Okay I'm not registered in this as of yet (going to give the quali a go later on) but this is what I think.
Budget Caps. This is a pretty fun way to set up a race but getting the car sorted to get the best performance out of it within the budget will take a lot of time fiddling around in the tune shop.
P/W ratio again has the same problem as the budget caps lot of time spent in the tune shop....but it creates the posibility drivers running lots of different cars for a particular race.
Personally I'd set a P/W ratio for the whole weekly race thang of around 2.0 - 2.5 then once a driver has a car they could race it at what ever race they liked with the driver changing it when the got fed up with it. (each driver could be asked to enter a FF, FR, 4WD car that they will use for the weekly race. The the type of drivetrain along with the P/W ratio could be used to add a bit of variety not to mention limiting parts each week suspension chassis, transmission)
It could run different P/W ratio's for the pros, ams & beginners this may get people wanting to work up the ranks to be able to use faster cars.
Tyres, run with some lower grip tyres, normal or sports or t2's and of course add in a bit of tyre wear here and there (sport tyres are a good way of forcing a driver to pit in a ten lap race but have around the same grip as a t2, I've been trying this out with a mine's skyline in the race of the red emblem where sport tyres force me to pit on lap 5)
Vary the race length's / types, hotlaps/races that involve the AI with closely matched cars/sprints (standing start to T1, T2, T3)
Displacement caps a good one...I've been thinking about a 2 litre long race series for a while now....although it does cut off some of the better cars in the game.
anyway enough of my bull****
