Pulley-mounted crank trigger w/stock Magnum EFI - trigger notches?

-

cudak888

1-Baker-11
Joined
Sep 30, 2014
Messages
703
Reaction score
51
Location
USA
Let me set the scene for you:

I have a 1989 roller-cam LA 360 with Magnum heads on it; currently set up with a carb. It is a runner/DD, and I'd like to EFI it on the cheap; hence, stock Magnum EFI.

Obviously, sticking on a beer keg intake (or the P4510016 if I'm ridiculously lucky) won't be a problem. But that's not the issue.

Problem is, as an externally-balanced LA 360 bolted to a 904 transmission, I have no crankshaft position sensor notch in the transmission (not a problem), the external balancing means I'm more or less stuck with my existing B&M 10239 flexplate (problem), and the early TBI LA block does not have a mounting pad for the stock Magnum CPS cast into it ("oh shiyte" problem).

With this in mind, it just seems sensible to follow the rest of the aftermarket and weld on a custom crank trigger on my pulley.

However, everything I've seen to-date in regards to crank pulley triggers have been for use with Megasquirt and other aftermarket EFI systems - 36 tooth, missing-notch trigger wheels. High resolution, but delivering a different type of signal than the simple 5-volt on/off sync designed for the factory EFI.

Now, if I were to design an 8-notch ring (designing and laser cutting it is not an issue) for my pulley, mimicking the Mopar flexplate, and run with a suitable 5V sensor, should I be reasonably right in assuming this would provide the Magnum PCM the same exact signal it would otherwise receive from the pulse ring on the stock Magnum flexplate?

-Kurt
 
Last edited:
Sure you could. Your machining would have to be top notch (pun intended) and you could even use the stock crank sensor to generate the signal. The window size will have to be the correct size relative to the diameter of the wheel, which will be different from the oem magnum flexplate but with some math you should be able figure it all out. Good luck.
 
Sure you could. Your machining would have to be top notch (pun intended) and you could even use the stock crank sensor to generate the signal. The window size will have to be the correct size relative to the diameter of the wheel, which will be different from the oem magnum flexplate but with some math you should be able figure it all out. Good luck.

Got it. There's a company I use to laser cut parts now and then; the results have always been within 0.5mm; probably tighter. This was for a stainless-based tool.

Theoretically, the notch size could be calculated by reducing the size of the original flexplate holes at ratio of the original flexplate diameter vs. the crank pulley diameter, correct?

I'd want to stay away from the OEM sensor for both space, neatness, and reliability. There are a few aftermarket, adjustable crank trigger sensor mounts out there that are affordable enough to make it worthwhile just to avoid headaches, and allow for excellent positioning options.

I don't think I'd want to rely on creating an adapter to work with the fairly flimsy factory mount - which isn't really optimized for use around the crank in the first place. It makes sense to splurge in this one area, IMHO.

-Kurt
 
Last edited:
Got it. There's a company I use to laser cut parts now and then; the results have always been within 0.5mm; probably tighter. This was for a stainless-based tool.

Theoretically, the notch size could be calculated by reducing the size of the original flexplate holes at ratio of the original flexplate diameter vs. the crank pulley diameter, correct?

-Kurt
Correct. The window size (in my little brain at least) should be proportionally shrunk the same percentage as the overall diameter. I can’t remember if they trigger on leading or trialing edge but there are pictures of Oscope graphs floating around on the Dakota forums that may help. I’ll see if I can find one. Maybe @KitCarlson would know.
 
Correct. The window size (in my little brain at least) should be proportionally shrunk the same percentage as the overall diameter. I can’t remember if they trigger on leading or trialing edge but there are pictures of Oscope graphs floating around on the Dakota forums that may help. I’ll see if I can find one. Maybe @KitCarlson would know.
I don't have direct experience with mopar trigger wheels and sensors. I do have experience with the application of Hall sensors with targets. I do ground up ECU designs, including reference sensing, but I make what I want, and code to go with it. In your case the code is likely fixed, so the sensing means needs to replicate original signals in all respects.
The first thing, is capture cam, crank, and ignition primary signals on a logic analyzer along with timing measurement from timing light. That provides all necessary information to replicate, and properly setup.
Next thing is to accurately measure trigger wheel, and see how that compares to sensing signal. Typically widows shrink, because sensor triggers before and after tab, because sensor does not trigger at center point, it has diameter. Sensors vary in size too. Most Hall sensors sink current, go low with tab of target. Target should be anealed low carbon steel, not stainless.
 
I don't have direct experience with mopar trigger wheels and sensors. I do have experience with the application of Hall sensors with targets. I do ground up ECU designs, including reference sensing, but I make what I want, and code to go with it. In your case the code is likely fixed, so the sensing means needs to replicate original signals in all respects.
The first thing, is capture cam, crank, and ignition primary signals on a logic analyzer along with timing measurement from timing light. That provides all necessary information to replicate, and properly setup.
Next thing is to accurately measure trigger wheel, and see how that compares to sensing signal. Typically widows shrink, because sensor triggers before and after tab, because sensor does not trigger at center point, it has diameter. Sensors vary in size too. Most Hall sensors sink current, go low with tab of target. Target should be anealed low carbon steel, not stainless.
Thanks for chiming In. I knew you’d have good info to share. These don’t trigger off of a tab though. The wheel is solid and a window passes the sensor to trigger it.
1BE63D68-D0C5-4732-9BCD-9D85D543701C.png

Windows (8 of them) can be seen around the outer diameter.
 
