Overrun Timing Control

[Jenno007]

Hi guys,

Another thread regarding retarding timing on overrun.

I have a car that has been professionally tuned and is running great, however I would like to have a little play around with getting the "rally car" pops and crackles on overrun.

The car is very rarely street driven (very rarely driven), and I understand the accelerated wear on the exhaust + cat.

The car currently has the overrun fuel cut turned off (does induce a little bucking around 1500 rpm in first but I just put the car into neutral before coming to a stop to avoid this). Having the fuel on overrun made no difference to the exhaust note so I'm hoping if I retard the timing that will have the desired affect.

I'm just wondering the best way to do this. If I have a big difference in timing from 0% throttle (eg -10deg) to the next increment in the ecu, say 5% throttle (eg 10deg), will that have any undesired affects? I thought I would ask here before I play around.

Cheers

 

[Adamw]

You should only need to adjust the cells that will only be accessed during overrun.  If your ign table has TP as the load axis then the cells I have highlighted in the table below would be what I would adjust.  There should be no problem having big changes/increments between these cells in this area as they will only ever be active during overrun where cylinder pressure is very low and there is almost no combustion. 

[Jenno007]

Thank you Adam.

Could you advise a reasonable starting point, and a figure you wouldn't want to go past? Or is this fairly safe from an engine point of view.

Cheers!

[Jenno007]

I also note when my car was tuned, the timing is dependent upon MAP, not throttle %.

If I change the axis and alter the throttle position settings, I believe that would mess up the timing for all other conditions? Seems I may not be able to do this without a professional retuning the timing? I'm hoping I'm wrong...

[Simon]

You can still do this as on overrun you will generate more vacuum so should be in a similar place in the map (top right corner) where you can retard the timing.

The other option would be to activate a 4D overlay table that is based of TPS and RPM as a condition (use a virtual aux) 

[Jenno007]

Thanks Simon. The 4d table sounds a little complex for my knowledge so I might just take your advice regarding using this table.

I guess I just take some logs to see what the MAP is at light throttle vs 0 throttle so I know what cells to adjust in the table.

Would it be disastrous if I had -ve timing whilst I am light throttle? Just incase I get it wrong at first.

[Adamw]

5 hours ago, Jenno007 said:

I guess I just take some logs to see what the MAP is at light throttle vs 0 throttle so I know what cells to adjust in the table.

Get someone to take your car for a short drive with you in the passenger seat with your laptop.  Open the ignition table and turn on the snake trails, these will show you the cells that the engine travels through during the last few seconds of operation, watch it during overrun and you will have a pretty good idea of which cells you need to adjust and if they are ever close to being accessed during normal driving.

 

6 hours ago, Jenno007 said:

Would it be disastrous if I had -ve timing whilst I am light throttle? Just incase I get it wrong at first.

No it will just drive like shit.

[Jenno007]

Made the changes and I've got the desired effect! Sounds great. Thanks for the help. Throttle response seems to be a little worse but I'm happy to pay that price as it's a very laggy car anyway (no boost until 3.5k)

Two quick questions: 

Q1. have I taken too much timing out at high rpm? I've only tried it from 3000rpm down to 2000rpm and it sounds great

Q2. some of the pops are a little loud like a backfire, if I take some fuel out I'm hoping that will fix it a little. Any suggestions on how much I would lower the fuel values, and what region (difference between MGP vs MAP as I understand will be the atmos pressure?)

Thanks