Noo nooo man delete that video asap omg hahh. I appreciate the shout out, but we may be misleading a lot of folks out there with the info given in the video. I mean maybe I wasn't clear enough when I talked about the diff on these old rwd cars. I was talking specifically about the Sierra Cosworth, which comes equipped (in this game only actually which surprised me) with a viscous differential. The viscous differential has nothing to do with the accel and decel differential ramps you see in the other cars.
What you're doing in the video is setting the first slider to a loose setting (power ramp_acceleration diff). What this does is not allowing the wheel under load to spin faster than the other in the same axis, which may give you a false* feeling of superior control of the back end on slippery roads (mud_ wet mud; gravel_wet gravel; snow; ice; wet tarmac; other), but will also make you lose a lot of time, because it won't allow the car accelerate to the max until there's no more slip.
When racing, regardless dry tarmax or dry gravel_mud, you must use a blocked differential. It will always lead to more understeer and the car will be prone to spin the wheel too much when in the middle of the act of negotiating a corner, but will give you more stability the moment you enter it, and with the proper throttle_steering skills, allow you to exit it faster gaining tenth by tenth corner after corner, because it won't hog your engine by stopping the spinning wheel under load until it matches the other. This is why rally drivers use the handbrake that much. It helps you overcome the high blocking setting on the accel diff, for then taking advantage of the unleashed power it grants you in the middle of the corner that is being negotiated by allowing the spinning wheel to continue to spin, basing everything on pure skill of your throttle/steering skills.
If you have found that you're so confortable with a very loose diff like you did in the video and on your personal testings after my post, remember that it will come to the cost of so much time lost, so I'd advise, at the very very least, you bring back that slider to the right again up to its middle setting. Still, and with that false* feeling of superior control of your car, you will continue to loose time until you bring back that power ramp to a tight angle (the slider more to the right).
The second slider you're also wrongly setting it to very loose, or the loosest setting, is the deceleration differential, which should also left it to kind of very blocked. This will give you stability under breaking before a corner with the con of more understeer, but the pay off is positive given the nature of a wrd car, and because the handbrake is, again, key for rally racing, so use it. Actually, I don't understand how you didn't spin when breaking before every corner in the video with the decel diff set to loose : - D
The preload you can play with it regardless. More preload will give you more stability, less more agility, but on rwd cars is recommended to leave it kinda high.
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* Try a full loose accel. diff setting on dry tarmac and find out why that feeling of better/superior control is completely false, and find yourself spinning around everytime you touch the throttle under evry grippy conditions such as that one on that surface, because not allowing the spinning wheel to continue to spìn under grippy conditions will lead to inevitable spin, while a blocked setting won't regardless you have more power on the throttle when exiting. It is the nature of gravel_mud racing on gravel_mud tyres what has given that false feeling of superior control without spinning even on a loose power ramp setting and on a rwd car. The moment you do the same with the same accel diff setting on asphalt and with asphalt tyres, specially if they are performance tyres, then you will spin all around the moment you exit a corner most probably.
Remember. Loose accel diff setting only under very very slippery conditions regardless it is gravel or tarmac. And still, don't open it to the max. Leave it on a balanced setting.
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Hope this wall of text (sorry) helped clarify matters. You must use blocked accel diffs when racing regardless the power of your car.