Autosport Labs

Hi everyone,

New guy on the scene with a difficult engine for Megajolting. In order to build up the tension I will do a bit of introduction first and save the questions for last .

I have a Saab Sonett III for which I am gathering parts for an engine with quite a bit more power than stock. One of the more difficult things to build is an inlet manifold for two Weber 40 DCNFs. These were used by Saab for a short while on their rallycars in the early 70's, but sadly these manifolds do not fit under the bonnet of a Sonett. In order to fabricate one myself it would be very convenient to get rid of the dizzy and create more space at the rear side of the manifold. By chance I came across the Megajolt scene and decided to give it a try.

The Ford V4 Cologne engine that was used by Saab is a very difficult engine to convert; no crank pulley, but one on the balance shaft; this is gear driven 1:1 from the crank shaft so I will use the pulley anyway for attaching the trigger wheel (as long as the gear wheels and balance shaft bearings are ok it should give no timing distortion). In very close proximity of this pulley are the pullies of the cooling fan and the water pump, so the mounting of the VFR-sensor is a challenge by itself. There is no room for bolting a trigger wheel directly to the crank gear under the front plate or fabricating an extension and putting it outside; the balance pulley is in the way . Nevertheless, in order to sort out the placement of the sensor properly I ordered a Ford EDIS set with all electrics included.

For simplicity I aim at using the crank pulley for the sensoring, but if this won't work there is the option of using the flywheel. I have one that has been lightened and in the area where material has been removed I could drill 36-1 holes, neatly separated by 10º, and put in 35 small bolts, using the heads for sensing.

Now the questions:
1: What gives the firing signal in a trigger wheel, sensing a tooth or sensing nothing (the space in between)? I guess it is the teeth, but I'd rather be sure.
2: If it is the spaces in between that trigger the firing signal, should it be ok just to drill 36 holes and remove the steel between two of the holes to emulate the missing tooth?
3: Should the size of the "teeth" (in this case the bolts) grow when the distance from the centre point of rotation grows? I guess not, but as a newbie in this matter I might think along a very wrong track.

Thank you all in advance,

Alex

Hi,
I'll answer the best I can, the firing signal is generated by the MJ counting the teeth on the trigger wheel after the missing tooth to determine where TDC is. In your case the MJ would count 9 teeth after the missing tooth (90 degrees crank rotation) where your engine would then be at TDC. The spark would then be fired at the appropriate time according to your ignition map.
The sensor just senses metal going past it to generate the signal, I don't think it will matter if the metal is in the form of teeth on a trigger wheel or the solid bit between holes on a flywheel. If you are going to use the flywheel, I would have a look at some of the later Ford engine flywheels to see how the engine designers used the flywheel to generate the crank signal.
As for size of teeth and spacing I would personally keep the tooth size and gap size equal if possible, if anyone out there knows differently I don't mind being corrected. Hope this helps.

Andy

Hi Andy,

Thanks for your reply. Having the "teeth" and the spaces in between equally sized might be the ideal situation, but almost at the edge of a flywheel a stretch of 10º is already quite a distance. The easiest (and likely cheapest) way of using 35 bolts might not work then. I have seen a picture of a Peugeot flywheel that was also used for crank sensing and I will have another look at it. If you have access to Ford pictures or whaterver it would be very educative. When I have the Edis and MJ together I will do a test with a wooden flywheel with bolts attacthed to the electric drill (though that might not be until after this winter...).

Alex

Hi Alex,
I don't have any pictures of Ford flywheels, but I'm sure something would come up if you put "Zetec flywheel" into Google. Have you thought about having circles machined into the flywheel to replicate gaps between teeth, it would be easier to control the spacing that way.

Andy

Not much to see on the Zetec flywheels. The pictures I found did defintely not show signs of incorprated trigger wheel surfaces. The lightened flywheel I bought a few years ago already lost its "meat" that could be used for circles, so that's why I thought of the bolts as actuators. But if that doesn't work I could use the stock flywheel of the dummy engine I use for fitting for that. But first things first; let's wait for the postman with the EDIS-set.

Alex

Alex B wrote:Not much to see on the Zetec flywheels.
Alex

This any help?

Yes, it surely is. Now I know that I can use bolts with small heads if I am going to pursue this method. For starters I will go for a trigger wheel on the balance shaft pulley. I made a bracket for the sensor and the next step is getting the trigger wheel installed. Progress is made, slowly but gradually.

Alex

regarding triggering...
the VR sensor is not triggered by either the space, or the tooth, in themselves.
it is triggered by the transition between being proximate to metal, then not proximate to metal, or the reverse.
so, if you had the VR sensor positioned near a spinning, smooth piece of steel, like the crank damper, then there would be 0v out put from the sensor. similarly, if you hold it in the air, there will be a 0v output.

starting with a space next to the VR sensor, the approach of metal causes a voltage spike from 0v, to some negative voltage, and back to 0v. starting with a tooth near the sensor, the approach of a space causes a +ve voltage spike whih also falls back to 0v. it is the fall from 0v upon the approach of a tooth which is used by EDIS for its internal counter.

note that the counter is triggered by a fall from 0v to -300mv ie quite a small drop, which means that you dont need to generate a particularly strong signal at all for EDIS to work. in any case, the peak voltage increases with speed, so there is in fact no single voltage associated with any particular setup.

if you had a wheel with one tooth, the VR output would be 0v for almost all the time, then drop to some -ve value as the tooth approached, triggering the counter in EDIS, then return up to 0v, then immediately have a +ve voltage spike as the back edge of the tooth passed by. if you had a wheel with one single hole drilled into it outer edge, the VR output would also be 0v for most of the time, but have a +ve voltage spike as the hole approached, then drop back to 0v, then an almost immediate -ve spike as the other side of the hole approached, whcih would trigger the EDIS counter.

point is that it really makes no difference whether you have lots of hole, and little tooth, or lots of tooth and little hole; either way, the arrival of the tooth will cause the required -ve voltage spike. when i say 'arrival of the tooth' that includes the approach of the 'back' edge of a hole which has been drilled into an otherwise smooth crank damper/pulley. it seems the only limit is whether the tooth is sufficiently distinct as to cause the required -300mv voltage spike. as things get smaller, the problem seems to be twofold: if the space isnt big enough, then the VRs coil will sense adjacent teeth simultaneously if the tooth isnt big enough, the induced voltage spike could be too small. from that i deduce that the safest approach is more space and teeth with a steep side eg vertical.

having said that, it seems that all manner of teeth shapes, spacings, and space/tooth ratios work perfectly. there is a photo in a recent post on Installation forum of a crank pulley with holes drilled into it, rather than teeth. what a neat solution!! it saves you buying or making, and then centering a trigger wheel, and costs nil. very inspired! the oscilloscope waveform there shown is textbook EDIS.


to answer alex's initial questions:
1. what triggers the edis...see above!
2. yes, absolutely drilled holes will work... see http://www.autosportlabs.org/viewtopic.php?t=1968
3. no need to make the holes proportionately bigger - as long as the holes are distinct from the teeth - even if achieved by drilling holes - that will be fine.

regards
alexander.