Knock control setup question - too aggressive after poly bush?

[M1tch]

Hi all,

Just logged some data from some 1/4 mile runs yesterday, I have been tweaking the knock control for a while to get the cylinder trims about right and adjusted the knock threshold a bit as well as the gain (currently at a gain of 5).

After checking the logs from one of the runs yesterday (not been able to fully run through all the RPMs on the road) I can see that its pulling about 5 degrees of timing out at the top end as its over the knock threshold.

I have recently changed over to some stiffer poly engine mounts which I would guess are now adding additional vibration into the system. From what I can see as the cylinders are showing fairly equal 'knock' that its not actually knock its just more vibration so I am thinking that I perhaps just need to dial down the gain slightly to reduce the signal. I can then log a run in 2nd gear on the road and see where that peaks at and adjust where needed - might also dial back cylinder 1 slightly as its consistently higher.

You can also see in 3rd and 4th gear the boost level is fluctuating and not actually reaching boost target probably due to the timing being pulled. You can see that the fluctuations are about the same time as the knock threshold is exceeded - 5 degrees of timing is being pulled out meaning that the engine is being given around 7 degrees advanced at around 13 psi.

Current cylinder level gains are (might actually just reset to just 1 across all cylinder):

1 = 1.00

2 = 0.80

3 = 0.80

4 = 0.85

Just wanted to sense check that this 'knock' is actually just mechanical noise rather than true knock as its gradual vs the RPM - note that I am seeing the same trend when I ran at spring pressure (7psi) compared to 12-13psi in later runs.

[M1tch]

Have just adjusted the gain to 4 rather than 5 and adjusted cylinder 1 to 0.95 rather than 1.00 so its more inline with the others, boost was stable at around 12psi (map of 180kpa) across the rev range. There is a minor exhaust leak so I think there might be some rich misfire up top, not sure whats going on with cylinder 3 but at least I know where the rest of the knock levels are at. Noticed that the hint in the help file is for the knock threshold to be about 20% higher than the highest non knock level.

I think I might drop the gain down to 3 so that there is more headroom - although looking at another 2nd gear pull the highest knock apart from cylinder 3 seems to be at around 400 - again need to check why 1 cylinder is spiking up a bit as I don't think its going to be knock. Cylinder 3 was showing consistent 'knock' level of around 800 as can be seen below- its not spiked though. Perhaps I should adjust the cylinder 3 level gain to say 0.75 rather than 0.80 - I can see it is slightly higher on the runs on Saturday as well.

 

[Adamw]

Chassis related stuff definitely influences both knock volume and frequency so you have to re-verify and/or adjust after any changes like that.  There is an example with frequency analysis in one of the Motec documents where from memory a rollcage and exhaust was fitted to a mitsi evo and it changed the knock frequency for best SNR.