MicroSquirt.com read only forum (Jan.15,2018)

I've just come across the cb750 post, and I am going to update my ini file and see what more negative offset will give me.

That post brought about some links that I had not seen anywhere else.. Would have been nice to know about them a few days ago...
Need to search more.

Thanks again, I got it running.

My trigger offset is -315 using rising edge and calculated.
Switching to falling edge only moves the timing 10 degrees.
Do I have a setting wrong? The lump on the flywheel is just passing the sensor for each cylinder when it is on TDC.
I don't see why it should need more than 5-10 degrees offset.

That's just it, you can't set up to start charging the coil and firng spark unless you are far away from the point at which you are going to fire. The Dual spark option sets up for firing spark 2 tach (cylinder) cycles ahead, that is way after tdc.

Al

Sometimes it is hard to get your head around this trigger stuff, until you see it graphically.
Last night I was running a 24-2 wheel on my test stand. I wanted it to fire on the 14th tooth after the missing teeth and fire the #2 output 90 deg later. I set skip teeth to 13 and sure enough it fired on the 14th tooth and 90 deg later on the 20th tooth. When I tried to use the trigger wizard I could advance the timing but could not retard it. Then the light bulb finally came on. I set the skip teeth to 12 which made it fire on the 13th tooth. I then set the trigger offset to –15 deg which moved the trigger point 1 tooth(360/24=15) to the 14 tooth. Now with the trigger wizard I Had 15 deg of retard in the trigger wizard. I would assume that if I needed more retard, I would just lower the skip tooth value and set the minus trigger offset accordingly.
Now I have a little better idea of why you need to use negative trigger offset when using the dual spark, trigger wheel mode.

old guy wrote: Then the light bulb finally came on. I set the skip teeth to 12 which made it fire on the 13th tooth. I then set the trigger offset to –15 deg which moved the trigger point 1 tooth(360/24=15) to the 14 tooth. Now with the trigger wizard I Had 15 deg of retard in the trigger wizard. I would assume that if I needed more retard, I would just lower the skip tooth value and set the minus trigger offset accordingly.
Now I have a little better idea of why you need to use negative trigger offset when using the dual spark, trigger wheel mode.

Yep, you understand! You need to go to the tooth past where you want to be then increase the minus trigger offset. This makes the embedded code implementation simpler in that there is always the target tooth far off in the future and the minus offset brings it in closer.

- Bruce

I can only retain the trigger logic in my head for 15 minutes at a time, so I know how confusing it is. But I wanted to clarify something. Skip teeth should not have anything to do with timing offset. Skip teeth must be set to however many teeth you want to constitute a tach cycle - that is the number of teeth between tach pulses which is normally no_teeth / (no_cyl/2). In crank degrees it would be 360deg / (no_cyl/2). You don't tune or fiddle with this - you only set it based on no_cyl, wasted/ sequential and 2 vs 4 stroke. The spark should fire once every skip teeth. The term skip teeth is a holdover from the first wheel decoding, which simply threw out (or skipped) every skip teeth and otherwise used the same logic as was used with a distributor reluctor. It should really be called no of teeth between sparks or tach pulses.

What you were talking about sounds more like delay teeth. This is what is meant to allow you to go past the missing tooth by however many teeth it takes to put you in a position where you can use a negative trigger offset (in dual spark mode). This is to avoid repositioning the VR sensors for people who don't have room or a bracket mounting place to do this.

Note there is something called skip pulses - this has absolutely nothing to do with wheels or spark timing. It simply skips the first nskip tach pulses in case you have a flaky starter.

Al

Your right, I got my skip teeth and delay teeth mixed up.
Actually it is set at delay teeth 14 which makes it fire on the 15th tooth.
Then the -15 trigger offset bring it back to 14.
Like I said it is much easier for me to visualize this stuff

grippo wrote:I can only retain the trigger logic in my head for 15 minutes at a time, so I know how confusing it is. ...

Thanks for the post as I am struggling with M-0 offsets

I have 4 teeth on a four cylinder engine with a cam sync too. So I thought I would be using the M-0 part of the dual spark module, but I'm not really sure.

In the Advanced Ignition Options, what settings do I use in the Dual Spark options, the names don't seem to relate to the descriptions I have seen in the manuals,

My skip teeth are 2, the first pulse is at TDC (1 & 4) and just before (37 deg.) the next tach pulse (TDC 2 & 3), the cam pulse makes, which as I understand triggers "output1", then it skips a pulse and makes the next tach signal "output2" 180 deg. later.

