For SRX I used a simple system of 2 sounds - they're both a looping engine sample at high revs, but one is more aggressive (the ON throttle sound) and the other is muted and quieter (the OFF throttle sound).
I simply play them both together, then as the player varies the throttle I adjust the volumes in realtime - at 100% throttle the ON sample is at 100% volume, at 50% throttle both ON and OFF are at about 75% volume, and at 0% throttle ON is at 0% and OFF is at 100% volume.
To get the pitch right I simply calculate which gear the car is in, dep. upon speed, then work out the required pitch for that gear & speed and adjust both samples to match. The sample is the engine at high revs, so most of the time it is being played slower than normal speed.
I've added a few other tweaks - as the car goes up the gears the baseline for the pitch rises, meaning each higher gear has a slightly higher pitch than the last, and first gear plays at a slightly different rate again, merely to get it to sound right.
I measure the sideslip of the car (relative movement in the X axis of the vehicle, which is used for the volume of the tyre scrub SFX also) and increase the engine pitch according to this sideslip, so as the player powerslides the car the engine revs appear to increase slightly as the car gets more sideways - this is done only because it sounds right when playing, not because it's physically accurate.
I also drop both ON and OFF volumes to 50% for a few frames when the speed reaches a gear change point, to simulate the driver lifting off the power for a brief moment.
Finally I build up a turbo over-run counter whenever the player has 100% throttle - when the player releases the throttle I check this over-run and if it has built up beyond a threshold point I play a random exhaust pop / bang sample (at random volume, so sometimes you get no pop) - this simulates the build up and release of turbo boost pressure, but the threshold ensures it doesn't happen every time the player comes off the throttle, just after a sustained period of acceleration.
Hope this helps.
I simply play them both together, then as the player varies the throttle I adjust the volumes in realtime - at 100% throttle the ON sample is at 100% volume, at 50% throttle both ON and OFF are at about 75% volume, and at 0% throttle ON is at 0% and OFF is at 100% volume.
To get the pitch right I simply calculate which gear the car is in, dep. upon speed, then work out the required pitch for that gear & speed and adjust both samples to match. The sample is the engine at high revs, so most of the time it is being played slower than normal speed.
I've added a few other tweaks - as the car goes up the gears the baseline for the pitch rises, meaning each higher gear has a slightly higher pitch than the last, and first gear plays at a slightly different rate again, merely to get it to sound right.
I measure the sideslip of the car (relative movement in the X axis of the vehicle, which is used for the volume of the tyre scrub SFX also) and increase the engine pitch according to this sideslip, so as the player powerslides the car the engine revs appear to increase slightly as the car gets more sideways - this is done only because it sounds right when playing, not because it's physically accurate.
I also drop both ON and OFF volumes to 50% for a few frames when the speed reaches a gear change point, to simulate the driver lifting off the power for a brief moment.
Finally I build up a turbo over-run counter whenever the player has 100% throttle - when the player releases the throttle I check this over-run and if it has built up beyond a threshold point I play a random exhaust pop / bang sample (at random volume, so sometimes you get no pop) - this simulates the build up and release of turbo boost pressure, but the threshold ensures it doesn't happen every time the player comes off the throttle, just after a sustained period of acceleration.
Hope this helps.