4.2L crank + aftermarket cam= ??? in setting cam sensor

[SilverXJ]

I've been thinking about this for a while. As far as setting the cam position sensor on 00+ XJ, TJ and 99+ WJ maybe the factory method isn't the best. The FSM tells you to stab the cam sensor in at TDC on compression on the #1 cylinder then do minor adjustments with a DRB scan tool. As far as the FSM is concerned it is referencing TDC and a static point in valve timing. Using the 4.0L crank, stock cam and stock non-adjustable timing set. My thoughts are that if you change the cam and/or add an adjustable timing set you change the valve timing events. That will change the optimum injector firing time in relation to the valve events. The way the FSM does it is to sync the cam sensor with the crank then fine tune it exactly to the crank using the DRB.

Maybe this isn't ideal for out application. Perhaps stabbing it at TDC is good enough to get us in the ball park, but what about the fine tuning previously done with a DRB? The what happens when you mode the crank sensor to adjust timing? One method I have had suggested to me is to loosen the cam sensor just enough that you can turn it by hand with teh engine running. Mark a single point on the can sensor drive to reference off the engine block. Then turn the sensor clock wise until you hear and/ or feel the engine idle start to degrade, mark that spot then turn it counter clockwise and do the same. Find the middle of those points and position the mark on the drive there. My only concerns are a)that you might be changing the ignition timing a bit, but I don't know how much of a role is played by the cam sensor in timing events and b) what would keep the PCM happy.

On the factory cam(going off the numbers I have) the intake valve opens at 14* (BTDC) on seat to seat and -20* (or 20 ATDC) at .050" timing. On the Hesco cam I an running 4* advanced the intake valve will open at 26*(BTDC) seat to seat and 4*(BTDC) at .050" timing. So, numerically speaking, in my case would it be better to advance the cam sensor 6* (as the difference between the stock and Hesco is 12* and divide that by 2 to take in the fact that the cam turns 1/2 speed of the crank)? Then possibly advance it another 3* for the 6* I advanced the CPS? Total of 9*.

Thoughts?

[jsawduste]

Think you are on the money.

The adjustment process you mention is exactly what i used to set the injector timing on my OBD1 engine. Might be a small change in ign. timing for it more then made for in injector timing.

As you mentioned........The FSM is written with the specs of a stock engine. Changing any of the parameters changes the dynamic values. Thus the adjustment methodology needs to be changed as well.

[SilverXJ]

Then again, how sensitive is the adjustment? Our engines are sequential fire, but other engine get by with batch fire. So if the indexing of the sensor is off a little bit would it even matter that much?

[dwg86]

My only concerns are a)that you might be changing the ignition timing a bit, but I don't know how much of a role is played by the cam sensor in timing events and b) what would keep the PCM happy.
Thoughts?

Everything I read says the ignition timing is not effected by turning the distributor or "cam sensor". The only way I can see the ignition timing can be changed is moving the crank position sensor. I think the toothpick method works good. It got me within 2 degrees or units or whatever the DRB is looking at.

[SilverXJ]

Everything I read says the ignition timing is not effected by turning the distributor or "cam sensor".

Same here.

I think the toothpick method works good. It got me within 2 degrees or units or whatever the DRB is looking at.

I use the toothpick method and haven't had a problem. I'm talking more along the lines of fine tuning the position. And perhaps what the DRB wants isn't correct for out application.

[gradon]

The is an area that is an unknown variable to me as well cause I'd like to know with certainty that it was spot on instead of off a few degrees. At least I'm using a Mopar cam, so has somewhat of the factory backing.

