Let's discuss spark advance
 

As some of you know, I recently came into a good deal on an MS3, and so I installed it into my bone stock, naturally-aspirated 1990.

For the initial base-tune, I grabbed an MSQ file from a reputable user here, visually compared the major maps (VE, ignition, etc) to those from a DIY-supplied MSPnP2 map, and judged them to be similar. So I loaded 'em up, hand-tuned the idle a bit, let TunerStudio work its magic on the VE table for a while, and wound up with a tune that's running quite well.

Except that after a tank or two of fuel, I noticed that my fuel economy seemed to be in the shitter as compared to what I'd expect.

One thing I hadn't yet touched was the spark map. I did spend some time fine-tuning the trigger latency, so I know that my actual ignition advance is the same as my commanded advance across the entire RPM range (at least, when the commanded advance is locked to 15°), and I have no reason to suspect that anything is malfunctioning there. I've also doweled the engine, and so I know that my crank pulley is aligned with the crank.

Then I remembered the thread where we were mocking TDR and their "free tuning" thread. And this letter, alleged to be from TDR to Bill Cardell at FM, concerning an analysis of some Dynotronics tunes: Letter To Flying Miata. Re: DP Tune. And FM’s Response…

Specifically, this paragraph:

6.Timing has been cut on the fuel efficiency cruising tables, this causes the car to run low timings at cruise (40’s vs 50’s) decreasing fuel economy and making the car feel slow and heavy

Now, before we go debating who is an asshat and who is not, it got me to thinking. Cruise timing in the 50s? That seems like a lot, but what if it's at least leaning in the right direction.

So here's the spark advance table that I started with, based on copy-and-paste of base tunes from reputable people:

https://www.miataturbo.net/attachmen...1&d=1353963410

Cruise timing in the 30s. The LOW 30s. So over the past few days, I've been gradually adding advance a few degrees at a time. Here's where I stand at the moment:

https://www.miataturbo.net/attachmen...1&d=1353963410

I'm not done yet, nor do I claim that this map is optimal, smooth, useful, etc. But what I do know is that the engine isn't pinging yet, and that's on craptastic 87 octane Californigas. And based entirely on the seat-of-pants dyno, I'm pretty sure that the engine is making quite a lot more power at half-to-full throttle. It's definitely climbing hills faster than it was a week ago.

What's frustrating me is that I'm having a dickens of a time finding any hard data on this topic. I've come up with tons of reference material discussing the underlying theory of ignition advance vs. BMEP, and a lot of guys disagreeing about where to set the static timing on the vacuum-advance distributors of their carbureted V8 engines, but absolutely nothing to the effect of "We put this B-series Mazda engine (or any similar 16v 4 cyl engine) on the dyno, and found that best efficiency at 3,000 RPM at 50% throttle was achieved with X amount of spark advance, and that at 100% throttle, the engine was detonation-limited to Y amount of spark advance."


Am I really the only person who is curious about this apparent dichotomy?

are you reducing fuel like crazy when you're advancing that far?

On my 9:1 engine I found that 34* of advance on 87 octane in cruise was optimal. on the 10:1 engine in 31* of advance. I tried going further and found detonation, took away fuel and detonation stayed, added fuel and detonation became more severe. I've found that I can get 34mpg at 80mph in my 10:1 engine with the headlights up on the attached MSQ.

I can't upload my VVTuner table because it's the wrong extension.

My system to determine peak fuel efficiency:

1. Place brick on throttle
2. adjust spark
3. adjust fuel
4. Does KPA raise or fall?

If I have time on the dyno on 12/15, I'm going to play with this and see what I can find. Considering my current phenomenal MPG numbers, I expect to find no improvement. I think a piston that displaced combustible air to the exhaust side of the chamber would yield a significant increase in fuel economy.

I agree with you, there is virtually no discussion of fuel economy tuning on the interweb.

I'm not sure I understand the reasoning here. Reducing fuel "like crazy" would tend to lower the detonation threshold at WOT, would it not?

I am running closed-loop across the entire map, targeting 14.7 in all conditions except the very top row, where I'm in the mid 13s. I could probably go a tad richer in that zone, though for the moment I've been concentrating mostly in the non-WOT zone, as that's where the car runs during an emissions test.

This isn't just about fuel economy, it also about torque. Although I have no dyno numbers, I do live in a very hilly area. It's quite obvious that power has been significantly improved at (and near) WOT by running roughly 10° more advance than the baseline map I started with. (Again- naturally aspirated, stock C/R, 87 octane.)

When I have a free moment, I will pull the valve cover and visually inspect the cam positioning just to be absolutely certain that it's correct, however I have no reason to suspect that it is not.

Yes, but also lengthen burn duration, creating realizing more output by approaching LPP at the given AFR and piston speed.

Hustler, how do you tune part-throttle areas with your system?

I really want more mechanical octane in my life.

Brick on the throttle then load the dyno to hit the RPM cell I want. I want to make a boat-throttle cable adapter so I can sit at the dyno computer, outside of the car and do it all there. Man, that would be so accurate.

I basically tuned cruise in my car out of boredom and it took 100x longer than it could have at the dyno.

Joe, was your bone stock car running stock ECU with EGR?

Can you compare cruise throttle angles before and after the MS3 swap? Did the throttle angle increase or decrease?

did 90's come with egr?

Ah, I see where you're going.

As I said, I'm targeting 14.7:1 in most of the map, which I ASSUME would also have been true of every one's basemaps. (Why would you want to run richer than 14.7:1 in non-WOT cruise?)

At WOT, I am running in the low to mid 13s. Depending on which textbook you believe, this is either exactly at or slightly leaner than the optimal mixture for maximum flame-front velocity. I can certainly richen the top row and see if it starts knocking. But at the moment, I can floor it at 2,000 RPM in 5th and not get any knock.

EGR was not introduced until 1994. So no, I have never had EGR on this car.

Until installing the MS3, I had neither an analog TPS nor an accurate means of logging anything. I could certainly cobble together a parallel harness to treat the MS3 as a datalogger and re-install the stock ECU if it is felt that this would yield statistically significant data, though I must admit that my desire to do that is not great.

Would you mind doing this with your original map, vs current?

aren't there OEM ecu maps floating around somewhere?

at least I thought I saw some posted some time ago. you could glance at those and get a reference point?

This is something I can probably whip together pretty easily. I will do that tonight.

Wasn't sure on EGR.

OK so I was curious because some efficiency gains can be had by running at higher throttle position for the same drive torque (which is the case with EGR--which you don't have). Pumping losses decrease relative to the work of the air/fuel charge. But you still need optimal timing to provide the best power for the fuel injected or you're just wasting part of the squirt.

Exactly.

Assuming that my target AFR must be 14.7:1, ignition advance is really the only variable that I have control over.

But this still doesn't answer the underlying question, which is why my engine seems to like more advance than most of the basemaps I am seeing for '90-'93 Miatas, given that those engines all operate under the same constraints as mine.

I was running 15.5 AFRs for economy. I haven't a clue what to do about the cruise timing. I'm paying attention, though.

It's not that simple, it's more dynamic. AFR, spark angle, and piston speed must all be considered when looking for the minimum duty cycle to sustain speed at a given MAP. For simplicity sake, keep piston speed or RPM constant and adjust AFR and spark angle to find the lowest overall PW or duty cycle. It's easy as hell to tune for power and drivability, tuning fuel economy is an art.