Am I on the right lines? My delay pulse = 0 , because it occurs at TDC, but what is my offset? Does it not relate to the 37 deg?

PS I keep trying to read the manual or the sections, but it's quite hard navigating and searching for the relevant bits, for a noob anyway.

You have made a pretty good first cut at this. Since you have a cam synch you should select Dual Spark: falling cam synch under Advanced ignition options. The important thing here is that the cam synch comes in after the last tooth and before the first tooth - it must be inside that interval by enough to make up for any cam or crank twist at high load. Also I am going to assume your 4 - 0 wheel is crank mounted, so your skip teeth = 2 = 180 deg is correct. That leaves Delay Teeth and trigger offset. I would try the following setup:

Delay Teeth = 1 and trigger offset = -53deg (atdc).

In this I assumed your no. 1 tooth comes in at 37deg btdc, but you want a negative trigger offset with dual spark, so if we delay 1 tooth(90 deg), we can use a trigger offset of 37 - 90 deg = -53deg. This should work with the following timing diagram:

------------T1----------T2---------T3-----------T4----------T1----------T2
------------^---|-------^---|-------^---|-------^---|-------^---|-------^
----------------tdc1--------tdc2--------tdc3--------tdc4-------tdc1-------
-----cam-------------tach1--------------------tach2--------------------tach3

After tooth1 comes in you delay 1 tooth and start counting tach pulses from there (T2/ tach1). You have passed cylinder 1 so you set up for dwell and spark 2 tach pulses down (tach3), on the 3rd cylinder in the firing order on output 1. This will occur somewhere table adv deg before the rightmost tdc1. Setup for the second output starts at tach2 and fires before tach 4 (not shown).

Hopefully you don't have an odd fire engine.

grippo wrote: but you want a negative trigger offset with dual spark, ...snip....
Hopefully you don't have an odd fire engine.

OK, now I get a little more, what's nice is I had a diagram too, but didn't know how to express it in the post...another thing to chalk on the board.

This is what I did FWIW, and how I understood things

--T1----------T2----------T3----------T4----------T1-----------T2-----------T3-
-tdc1--------skip--------tdc3--------skip---------tdc4---------skip--------tdc2-
tach4-------------cam--tach1--------------------tach2--------------------tach3
--^---143˙--------^-37˙-^--------180˙----------^-----------180----------^--

My mistake was not understanding the negative offset & delay concept properly (especially as I thought the delay value = 0, 'cos it happened at TDC). Although I knew the cam sync is a reference pulse for all the other tach inputs, I should have started from the tach point after the cam sync...like the manual/help section said.

I wouldn't have got the delay teeth bit without some help, but if I understand better, I have

-------tdc1,4----------------tdc2,3-----------------tdc1,4----------------tdc2,3---------------TDC
-------input1----input1----input1----input1----input1----input1----input1----input1---CRANK
----------^---------^----------^----------^----------^----------^----------^----------^-------CRANK
--input2--------------------------------------------------------------------------------------input2----CAM
---^----------------------------------------------------------------------------------------------^------CAM
---^-delay=1---tach1-----skip------tach2------skip-------tach3------skip------tach4-^-delay=1---COMBINED SENSED SIGNAL
---^-----------------l-----------------------l-----------------------l------------------------l---------------delayed tach output
---^---delay&offset-----l-----------------------l-----------------------l-----------------------l---------delayed tach & offset output
---^------------------OUTPUT1------------OUTPUT2------------OUTPUT1------------OUTPUT2----

and now if I was watching on a scope I would see all the outputs happen after a tach event, and if I altered the offset I could move these outputs incrementally? ignoring any spark advance values.
And if say I altered delay teeth to 2, the OUTPUTS would jump 90 degrees to the next input event, which would be a useful way to get round this problem... you have a spark advance table with the right values in it but you are firing on the wrong cylinder (out of phase), so I could increase the delay teeth as opposed to switching plug caps.

Am I right in thinking (MicroSquirt) that you can use the offset value to tweak when the injection happens (as linked to OUTPUT event) relative to TDC, or am I getting ahead of myself?

Either way, many thanks for your help and patience, but I doubt it will be the last time that I post to the forum. Hopefully one day when I understand things better I can contribute, and help others.

PS It's not oddfire.