Ignition cut vs Fuel cut for shifting?

[TurboS52]

I have a DCT transmission and when I upshift I have the trans module (DKGecu) send a shift cut signal and I am wondering what people recommend.  Do I cut fuel or ignition.  It is a Turbo application and the shifts are fast 50ms.  I am thinking if I cut fuel it will run lean and could cause detonation and if I cut ignition I have heard of damaging the turbo from the unburnt fuel igniting in the turbine housing.  Just looking for input.  I have been using ignition cut for a while but wondering if I should be using fuel instead.

 

Thanks,

-Mike

[Adamw]

Fuel cut will not cause it to run lean - there is no combustion at all when you cut the fuel.  i.e a "20% fuel cut" doesnt mean you are removing 20% of the fuel, it means you are cutting 20% of the combustion events.  

Because fuel is injected and sitting in the intake port quite a bit before combustion, a fuel cut is not instant torque reduction - you still get a cycle at full power after activating the cut - similarly when the shift has finished and you want to restore power, you dont get any power for a full engine cycle after reactivating fuel.  This can make the shifts feel much slower - at least with motorsport gearboxes, I dont have any experience with road car shifts.  

Fuel cut is safer for some engines that have trouble with valve control or hydraulic valve lash control.

Fuel cut is quieter - generally no bangs. 

Correspondingly,  Ignition cut is much faster, but noisier, can cause issues with valve control, and potentially harder on lambda sensors etc.  I havent seen any issues with the turbo from ignition cut gear shifts.  

 

[TurboS52]

Excellent explanation.  I appreciate it.  I will continue to use Ignition cut for shifting since it is 50ms shifts.  I went out and got the timing just right and it feels perfect.

Thanks

-Mike