Thread: stock suspension and drive set up for strip, vega.

Tweet

"the car was squatting so hard that the pinnion was hitting the floor board."

Install a pinion snubber.

I'll explain a little about engine angle for the younger readers:
When the factory designed your grocery-getter, they cut the carb mounting surface on a front-down angle so the carb could sit level and the tailshaft of the trans could be lower in the car to provide legroom for passengers. This angle was usually 2 to 3 degrees.

When doing an engine swap, like Dave said, it doesn't matter about the rake on the body. What does matter is the relationship of the crankshaft centerline to the pinion centerline as viewed from the side of the car. You want these centerlines to be PARALLEL to each other. Not pointing at each other necessarily, but parallel with each other like railroad tracks with the car cruising under minimum power. To achieve this, you would usually rotate the differential housing to the point where the crank and pinion centerlines are close to parallel, with the crank centerline at maybe 3 degrees down bubble and the pinion centerline maybe 2 degrees up bubble. This will allow 1 degree of pinion rotation upward under cruise power and the u-joints will be aligned for best power transmission and minimum wear.

Now, the kicker comes in when we begin drag racing the car and the pinion climbs the ring gear violently, upsetting the parallel relationship we had at cruise power. The u-joints are no longer transmitting all the power generated by the motor and in severe cases can break. That's why the rules specify a driveshaft loop (at 13.99 with slicks), to prevent the driveshaft from dropping onto the strip surface or augering up through the floor of the car to injure the driver. (360-degree loop).

So, what do you do? Well, you find a compromise depending on the type of rear suspension you're using. You install the differential housing (pinion) at somewhere between 2 and 8 degrees different angle from the crank centerline so that under extreme power, the u-joints line up parallel. In other words, with a leaf spring car, you might install the pinion at 5 degrees down bubble with the crank centerline at 3 degrees down bubble. This would allow the pinion to climb the ring gear by 8 degrees to line up the u-joints parallel.
You could also install a pinion snubber to limit the pinion rotation and use less differential of installation angle.

Different rear suspension systems will allow more or less pinion rotation. Leaf springs are the worst case scenario, then ladder bars, then 4-links. Dave Morgan lays all this out for easy understanding in his book.
http://www.amazon.com/Door-Slammers-.../dp/0963121707

A little more about engine swaps: It doesn't matter where you install the motor left to right in the car. Oftentimes you have to move it to the passenger side to clear steering or whatever. What does matter if that you install it with the crank centerline PARALLEL with the centerline of the car. Do no point the transmission tailshaft at the pinion unless it just happens that way due to pinion offset in the differential. Some Mopars have been found to have the motor offset to the passenger side by as much as 3 inches. As long as the motor/trans is parallel with the centerline of the car, the angles of the u-joints will cancel each other out and the joints will live a long and happy life.