Thread: Let's talk about ignition timing

So I've been thinking about this for the past month or so. I haven't been back to the track so it's all paper work so far, no DSM specific test and tuning... YET But understanding what you?re doing before actually doing it always yields better results. Read what I've got, add to it if you've got anything else.

We want to achieve peak cylinder pressure at 15* after top dead center (ATDC). We adjust ignition timing to achieve this. Variables to consider while adjusting ignition timing are piston speed, flame front speed (Air Fuel Ratio and charge density), cylinder head and piston top design, and volumetric efficiency (VE).

Piston speed - The speed of the piston moving inside the cylinder. Obviously this is RPM dependant and only RPM dependant. Ignition timing must adjust constantly to account for the speed of the piston increasing constantly as rpm rises. It takes time for a gasoline mixture to burn and while it is burning, the piston is always moving. On top of that, the speed of that piston is always changing.

Cylinder head and piston design - These are beyond our control for the most part, we work with what we've got or we buy/modify to achieve a goal. I can't imagine anyone but Shep and the other single digit guys worrying about this. Most modern cylinder heads are of the fast burn design so there's not much to talk about but I thought it should be mentioned :D

Flame Front Speed - Oh God, here we go. Flame front speed is how fast the gasoline and air mixture burns inside the combustion chamber. Imagine a cloud of gas vaper in the open air. It burns from one side to the other, extremely fast. In a combustion chamber, it burns from the source of the spark to the piston face. The spark plug obviously starts the explosion and the flame front hitting the piston top and pushing it down is what makes us go. So the speed of the flame front determines when peak cylinder pressure is achieved. Air fuel ratio in part determines how fast the gas/air mixture burns. The fastest flame speeds are seen between 11.5:1 and 13.3:1 for gasoline and the speed drops off dramatically on either end. Those numbers represent rich best torque and lean best torque. Today?s mean best torque, sort of an average, is 12.2:1. Adjusting the fuel delivery calls for an adjustment to ignition timing to accommodate the increase or decrease of the flame front speed.
The other variable to flame front speed is the density of the charge. Higher levels of boost cram more air into the combustion chamber for every rev. Assuming the AFR does not vary, the more air, the faster the burn. Many things limit how much air you can push into a combustion chamber. Boost levels being the simplest. If you increase boost, some change to timing is needed. How much, I honestly don?t couldn?t tell you. Also, EVERYTHING from the valve port back to the air filter restricts and directs air into the engine. Getting the exhaust out of the chamber is obviouly a big deal here too. If all of the hot exhaust gasses aren?t expelled, that takes up room that could be used by fresh air for the next combustion cycle. Hence the great power increases we get from free flowing exhaust systems. When we modify these parts, we are increasing the efficiency of the engine to consume more airflow per revolution (This is why some of us get 34+mpg on a highly modified street car; dramatically increased efficiency). Increasing the airflow per rev increases the speed of the flame inside the combustion chamber, there for calls for an adjustment to ignition timing. After all, a denser charge burns faster.

Volumetric Efficiency - The last paragraph started touching on VE, even though I didn't come out and say it. Simple, the amount of air that gets into the CC during one intake cylce compared to the amount of air that could possibly fit in the CC. Our only worry with VE concerning ignition timing goes back to charge density. When piston speeds increase as rpms increase, there is less and less time for the combustion chamber to fill with air. The valves are also opening for a shorter amount of time also limiting the amount of air that can flow through them and into the combustion chamber. VE obviously effects charge density in the sense that as VE decreases, the amount of air entering the engine and your charge density decrease and the speed of the flame front decreases. If you've dynoed your vehicle, you can see where torque peaks, then drops back down. Assuming your boost and AFR levels have stayed the same and your timing is somewhat optimized, this is a good indicator of where VE starts to diminish. Ignition timing needs to adjust as VE starts to drop to accommodate a slower burn. This gives the piston more time to cover the distance from ignition to 15* ATDC before peak pressure is achived.


Ok, this is unorganized and possibly just rambling but I wrote it out without notes and without a draft. It's just what was on my mind. If you want to add to it, please do!! This crap is complicated and the more info the better.

Originally Posted by Shane916

:shock:

great post there shane.....

thanx for your very informative input...

Good Lee!!!

You going to teach a class at AR???


Geometric Progression is fun!!



HAHAHAHA!!

Originally Posted by OldDirtyNoSeven

Good Lee!!!

You going to teach a class at AR???


Geometric Progression is fun!!



HAHAHAHA!!

Sh!t I guess so. No one wants to talk about tuning with me here, maybe my class will lol

Yes.. you should advance timing as your VE starts to diminish.. BUT.. you have to take into consideration knock.. By the time your HP and TQ numbers are starting to drop your winding that engine up pretty high.. And your probably already tuned as close to your knock threshold as you wanna go.. Soo.. to advance timing at that point would you have to reduce boost? Add fuel?? You advance timing too far and the flame front starts to contact the piston crown and heat things up even more.. Where are the most gains to be had?

I never addressed any tuning angles, only the physics of ignition timing. I guess we can start.

you have to take into consideration knock

Hence the term, knock limited engine and the only reason we need to worry about such issues. Old V8 guys can simply advance timing until they start to loose power then back off. Must be nice, they don't have to understand isht.

By the time your HP and TQ numbers are starting to drop...

Yes, HP drops with TQ because HP and TQ are directly related. hp=[t/(ft*lbf)][w/(rev/min)] / 5252 If you're talking about max performance of an engine, you can go ahead and use the term HP. Besides that, it's useless. If you're using a dyno or old dyno sheet to assist in tuning, you're worried about TQ at certain rev/min points. Ignore HP, it's only a calculation. When you're tuned, bring HP back up and brag about it :P

Soo.. to advance timing at that point would you have to reduce boost? Add fuel??

Yes, to achieve optimum ignition timing on a knock limited engine, you must sacrifice your boost level according to Boyle Mariotte's law, or your AFR according to Joseph Black and his theory on latent heat of vaporization. Both obviously have to do with heat in the combustion chamber. The only other option is to use a fuel with more resistance to detonation. When we've got so many availible to us, that's the only way to go IMO. Run 20psi+ on 100+ octane fuel at the track and run low boost on the street. If you race on the street everyday, you've got other issues.

You advance timing too far and the flame front starts to contact the piston crown and heat things up even more..

Obviously, you do NOT want to advance timing to the point where peak cylinder pressure is occurring later than 15* ATDC. Timing can and should be tuned on a loaded dyno which is hard to find. You can hold the engine at one rpm point and advance timing until you're not longer gaining power (if you can reach that point without inducing knock and requiring a higher octane fuel), then back off slightly for a safety net. If you're patient and make many, many runs with very small adjustments, you can tune timing on the street/track with a good EMS. You'll find that same point where you're no longer yielding gains. Unfortunately, anything after that point can and will result in sever engine damage. Fine tuning timing on the street is difficult, dangerous, and not for the faint at heart.

Where are the most gains to be had?

If I knew this, my first post would have been a lot shorter lol

im putting my car on the dyno this saterday ( if plans dont change). obviously these will all be full throttle runs. how would you go about tuning for part throttle and low boost situations where knock is not a problem? i want my car to run perfect all the time, not just under full load situations.