Weird Automatic Shift Logic (Did I Break It?)

I'm having some trouble getting the automatic transmission to shift appropriately, and figured I'd ask here if I'm doing something wrong. Before y'all laugh at me for making the most ungodly Frankenstein combination of parts, I am well aware that I'm pushing the limits of what the devs intended for the parts configurator.

Here is my situation: I'm trying to build a diesel drag car using the Gavril D15. Base model Fleet D15, 6.0 diesel V8, 4spd drag transmission, some chassis mods (suspension, diff, tires etc) but those are irrelevant here. I am also using the race ECU and have the rev limiter at 4500 rpm, quite a bit higher than stock.

The problem: On launch, the gearbox will hold first as intended, then when shifting into second it will immediately shift into third for a moment before shifting back down again. Same thing happens on the 2nd/3rd shift too, it double shifts then has to downshift to get back into the correct gear. Using the Shift Decision Debug I can see that at idle the shift-up point is around 3500 rpm, and when I'm on full throttle, the shift-up point moves to 4300 rpm. However what's actually happening is it's shifting up at ~3100 even under wide open throttle. The problem persists no matter the stall speed of the torque converter, or final drive ratio.

Anyone got any ideas what's up? Or was this transmission just never coded for use in a low rpm diesel application? Any help would be greatly appreciated

 

I think this might be the cause, or rather the transmission not being adapted to petrol & diesel engines at once, but just one of them. I tried to build such a drag car yesterday & also had the same issue with the transmission upshifting more than it needed to

 

I see, do you know of any way to change that? There must be some parameters in the code that can be changed to suit a certain rpm range