Advancing the cam sensor - How much?

B

Blazer406

Mechanical Engineer
Joined
May 2, 2002
I can reason that the cam sensor tells the ECM where #6 is to start the firing sequence of the injectors.... If I understand this.... the theory is to inject the fuel at the correct time to not allow any puddling of fuel and maximize the efficiency of the fuel being sucked into the combustion chamber. With that said.... I could argue... that I buy into the concept that a larger cam has the intake valve opening sooner because of the increased duration...... and that all things equal... it would seem logical that you needed to advance the cam sensor timing by the same number of degrees that the intake valve has opened sooner....

Example:

Not sure of the stock cam specs.... but for arguments sake lets say it is basically 194 @ .050 on the intake.... lets say I plan on running a 218 cam...... for the sake of argument... lets say they are installed on the exact same intake centerline.....so therefore there is 24 degrees total difference in duration (@.050).... therefore... since they are on the same centerline....the intake valve (could confirm with cam card) opens 12 degrees sooner........ so if I want to inject fuel at the same time relative to the opening of the intake valve... I would need to "advance" the cam sensor by 12 degrees.

Did I hit my head? Or does this make any sense?

I found an interesting thread on this over on another board. It says.... it can affect the way it runs... especially at a higher RPM..... but shouldn't affect the timing at all of the injecotrs.... this subject gets muddied yet again....

cam sensor timing thread
 
The Casper's Electronics Cam Sensor Cap comes with slotted holes where the cap screws down to allow for adjustments such as you are asking about. John's directions that come with the cap doesn't tell you how much to adjust but I'm sure thats because that amount depends on the cam's specs. Seems like you have a grip on the concept behind it. If you use the Casper's piece you can make the adjustments without having to loosen the hold down bolt.
 
Evidently..... on a car with a sensor adjusted nearly all the way out of whack.... like at the edge of acceptable adjustment on the correct cylinder...... that there can be some drivability issue... popping... etc... above like 3000 RPM's..... seems to happen more on cars with larger cams....... that if you advance the cam sensor some..... it can affect this issue above... sometimes eliminating it.....

I am pretty certain.... that this thread (the one I posted the link to above) does have some extremely valuable content.... as the guys are bench testing these various theories using an oscilliscope.... so you can follow exactly what these adjustments are or are not doing when they make them.

A good read none-the-less. They had a double posting issue sevaral times in the thread... that makes it a little hard to read.... but I got thru it....
 
Nasty Wendy said:
The Casper's Electronics Cam Sensor Cap comes with slotted holes where the cap screws down to allow for adjustments such as you are asking about. John's directions that come with the cap doesn't tell you how much to adjust but I'm sure thats because that amount depends on the cam's specs. Seems like you have a grip on the concept behind it. If you use the Casper's piece you can make the adjustments without having to loosen the hold down bolt.
Click to expand...

Per one of the articles on GNTTYPE if you don't have a fancy cap........

Jim Frankovich has done some measuring and calculating and has the following input:

20º = 1.160"
21º = 1.218"
22º = 1.276"
23º = 1.334"
24º = 1.392"
25º = 1.450"
30º = 1.740"
Every degree figures out to be about 0.058"

You just make you a new piece of tape... and set the cam sensor normally.... to the differnt mark...
 
All this junk begs the question in my mind...... why didn't GM just put a signature window in the window on the balancer so it knew where #6.... or #1 or whatever was so it could fire the injectors off that signal. Would have made for a simpler system IMHO.... one less sensor to "go-out".
 
I would imagine that if they had used the balancer alone with no input from a cam position sensor they wouldn't be able to determine if #6 (or whatever #) was on the intake or power stroke.
 
The TBs.com thread was very interesting but it got flaky when mistakes kept exposing themselves with how he had his bench set up. Made me wonder if anything worked the same on his bench as in real life on an actual car. I know that my car runs different when my cam sensor is moved......and I'm not talking about a whole lot of movement either.
 
I know I was experiencing tip-in stumble issues with mine set exactly to factory specs. After rotating it counter clockwise a couple degrees, it cleaned right up.
 
I thought I read.... that the injector timing was triggered off the crank sensor once the order was initiated by the cam sensor (#6 pulse)..... and that there was a programmed "delay" on both the crank sensor and cam sensor... separate delays..... programmed into the chip.....in one particular cell (each.... respectively)

If this were the case..... surely you can alter this data value to "advance" when the injectors fire.....

I went back and found the quote off the thread:

There is a correction in the chip for this called 'time domain correction to spark' at $3164 in the chip. Just as the crank sensor has a delay, so does the cam sensor. But the thing is, they don't have the same delay. So what happens when I rev the engine up, and being that the cam sensor is way out of wack at this point, I can get the engine to stumble. And this is because the order of events is changing.

