>> In other words you think the angle should be UP? <<
Yes, as compared to the horizon. Like I said, the pinion angle is measured against the power angle, not the horizon.
That is the key point.
In most cars, the power angle is about 5*. That's why the pad on many 4 barrel intake manifolds is milled at an angle. That keeps the carb flat, even though the engine is tilted. It seems that many CObra's have a power angle much lower, like 1-2*.
Under power, you want the power angle/ driveline angle to be the same as the pinion angle. That doesn't mean they'll be perfectly aligned with each other, just at the same angle. The greater degree of disparity between the two angles, the more power loss you have. And the greater probability of wheel hop.
Think of the drive line and pinion as a side view. The drive line is angled at 5*, pointed up toward the front. The pinion is also angled, pointed up toward the front. Under power, you would want them to both be pointed up at exactly the same angle. In this case, 5*.
Under launch, the pinion angle will change, moving upward. The trick is to figure out exactly how much. So, lets say 3*. Since you want the angles to be the same under a load, you set the pinion angle at -3* in relation to the drive angle. Which is actually +2* from the horizon.
An IRS car has a fixed differential. That makes it easy. Measure the drive line angle, then set the pinion angle exactly the same. The easy way to measure the drive line angle is across the face of the front pulley or balancer.
Also, don't confuse pinion angle with offset. U-joints need a little offset to keep the bearings lubed and alive. As the rear moves up and down, the angles of the u-joints will change, which keeps them alive. With IRS, The odds of getting a perfectly straight line from the pinion to the trans is pretty slim, but I suppose it could happen. If it did, probaly the easiest thing to do would be to cock the driveline 1* to the left, and the pinion 1* to the right. Angles would still be equal, but it would have a 2* offset.
Carroll Smith's series of books starting with, "Engineer to Win" are an excellent resource for this sort of thing.
Go to the other Cobra site and do a search on "pinion angle" and you'll get about 18 results from the last year.
Also, do a google search, and you'll find some excellent resources, complete with pictures and diagrams.
"Ideally, the angles between the transmission output shaft and driveshaft, and between the driveshaft and the pinion will be equal and opposite."
http://www.carcraft.com/howto/91758/index.html
"Pinion angle is the difference between the driveshaft angle and the pinion angle on the differential."
http://www.wolferacecraft.com/pinionangle.aspx
I especially like this one: Myth #3: The garage floor is the correct reference point for
measuring the pinion angle.
Straight Scoop: You've got to be kidding, right? The garage floor
doesn't have anything to do with anything.
http://www.buickperformance.com/Pinion.htm