Digital ignition curve

[408swinger]

to clarify... would advancing or retarding timing just off idle enhance driveabilty , improve idle or reduce engine heat etc...
Centrifugal advance can only advance but digital allows you to do creative things.

[groberts101]

With advice like that I'd be looking around for new "ignition guys". I have used and will continue using msd's programmable 6 and 7 series boxes on all my stuff. Everyone who's taken my advice and moved away from the ole' vac pot arrangement towards map based programmable stuff wishes the'd made the switch long ago.

To answer your question.. no the curve doesn't need to be linear.. only based on load and what cylinder pressure the fuel will allow. Cool the intake charge and you can push the tune harder without the negatives. More aggressive idle timing increases manifold vacuum, makes the carb easier and more responsive to the tuneup and also cools the motor at idle.

For instance.. most of my tunes over(some well over) 11:1 scr and only moderate/midrange cams(relatively high dcr) running street gears and full interiors start with REALLY fat timing numbers(50 or more) at idle and quickly bleed off cylinder pressures by retarding upwards of 8-12 degrees at around 1,700-2,000 rpm. From there you just progressively pull timing for either WOT torque output(which is still one and the same as cylinder pressure).. or for best in-car manifold vacuum gauge readings(which again denotes cylinder pressure and overall engine efficiency levels) during light to moderate throttle angles. MAP's really shine here as the vac pot is always a futile exercise of robbing peter to pay paul.

The thing most people get caught up on and end up giving away throttle response power and efficiency over is getting too fixated on the timing numbers an engine wants or prefers to make max torque or peak horsepower. Far far more to it than that when you consider an engine spends very little time at each peak and it takes time to get there. It really all boils down to mixture density and squish velocities when it comes time to dial in ignition timing for any particular load range and piston speed. So what may be perfect timing for max torque at say 4,000 rpm don't mean squat when an engine begins it's acceleration curve from idle rpm. Even at WOT fuel molecules are very sparsly packed into the chamber and mixture velocity is maybe 1/10th of what will be coming into the picture at 4, 5, or 6,000 rpm.

So ideally the digital curve is a reverse slope with timing being pulled as density and mixture speed increases within the cylinder. What you will also find is that all the extra timing during initial engine acceleration will gobble up more fuel and require larger faster pump shots, which of course also increases throttle response and hits the converter and tire harder.

[408swinger]

"So ideally the digital curve is a reverse slope with timing being pulled as density and mixture speed increases within the cylinder. What you will also find is that all the extra timing during initial engine acceleration will gobble up more fuel and require larger faster pump shots, which of course also increases throttle response and hits the converter and tire harder"

Are you suggesting this to make the car more driveable ?

[groberts101]

"So ideally the digital curve is a reverse slope with timing being pulled as density and mixture speed increases within the cylinder. What you will also find is that all the extra timing during initial engine acceleration will gobble up more fuel and require larger faster pump shots, which of course also increases throttle response and hits the converter and tire harder"

Are you suggesting this to make the car more driveable ?

That's just another byproduct of maximizing idle/light cruise manifold vacuum because it denotes efficiency.

I build ALL my stuff.. street or race only.. to maximize power at idle to past redline. I do not detune an engine simply because "it already spins the tires too easy as it is". Aside from maybe a dedicated bracket racer where utmost consistency is key, I prefer to make the engine "be all it can be".. then tackle traction or driveability issues as completely separate issues altogether. Or IMHO, simply learn how to pedal the tire against the tach like a good driver should be doing anyways rather than purposely hamstringing the motor like some kind of poor-mans traction control.

I personally spend my hard earned cash to make a motor as powerful as possible.. and why people choose not to tune for what the fuel will allow(+ some extra ambient temp and passenger weight margins for good measure) just blows my mind most of the time.

[408swinger]

Seeing as how its digital I can have 2 maps. 1 for street driveability and 1 for track. Not too hard to change once you have figured out each one.

[BigBlockMopar]

I always run with 2 timing tables just to try things on one table and compare it to the other while driving and see which performs better.

Engine rpm and load-ranges at the street are not (quite) the same as at the track (circuit or dragstrip).
So, IMO, unless you run a different fuel at the track, the street and track tune can be both inside 1 timing table.