12v step down voltage to 6, 9v 10amp

Im trying to make a methanol injection kit for my car and im trying to control pump speed with different voltage supplies. My ECU has outputs than can be set to any rpm and my goal is to activate 3 relays at 3 given rpm to voltage control pump speed as revs increase. I know very little about electronics and would appreciate any help finding a step down to 6v and 9v that will handle the approximate 10amp draw from the pump. Then the final relay will be the full 12v supply for max flow at higher rpm. The only step downs I can find are for milliamp items such as usb and phone chargers. Im going to show my electronic ignorance here but would there be an issue with the pump receiving the 6, 9 & 12v simultaneously? Because when the engine is at high revs all 3 relays and both step downs will be engaged. Any help would be amazing if possible! Cheers

1978 ford capri, high cr cosworth yb on bike carburettors and eaton supercharger

One dc-dc converter with (say) 8V to 16V input and AT LEAST 10A output AND suited to driving motors for the voltages you require. If by 12V you mean 12V, then 2 relays to switch in (or out) the extra resistors that set the converter voltage, plus a 3rd to switch power to the converter. If 12V means the 12V system's voltage (ie, up to 14.4V etc) an SPDT relay can switch the motor between raw 12V (14.4V etc) else the dc converter output. The above means there is no interconnection of different voltages which (yes...) is a BIG problem. A diode could isolate the voltages, but why use three 20A rated diode that probably drop 1V and hence dissipate 20W when used? You could have a voltage that varies with RPM - I've merely described your 3-step implementation.

PWM motor controller. Changes the speed of the motor by varying the duty cycle of the voltage applied to the motor. Along the same lines as Oldspark mentioned, have your ECM switch resistances to vary the duty cycle/speed. From what I've read, simply powering a motor with less voltages doesn't work efficiently as a method for speed control.

x2. Voltage can control the speed; it depends on the motor. But as Ween said, PWM is generally the way to do it.