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Power Steering box ratios. Most GM cars & light trucks:
Ratios & the Number of Turns. A big question for most people !
12.7 to 1 (Commonly referred to as 12 to 1
12 to 1 is the fastest or quickest ratio Power box used in any GM vehicle. In the 80s Chevy Z-28 was GM Introduced the 12 to 1 quick box
It only had a pitman shaft travel (rotation) of 68 degrees. We refer to this as a restricted travel box. This box turned approx. 2.5 turns lock to lock. This is fine for most late models but not for 60s & 70s cars.
The 1964 to 72 A Body GM Cars need 83 degrees of pitman shaft travel to have the factory full turning radius! 12 to 1 Will have 3 turns of travel lock to lock in an un-restricted box (Has no stop limits). So a 12 to 1 box can have from 2 1/4 to 3 Turns lock to Lock, one can't tell the ratio or how quick the ratio by the number of turns alone. The 16-13 to 1 variable ratio box and the 14 to 1 (fixed ratio) have about the same number of turns, giving they have the same internal stops. The Variable ratio box has a longer center tooth on the pitman shaft than its side teeth. Making the ratio slower in the straight ahead travel, then quickening up for parking & U-turns. The 14 to 1 ratio having about the same amount of total turns is muck quicker for highway lane changing. In the 90s GM switched to the 14 to 1 ratio for pickup trucks.
14 to 1 Box will have a 3 1/4 turns lock to lock (no stop limits) and about 2 1/2turns with high internal stops... This is a fairly new ratio for GM, most pickups 1992 and newer have this ratio.
16-13 to 1 Box Used Mid 70s to 91 for most cars & light trucks. 63 Buick Riviera Might have been first to have this ratio ???
17 to 1 Ratio is over 4 1/2 turns lock to lock, in a box with no stops, used from 1959 to mid 70s in most cars & trucks..
The only way to know the true ratio is to measure the distance of the worm thread (pitch) & to look if the teeth depth in the worm nut or the pitman shaft. all straight ratios have the same length teeth 12, 14 & 17 to 1 ratios..
The distance (thread pitch) between threads.
12 to 1 is approx. .650 inch thread pitch.
14 to 1 is approx. .590 inch thread pitch.
17 to 1 thread pitch is approx .470 .
16-13 to 1 Variable ratio is about the same as 14 to 1 (but the center tooth is longer than the side teeth. This is what makes it slower for lane changing !
Manual Steering ratios:
Most GM cars & light trucks
26 to 1 Standard ratio 6 1/2 turns lock to lock (with pitman arm un-hooked) 20 to 1 Quick ratio 5 1/2 turns used (64 -72) A body GM cars & some others as quick ratio option.. 16 to 1 Ratio (Corvette 53-82) 3 3/4 turns. 16 to 1 is very hard to turn in heavier cars recommended for use in Drag cars & dirt track ..
We hope this clears most questions on ratios & the number of turns: