The following is my understanding of how engine tuning should be carried out. I by no means am a wizard of engine tuning, but here is what I have gathered from 2 years of high school automotive, and discussing engines with my Mechanical Engineering professor about F.I. and N.A. IC engines, as well as the Mechanical Engineering director for the Mini Baja Collegiate racecar team at my local college, and talking with the Roanoke automotive machine shop technicians. A lot of this theory also comes from Jeff Hartman’s book, “Engine Management Systems.” If some of the info seem is elementary, I’m not saying it to insult you, (which is how a lot of people interpret things here at G2ic), but rather I’m just putting it out there because it raises a valid point.
Where I would personally start in configuring the ignition timing, is making sure that I am using gasoline that would give me room for early tuning mistakes. (I.e, 112 octane gasoline. It’s about $6.90 per gallon here at Virginia International Raceway.)
Now, tuning the ignition timing, I believe should be looked at in conjunction with Volumetric Efficiency, the Air/Fuel ratio, boost PSI, elevation, ambient air temperature, a well as other factors. Then computing the values in useful equations is the next step.
Now for a little review: “The most important step in tuning an engine is establishing the required ignition advance. An engine with too much timing will detonate, regardless of how much fuel is thrown at it. An engine with too little timing will perform poorly and overheat the exhaust in short order. If the exhaust headers are glowing, you know there is too little timing; if the engine is knocking, timing late may be too advanced.
A critical overarching requirement, however is to also avoid engine damage by delaying light-off to the degree required to avoid having unburned portions of the charge explode as combustion pressure and heat build during the burn (as the piston may still be compressing the mixture.) The optimal amount if timing varies INVERSLEY with volumetric efficiency because the denser mixtures burn faster and require less lead-time to achieve the 15-degree peak.
Therefore, engines need more advance at low-load, narrow throttle settings when VE is poor. As the throttle opens and the engine speeds up toward peak torque-peak VE, combustion speeds up along with VE, and thus requires LESS spark advance.
If the engine is turbocharged, the onset of boost increases the effective compression ratio, and combustion speeds up dramatically, requiring less timing, and simultaneously increases the risk of detonation. Maximum timing at 1.0 bar boost is typically in the range of 23 degrees at ay engine speed, and 2.0 bar boost might only want 22 or even 21 degrees of timing advance. All things being equal, bigger engines need more timing advance because it takes the flame longer to burn it’s way across a large bore than a smaller one. Engines with small combustion chambers [like the b18] need less timing. (Opposite reason.)
If you were to make a table of optimal spark timing, the advance values will correlate highly to the VE of the engine and take into account time per degrees of engine rotation at various speeds, since denser charge mixtures at points of higher volumetric efficiency burn faster and require less spark timing.”
The high-octane starting point gives the tuner a larger range of settings to play with. The octane rating not only affects the BTU values, but also the fuel burn rate. “Special cameras recording the combustion event through a porthole in a cylinder reveal that flame-front speeds for gasoline-air mixture vary from 20 feet per second to more than 150 feet per second, depending on air/fuel ratio, density, compression ratio, turbulence among the exhaust gases, an combustion chamber design. Flame speed is fastest at rich mixtures near 11.1:1, falling off dramatically in both the rich and lean direction from this point (especially in the rich direction). The slower the flame front, the greater the chance of abnormal combustion.”
“Start with a timing map KNOWN to be safely on the rich side. Even then you want to gradually “sneak up” on boosted conditions, while analyzing the data, and correcting encountered speed loading points, and points beyond them at higher boost you have not yet actually encountered when timing and air/fuel ratios are headed in the wrong direction (one ting you can count on is that higher boost levels will NOT want less fuel and more timing.)” Jeff Hartman, EMS Tuning 101, Ch 13.
I hope that this was a helpful explanation. I definitely recommend the Engine Management Systems book, as a lot of the above material was taken from the Chapter 13, EMS Tuning 101.
I’m sure others might have constructive comments about flame speed and ignition timing curves on FI engines.
Happy Boosting,
-Andrew