So if the OP was to shrink the diameter to the size of a crank pulley do you think the relationship of the window size to diameter of the wheel will be a constant ratio?
 
I don't have direct experience with mopar trigger wheels and sensors. I do have experience with the application of Hall sensors with targets. I do ground up ECU designs, including reference sensing, but I make what I want, and code to go with it. In your case the code is likely fixed, so the sensing means needs to replicate original signals in all respects.
The first thing, is capture cam, crank, and ignition primary signals on a logic analyzer along with timing measurement from timing light. That provides all necessary information to replicate, and properly setup.
Next thing is to accurately measure trigger wheel, and see how that compares to sensing signal. Typically widows shrink, because sensor triggers before and after tab, because sensor does not trigger at center point, it has diameter. Sensors vary in size too. Most Hall sensors sink current, go low with tab of target. Target should be anealed low carbon steel, not stainless.

Thanks for chiming in, Kit.

If I recall correctly (and this is from a long time ago, so take it with a grain of salt), the OBDII PCMs are flashable, but primarily for the purpose of removing unnecessary sensors, rather than altering the code that reads the Hall output.

I obviously won't have a chance to capture these signals, but I might have the chance to do so off a second-gen Ram - it's dependent on a neighbor's potential trade. While I have a reasonable grasp of what the logic analyzer will do (particularly given the data @TT5.9mag linked to at DodgeForum), I've never captured data with one before - nor do I know what would be an affordable unit.

Any links you might be able to point me to? I did see the SmartScope in that thread, but would like to hear your opinion.

I can get the ring laser cut in annealed 4130. Not a problem.

Found someone running a similar system on a /6 using one of those red generic hall sensors. I take it the choice of sensor will have an effect on the waveform just as much as the ring itself.
1966 Dodge Dart - Turbo slant six-Page 10| Builds and Project Cars forum |


So we're aiming for a nice square wave; nothing surprising there.

The back of my mind is wondering whether these printouts would be enough to get a rough idea of where to clock the ring.

-Kurt
 
Window makes high pulse. My best guess is keep window size same, don't shrink it, shrink metal part between. Shrink of window may lead to loss of pulse. There are 2 pulses per cylinder, roughly 45 degrees apart, less pulse widths. Similar to my system 45 degrees is used as advance reference, actual desired timing derived from that. In my system tab gives me high and low, so that identifies. The CMP is crucial with 2 pulses, it sorts out correct one. The problem with that, CMP updates per revolution. Since wave captures done on a engine not running correctly, we can only guess. Sync out, would be troublesome....
 
Trigger point likely as high goes low, that is driven edge. Going high is by pull-up, slower and less precise. Lookin at windows in relation to crank timing may give clues. In my system one edge is base timing, the other is max advance. So, one window pulse may be base, other max advance. Since CMP is related to rotor phase too, seems crucial setting. I have no idea about setting that. My work has been mostly direct fire.
 
Trigger point likely as high goes low, that is driven edge. Going high is by pull-up, slower and less precise. Lookin at windows in relation to crank timing may give clues. In my system one edge is base timing, the other is max advance. So, one window pulse may be base, other max advance. Since CMP is related to rotor phase too, seems crucial setting. I have no idea about setting that. My work has been mostly direct fire.

Well, I can't see there being much of a problem cutting out two test triggers - one proportionally sized, and one cut to the original flexplate size, and giving both a go.

However, with the discovery of that factory CPS mount, I'm now wondering if it'd be possible to use a neutrally balanced Magnum 5.2 flexplate with a converter balanced for the roller cam LA360.

-Kurt
 
That sensor mount that hipotec sells is probably the easy button here. If you’re using a 727/904 you’ll have to notch the bellhousing but that’s easy. And just use a stock magnum flexplate. I believe all of them are neutral balance as the balance was in the converter.
Edited to add:
I had to elongate one hole on the flexplate to match my 904 converter before I used the 46re.
 
That sensor mount that hipotec sells is probably the easy button here. If you’re using a 727/904 you’ll have to notch the bellhousing but that’s easy. And just use a stock magnum flexplate. I believe all of them are neutral balance as the balance was in the converter.

I've never been one to buck the easy button if it gets me where I need to go with the least amount of fuss - provided it's not outright hackery. I like the simplicity of that bracket, and it doesn't look overly fussy.

Notching the bellhousing is easy, but my car is an ex-318 with its original 904 at the back. As such, my converter is still neutrally balanced; I used a B&M flexplate to get the roller LA360 to play nice. It's going to take another converter or I'll have to weld on some weights to the existing one.

-Kurt
 
Last edited:
I've never been one to buck the easy button if it gets me where I need to go with the least amount of fuss - provided it's not outright hackery. I like the simplicity of the bracket though.

Notching the bellhousing is easy, but my car is an ex-318 with its original 904 at the back. As such, my converter is still neutrally balanced; I used a B&M flexplate to get the roller LA360 to play nice. It's going to take another converter or I'll have to weld on some weights to the existing one.

-Kurt
Me either. If someone makes a nice,well thought out piece that I was planning on making and it saves a bunch of custom work down the line, to me it only makes sense.
 
I went down this road because of the lack of a flywheel solution (LA balance with a tone ring). Build a set up, ring on the crank pulley and a mount, but haven’t used it yet. Too many other projects.
 
I went down this road because of the lack of a flywheel solution (LA balance with a tone ring). Build a set up, ring on the crank pulley and a mount, but haven’t used it yet. Too many other projects.
4 speed? 5-6 speed? Can you post a pic of your tone ring?
 
-
Back
Top