I need help changing remote frequencies on a remote control

we are building remote control cars in my tech systems class, there is only 5 of us inlcuding the techer, but the problem we are having with the cars is that since they are so cheap they are all on the same frequency so 1 remote controls all 5, we want to "race" them and have little competitions with them, but we can't if the frequencies are the same, so i was wondering if anyone knew what we would have to do to change them, i will include a diagram from the car and the remote of the boards any help would be appreciated

thanks Keith, Kyle, Kyle, Dan

I was thinking you could replace the crystal (CRY) in four of the transmitters but it appears the receivers dont use matching crystals.

From your schematic it looks like the receivers use inductive tuning, you could try tweaking the variable inductor ( L1 ) to tune the receivers to match the transmitter frequencies.

I agree with nettron1000, but in receiver need more tank circuit for increasing the selectivity. This receiver with only one tank cct. is relative wideband receiver, and can work with any signal in CB-band.
(I've seen a moving bear from Taiwan, who jumped from table when a taxi-driver say "break")

Thanks Sebi, that original schematic was hard to look at.