PLEASE SOLVE MYSTERY OF THE TOOTHED CRANK TRIGGER

[TJM]

We have a 60-2 wheel, a 1984, SBC V8 So 6.9.4 in the New Manual says:

A. Should we pay attention to the suggestion that [ by the manual page 123 ] our sensor should be 40 degrees from the Tooth#1 at top dead center. Makes no sense?
Or
B. Do we follow the instructions on page 124, and use 80 degrees ?? Which makes far more sense?

And just what is this Two Tooth Gap Telling the computer ?? : “ Hey.. HAL... we just passed #1 TDC. “

Must be? Then if so, the Chevy = V8 and a 60 - 2 [ 58x] wheel, HAL watches 15 gaps pass and fires plug #8??? How’s this work?

I hope the answer is a simple “B” and a Yes

[have 1/2 Moon cams signal generator]

TJM

[grom_e30]

when your engine is at tdc on cylinder 1 what number tooth is the sensor over?

[TJM]

with the 58x it would be the 13th~14th

TJM

[grom_e30]

well you can do it this way 360/60= 6* per tooth, if the sensor is over the 14th tooth, thats 13 gaps back till the first tooth after the missing gap (tooth 1), so 6 * 13 = 78* tooth1 angle.


so the ecu knows that 78* after the gap, (tooth 1) is tdc on cylinder 1 and knows to fire the spark, then 90* after firing cylinder 1 its time to fire the next cylinder , and another 90* its time to fire the next cylinder and so on.

[jsmcortina]

The manual doesn't say it SHOULD be 40 deg or 90 deg. Those are EXAMPLES only. Do as the other posters said - enter the actual tooth#1 angle for your engine.

James

[TJM]

Thanks James & Grom,

Semantics suck, and I hate demonstrating my stupidity in public, but here goes:

While the computer can cope with any sensor /tooth phasing, and 40 degrees is suggested, my poor old brain is trying to process what the sensor’s signal is telling the computer? Just one of those Alzheimer’s moments.

In my words: Any tooth is fine. The tooth represents an arbitrary number. That Number has NOTHING to do with Crankshaft degrees or anything other than a reference for the computer to know #1 TDC is occurring?? Type in the correct Tooth #1 angle into Tuner studio and life is good?

Did I say that right?

I have seen too many pistons with holes in them to just try and be smart about timing methods that do not “click”?

Thanks for the patience......

TJM

[16vboost]

and I hate demonstrating my stupidity in public, but here goes:TJM

haha, I know how you feel.

Here's my perspective as someone who didn't write the code. You're exactly right and the tooth angle is arbitrary (theoretically). BUT, the code just happens to work better when the missing tooth is 40-90 degrees before TDC. Mine is set up at 110 and works fine.

-Alex

[elaw]

First, it would be easier if you told us what type of Megasquirt system you have and what manual you're looking at! I looked at a couple of the many manuals linked at http://www.msextra.com/doc/index.html and didn't see exactly what you're talking about (the suggestions on pages 123 and 124)... frankly I don't have the patience to look at every single manual to find what you're referring to.

But it sounds like your basic understanding is correct. As the engine is rotating, the MS will receive a series of pulses from the crank sensor - one for each tooth that passes the sensor. So every time the MS sees a pulse, it knows the engine has rotated by the number of degrees that the trigger wheel teeth are spaced apart. But without one more piece of information, the MS doesn't know *specifically* what position the engine is in... just that it's rotated a little farther from the time the last pulse came along.

That's where the missing tooth comes in. When the MS receives a pulse that's delayed a lot longer than the one just before it, the MS realizes that the "missing tooth" has just gone by the sensor, and the pulse that just came in represents "tooth #1".

By you entering the correct value in the tuning settings, as soon as the MS identifies "tooth #1", it now knows the exact rotational position of the crankshaft. If you set tooth #1 angle to 50 degrees, once the MS sees tooth #1 it knows the engine is 50 degrees BTDC on cylinder #1. And once it knows that, every time another tooth pulse comes in the MS knows exactly what the crank position is by adding the number of degrees per tooth to whatever position it computed last time.

For example, if you have a 36-1 wheel on the crank (so a tooth pulse is generated every 10 degrees) and your sensor & wheel are set up so tooth #1 is at 50 BTDC, here's what happens: once the MS identifies tooth #1 (by sensing the gap before it), it knows the crank is at 50 BTDC. When the next pulse comes in, the MS knows the crank has rotated another 10 degrees and is now at 40 BTDC. The next pulse tells the MS the crank is at 30 BTDC and so on until the engine comes around to 50 BTDC again and the cycle starts over.

