V8 RPM Signal

[PaulR]

Noticed on different a few different holden v8s the rpm signal isnt very linear under load. Ive done a few logs now on v6 engine and much better. Not sure if its do with V8s having a Dizzy and V6s being crankshaft sensor, or electrical noise? The Cars seem to run well, no missfire etc Anyone else come across this?

[Dylan]

I have noticed that on the V8's. I've logged a couple of V6's but cant really remeber there RPM trace.

[vlad01]

V6 are pretty much dead straight like this.

[PaulR]

V6 are pretty much dead straight like this.

Thats a good log, bet some time has gone into that.
I noticed on the v8s that the RPM signal is more straight in lower gears when the engine revs up quicker, worse in higher gears and more load?

[vlad01]

Likely timing scatter from the fact its a dizzy thats driven by a gear which is driven by the cam , gears have lash and so does the cam due to the timing chain and sprocket. V6 is a solid fixed sensor reading the crank so there is next to no error in timing.


Thanks for the compliments, that was actually early stages of my tuning the VR, its a lot more refined now, there was several 10's of logs after this one and even many hours on the dyno.

Can't say how many 100s of hours was spent on this tune lol

[kojab]

Logging rates (frames / seconds) also have a great influence on how smooth or rough the RPM signal appears. Notice how the Wideband reading looks ragged as well.

[antus]

Yeah, but it does not average the rpm between frames or anything so the data shown is what the ecm was thinking at that moment, the dizzy lash would be the biggest factor. A faster scan rate would not flatten the curve - the data points would be as far off the average and it would just look a little more rounded instead of jagged. Having said that if you went even slower you might miss the lash entirely even though its still happening.

[Holden202T]

yeah its the dizzy/cam/crank play thay does it, I had the same results with my 202

below is a log with the dizzy.

dizzy trace2.jpg (132.07 KiB) Viewed 6570 times

then a log with crank mounted trigger wheel.

the dizzy is noticeably worse at high revs.

that said it doesn't seem to make a bit of difference to the motor, but I'm sure if you measured it with a dyno or something it might show some differences.

[Gareth]

Has the 202 got straight cut cam gears?

[Holden202T]

yep and I know from when I built the motor that there is rock in them!