The Slant is back, and working well enough to begin setting the timing and fuel maps. I jsut read Doc's post on timing, but I would like to get into this a little deeper, and discuss timing theory, and how it effects the cars performance, idle, emissions, transition from one speed, or throttle setting to another etc.

Setting up a distributor like Doc describes in an earlier post, sets the stage for alot of unseen modulation to take place, and it is the in between times that I am interested in. Let's assume the engine idles best at 8 BTDC. And let's assume the vacuum pot pulls in another 18 total. So now we have 26 degrees. And then assume 15 in the centrifigal, that starts at 1300, and works in total at 2200 RPM. This would put you at 41 total at 2200 RPM. That is all fine. But what happens when you step on the throttle, and the vacuum drops part way out. Let's say it goes to 9 inches mercury. That will leave some vacuum advance in, but how much. And when does the vacuum advance start to add in advance as you accelerate. If the vacuum drops below 8 inches, in theory, it adds nothing, but the centrifigal does, and then so does the vacuum advance as it comes on line as the vacuum rises.

The thing is, if you are programing a spark map in an ECU, then you have to try and figure out what the advance looks like across the entire operatin range. It seems as if duplicating the old mechanical set up would be a good starting place, if you could do the interpolation of how all that stuff worked together.

How do shifts in timing affect the engine. How big a jump can you make from RPM range to RPM range without getting a hickup in the transition. Of course the spark map is interpolative, and it does the math, and works out an average as it moves from one cell to another. It does not just jump up all at once.

Any ideas you want to toss around? I would like to hear them.
Sam

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Vacuum advance cans are pretty linear.....

They usually have specs like:

starts @ 5"
10 degrees advance (can either be distributor or engine)
all in @ 15"

So in the above example you'd have 5 degrees @ 10"......

The adjustable cans move the range so you can have 6-16" 7-17"....... and there's even some cans that have limiters so you can limit them...

(but remember the slope stays the same. So if you limited the first example to 5 degrees, you'd get that at 10"......)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

But, if you hook the vacuum pot up to ported vacuum, then the vacuum advance does not show until you begin to see vacuum above the throttle blades. When does this happen? At what RPM does it happen? How can I relate this to the timing map/matrix of EFI. I ask this, because we are pretty sure the vacuum pots work well enough, and might make a decent model to follow when creating a map.

Based on Doc's advice I set the cells that the engine lives in during idle at 10 degrees. The default calibration from Accel had them at 17 degrees, and the car would not idle down enough like that. I left the other cells with lower vacuum readings, and boost at 17. I subsequently discovered while driving the Dart after that, this evening, this extra 7 degrees in the lower vacuum cells created serious detonatation. While drifting at 1000 RPM in high gear, if I steped on the throttle, it would pre-ignite like crazy. It sounded like a can of marbles. If any cell in the 1000 to 1250 RPM range has any value above the base amount; regardless of the map reading (vacuum or boost) the engine is seeing, it is a candidate for pre-ignition. I am wondering if it would hurt anything to actually retard the timing maybe 5 degrees in the low vacuum cells at 1000 RPM. I have set these cells back to 10 degrees, for now, but have not tried it as of yet.

I intend to take each operating range of the engine and play with it in this regard. Hopefully, I will see a gradual improvement accross the entire operating range. I hope you don;t get tired of my feedback here, and I hope those with opinions will speak up.

Sam


Sam

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Ported vacuum would be throttle position dependent, not rpm....

With a TPS, you could make some rule up that vacuum is 0% at X degrees and 100% at Y degrees. Make it linear and multiple the manifold vacuum by the % you obtain above to simulate the vacuum a vacuum can would get. Then simulate the vacuum can.

Not sure what X and Y would be, you need to look at a carb to make a guess on when you think the port is exposed enough to see manifold vacuum

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

But when the maifold vacuum drops, as in, when the throttle is opened, the ported vacuum would not see vacuum. The vacuum would then build as the RPM built, (would it not?), given the same throttle postion. This has alwasy been my understanding, anyway. So, When you first open the throttle, the dist. vacuum pot would not see vacuum. Or, when you come to a hill, and lay into the throttle a little, two things happen, the power valve opens, and the timing drops out as the vacuum on the canister drops. This is what the fuel and spark map need to emulate, with more complete control of course. You can be much more precise about how much and when.

