A Differing of A Pinion.
by Ben Pila Jr., Vice President

This month I decided to write about the rear differential ratios for Zs of the 1971-83 vintage as there was talk among some club members. Why change your rear diff ratio. The reasons are simple. To allow your engine to perform better which in turn translates to faster acceleration and better driveability. Here's an example. If you have a 240Z with a manual 4-speed and the original diff ratio of 3.545, your rpms are not too high on the freeway. The 3.545 ratio diff was determined by Nissan engineers to be the best for all around driving. Now if you converted to a 5-speed, you have much less rpms on the freeway but 1st through 4th remain the same. So to take advantage of the new 5-speed. Changing the rear diff ratio from the 3.545 to the 3.90 makes a very noticeable improvement in the acceleration in each gear. With the 3.90, your rpms drop less between shifts so the power band is already kicking in and you feel it. The freeway rpms isn't really changed much. There will be some more rpms at freeway speeds but with the stock rear and the new 5-speed, the rpms are lower than needed anyway. Were looking for a major improvement here and trying to get more zip out of your Z. Note the following and refer to the accompanying chart. There are two sizes of differential housings used in the Zs and some other vehicles that we are concerned with. The number in "R180" refers to the diameter of the ring gear which is 180mm. Same goes for the R200 which is stronger than the R180. For most non raced early Zs with the R180s, you can stay with this size diff as it should hold up fine. If you will autocross, drag race or have a high output, modified engine, you might want to convert to the R200 larger diff. The R200 is physically larger, 25 lbs heavier and stronger but will fit into an earlier Z if you use the crossbar from a later R200 280Z. Of course if your Z has the R200, keep it.

See the accompanying chart for your current ratio. Also, note that the chart shows other ratios and which vehicles came with them. Most of the diffs in the chart are interchangeable. Some like the 4x4 trucks (their front diff is a Z diff mounted backwards,) can be put into a Z if the output flanges are swapped with the ones from your old diff. There are some 4x4 diffs that have output flanges held in with a circlip rather than a bolt and press in. The ones with a bolt are what you need. The press in axle diffs aren't compatible.

If you are curious of how your rpms will be affected, use the following formula and plug in the numbers. "Tire diameter" is in inches and is often in tire makers brochures. "Ratio of diff" is the rear end ratio from the chart and "ratio of gear" is that of the transmission gear you are in (usually listed in a service manual.)

Let's try an example. A stock 240Z with a 4-speed.

RPM=2939 which isn't too high. Now check out the following. With a 5-speed the RPM at 65 drops to 2539 which is much lower than necessary. So let's change to the 3.90 and the 5-speed and we get 2944 which is just the same as the 4-speed but now with each gear change, there is less drop and better acceleration. If you wanted to, you could try using different info for the trans, diffs, and tire sizes and try them at different speeds. But many of you don't like math anyway so you could just take my advice and install the 3.90 rear end ratio and really enjoy the ride.

If you have any questions, comments or suggestions on this or other tech articles feel free to e-mail me at benpila@juno.com or write to the Z Club attention Tech writer.