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Discussion Starter #1
what exactly is timing? first ill give you my guess. is it the time interval between the injector releasing fuel into the cylinder and the spark plug firing? hehe, nice guess eh? :E also, i know there is a way to manipulate it. i remember watching a friends dad with a little light gun "checking the timing" of his hot rod. is this the same thing? thx.

solar
 

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In a spark-ignition piston engine, there are numerous parameters that can be termed "timing": valve opening and closing, spark firing, injector firing etc.

You are likely referring to the most commonly described one, spark timing. In the engine, there are 4-strokes (4 cycle engine...): intake, compression, power & exhaust. On the intake stroke, the piston moves down with the intake valve open. The vacuum created (the source of "manifold vacuum" BTW) draws in fuel and air. The valve closes and the piston is on the way back up to compress the mixture. In a theoretically "ideal" engine, the piston would reach TDC (top dead center), the spark would occur and in an instant, the fuel would burn 100% and the cylinder pressure would skyrocket, forcing the piston down the cylinder.

In the real world, time is required to start the mixture burning and the burn itself can take some time. At 6000RPM, the crankshaft makes 100 complete turns every second. 10 degrees of crankshaft rotation occurs in about 278uS (two hundred and seventy eight millionths of a second!). By the time the mixture has burned to increase cylinder pressure appreciably, the piston would have already moved far down the cylinder again. For this reason, the engine management system "advances" the spark timing. When you were a kid and threw snowballs at schoolbuses, you'd have to lead the bus a bit when you threw so that by the time the snowball got to the bus, it was there to hit, not further down the road. Same with this: because the fuel/air takes time to start burning and time to complete burning, the "fire is lit" ahead of time so that pressure is maximum when it's needed: after TDC.

Advancement occurs with respect to the crankshaft angle at TDC on the compression stroke: TDC is 0-degrees. 10oBTDC is 10-degrees before top dead center. By firing the plug this early, the piston just comes through TDC when the cylinder pressure is maximizing. The intent is to maximize the "area under the curve" using pressure and crank angle as the metrics. By doing this, the work done on the piston by the expanding gasses is maximized and so thus is efficiency.

Engines have differing spark advance requirements based on a number of factors, all of which are tied into the engine management system: load, engine RPM, coolant and intake air temperatures, EGR contribution, knock retard etc etc. On older engines, the timing was determined by the base setting (where the sitributor is turned), calibrated bob-weights that would advance the timing as RPM increased (called "centrifugal advance") and by a vacuum canister that altered timing in response to engine load ("vacuum advance"). The advent of microcontrollers powerful enough to handle the computing chore, all of this and more was moved into the hands of the PCM.

HTH.
 

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and for some reason, some people think that if you run 101 octane gas, it will always increase HP.

its race gas for a reason. because you use it when you have a big advancement.


some terms that i (and other people) use to differentiate between the different "timings"

"cam timing"= cam phasing
"injector timing"= injector firing
and spark timing is just timing, or "firing" ... this is when the spark plug fires with relation to TDC. your friends dad was aiming the light at a mark on the crankshaft dampener. the timing light was hooked to the #1 spark plug wire and flashes whenever the #1 plug fires. "dad" could then tell what his timing was by seeing where the mark was located when the light flashed and could adjust accordingly...
 

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Discussion Starter #4
jebus...

thx guys. hehe, if only i understood it all....LOL!!

solar
 
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