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Hey,

 

My 280z has a 3UZ engine swap with the auto transmission and i am preparing to convert to a manual. I am having issues deciding what gearbox to use as the ratios on offer dont seem to work (in my mind atleast)

 

The car currently has a 3.34 diff (but was hoping to put in a 3.54 subaru LSD diff at same time) and is running 16" wheels.

 

There are various conversion kits on offer for the following boxes and these are a few examples of speed for each one, I guess i am after input on if you think the ratios will work in day to day driving or will i just shift from 2nd to 6th and that be it?

 

Most are similar close ratio 6speeds, the RX7 box is the most promising but they are much more expensive. Do you think i could like with the close ratio 6 speeds?

 

Lexus is200 - 70mph in 6th = 3000rpm

1st 29mph

2nd 52mph

3rd 76mph

4th 93mph

5th 113 mph

6th 130 mph

 

Mazda rx8 - 70mph in 6th = 2900rpm

1st 29mph

2nd 49mph

3rd 67mph

4th 93mph

5th 111 mph

6th 132 mph

 

BMW 330D - 70mph in 6th = 3000rpm

1st 26mph

2nd 45mph

3rd 69mph

4th 94mph

5th 115 mph

6th 135 mph

 

RX7 5speed - 70mph in 5th = 2700rpm

1st 33mph

2nd 55mph

3rd 80mph

4th 111mph

5th 140 mph

 

Thanks for taking the time to read!

 

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Compare the ratios to a "normal" 240Z or 280Z.  1st gear is where you'll first notice the ratios.  If you live in a hilly area you'll want something around 12:1.  If you live in a flat area, with a big torquey engine, maybe 11:1.  And, "16 inch" is the diameter of the rim, not the diameter of the wheel.  

 

240Z - 3.592 x 3.364 = 12.0

 

280Z - 3.321 x 3.54 = 11.75

 

280ZX - 3.062 x 3.9 = 11.94

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https://www.blocklayer.com/rpm-gear.aspx

 

This calculator is pretty helpful for getting the information you want. You'll need the ratio of the gears, the final drive in the car and the overall tire diameter. You can see where each gear redlines and where when you shift where the RPM's drop down to. In this chart, it seems to show the max speed of each gear, so to go from 2-6, you'd be redlining 2nd gear to get into 6th and not bog the motor. 

 

Since these are all factory gear boxes, the ratios will all feel about right. Biggest variance will likely be what rear end gear you go with. I'd go with whatever transmission is easier to source and install. 

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