View Full Version : Spark vs. VE vs. too cheap to buy dyno time
09-10-2005, 03:11 PM
Hey gang. Short of going to a chassis dyno which is of course the right way to determine spark advance... would you say that the timing that requires the least fuel to get a given AFR for given bin is optimal?
I've been running my Kenny-Kopied spark map from his short lived 8V 531. Much was made of its superior efficiency, so that got me thinking, if I've got a stock plain jain B230FT, I probably need more timing.
So - I advanced my cruise timing from 52 (3000RPM, 60-90Kpa) to 62. Running a short DL from a cool day through Vexme got me the following results:
Kpa Old New
60 61 58
70 61 59
80 66 63
90 68 64
Advancing the timing got me a reduction in VE of 2 to 4 for the same AFR. Less fuel = same "output" = more efficient?
Whattya think? I'm always trying to come up with a workaround way to nail down timing.
09-10-2005, 04:16 PM
Are you talking about purely off boost economy tuning? Hurm, it's an interesting concept. No doubt that changes in the ignition map will change AFR, but I don't think it's that that much is it? More advance means the burn starts sooner, and lasts longer than it would with less advance. Wouldn't burning longer require more fuel? Your theory states that it requires less fuel, which doesn't make much sense, at least not at first glance.
Eh, I'm not in the right mood to think any more about this, but I hope other people chime in with more advice. More efficiency off boost is something that every car can benefit from.
09-10-2005, 05:00 PM
Spark has little to do with VE. Let me rephrase that: You won't be able to tell the optimal spark advance by tuning it for the lowest VE entry in MS. That would suggest that spark advance alters AFR, which it doesn't. At least, not the way you'd hope it to. A misfire or incomplete burn will give a lean reading, for example, but moving the spark around 5-10 degrees won't do a thing for the actual AFR.
Typically when the engine is most efficient VE-wise, it requires less spark, but that's about as far as you're going to get by studying the VE table. Less VE = less cylinder filling = slower burn = more advance needed.
09-10-2005, 05:04 PM
Actually, thinking about it for a few minutes longer, altering the spark MIGHT alter the VE, IF it alters the scavenging of the exhaust header. Since I doubt this is a consideration in a turbo, I stand by my earlier statement that any changes to spark you make will be too hard to diagnose in the VE table, and it's useless to try.
Base your spark requirements on your VE table, weighing advance against VE, but don't try tuning your spark by aiming for the lowest VE for a particular AFR.
09-11-2005, 11:38 AM
Thanks guys. Makes more sense after I've slept on it. There's no measure of torque involved in what I've done, so all I've done is add less fuel and adjusted timing to keep the AFR where I want it. Whos to say if that's the optimal timing. :e-shrug:
//edit - alternate translation - maybe my changes have me cruising at 80Kpa instead of 70Kpa, after removing fuel and power from 70.
09-11-2005, 01:08 PM
if the correct adv. burns the entire charge there will be less o2 in the exhaust
09-11-2005, 04:15 PM
An earlier advance will always burn the fuel more completely in the chamber. It might also detonate, and it still hasn't changed the air/fuel ratio. In addition, it should continue to burn until the combustion event is finished - just because the exhaust valve is open doesn't mean it stops burning, and with a wideband being so far downstream it's surely not still burning by the time it hits the sensor.
09-11-2005, 04:23 PM
the O2 reading is a percentage of gas content post turbo. It keeps burning as matt says why do you think pulseairs work? ;-) Otherwise o2 readings would be totally redundant. Timing also has very little effect on CO and HC unless it's so retarded the manifold can't burn all of the fuel (or you create a miss). That's also why egt's skyrocket when timing is over-retarded.
Very important concept to wrap one's head around.
edit: also as Matt said the actual ability of spark timing to modify ve is very, very small, especially on a turbo where scavenging is limited/arguably nonexistent. Even in the case of a motor that depends largely on manifold resonance- the resonance timing is dictated far more by cam event timing that combustion timing. The pulses will exist as long as there is combustion pressure, they might vary in intensity a tiny bit, but we're talking a MAJOR timing change for that to be significant.
09-12-2005, 12:08 PM
Thanks guys. It appears that I'm running higher Kpas than before, so my timing increases must have reduced power. (Haven't changed fuel yet, so it's running a touch richer than before, ruling out being "too lean").
The good news, at least, is that I can run 62* timing. Before revising my TA, the highest I could make it run was 37*.
Time to wrangle myself some dyno time.
//edit - It keeps burning... Timing has very little effect on CO and HC unless it's so retarded the manifold can't burn all of the fuel
I think that is the concept I hadn't clued in on; the fuel/air, once lit, will keep burning until it's all gone, or as gone as it's going to get...
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