Hello! Another technical thread in which you, dear reader, can help me out by correcting my misunderstandings.

This week - ignition advance.

Note: This discussion is only about crank speeds BELOW 1000rpm. I do not care how much you wish to share on knock sensing, MBT, or advanced advance systems - start your own thread if you wish to discuss this

The problem: We use an Omex 200 ECU on our car, which is, essentially, just an ignition map, without correction for temperature or other clever stuff, and as the car is on carbs for power reasons it doesn't do any fuelling either. It was set up on a rolling road, and has not given any problems at all whilst driving.

However, a few times it has been hard to start - especially when the engine is hot or when the battery voltages are a bit low late in a weekend, and I suspect this might be because of (or at least hindered by) the ignition mapping.

When the car was set up it was programmed with (from memory) about 20° ignition advance at cranking speeds (400rpm), perhaps a little bit more with more throttle. This is opposite to what I thought was correct, as with lower engine speeds you want less advance so that the flame front and peak cylinder pressures occur slightly after TDC, and therefore run the engine. The engine, when it's hard to start, coughs and backfires, and I suspect the combustion is occuring too early at such low crank speeds.

However, the spanner in the works is that on the rolling road it's difficulty starting was noticed, and advance was INCREASED to rectify it, and at the time it seemed to work.

Thus I am hoping that some people here understand ignition and combustion at a complex enough level to not only tell me which 'way' is correct, but also why (the why is very important, and without it I'll probably ignore your answer as being without any substance).

My feeling is that because of the low crank speeds, and the finite combustion time, LESS advance would help regardless of engine temperature (hot starting) or throttle opening (sometimes starting at full throttle will help clear any flooding). So I have changed from 20° to 8° but have yet to try it as a few engine sensors are currently removed. However, my testing could only cover a tiny range of real life situations, because it's hard to simulate stalling after a spin on a hot summers day and flooding the engine, and then desperately cranking the engine whilst madly playing with the throttle as the driver's presence of mind fails him.

well

I know that the more advance the engine has, the more difficult it becomes for the engine to turn over while cranking. Too much advance, and it can kick back on the starter, or cause it to get caught up and then turn, slow down, then crank some more etc...terrible way to describe it, but trying to be more descriptive than just saying the starter is laboring to turn the engine over more than usual.

If the engine is flooded, the best thing to do is to hold the throttle open and crank, avoid pumping the gas as that just makes it worse. (meh, read the end of your post before reading the bit where you said the same thing I just did here)

I also believe that less spark timing would be easier to start the engine, seeing as too much advance will bring it to almost pre-ignition time frame, and will make it hard to turn over.

I'd say go do some doughnuts and get it nice and hot and stall it and give it a go. Giving it more spark advance really does not seem logical, and possibly has something to do with being flooded. If you had your foot in the gas to clear the flood, then possibly it was advancing the throttle due to a throttle position sensor or something?

Quote :

When the car was set up it was programmed with (from memory) about 20° ignition advance at cranking speeds (400rpm), perhaps a little bit more with more throttle. This is opposite to what I thought was correct, as with lower engine speeds you want less advance so that the flame front and peak cylinder pressures occur slightly after TDC, and therefore run the engine. The engine, when it's hard to start, coughs and backfires, and I suspect the combustion is occuring too early at such low crank speeds.

if that is true; then that could be the reason in the low RPM "high" load you would take away timing for the reasons said. so if the person who set it up added timing that could be the reason. I have a high Hp NA car 250hp 2.0 liter and thats the way its set up; cranking is around 18 degrees and at 50% throttle is pulled back to 6-5 degrees.

out of curiosity, what is the ECU using to change the spark table? Is it simply using engine rpm, or does it have other sensors? I'm curious what it used when you said it added spark during cranking.

I talked to my dad about it today, because I am curious about this as well. 20 degrees spark during cranking is not too much, but maybe a tad towards the high side. However, its no where near too high, and you should really never have any trouble starting because of it. Higher spark during cranking does add extra load to the starter however, and if you can calibrate it to start with less spark, you might be better off to do so.

I guess to figure out why it was having difficulty starting/why it would be remedied by more spark timing we need to know what sensors told it to increase the timing.

The only inputs are throttle position sensor (load) and rpm.

I've spoken to the person who mapped it, and I'm trying to speak to Omex, who make the ECU, but I've already put the cranking ignition advance at 8° regardless of throttle. Once it fires and idles (at about 1500rpm) it uses the original mapping where it works fine.

At the moment the gearbox is in bits (changing bolts on the output flanges and both inner CV joints, which requires the removal of the differential), the front brake discs have been sent away for copying, and the oil pressure sensor is away being replaced under warranty, so I can't start the car up at the moment to see if it's easier to start.

Just in case I've made ECU maps with cranking angles of 20, 10, 8 and 5 degrees (all regardless of throttle position) so I can quickly load a new one in...

Fun and games!

When cars first started ECU's, but still had carbs on them, they would revert back to the original ignition timing that is pre-programmed into the car for startup. This is the base timing that you use to set the timing on the car at idle. Once it fires it goes back into the normal ignition operation.

If you had it set up before that the TPS was affecting the timing during cranking, it possibly saw a lot of throttle, and very little engine load (cranking speed), and cut the timing way back.

If the timing wasn't locked out during cranking before, I think you're definitely on the right path now.

It is logical that when the engine is hot, it’s hard to start with such high advance in low revs, for the reasons you have already mentioned. With already high temperature in the cylinders (after a spin that causes a stall), the combustion and the expansion of the gases happens much faster, comparing to a cold start.
So actually with that high advance you may not have problems in cold start, but in hot conditions it’s more likely that you have.
I think you are heading to the right direction by setting the advance lower at low rpms. Anyway you don’t need power down there.
Also richer air fuel mixture would help the combustion happen a bit later, hence the increased advance as you press the throttle.