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Old 09-04-2015, 01:22 PM   #1
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Default Dumb question about crank and cam angle sensors

I've got what is basically a built 99 engine with the CAS from a 97 at the back of the head.

I assumed this entire time that the cam angle sensor was controlling the ignition timing and that the crank angle sensor was only used for VVT. I was basing this off the observation that the cam angle sensor has to be aligned correctly to get the right ignition advance. Last night I was under the car and I noticed that the harmonic balancer had 4 notches on it. And looking at 949 racing, they sell harmonic balancers with 4 notch trigger wheel options for 96+ cars.

I am confused, what does this thing do on non-VVT cars? I realize it's supposed to help with misfire detection or something, but I assume that none of the aftermarket ECUs do that. Is the 4 notch thing on the 949 racing harmonic balancers just for compatibility with the stock ECU?
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Old 09-04-2015, 01:39 PM   #2
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In '95.5 - '97, the crank sensor was used only for misfire detection, while the CAS was used for the staging and timing of both the fuel injectors and the ignition coils. The CAS in all NA cars outputs two signals, which are conceptually comparable to combined crank and cam sensors in the NBs. One signal is four evenly-spaced pulses per cam revolution (one pulse per ignition / injection event) and the other has two uneven pulses, from which absolute engine position is determined.) Thus, an NA can derive both the timing of the engine as well as the phse of the engine from just that one sensor, with the disadvantage of having some jitter introduced by the cam belt.

From '99 onwards, the crank sensor became the timing reference for fuel injection and ignition, while the cam sensor provided the staging reference (eg: so the ECU can tell which crank pulse corresponds to which cylinder.)

On the VVT cars, the cam sensor additionally allowed the ECU to judge the exact angular position of the intake cam, by comparing the phase angle of the cam sensor to the crank sensor.
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Old 09-04-2015, 01:58 PM   #3
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<p>So the 4 tooth wheel is one pulse per cylinder basically, tells the ecu when each piston is at top dead center? Cam sensor determines what cylinder it is?</p><p>I think we could definitely use a Crank/Cam sensor megainfo thread. Including all of the viable options for megasquirt users. I know the information is out there if you search, but could definitely be a good stickied topic.</p>
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Old 09-04-2015, 02:02 PM   #4
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Originally Posted by aidandj View Post
So the 4 tooth wheel is one pulse per cylinder basically, tells the ecu when each piston is at top dead center? Cam sensor determines what cylinder it is?
I assume that, in the above, you are talking about an NB.

The 4-tooth crank wheel is actually two teeth per cylinder, for rather obscure reasons having to do with the fact that the crank doesn't rotate at a constant angular velocity and so it's necessary to do some error-correction. But conceptually, you can consider it to be two PAIRS of teeth, and for our purposes, it is functionally equivalent to one tooth per cylinder.

And it doesn't tell you when a piston is at TDC, rather it tells you when the piston is at some specific angle before TDC, as the ECU needs some time to start charging up the ignition coil (the dwell time) prior to ignition. I can't recall the exact angle, but I think it's something like 60 BTDC or thereabouts.

And, yes, the cam sensor is there so that the ECU can figure out which pair of teeth on the crank corresponds to which cylinder.



If you were talking about an NA CAS, then the above still applies, just replace "pair of teeth" with "tooth", as the NA CAS doesn't have the double-pulse on the crank signal. Otherwise, same exact operating principle.




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I think we could definitely use a Crank/Cam sensor megainfo thread. Including all of the viable options for megasquirt users. I know the information is out there if you search, but could definitely be a good stickied topic.
Assuming this thread goes in a useful direction, I'll probably make a few edits and then stick it in the "important threads" list.
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Old 09-04-2015, 02:35 PM   #5
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<p>Please edit for correctness.</p><p>From the slight amount of reading on the subject that I've done the megasquirt options are as follows:</p><p><strong>90-93 (B6):</strong></p><p><em>CAS on rear of intake cam,</em></p><p>- Stock CAS can do sequential fuel+ign</p><p><strong>94-97 (BP):</strong></p><p><em>CAS on exhaust cam? Crank sensor?</em></p><p>- Stock NB wheel?</p><p>- 36-2?</p><p>- 36-1?</p><p><strong>99-00 (and MSM) (BP-4W):</strong></p><p><em>CAS on front of intake cam, Crank Sensor</em></p><p>Stock wheel + CAS? Something about cutting off a tooth?</p><p>36-2 wheel?</p><p>36-1 wheel + another sensor?</p><p><strong>01-05 (except MSM)&nbsp;(BP-Z3):</strong></p><p><em>CAS on front of intake cam, Crank sensor</em></p><p>- Stock? Does it allow for seq ign?</p><p>- 36-2 wheel?</p><p>- 36-1 wheel?</p><p>&nbsp;</p>
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