car getting loose on shift

One problem is way too much tire pressure. Wow 6.5 is high for a 2600 lb car. Drop them down to 5.75 and see what happens. The other thing is to have a retard come on for a second right at the shift and ramp back out. What does your driveshaft RPM trace look like on the data logger at the shift? does it hump way up? If so then first see if bring the tire pressure down reduces the hump and if that doesn't work then bring on some retard at the shift until that driveshaft hump becames a small hump.

 

If the tyres have too much pressure then the car is riding on the middle so less traction. If the tyres are too low in pressure then the car is riding on the outer edges. For the weight of your car the best traction with those size tyres is right around 6.0 psi BUT that maybe too much traction and can cause paddling/basketballing on launch and keep the driveshaft rpm too low so you have to change depending on the track. I would try 5.75 psi and see what happens.The other thing is the compression side of the shocks maybe too loose so the rear of the car is dropping on the shift and unloading the tyres or the rear springs maybe too light causing the same thing.

 

Try tightening up the compression side of the shocks and drop that tire pressure down to 5.75 and give it a run and see what happens. Are you sure of where your IC is? Is that a guess or is that measured?

 

If you compression is that soft then that maybe a contributor to the problem along with the tyre pressures being way too high.

Let me explain how I understand the rebound (extension) and the compression sides of the shocks work for a blower car.

The rebound's main function is to control the initial hit on the tires at launch. It works on Sir Isaac Newton's Third Law for every motion there is an equal and opposite motion. Right at launch probably just before the car actually moves when the tires feel that first hit the frame attempts to rise up away from the rear axle. The frame rising does an equal and opposite reaction of pushing down on the rear tires and planting them. If the rebound is too loose then the initial hit is too hard and it crushes and distorts the tires so you get immediate tire shake. If the rebound is too tight then you get no frame/axle separation and there is no planting of the tires so you blow the tires off right on launch. The other factors that come into play here is both the amount of power and if the car has a clutch or a converter.

Then you have the conpression side. This normally needs to be as tight as the track conditions allow so the car will not squat right after launch or at the shifts. If the car squats it takes weight off the rear tires. If the compression is too tight and the track is not smooth the car will start bouncing in second and third gear and will remove weight from the rear tires on and off. You will see this on the driveshaft trace as a series of humps. You can also hear it as chirping from the tires as the car is going down track. If this occurs then you need to back off on the compression one click at a time until you can tolerate it and the driveshaft trace smoothes out.

There are some other factors involved here but this is a top level explanation