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Curious things I noticed while Tuning a car the other day..

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Hey, Ok so one cool with with Megasquirt is that you can build your own plug and play ECU's. Here's a link for those interested:


Anywayz, was using it to tune our VS Commodore 5L Ute which runs a Locked Dizzy from standard which contains a hall effect sensor and is triggered by a External HEI ignition module. So ran it up on the dyno with the stock ECU just to get a baseline, Then pulled the stock ECU and fitted my plug and play unit, Got it fired up but was barely pulling any vacuum so I checked the timing and found it to be very retarded... I tried to change the trigger angle in the megasquirt software but It would not let me put in a negative number.. No biggy I just rotated the dizzy to suit... Anyway since Im just testing this PNP unit on this car I need to swap back and forth between the stock ecu and my one and I need to re-set the timing every time...

Just curious why this would be? is it simply because the stock ecu is setup that way and has a certain trigger angle built into it?

Second thing I noticed with this car once I had that sorted, I was going though and tuning my VE table first so I locked the timing to 15 degree's... All was sweet but I noticed when I was doing some power runs the ignition was missing alot, At the time I thought I might need new plugs or just re-gap so I continued on. But then I got to the ignition tuning and as I was tuning I noticed it stopped missing completely... I did another power run with the timing locked to 15 and coughed and splutterd as before, But with the timing more advanced it had no probs...

Just wondering what the deal is with that exactly? What causes it to miss when the timing is too retarded?

Could you upload a screenshot of your ignition setting (ignition options/wheel decoder) in Tunerstudio ?

Normally (depending on your setup) you should be able to set the trigger offset to +/-20° in this menu.

Also, have you tried to use the diagnostics tool to see if you got a rock solid signal ?

I'm asking that because the DIYPNP (like pretty much all megasquirt, except the new MS3 pro and PNP Pro) use an old signal conditioner (I'm talking about the electronic components here) and sometime it can be a little bit complicated so set correctly, so your problem could come from that. Bad signal = bad ignition.

I'm not familiar with Holden engine and their ignition in particular (being from the other side of the world and all) but if you explain me how everything is wired, I might be able to help you.

I haven't spent any time with the megasquirt ECU so can't really add a lot to how they deal with trigger setup. I know the old Link G1 ECUs didn't have an adjustable offset in their trigger mode so this may be similar?

If you needed to rotate the dissy to set the base timing then you may have introduced a problem with the distributor phasing. With a distributor the location of the rotor arm to the individual cylinder posts is critical. You want the rotor arm to be just about to leave the post at the most retarded timing you expect to run (perhaps 0 deg for example). This will allow the maximum amount of advance while retaining alignment with the post.

If the alignment isn't right, it's quite likely that as the timing advances up that the rotor arm will not align with the post and this can result in cross firing within the distributor cap. This will present itself as a misfire as you explain.

I could be off the mark but it makes sense if the engine runs correctly on the stock ECU.

Here is a screenshot of where you can adjust the offset of the trigger (depending of the trigger mode) in Tunerstudio (the software for Megasquirt ECU)

My guess is, in your case Reaper you use the Tooth #1 Angle (the bottom one) and yes in this one you can't use a negative value.

But there is a way around that. You start from 720° and subtract the value you need.

Attached Files

Remember that when your adjusting your base timing for the trigger offset to use a timing light to ensure the value entered in the software is a true representative value of the timing of the engine

The Holden V8 running a Delco is set to 10 degrees base timing and the reference angle is 60 degrees with the hall effect.

The notches in the chopper wheel are cut at 55 and 35 degrees.

When you move the distributor you are chaging the factory set base timing because the Delco itself on Limp Home Mode does not have any effect on the actual engine timing at all and it runs purely from where the distributor is set to.

Not having played with a Megasquirt, but knowing a fair bit about the Delco, I'd be putting he timing back to 10 degrees using the Delco with the diagnostic terminal shorted as per manufacturers specs. I'd then adjust the Megasquirt to suit the real base timing.

Thanks for the input guys,

Andre: So since ive changed the rotor phasing your thinking that when its at 15' advance it may be "too retarded for the new phasing" and causing it to miss altogether or crossfire. Yep makes sense.

Ludo: Was using basic trigger, so was using the Trigger angle/offset (though the trigger wizard) I just had another look in the software and in the help box it says you can adjust the trigger angle +/- 20degree's. But im sure the other day when I tried to enter a -ve number it popped up saying error you cannot use a -ve number with a basic trigger... Ill have to take a look at that again. Ill get a screenshot of the settings (on work laptop so dont have it with me)

Chris: Yep was using a timing light

Michael: Yep when I was switching between the MS and the Delco ecu thats how I was re-setting the base timing for the Delco (diag terminals shorted and to 10 degrees) But you are right, for the reasons Andre stated and it is what I had originally planned but Could not seem to do it with the software... But ill go back and double check that.

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Support/frm28392_tech_bulletin_rotor_phasing.pdf check this out.

Also I'm no sure i understand, you locked the timing and did a sweep? Since that timing it not optimal at 15* for full rev range I would expect for the car to lack power and stumble. Maybe I just misunderstood what you wrote.

Yea, well timing wasnt actually locked the map was just filled with 15 degrees, but same same...

I expected the lack of power, but it was really badly missing and jerking over the place, Thinking back now it was likely crossfiring to other cylinders

If it was crossfiring you'll need to fix that. The VN-VT V8s are pretty good and don't usually do that. I have only seen 1 instance of crossfiring on these engines. Maybe the distributor was turned so far that the plug leads had to much tension on them but still I doubt it. I'd just be putting it down to wrong settings in the ECU.

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