[SilverXJ]

My cam sensor failed today. Just the sensor, not the drive. Came back from lunch and the MIL went on. Scanned codes after work and P0430 comes up, cam sensor circuit failure. Figure I would take care of it once I got to the evening job at the machine shop. No such luck. The Jeep just didn't want to start. Popped the hood in the rail. Plug as firmly in the socket, so it wasn't a lose connection. Remove the cam sensor and the magnet was sitting in the center of the stator. No wonder it wouldn't work. The rare earth magnet just became unglued from its backing plate. Got spare sensor off of the spare drive keep in the trunk and installed it. Cleared the codes and it fired right up. Incidentally if the cam sensor is faulty you might get codes P0351 P0352 and P0353 which indicate a coil problem; the same codes I got last time I had a CPS failure (No CPS code though). Glad it was an easy fix and I had one on hand. The sensor that was on there has now been fixed with some JB weld. I should have checked the sensor earlier as a few times I had a back fire/no start on the first cold start. That should have told me something, but I figured it was something I did on the programing of the AEM as it coincided with the back fire start issue.

Anyhow, the reason I pulled this thread up is because it got me thinking on correct timing of the cam sensor to crank sensor. Of course the injector firing is based on when the intake valve opens. I have heard that in stock configuration the injector fires while the valve is closed and right before it opens. However I don't know if this is true or not, but if anyone knows different let me know. In my case the stock cam opens the intake valve opens 12.4* BTDC. The Hesco cam opens the valve 22* BTDC (or in my case 26* BTDC because it is advanced 4*). So, the injector is now firing 13.6* AFTER the valve location in a stock configuration. I initially though that I would have to divide 13.6* by two since the cam is moving at half the speed of the crank shaft but then I realized that the cam sensor/ oil pump drive gear is 1/2 the size of the cam gear. So, the cam sensor stator is moving at the same rate as the crank shaft. Theoretically all I would have to do is set the cam sensor in its stock location then advance it 13.6*. Since the drive rotates clock wise I would just need to rotate it 13.6* counter clock wise to get it in the correct position. However, that is based on the fact that the injector fires on the intake valve opening, not some other event like after overlap. Now 13.6* maybe a bit difficult to determine on such a small circle such as the cam drive, but if I rack my brain enough I could probably bring back the college math and convert that to inches on that small circle. And we all though that we would never use that math. I know that I have actually used it quite a bit on figuring numbers out on my engine. I can't recall it immediately but a short refresher and it does come back.

[CobraMarty]

How small a circle? 13.6*/360* x circle diameter x 3.1415(pi) = for a 1" circle .119" Does that help? Is that about a toothpick thickness?

[SilverXJ]

Yeah, that is the equation I would have used. However, I just placed the mark on the harmonic balancer at 13* BTDC then set the cam sensor in as usual with its holes lined up (tooth pick method). I haven't noticed any difference in idle though or any where for that matter. I also played with rotating the sensor with the engine running. I found that if I rotated the sensor the same amount clock wise (retard) that the engine didn't like that much. Rough idle, mis fires, etc.

This also brings up another issue. With the AEM when you retard timing it retards it on both the crank and cam sensor outputs to the PCM. This seems like a bad thing and i should probably unhook the AEM from the cam sensor. However, I wonder how the PCM would react if it saw the two signals varying from each other.

[CobraMarty]

Why are you retarding with the AEM? Aren't you N/A?

[SilverXJ]

I am but I have 6* added by the CPS sensor mod and another 6* added by the PCM program and there is a slight bit of ping.

[I6FAN]

silverXJ wrote:

I initially though that I would have to divide 13.6* by two since the cam is moving at half the speed of the crank shaft but then I realized that the cam sensor/ oil pump drive gear is 1/2 the size of the cam gear. So, the cam sensor stator is moving at the same rate as the crank shaft.

I thought the cam-to-distributor/oil pump was 1:1; which makes it half of crank speed also. One turn of a distributor completely fires all six cyl. which takes two crank revolutions???

[SilverXJ]

I thought the cam-to-distributor/oil pump was 1:1; which makes it half of crank speed also. One turn of a distributor completely fires all six cyl. which takes two crank revolutions???

