - spark timing is typically measured at idle as a "base" setting with advanced/retarded being a comparison to that. This dates to distributors that were adjusted to spec. at idle. (when not an absolute measurement vs. TDC)

Once off idle spark timing is advanced on a curve, mechanical distributors had a "full advance" limit while electronic systems are infinitely flexible. Vacuum advance/retard systems were used with distributors to compensate for load on the engine, throttle position vs. rpm is the main reference used for the same purpose with current electronic systems.

While spark timing adjustment is now unlimited, the effective range is still limited by other factors - the fuel air mixture has to be appropriate for ignition & not have ignited on it's own to list basic limits.
 

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Discussion Starter #5
Ok, thanks team. Yeah, I was referring to mention of retarding or advancing timing in literature I've been reading with respect to engines in general.

I was somewhat sure it was referring to spark timing, but as others have mentioned, it depends on the context in which it is mentioned to get the absolute meaning.

And yes, I am aware (at a basic level) of camshaft timing which I believe can be specified in duration (how many degrees of a camshaft revolution it stays open), start (where in the cycle relative to TDC? the camshaft opening starts), and lift (how far the valve opens at its peak?).

Once again corrections to any of the above are much appreciated.
 

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C2H5OH
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Far as Ignition Timing, give this a whirl --> https://drive.google.com/file/d/0B4sorVY7Ysh4bWx0bXdJTFB2NGM/edit?usp=sharing

The goal for power is to balance the Peak Pressure Point and negative work, or to keep the PPP at a point far enough from TDC where the most force over time is put into rotating the Crankshaft.
On average, that is about 13* ATDC. This is due simply to Geometry, the Lever Principal.

The problem though is that the tighter the mix is compressed, the faster the burn (at equal Lambda's). So either less lead is required or more fuel (used to absorb the excess heat and slow the burn). The fuel scenario is the most common used and this is evident by seeing how rich WOT is tuned.
 

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Discussion Starter #7
Excellent thank you! Let's see if I understand a few points from this example.

Let's say the "nominal" timing is 13* ATDC (i.e. piston on the way down -- power stroke right?). If that was timed BTDC, my goodness that would be a mess wouldn't it? Piston's coming up, period. Flame front hits it, I would imagine ouch (piston? connecting rod? rings? piston pin?).

Anyway, back to my original question, if 13* is "nominal" (correct term?), then if that's retarded, it would fire at some value < 13* ATDC? Say 11* ATDC?

Likewise same for advancing -- > 13* ATDC?
 

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C2H5OH
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Advance - the PPP moves further from TDC. So if optimal is 13*ATDC advancing it would be 14* or more.

But in the context most talk, they use ignition timing and not PPP (which is tough to find and requires sensors cars just aren't equipped with).
So lets say that the engine wants 10* at idle. This means the ignition fires 10* BTDC. The PPP is probably something like 25*ATDC.
If you advance the timing, say to 15* that will also move the PPP, but not in an equal amount. 15* might result in a PPP of 17*ATDC. How tight the mix is, how large the chamber is and the type of fuel all come into play.
PPP and ignition timing are related but depend on variables. Also, at idle you don't need full power so timing can be retarded to save ware-n-tare and fuel.

And ignition timing becomes more critical as RPM increases. Here's an example chart I'd made for something else,



The Y axis is seconds and the X is *ATDC.
If you think about the slope of the lines you'll understand why proper ignition timing at high RPM becomes more critical.
2* at low RPM won't be that noticeable. But at higher RPM it becomes critical.
When it comes to ignition timing, milliseconds matter. In the racing world, the wrong resistance spark plug wire can cost power.
 

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TooOld I keep seeing your name everywhere asking lots of questions.
I am mostly self taught like you. I recommend:
Engine Management Advanced Tuning by Greg Banish and
Performance Fuel Injection Systems by Matt Cramer and Jerry Hoffman
These two books go into a lot of detail on how EFI systems work, as well as how tuners modify these systems. Much more information than anyone on this site will be able to provide you. Also much more info than you will find for free online. I bought them as Kindle books.
Engine tuning has a steep learning curve, but familiarity with physics and chemistry go a long way in softening that curve.
Why the sudden interest in cars? (If you are indeed old as your forum name implies)
 

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^x2 Those are the same 2 books I refer people to read. Both good reads with in depth knowledge. I found them very easy to follow and understand. They break it down to a beginners standpoint and build it up.
 

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For a simpler piece of info., "advanced" spark timing means the spark event happens further in "advance" of TDC - longer before the piston is at the top and starts moving down again.

As iminhell was mentioning, the burn event takes time (variable time) and is initiated BEFORE the piston is at TDC.

For a side note, some 2 cycle engines can run "backwards" and that was used in some production machines using electronics to stop the engine & restart it backwards for an inexpensive reverse drive mechanism. (snowmobiles)

For a Ski-Do I owned, pressing the reverse button at idle would "kill" the spark and engine and as it slowed to a halt the spark would be initiated again "advanced" enough to start it running in reverse direction. It then adjusted to a normal (backwards) timing for that direction until you repeated the process to restart it again for forward motion. Works well for port or reed valve intake types, not a rotary or any other mechanically valved system.
 
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