Project Ruze The UK's first all-Japanese RX7 V8?

Propshaft & Differential

There’s a fundamental problem with mixing gearboxes and differentials from different manufacturers in a car. Some manufacturers use high gear ratios and a low final drive, and other manufacturers use low gear ratios and high final drives. If you mix the two, you can end up with both low ratio gearbox and diff (bugger all acceleration but very high theoretical top speed) or both very high ratio (crazy acceleration, but a limited top speed, and very high rpm cruising speed). Unfortunately Toyota use high ratio gearboxes, and Mazda use high ratio final drive gears in their differentials.

The stock manual RX7 diff is 4.1:1, which is the diff I had on the car when I bought it. According to Wikipedia, the ratios in the R154 are like this:

  • First Gear: 3.251:1
  • Second Gear: 1.955:1
  • Third Gear: 1.310:1
  • Fourth Gear: 1.00:1
  • Fifth Gear: 0.753:1

Knowing the size of your wheels, you can use these figures to calculate rpms in a particular gear for a set road speed, say cruising on the motorway at 70mph. Ideally you’d want that to be quite a low rpm level in top gear for better economy, especially when not using a high revving engine. So, for a nicer driving experience, I’d like a bit of a smaller ratio in the diff – Toyota diffs tend to be ~3.5:1 or thereabouts.

You can do this a few different ways, if you’ve got money to burn you can buy the HKS final drive set which is 3.7:1, or you can change the whole rear end for a Toyota or Ford setup, which some of the higher power RX7s seem to do. You’re looking at around £2k to do that though! I decided a nice compromise was to fit a diff from an automatic RX7 – still an LSD, just with a few less teeth on the final drive gear. Its ratio is 3.9:1 which is not a huge difference but should make everyday driving a bit more comfortable. I picked one up cheaply from FDUK.

Now, people will tell you they are just bolt-on, and you can happily swap them out relatively easily. This actually isn’t the case, as the companion flange (where the prop connects) on the diff is different. The flange has a single-use crush tube behind it, and the bolt holding it on seems to be done up to a million ft-lbs so swapping the flange is not easy either.

Here are the diffs side by side, auto on the left and manual on the right:

Diffs!

 

 

 

You can see the companion/prop flange is set further back on the auto, to the point where the nut protrudes. The PCD of the bolt holes on this flange is larger too, so even if like me you do have the flexibility of being able to move the gearbox further back, it won’t bolt up.

The flange being further towards the rear of the car is actually an advantage to me, as the further back I can get the gearbox the better, from both a shifter position point-of-view, and a general weight distribution one. It just meant I had to change the flange on the custom tech2 prop to one from an auto RX7. So I bought an auto prop and as it had a staked U-J rather than one held in with circlips, I hacked it up…

IMG_20130423_193716 Auto Prop  IMG_20130423_193802

The last photo shows the differences. On the left is the Mazda auto flange & UJ. The UJ is smaller (and in my mind smaller = weaker) and the holes in the flange are further apart. On the right is the significantly more beefy UJ that came with the tech2 prop, and the flange for the manual diff. I sent these off to the machine shop to have the UJ holes in the auto flange modified to take the tech2 UJ, and to be swapped over. Here’s the end result:

Modified Prop Flange

That’s the Mazda automatic prop flange on the tech2 UJ, which I then pressed back into the end of the custom prop. With that all back together on the prop, it bolted straight up and fit like a glove. So the net result is that I have a lower ratio diff, have retained my torsen LSD, and the gearbox is further back.


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I'm the one actually trying to build this thing!