Now of course the Megasquirt doesn't do things just in 10-degree increments... it computes the engine position much more accurately using math against the timing of the tooth pulses. But that's beyond the scope of this discussion.

So the bottom line is if you correctly identify the position of tooth #1 relative to TDC of #1 cylinder and enter that into your tune, the computer inside the MS will work everything else out.

The reason you'll see "recommendations" on where tooth #1 should be even though theoretically it can be anywhere relates to timing accuracy. As I mentioned above, the MS only knows the engine's *exact* position when it receives a tooth pulse... every 10 crank degrees in our example. All the rest of the time it's estimating the position based on the timing of the last two tooth pulses received. The estimates are never 100.00% accurate as the engine speed is always changing, but they're pretty close since you have a tooth every 10 crank degrees and how much can the engine's speed change over 10 degrees?

But wait a minute... there is *not* always a tooth every 10 degrees. A 36-1 wheel has one tooth missing. So in one case, the MS doesn't see a tooth until the engine has rotated *20* degrees instead of 10. Since more time has elapsed between the two tooth pulses, the engine has had more time to change speed, and the timing accuracy will be less in that region.

Because of that effect, it's best if you can have the missing-tooth gap occur at a time when an ignition event is not going to occur. Since most engines run 45-50 degrees of timing max., having tooth #1 at 50 BTDC means the gap in the trigger wheel will have already gone past once the crank reaches the spark range, and timing will be most accurate.

Of course putting tooth #1 farther BTDC would be fine too, but if you advance it too far you may put the gap into the spark-firing range of the previous cylinder.

[dragbug]

Quick question. What if the sensor is not directly centered over a tooth when at TDC? What if the center of the sensor is half on/half off a tooth? Will that cause an issue? If not, what tooth angle do you tell MS when the sensor is over partially over tooth 4? Would the tooth angle be 30* or 20*?

[jsmcortina]

Quick question. What if the sensor is not directly centered over a tooth when at TDC? What if the center of the sensor is half on/half off a tooth? Will that cause an issue? If not, what tooth angle do you tell MS when the sensor is over partially over tooth 4? Would the tooth angle be 30* or 20*?

The tooth#1 angle can be adjusted in tenths of a degree. Make sure that the commanded timing matches strobed timing - use whatever angle required to achieve that. My engine happens to use 39.0 deg.

The absolute golden rule is that the actual strobed timing must match what is shown in TunerStudio. i.e. if you put 23.4 deg in the timing table and TunerStudio is then showing 23.4 deg (there could be other adjustments) then you MUST see 23.4 deg on the strobe.

James

[dragbug]

Quick question. What if the sensor is not directly centered over a tooth when at TDC? What if the center of the sensor is half on/half off a tooth? Will that cause an issue? If not, what tooth angle do you tell MS when the sensor is over partially over tooth 4? Would the tooth angle be 30* or 20*?

The tooth#1 angle can be adjusted in tenths of a degree. Make sure that the commanded timing matches strobed timing - use whatever angle required to achieve that. My engine happens to use 39.0 deg.

The absolute golden rule is that the actual strobed timing must match what is shown in TunerStudio. i.e. if you put 23.4 deg in the timing table and TunerStudio is then showing 23.4 deg (there could be other adjustments) then you MUST see 23.4 deg on the strobe.

James

Thanks James! That's what I was hoping for.

[TJM]

[quote="elaw"]First, it would be easier if you told us what type of Megasquirt system you have and what manual you're looking at! I looked at a couple of the many manuals linked at http://www.msextra.com/doc/index.html and didn't see exactly what you're talking about (the suggestions on pages 123 and 124)... frankly I don't have the patience to look at every single manual to find what you're referring to.

It is a MS3X/V3.0 and we are using the New and Vastly improved manual by Mr. James Murray. Frankly, I could not have phrased this using the old directions.

I have a greatly improved understanding and thanks for the input.

Our first partner was cavalier.. and melted one set of pistons and grenades other motor 1st time out due to misunderstanding R.R. oiling... He went back to drag racing $10,000 lighter for the experience? I don't want to repeat that performance.


Getting the same answer a couple ways is sometimes the only way to understand... To quote G.B Shaw.. "The single biggest problem with communications is the illusion it has taken place " !!!


