Because 4 degrees is not Optimal. With 4 degrees of drivline angle and a good stance like you'll see on my car or bad penny, etc. You will have an UPWARD driveshaft angle with equal and opposite drivline/pinion angles. That means your working angles are getting severe for high speed run out. All you're doing by pointing the pinion down is making the working angles as small as possible. I was able to achieve under 3 degrees of working angle where with equal and opposite you would have 7-8. The driveshaft will actually slope down towards the rear a little which is optimal. Personally, if you are in the fabrication phase? I'd cut that tunnel in a heart beat and set that thing up with equal and opposite numbers. You're going to need 2-3(approx) degree driveline angle to do so. That would allow you to get your working angles between 1-2 degrees which will make your car run out super smooth until you run out of horsepower. I'm not saying it's not possible with 4 degrees of driveline angle, I'm simply saying it's the second best way to skin the cat. Take a look at a modern corvette/camaro etc and take note of the driveline slope.
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Todd
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