Right why am i bothering to mess around with my megasquirt settings?
Answer: I have read lots of posts on this forum and others about megasquirt not being able to idle smoothly at AFR's much higher than about 13-13.5. It seems that some people have no problems at all whilst others (me included) have been. At first i thought this was due to people using EFI heads and others using spacer plates but actually i believe it is due to people using different injectors, some with dead times closer the MS default 1ms.
My car idled fairly well at 1000rpm and 12.8:1, i could get it to 13.5:1 but the idle would be very weak casing the car to stall easily, it also over fuelled on light lift off (im yet to see if this has been improved but this is another common symptom).
What is injector dead time?
Theres lots of blurb about this in the interweb. But heres and brief and i hope simple explanation:
Injector dead time is the time it takes for fuel to start squirting from the injector AFTER the injector has been energised. Like a tap, water doesn't squirt out the second you turn the knob, it can sometimes take 1-2 turns before water comes out, the time between you first turning the tap and water coming out could be called tap dead time.....
Injectors also don't stop injecting fuel instantly (although they stop much faster than they start).
Injector dead time is heavily effected by fuel pressure and voltage, fuel pressure should be taken care of by your pressure reg, but voltage (which changes a fair bit) needs to be taken care of by a look up table or a constant value.
Megasquirt uses 0.2ms per volt as its standard constant. So for each volt the opening time is reduced by 0.2ms...
(and seeing as we love examples!)
An injector dead time of 1.1ms @13.2V and a voltage correction value of 0.2ms/V would change to 1.3ms @12.2V and 0.9ms @ 14.2 volts....
Whys it important?
Well, your ecu needs to know an injector dead time because it needs to know how long to open the injector for. Its not overly important at higher pulse widths as and variation is easily sorted by your VE table, but at lower pulse widths where the injector isn't injecting very much fuel at all a small change in pulse width (0.1ms) can be the difference between a squirt and no squirt. The injector dead time is added to the PW.
This would explain why my idle had to be rich, as a leaner idle meant small pulse widths and not very much fuel, hard to control and difficult to achieve a lean smooth idle.
How do you find dead time?
The easiest way is to look for a manufacturers figure... however i really couldn't find mine (Bosch 360cc), Siemens Dekas are fairly well published. But it never hurts to double check.
Another method uses an Oscilloscope, and another mean playing with number of squirts and some other jargon... these methods are at best approximations.
I measured the actual dead time of mine using my MS ecu, and some jam jars.....
This was only a first attempt as i will be changing injectors soon and will do it all again properly.
How i did it...
I wanted to use my megasquirt in out put test mode to gradually increase the pulse width until fuel JUST started coming out this pulse width is my dead time.
I did this in the car, i would have been better doing it with a power supply so i could alter the voltage more and get a more accurate voltage correction value.
I removed my fuel wiring and injectors, stuck the injectors back into the fuel rail and then shoved the free ends into some jam jars. adjusting my fuel reg to 3bar (running pressure) as i was getting a value of nearly 4bar with no inlet vac attached.
Heres some pics
Wiring removed
Injectors in a jar using the heater box as a handy stand
and a pic of my Ohhhh so professional set up
And thats about it.
I ran the fuel pump and tested the injectors, starting at the standard setting of 1ms... no fuel, and slowley ramped up to 1.3... fuel JUST started coming out at 1.4ms, so thats my dead time.
Now the voltage correction.
In an ideal world this would be done using a power supply to give the base figure of 13.2V MS wants. However this was just a trial. So i turned every thing on, lights, fans etc. and got my voltage down to about 11.4v from 11.8 this was just enough to give a dead time variation of 0.05ms (1.45 instead of 1.4) and from that i now have a voltage correction value of 0.125ms/V.
From this i have a dead time of roughly 1.3 at 13.2V (this is still a rough figure, but its good enough to see if its improved anything).
Some pics
The result!..!
Fantastic, reducing my VE table around idle now gives a smooth idle at around 890-920rpm (need to turn this up!) and at an AFR of 14.5(ish) doesn't stall as easily and seems much smother
My injectors aren't amazing, they don't appear to atomise the fuel very well at small opening times and one injector seems to have a slightly lower dead time... but hey.
nearly back together
and a good figure at a steady idle
Some different injectors and some fine tuning should see the idle become even better.
Obviously my VE table now needs some adjusting
I've just upgraded to sequential 3.1.0 code so will be trying semi sequenial (and fully sequential when i have a cam sensor) as soon as my maps a bit more refined. This should help make for a smoother idle and better fuel consumption etcetc.
Cheers for reading and i hope it helps someone! if any ones got any suggestions/corrections please post them up as you'll be helping me for next time.