Advancing ignition timing - what it means for your cylinder

[B-Man]

There's no real way to see the limit of timing vs cylinder pressure. You just kinda have to know from experience what's too much or do you normally encounter knock before that happens?

[i3igpete]

note that there was no knock in any of the simulation cases. knock pressures are much higher because there are multiple flame fronts.

[KeithMac]

10 degrees of timing makes a hell of a difference on my car, using straight pump fuel I have to pull 17 degrees at some points in my map to avoid knock showing on the logger.

I`ve recently been adding 20% methanol (by volume) to the fuel in the tank and it`s night and day difference, can add 10 or more degrees of timing back in and the car flys!.

[SnakeSkinner]

not when I first got into the 8's, I ran 8.81@166mph with no timing control whats so ever (955 whp). and yrs worth of running 9's, and 10's in meny cars.

I only put on the ITC when I put in the 1000 cc injectors (pressure cranked up and 2 625ML meth nozzles, making 1093 whp at that time, and running 8.74@168 with lots of tire spinn), and it still was fine with it set to 0 it just made a little more power with a few deg of timing PULLED out on the dyno.

[J. Fast]

There's no real way to see the limit of timing vs cylinder pressure. You just kinda have to know from experience what's too much or do you normally encounter knock before that happens?

Well it depends, one thing to note is that peak torque may or may not be reached before an engine starts to knock. I have tuned engines where knock has occured before MBT is reached. I have also tuned engines where there's a significant amount of room for more timing advance between where peak volumetric efficiency occurs (which is peak torque) and when knock starts to occur. In these engines, if you add timing after MBT, the engine will lose significant power, but may still not knock. So, it is not safe to say that peak torque will be reached just before the point that an engine begins to knock. This means that even if you have a good method of knock detection, adding timing up until the point just before knock occurs will not always result in an ideal torque output, net the most power, or maintain the highest area under the curve.

The best way to establish an optimum spark timing mark is to inspect your plugs. There's a test called a spark plug hook test. You warm your car up, gap your plugs, do a full pull and then pull the plugs and inspect them. Where the ignition signature lies on the sparkplug hook dictates wether your timing is early, late, or dead on.

Jeremy

[J. Fast]

It entirely depends on the fuel that you're using.

For example if you're running on natural gas with 8 to 1 CR, it will never knock, even with lots of boost. You'll just loose power, get hot and become so jerky it's undrivable.

It is true ignition timing is dependent on fuel type due to different ignition points and kernel speeds. However, when utilizing any fuel you may revert to the spark plug hook test. The ignition signature on the spark plug grounding electrode will allow you to determine the optimum ignition time, regardless of fuel. As illustrated here in my EFI University Notes:



Jeremy

[akotten]

Consider the simulation I ran with a predictive combustion model. Though it's just a simple 4-cylinder model, the same applies for all gas engines. Spark timing has been varied from -18 to -10 degrees. a/f ratio is the same throughout. you may be wondering why the peak power does not go up much. recall from your thermodynamics class that the integral of the PV diagram is the actual work produced by a heat engine. timing increases power by pushing the peak in the corner higher and higher. boost adds power by shifting the entire curve upward.

That's interesting because we went over that exact concept (PV diagram) this morning in thermodynamics lol. What's your major?

[i3igpete]

Mech E. you'd be surprised how much you use engineering in some fields of work, and absolutely none in others. i guess i'm one of the lucky ones

[akotten]

Mech E. you'd be surprised how much you use engineering in some fields of work, and absolutely none in others. i guess i'm one of the lucky ones

Cool same here. Feeling like there is no end to this major atm though. Thermo, statics, diff eq, and tech comm...all kicking my ass.

[i3igpete]

junior year will be even worse