TJM

[jsmcortina]

When setting the motor up, DO listen for knock on the dyno and DO pull plugs and take an "old school" plug reading. I've been blinded by science before and skipped the basics, I don't want to repeat that.

James

[TJM]

When setting the motor up, DO listen for knock on the dyno and DO pull plugs and take an "old school" plug reading. I've been blinded by science before and skipped the basics, I don't want to repeat that.

James

The TRUTH sir.. the absolute truth. Running E85 does not make life easier reading plugs... not to mention the limited heat ranges available in Resistor plugs...

Which brings me to a crossroads in my own thread.. With the LS coils mounted low on the frame and plug wires 24+ inches away from any I/O and all I/O in shielded cable.. can I get away with racing plugs again ??

I have a sizable investment in plugs!! Seriously about $300.00 worth of Non Resistors...

TJM

[elaw]

The only way you can know that for sure is to try it.

Get the engine running first on resistor plugs, and make sure it runs reliably and there is no excess sensor noise under all conditions: cold/warm, low/high RPM, low/high load. Once you're sure the engine is solid (and have taken some logs so you can compare afterward), you can try the non-resistor plugs and see if they cause any problems.

[TJM]

The only way you can know that for sure is to try it.

Get the engine running first on resistor plugs, and make sure it runs reliably and there is no excess sensor noise under all conditions: cold/warm, low/high RPM, low/high load. Once you're sure the engine is solid (and have taken some logs so you can compare afterward), you can try the non-resistor plugs and see if they cause any problems.

Had not thought of that? With all the painfully "noise" we observed trying to get a clean RPM signal when using a 4 PIN HEI [terrible, TERRIBLE idea by the way] I guess it would show up pretty quick??

I will going to the Dyno in a week or two. Once we get the basic tune in, I guess even over the road driving would show that issue?

It's not what you'd call a street legal car in Ga... I mean it meets the letter of the law, but it is pretty thinly disguised racer... ?

Speed legal drives shouldn't hurt much through?

TJM



.

[TJM]

Life is not good...............

The cold facts:
Ms3x/V3.0 ,
SBC,
LS3 COP,
Sequential.
Holley 58X toothed wheel,
EFI Cam phase sensor.
Set Tooth #1 at TDC, checked Cam sensors dead nuts.
Set sensor tooth wheel at 24th tooth.[ 144 degrees ] [set up allows Range is 16 to 28 teeth? ]

IGN settings:
Spark mode = Toothed wheel
Trigger wheel arrangement = Dual wheel with missing tooth
Trigger wheel teeth = 58x
Missing teeth = 2
Tooth #1 angle = 144
Main wheel speed = Crank wheel
Second trigger active on = rising edge
Checked with timing light.. all is good dead on 10 degrees...
Went downhill: Backfiring through the throttle body as if timing is massively off???
Fiddled with timing seemed to improve with -10 disagrees advance???
Something not as I think it????

TJM

[billr]

"Something not as I think it???? "

Yes, of course, otherwise you wouldn't have a problem! Are you using a "dial-back" timing light? Checking the timing with a simple light seems to work better, less chance of confusing the light electronics or the operator. Are you saying you have run with "fixed" timing at 10 BTDC and the light shows it staying steady at 10 BTDC all the way up the rpm range? I think there is some issue with LS coils "false firing" if the dwell isn't set correct...

[elutionsdesign]

A good, cheap, no other controls but a single button timing light is your best friend.

[TJM]

"Something not as I think it???? "

Yes, of course, otherwise you wouldn't have a problem! Are you using a "dial-back" timing light? Checking the timing with a simple light seems to work better, less chance of confusing the light electronics or the operator. Are you saying you have run with "fixed" timing at 10 BTDC and the light shows it staying steady at 10 BTDC all the way up the rpm range? I think there is some issue with LS coils "false firing" if the dwell isn't set correct...

Simple strobe, nothing fancy.

No.. the 10 degrees was what we interpreted as the "Check Timing" to be sure the "light" agreed with the Computer setting?


To me this is the equivalent of setting timing with a Dist, by disconnecting the vacuum advance.

Sadly.. No run .. no full advance???

It sounds to my old ears that we are WAY out of time..

Probably should have opened with this: In Tuner Studio... Should we be in " POLL" instead of "Rising Edge".. for the second trigger?? Smart young God Son says No.. ?? I am conflicted and Say yes??



TJM