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Forgive me, I did not read through all your posts. The spark map you are working with - is a table of spark vs what? Vacuum, MAP, Load? I can email to you a few OEM spark tables used in Ford 4.6s. The primary factor that changes spark requirement on any engine is head design. There is a lot that goes into calculating spark at the OEM level, but you need to know two things: what spark will give you maximum brake torque (MBT) and what spark will give you borderline knock. The OEMs determine this for every engine in a lab. The best you can do is determine what gives you WOT max power on a dyno and this will approximate MBT. You will have to construct the rest of your spark map using trial and error. If you have unlimited octane, then borderline knock will equal MBT. At light loads, you can run relatively high spark which for your engine will typically be in the neighborhood of >40 degrees. With your ECU, can you modify spark based on coolant temp, intake air temp, air fuel ratio? Is this ECU designed to use an oxygen sensor? Do you have any kind of closed loop idle control of air, fuel and spark?

Mitch

Mitch, Thank you very much. This is the kind of info I am looking for. I would very much like to recieve any helpfull information you have. The Accel ECU is very powerfull, and very flixible, and has pretty much all of the OEM style trim table; everything you mentioned plus many more. I simply don't know how to use them yet. Please share more if you have time.
Sam

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I'll send you a few screen shots of different things. There's a lot more to tuning than most people think. It is easy to put a car on a dyno and tune it to make max power, but quite another thing to make it start, run and drive well... and make it last. Does your ECU give you the ability to datalog? Is this a fuel injection setup?

FYI, there are about 4000 - 5000 different calibrations in a typical OEM computer.

Mitch

Am I to understand that the timing is always just below the point of pre-ignition at all times? Does the mean you can dial in the timing until you get preignition, and then take out a few degrees? Yes, this is MPI, staged batch fire. Thanks for the offer. I am looking forward to learning more.
Sam

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One thing I've picked up.......... (meaning I don't know where the source is anymore)

Get your mixture correct before working on timing.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

You certainly cannot dial in timing without getting the mixtures right, but a fuel map that is drivable, and in the ball park should be possible as a starting point, don;t you think?

Sam

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Mixture and timing maps will be interdependent, so changing one will affect optimizing the other for best economy, driveability, and power.

I would start with a timing map that should be close, based on what you know from this group, then optimize mixture, then go back and play with timing again.

Lou

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Home of Slant6-powered fun machines since 1988

No - often the point where it makes peak power is a bit before the point of pre-ignition, and some engines the "advance it till it pings, then back off" tuning method will make less power. The reason is that too much advance can make the pressure go up on the compression stroke and reduce power even if it isn't having pinging issues. I've spoken to one tuner who had been dealing with a very detonation resistant engine and reported that the best power was way before where it started pinging - something like 10 degrees or more before, in fact. I think that was something crazy like a naturally aspirated engine with 9:1 compression on methanol.

The best way to optimize the ignition timing in the lower load regions (sometimes you can't hold an engine steady at full throttle to do this) is with a steady state chassis dyno. Set it to hold a given speed and keep the throttle at the same load point (either MAF or MAP reading), and adjust the corresponding cell up until the power starts falling, then back it off to the point where the power first peaked. That's the method they teach in the EFI 101 tuning classes, and what we've used to tune a couple of cars at work.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Thanks for the tips. Mitch, I can data log. Do you have a specific protocol that I could follow to work around here? Matt, how 'bout I drive down to Georgia and get a dyno tune session from you there. Are you working in a shop now that has a dyno?

Is there any way to feel horsepower in the "seat of your pants" while driving? What could you look for short of putting the car on a dyno. If you have a co-pilot running the laptop, what would indicate the need for more or less timing, and could you feel anything before preignition happened that would tip you off that you had advanced the timing too far. I know the fuel cells must be worked yet, but am just thinking off into the future here. I can see that without the steady state nature of the dyno, things would simply move out of any one cell to fast to get a handle on it.
Sam

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Afraid not - I haven't done dyno work since taking the EFI 101 class; it's usually my boss Jerry who does the dyno tuning, and we have that done at a Subaru dealer (!). I will be personally tuning my Dart there once it's back on the road, however.

About the only thing you could go by in a data log is the rate of RPM change combined with what gear you are in. It's not too hard to dial in the fuel part on the street since you can use a wideband sensor for feedback. But street tuning an ignition system is a nightmare.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair