Erratic RPM vs Time Graph

[Jayme]

Hi all, im very curious as to why the RPM vs time graph on v8's (well, the three that I have tried) is very erratic, esp in 2nd gear when compared to a V6, which is relatively smooth. example attached.
Could it be somehtign to do with the slack in the timing chain or the difference between hall effect dizzy and crank triggered DFI? The v8's seem to have a 1-2 degree timing variance with the timing light. I dont think this helps accurate fuelling either. is anyone able to shed any light on this?

[delcowizzid]

surely slack in the timing wont drop rpm levels though it would take lost pulses or something to make rpm drop.how many rpm are we talking here its definatly eratic looking and if the ecu was using that for its calcs one would think that the fueling would be all over the show if it dropped enough to go into a different VE cell them back up to the next cell

[VL400]

Is the graph from ALDL logged data or a external logger??

My mates valiant that has timing gears is smooth as so you might be onto something with the timing chain...

[Jayme]

all were aldl logged with efilive. a mate of mine has a set of straight cuts going in his 5L in the next few months, ill take a log before and after, that should be able to eliminate it.

[VNracer34G]

How about popping the dizzy caps off the 5L motors and checking for slop in the rotor on the shaft, play in the shaft itself in the body and backlash in the drive gear - cam. Any play in that lot will scatter things up.

The V6 has got a chain and a ballance shaft notoriuos for falsely triggering knock retard but no dizzy.

Hope this helps

[Holden202T]

the v6 also has a crank trigger directly connected to the crank so it shouldn't see the erratic timing ....

i can put a timing light on my motor in diagnostic mode and see the timing moving a bit (like 1 degree either way) which doesn't suprise me at all, theres the timing gears (straight cuts) which have play in them, then theres the cam gear to dizzy gear aswell ..... it all works hahaha

[Jayme]

well, I just went out and measured the amount of play in the distributor shaft, there is about 1-2 mm of end float and (measuring as if one whole rotation of the dizzy shaft is 360 degrees) 1 degree play in the rotation, with a further 1 degree after the end float lets the helical gear climb up a bit.
so double that to crank degrees and there is a 2 degree either way play not taking into account helical gear slip due to end float. so add helical gear climbing a bit up the end float and a total of 4 degrees either side is possible. This isnt even counting slack in the timing chain which I might measure this weekend.

[Holden202T]

sounds like your getting a bit sloppy

[delcowizzid]

well my V6 is doing this same thing but real bad maybe its not slack dizzy gears in your case.my rpm is all over the show i just logged it today for the second time tried wiggling the DFI module wires no different thinking CAS dfi or coils or its to do with the series one conversion into series 2 car maybe its a loose power feed to the dfi which i think feeds injectors voltage off the same wire too going to get into it this week and find it if it kills me LOL

[VNracer34G]

well my V6 is doing this same thing but real bad maybe its not slack dizzy gears in your case.my rpm is all over the show i just logged it today for the second time tried wiggling the DFI module wires no different thinking CAS dfi or coils or its to do with the series one conversion into series 2 car maybe its a loose power feed to the dfi which i think feeds injectors voltage off the same wire too going to get into it this week and find it if it kills me LOL

On a V6 i'd be chasing your earths first they're notorious for being dodgey at worst and inadequite at best, why not just run some extra earths to the motor (block-body, heads-body, intake-body) take radiator cap off and check voltage of coolant- sounds daft i know but its part of the reason they used to eat frost plugs.