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A little background: I have a project car with an EFI system from 1985 where TDC is (apparently) measured by a little gear in the distributor. I'm slowly switching over to an aftermarket ECU that requires crankshaft position information.

I don't think the output from the distributor can be used as-is with the new ECU, so I was planning to add a trigger wheel and sensor to the crankshaft.

Is there any benefit to a higher resolution reluctor/trigger wheel? 60 tooth vs 36, for example? Or is timing going to be just as accurate between the two? Price difference seems to be negligible.

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    Welcome to Motor Vehicle Maintenance & Repair! It's really going to depend on the vehicle in question. Some use more and some use less. Take for instance, the LSx engines from Chevrolet. There were two crank sensor models. One is 24 and one is 58 tooth count. These are not interchangeable without some major surgery (you have to buy a converter box, which isn't cheap). IOW: tooth count matters and must be matched to the computer you're using. Some reluctor rings have gaps of different widths which denotes specific places in the ring, which tells the computer where the sensor is at. – Pᴀᴜʟsᴛᴇʀ2 Jan 10 at 0:49
  • Ahh. Okay, that makes sense. This is with a programmable ECU, so I'll have to go re-read the spec and see what the requirements are. I must have missed it. Thank you! – 8bitartist Jan 10 at 5:29
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It somewhat depends on what the crank position information is used for.

60 would be better than 36, as the former only has 10 degrees of resolution, while the latter "60 tooth" improves this to 6 degrees.

Ultimately the ECU will probably use cam position for injection and ignition, so it may not matter.

A good aftermarket ECU will allow you to use either one, as long as you program the unit with the correct total number of "teeth".

  • Great answer. Thank you! Looks like I have the option of crank + cam sensor or just crank for this ECU. Will have to figure out something for the cam if I want to go with sequential injection. – 8bitartist Jan 10 at 5:39
  • @8bitartist - Sequential is far better than batch fire or TBI, that's for sure. – Pᴀᴜʟsᴛᴇʀ2 Jan 10 at 12:41
  • @Pᴀᴜʟsᴛᴇʀ2 - good to know! Sequential is on the wish list. Maybe I can use the sensor in the distributor. – 8bitartist Jan 10 at 20:26
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    Also keep in mind the cam sensor looks at trigger which is turning half as slowly as the crank, which effectively doubles the timing accuracy. With variable cams this is an absolute must. – SteveRacer Jan 12 at 4:41

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