So for a while now I've noticed that the RPM signal to the ecm is a little unstable. It'll go up and down like a saw from the oil pump and cam twist, and etc. So my question; can this effect the accuracy of the SA delivered to each cylinder? Can one cylinder be getting 20 and another 24? Maybe this is a reason why GM went to the cam chain optispark since it was a direct drive from crank being more accurate.

 

It'd be more of a timing chain issue IMO.
If the oil pump gears were doing it, I'd have imagined they'd have gone gerotor ages ago. Cam twist is from REALLY stiff springs from what I've heard.

I believe the optispark was more about lowering the overall profile of the engine, for being able to slide it back under the windshield, imagine a VI gen with a dissy and trying to change the cap and rotor?, brrrrrr, that lead anyone to swear....

 

Makes sence. Although cutting a little hole in your dash with an access cover to the firewall to adjust your base timing while driving.... now that would be funny!
I just figured it might be an issue.

 

So they put it behind the water pump! Great choice! IMO they just have just gone DIS and been done with it. They had the ststem ready to go (LT5)! They could've put the reluctor int the timing cover as they do on the Vortec truck. Besides, that would've made the DIS swap for us much easier!

 

Good question, AFAIK, yes, the changes you see in RPM will also be affecting the SA timing. The RPM is calculated from the number of 15.26 usec counts between each DRP.

The ECM will fire the coil at each DRP. The changing RPM is an indication of spark scatter.

To calculate the amount of scatter in degrees BTDC, take the difference in DRP counts from one DRP to the next and multiply by 15.26. That will give the varience in micro-seconds.

Then take the RPM and convert that to the number of micro-seconds between spark plugs firings.

Divide that by 90 and multiply by the varience and out pops degrees (I hope). I may have gotten that partially incorrect but you get the idea.

I posted a thread this past winter where I motored a distributor while connected to an ECM and scoped the EST line. I found the ECM would contribute +- one clock count of error (15.26 usec). This is to be expected and I was happy to see that.

RBob.

 

YES the distirbutor will cause spark scatter and up and down readings pull it out and shim the bottem gear on the shaft so there is no up and down play. also old worn distributors can have play with worn shafts and or housings.