Real cars: set pinion angle/finding "true horizontal".

[LDO]

I just read an article in the new Street Rodder. They mention 3 degrees up for trans and 3 down for diff. How do you find true horizontal...or does it matter? 

Thanks. Lee

[Ace-Garageguy]

A car doesn't need to be sitting level to determine these angles, as it's the crankshaft centerline that everything else is based on.

While setting this up, it IS good to start with the car at as close to the finished ride height and stance as is practical with the tools at hand...particularly if it's carbureted.

We usually use dummy coil-overs or something similar to achieve this when building a car from the ground-up, or if it's some kind of heavy mod / engine swap, the car can be ballasted to operational weight on its actual suspension (and the actual wheels / tires that will be used). Measure the angle the chassis makes with the ground, and when you jack it up on a lift, or put it on stands to work on, maintain this angle.

The tops of the valve covers are almost always dead parallel with the crank centerline, so they can be used for placing an angle finder to determine final configurations on engine and trans mounts to get the desired angle of the crank centerline (which will naturally be the same as the trans output shaft) relative to the ground. 

You'll note that carbureted V8 engines usually have an obvious angle milled into the top of the intake manifold that's not parallel with the tops of the valve covers. This is to allow the carb to sit relatively level most of the time (when the tail of the crank is lower than the front to achieve the proper angle) so the float can do its job correctly. Usually, when building a V8 powered car, using an old carb-style manifold with a level on the carb mounting flange will get you very close.

NOTE: DISREGARD THE MEASUREMENTS BELOW. THE ILLUSTRATION IS ONLY TO DEMONSTRATE THE ANGLE MILLED INTO THE TOP OF THE MANIFOLD FOR THE CARB BASE.

It's particularly important that the angles of the crank / trans output shaft and the driveshaft pinion shaft match when the vehicle is in its normal driving-down-the-road stance. Some suspension systems will allow the rear axle to rotate somewhat during acceleration / braking and bump / rebound (changing the pinion angle slightly) but you need to start as close as you can.

 

Edited February 25, 2018 by Ace-Garageguy

[LDO]

Bill- if you drink, I'm going to owe you a bottle of Scotch for all the professional advice you give.

 

Thank you. Also- I always wondered about that angle for the carb base. 

[Ace-Garageguy]

Glad to help. I added a little more material to clarify a couple points, but I think I hit the necessary spots.

[LDO]

Indeed. Thank you.

[cowboy rich]

48 minutes ago, LDO said:

Bill- if you drink, I'm going to owe you a bottle of Scotch for all the professional advice you give.

 

 

Let me know first....I need to pitch in on it!!