I questioned this when I was on the phone with jsawduste and though how the distributor would work if its spinning at twice the speed of the cam. I pushed it off at the time as the PCM is controlling when fire gets to the distributor. I was still scratching my head about it though.

My error was relating the timing set, which has to have the same sized teeth for the timing chain and uses different diameter gears, to two gears in contact (like a diff or the oil pump drive in this case). I recently put the cams I have in storage, but I had some pics of the cam and oil pump drive gears. I could only see half the gear, but they each show about 7 teeth on half the gear, equal tooth count. So, I had a massive brain fart and forgot that it was the tooth count that determines the resulting speed, not the actual size of the gear. Now I feel stupid.

Ok.. so I'll have to re adjust the cam sensor back ~7*. I'll just set the crank at 6* and drop it in with the toothpick.

I did find out that a timing light can read the secondary inductive kickback voltage when the injector coil's field collapses. That will give me an idea when the injector is firing.

[Retlaw01XJ]

....change the optimum injector firing time in relation to the valve events.....

Thoughts?

What IS the optimum injector timing??
I don't have a clue, so I asked Google...

Some basic guidelines from Megasquirt:
http://www.megamanual.com/v22manual/mfuel.htm

Injector timing is generally calibrated to the CLOSING of the intake valve....
>>> a brief introduction to the workings of MegaSquirt.
.......
However, sequential injection does not necessarily mean you are injecting into an open intake valve all the time. The intake valve is only open less than 30% of the time in a typical 4 stroke engine. Once you are trying to produce more than about 25% of maximum HP your injectors are firing for longer than the intake valves are open. If your maximum HP is correctly calibrated to a safe 80% duty cycle, your injectors are injecting well over 50% of the time on closed valves.

At higher rpms, it becomes increasingly difficult to inject while the valve is open.
.....
In any case, when fuel is injected while the valve is closed, it will simply stay in the port until the valve opens. In some cases, this period of time may allow enough heating of the mixture to better vaporize the liquid fuel, improving efficiency and emissions.<<<<

Another informative post by Ross at Motec:
http://www.motec.com/forum/viewtopic.php?f=39&t=769

>>>>>Injector Sizing
Typically in a production car an injector is selected to run at around 85% duty cycle at maximum fuel demand. This will normally allow enough resolution in the fuel table at idle speed to provide accurate fuel mixture adjustment and injection pulse times that are not too short. In this case the injection timing will have little effect at high RPM / Load because the injector will be open most of the time, by the time it closes it is nearly time to open again. Bigger injectors can be used which would give shorter injector duration at high speed and take advantage of the benefits of timing the fuel, however this will cause a loss of resolution and very short injection pulses at low speed. This will cause poorer idle quality and starting, particularly when the engine is cold. An alternative to this is to use secondary injectors (Hi/Lo).<<<<

What I've primarily learned from those articles is that:
- Injectors optimized for high rpm ( Hi flow, short pulse width) will be more difficult to control at idle and low speeds.
- Conversely, Injectors on the small side will need excessive pulse width at hi rpm.... a longer time than the intake valve is open, (which is typical).
- 'End of injection' is generally the timing setpoint.
- Spraying onto the back of an intake valve before it opens isn't a bad thing, and may even be beneficial.
- ***Injector timing depends largely on injector size and preferred rpm range.***

[SilverXJ]

I'm very familiar how idle and starting goes with an incorrectly sized injector. LOL.

I took a look at the timing again today using the timing light. At first I couldn't get the light to fire. It worked yesterday clamped to the injector wire. I tried every which configuration on the wire, but no go. I finally figured out that it had to be near the injector itself, not the wire. After I figured that out I was able to get some numbers. At idle the injector fires at roughly 43* BTDC. Bringing the RPM up to 2000 it was firing around 27* BTDC. This was under no load, so I don't know what it does with a load on it. Its surprising how early the injector fires before the valve opens.