Timing discussion.

[kjewer1]

This something I puked out on the DSMlink board, kinda went off on one of my tangents. I added it to my webpage, and thought it might be worth posting.



With timing, there is a mechanical limit for a given motor at each given rpm. Before you reach that, there is a point where you make the most power. I believe the old v8 guys typically go for timing that gives peak cylinder pressure around 15 degrees ATDC. The cool thing for them is they can advance too far and actually see power drop, while we usually blow the motor up before then and don't get to see this. So they know where it is, we don't. At any rate, that point that makes the most power is the goal.


Now, each motor/setup is going to require a different amount of timing advance to get peak cylinder pressure to line up with that ideal point in crankshaft rotation. The problem is running into knock before you get it that far advanced. If you are adding timing, and the ECU is taking it back, you obviously aren't getting anywhere. Race fuel will allow more timing, and typically gives enough knock resistance to get to that point you need to be at for best power. But of course changing the fuel so drastically also changes the motors required timing advance number (to get peak cylinder pressure at the same point), so its a bit of a moving target.


Most v8s need about 35 degrees advance to reach peak power with a well setup motor. That might even apply to all or most NT motors (thats about where the ECU runs our cars in the absence of boost). If we could assume that that doesn't change when you run boost on a given motor, we could say that anything less is a compromise to allow the use of boost without knock, with a net increase in power thanks to the boost. But again, forced induction changes the motors required timing, so its still the moving target problem. And I don't know what that point is on a FI motor. Perhaps with some 200 octane fuel we could find that point where more timing equals less power like the NT guys can.


So it all comes down to running as much timing as you can get away with without doing anything stupid with AFR or boost. AFR is adjustable, but there is a pretty narrow range where we want to operate. With the above mentioned NT motors they pretty much keep it to a .2 window. We tend to open that window up a lot more because AFR is another band aid we have at our disposal (we always go richer than NT motors). Boost is what makes power (or power potential to be more accurate), so you don't want to sacrifice that to make more power (but you want to keep it within reason based on the setup). So timing is what is left. And since odds are you can't approach that ideal point, you're left running as much as your setup and state of tune will allow.


One problem is that the knock sensor may not pick up on a bad situation when you go to far. I was at 28 degrees advance on pump gas (thought it was at 23, base was set to 10 after messing with the cam gears, oops) and didn't see knock until that last run when the motor let go. Had about 3 degrees max retard if I remember correctly. So you have to be careful, and like the other adjustable parameters, you have to keep it within reason even when you aren't seeing knock.


Now that I stop and take a breath, this is probably a bunch of diarrhea of the keyboard that has nothing to do with the topic. Oh yeah, so you can't simply assume that "increasing the timing will make the car faster." I guess that was my point.

[Urban_Squrill]

I just wish I understood some of that.

[JoeWagon]

Thanks for the info. I'd like to see some numbers on what kind of compromises between boost and timing (being limited by knock) results in the highest power output. I've heard the high boost, low timing works better for some people.

[guitarXgeek]

That's some good info, there! Just like joemathews, I'll be asking quite a few questions regarding timing and other things pretty soon, cause I'll have dsmlink sometime around Christmas . I actually have some questions about timing right now, but I'm in a hurry and I need to leave for dinner so the questions will wait til later .