Revisiting spark advance to optimize ...NEED SMART EYEBALLS....

[Spittybug]

Back from our little trip to New Braunfels for Wurstfest so I've now played with my model a bit.

• 13* fixed advance
• Vacuum advance cuts out with closed throttle (idle and decel)
• Added Hg inches (gauge) for reference to kPa for engine load
• Broke out the 3 separate sources of advance into separate tables to see effects (centrifugal advance (RPM based), vacuum advance (load based) and static)
• Set centrifugal advance to same as manual - all in by 4000 RPM, not 3000. Unknown if linear, most springs are not.

Does this look more reasonable? I know that just a couple of degrees of timing can have a 5% or so impact on HP, so before spending big $ on dyno runs I would like to get as close as I can. The values I'm actually running aren't far from this.....

[FABombjoy]

Back from our little trip to New Braunfels for Wurstfest so I've now played with my model a bit.

• Vacuum advance cuts out with closed throttle (idle and decel)

What RPM/sync strategy would retain the vacuum advance canister? If the distributor VR is RPM input it should be fully locked out.

You will feel that 20 degree border between 45/50 kPa - that's a light/medium load cruise area and any large timing transitions will be evident.

Assuming a 5-spd, between 30-40 MPH, and around 60 MPH (depending on gear & engine load) it may feel like the car is pulsing at times.

[Spittybug]

My distributor has locked weights and no vacuum advance unit attached. The goal of all this is to build the advance table based on the 3 part logic of a stock system. From there, it can be enhanced to improve upon stock.

I hear you about the +20* being felt. What are you suggesting instead, lower the kPa value at which the vacuum is cut off except at idle?

[FABombjoy]

Ahh, I see. Problem is that vac advance cutoff is more a function of TPS than MAP. You could bake it in to the advance table but you'd have to analyze your logs in a higher gear to see what your min kPa / RPM at low TPS turns out to be.

I would just render the table to match stock, set the idle zone to 13 w/ anti stall values around 5-600rpm, then adjust the decel range to encourage a return to idle if the engine wants to hang. Maybe drop the MIN function to remove the rounding, unless MS2 doesn't allow decimal values in the spark table.

I use the idle advance correction too, which gives a very steady idle (target +/- 40RPM) and helps alleviate the motors tendency to stick in higher RPM.

[Spittybug]

Problem is that vac advance cutoff is more a function of TPS than MAP.

Agreed. TP leads to MAP, so the challenge is to find the correct relationship.

What if set to idle advance on RPMs, set the table to 5000, 4000,3000, 2000 and 1000, -20* advance adder at all RPMs, condition manual, TPS below ~2%, RPM below 6000, load above ~5 kPa, coolant above 'warm' and time delay as short as it will go.... This *should* key the retard to the TP being essentially closed, not matter what RPM you are at, shouldn't it? Load is oddly stated as % rather than kPa in the dialogue window though... And I don't know if the time delay can be less than 1 second.

An interesting test.

[FABombjoy]

Some thoughts:

The original thermovalve/solenoid circuit solves a problem. Instead of emulating the stock solution, could this problem be solved with a different approach?
-Can you "light the cat" in a different way?
-Have catalysts changed since then, and is this still necessary if aftermarket cat?
-Did it really make a significant difference in cold emissions?
-How did OEM Volvo / Renault / Peugeot handle it? How did they do it in 1983? 84?
-Non-cat cars would not benefit from the added complexity

If you try your test I'd be curious to hear what it does.

[Spittybug]

I think you lost me there with the warmup stuff....

I'm not worried about the emissions angle of all of this. I have aftermarket cats & exhausts. The stock system didn't vacuum advance under two conditions; closed throttle and low engine temp, right? The engine temp condition was for cat warm up reasons, but the closed throttle reason is based on the resulting cylinder charge density and flame front propagation through it. A less dense charge needs the spark sent sooner so that it can propagate through the fewer molecules in the same amount of time. This goes away under more load since the air and fuel are rushing into the same volume much quicker and easier. Dense pack makes for easy combustion.

I'm simply looking for the best way to link the advance to the two factors that determine what it should be; RPM and load. The RPM one is easy since the spark advance table can change the amount of advance as the RPMs change (x axis). The only other linkable independent variable is load (Y axis) expressed in kPa. The problem as you pointed out a couple of posts ago is that this isn't truly an independent variable; it is actually dependent on the throttle position; the true independent variable. I can do some experimenting and using graphs like you posted determine the kPa values associated with idle and closed throttle decel, but it would be a better deal to be able to directly associate the advance to the throttle position itself. Hence the idea of using the Idle Advance Settings to retard the spark under closed throttle condition. I really only want to use the throttle position condition (not worried about load, current RPMs or temp), but if 1 second delay is the shortest available, that won't work I don't think. The added benefit would be to use the temp condition if this helped with emissions when cold.....

Sorry if I've restated the obvious, sometime I need to write things down to get them straight in my head. I'm also sure that some others playing at home are mystified by the nuances of timing and will see just how much flexibility EFI control can give them should they go that route.

[FABombjoy]

The stock system didn't vacuum advance under two conditions; closed throttle and low engine temp, right?

Right - cold cutoff for emissions, and low throttle cutoff for idle. Without removing vac advance at idle you'll never get the RPM under 2000.

I wouldn't worry too much about deviating from the stock emulated map in those low load / deceleration zones. I've never seen any evidence of misfire in my logs and my exhaust isn't poppy on decel. If you can get overrun fuel cut tuned up you won't even have a charge to burn half the time.

I'm actually running 2 different spark maps ATM. I found an MS bug if idle advance is enabled at startup: RPM target variable initialized to 0 instead of target RPM until closed-loop idle is first enabled. This causes an engine bog on startup for about 2-3 seconds. I'd fix it myself if they'd publish the damn source code.

In the interim, I created a second spark table that's active at TPS < 2% and has an exaggerated low timing ramp to transition the idle smoothly when rolling up to a stop. Once the bug is fixed I'll remove the table switch. But you could do the same if you have that kind of tableswitching in MS2.

[Spittybug]

I just realized that what I wrote didn't make sense..... why DOES the vacuum advance want to be cut under decel? Logic would say that it should WANT to be advanced because of the loose packing of the molecules. I'm guessing it is because we don't want 'peak horsepower' during decel and are deliberately sub optimizing burn? It's obvious we don't want it advance during cranking as this would make it very hard to start.

I suspect you are correct about using fuel cut on decel instead of timing.

Can you share your spark tables that you mention? I realize your boosted areas are not applicable to me, NA engine, I'm just more interested in seeing the 'smoothness' of your transitions and how quickly you bring in the RPM based advance.

Thanks!

[FABombjoy]

https://imgur.com/a/smVA9yT

Left is the < 2% TPS map. Most of the time its inactive so some of the values may not match up to the right. So far this seems to work well without any sense of oscillation and transition to idle seems smooth.

Using ported vacuum for advance was the best option based on the tech of the time. I don't think it was done intentionally for decel but was done for idle. The only way to make it work for idle meant retarding timing under decel at very low throttle. You really don't want to pull timing there unless you intentionally want a crackly exhaust note or to shoot fireballs.