Interesting?
 
i'm just curious why they didn't just design everything to go together with the balancer lined up right at TDC?
or would that have been too simple for the average normal dude working on his own car, which would have cut into the profit making capability of the GM service department?

now that i think of it, i think someone makes an aftermarket cap that is designed to do just that.
 
novaderrik said:
i'm just curious why they didn't just design everything to go together with the balancer lined up right at TDC?
or would that have been too simple for the average normal dude working on his own car, which would have cut into the profit making capability of the GM service department?

now that i think of it, i think someone makes an aftermarket cap that is designed to do just that.
Click to expand...

The Caspers cam sensor cap does just that... It has 25 degrees of adjustment in each direction. You put the crank at TDC and twist the cap all the way in one direction to get your 25 degrees. Once you lock the sensor bolt down, you rotate the cap back to zero and tighten the 2 screws... BAM you're done.
Unless that is, you have a tip-in stumble and need to go a few more degrees CCW. ;)
 
novaderrik said:
i'm just curious why they didn't just design everything to go together with the balancer lined up right at TDC?
or would that have been too simple for the average normal dude working on his own car, which would have cut into the profit making capability of the GM service department?

now that i think of it, i think someone makes an aftermarket cap that is designed to do just that.
Click to expand...

If you are referring to the caspers cap with the light in it..... according to the thread I posted from the other board.... basically you can't "advance" the cam sensor..... it eiter fires the injectors in the correct sequence.... or if you have the cam sensor out far enough it will jump to the next sequence.....which is 60 (camshaft) deg off on a even fire engine.

I guess I should clarify.... in theory.... although you are physically roating the cam sensor a few degrees if you choose to advance it.... it fires the injector at the same time.... unless you are close to the next "window" for a different cylinder.... then it jumps to that sequence.

Some still swear it helps if you find you have a tip in stumble you can't get rid of.

Don't think I won't try it if mine is acting an @ss.
 
From my understanding, the cam sensor is used to start the engine only. Once running the crank sensor and the ignition module "take over". BUT, for whatever reason when I set my cam sensor correctly, the engine won't start, sounds like it is way to far advanced. I have to move just till the light goes out. Car runs great and I know the cam sensor isn't exactly where it's supposed to be.:confused: Phil.
 
pdzz said:
From my understanding, the cam sensor is used to start the engine only. Once running the crank sensor and the ignition module "take over".
Click to expand...

Yeah well I once started my engine and didn't have my cam sensor tighted enough. After start up the cam sensor started to turn a bit and the car ran like doo-doo. Rev'd it a little and the engine popped and killed. Checked everything out and saw that the cam sensor had moved .... reset it and tightened it GOOD and car ran great.

I can't believe that the cam sensor signal is ignored after start up based on my experiences.
 
Blazer406 said:
I can reason that the cam sensor tells the ECM where #6 is to start the firing sequence of the injectors.... If I understand this.... the theory is to inject the fuel at the correct time to not allow any puddling of fuel and maximize the efficiency of the fuel being sucked into the combustion chamber. With that said.... I could argue... that I buy into the concept that a larger cam has the intake valve opening sooner because of the increased duration...... and that all things equal... it would seem logical that you needed to advance the cam sensor timing by the same number of degrees that the intake valve has opened sooner....

Example:

Not sure of the stock cam specs.... but for arguments sake lets say it is basically 194 @ .050 on the intake.... lets say I plan on running a 218 cam...... for the sake of argument... lets say they are installed on the exact same intake centerline.....so therefore there is 24 degrees total difference in duration (@.050).... therefore... since they are on the same centerline....the intake valve (could confirm with cam card) opens 12 degrees sooner........ so if I want to inject fuel at the same time relative to the opening of the intake valve... I would need to "advance" the cam sensor by 12 degrees.

Did I hit my head? Or does this make any sense?

I found an interesting thread on this over on another board. It says.... it can affect the way it runs... especially at a higher RPM..... but shouldn't affect the timing at all of the injecotrs.... this subject gets muddied yet again....

cam sensor timing thread
Click to expand...



I have known this for yrs. I have said it to some peeps and they told me it's not possible but alas somebody has been thinking :)

I noticed this when on the dyno one day that advanceing the cam sensor little by little the car started to make more power and response was even better.

The aftermarket ECM like AEM and BS3 have injector offset table that you can do this with aswell.

No the ecm looks at the cam sensor every time it comes around not just at start up. ever have one break the cap? It then goes it limp mode (batch fire)
 
After the car is running, unplug the cam sensor and see if the car still runs.
 
pdzz said:
After the car is running, unplug the cam sensor and see if the car still runs.
Click to expand...

it will continue to run but it run in Batch fire mode. without the cam sensor info it don't know where to start fireing the injectors @ #1 so it just fire them in a batch 2-6 1-5
 
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