Tranny compatability? '85 vs. later years?

[Steve Ostrovitz]

OK, here's the short question:

My question is this: Is there any reason NOT to use the '85 tranny in a second gen ITS RX-7

Here's the story:

I sent a non-ribbed box to a shop for forks and seals etc.... It was from an '87 parts car. Tech ordered forks and seals for an '87. Otherwise on the inside it looked pretty good.
Turns out the first gear fork is different. He tells me the only year that was different was '85 13B cars. So it looks like the parts car had a tranny swap done. So my question remains, assuming he gets the right 1st gear fork, is there a reason not to use the '85 tranny?

Thanks, as always,

Steve

[C. Ludwig]

I'm not a rules nerd but I'm going to steal their thunder here. It's illegal.

Chris

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Chris Ludwig
08 ITS RX7 CenDiv

[Silkworm]

And that's a great reason to not do that


PaulC

[Steve Ostrovitz]

Yes, I know it's not legal. I should have stated that in my post. We run legal cars every which way. My question is purely technical.

I believe that all of the internal parts, except for this fork, are identical however. But I am not sure and that's partially why I've turned to this forum.

So let me re-phrase and expand the question: Legality aside, is there an inherent difference, apart from the external ribbing, between an '85 box and later 5 speeds? If there are no other difference, then how significant are the external reinforcements on the case?

Thanks

Steve

[BrianB]

Steve,
Internally the gearboxes are similiar. The early boxes may have a smaller diameter tailshaft but the bearing surfaces are the same. The ribbing obviously reinforces it's structure for higher horsepower cars. You could use the front and rear covers from your blown 2gen, they will bolt right up to the 85 center section. The 85 box is illegal,3rd and 5th gear are a little lower,which will help if your car is running something less than a 4.88 rear.
BB

[C. Ludwig]

I was just playing around.

Chris

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Chris Ludwig
08 ITS RX7 CenDiv

[Steve Ostrovitz]

Thanks for the input Brian.

Just to make sure I understand (gear ratio stuff has never been a strong point for me), you're saying that if a car did not have the 4.88's in the rear then having the '85 third gear would be an improvement? (5th is a non-issue for the purposes of this question.)

BTW, no sweat Chris. This is mostly an academic question but I need to know the answer. I am building a new 2nd gen car myself and have a legal '87 tranny, so no other back of the pack guys have to worry about writing paper on me.

Steve

[BrianB]

Steve,
The 85 has a 1.419 3rd gear ratio,the 87 has a 1.366. In terms of rear end ratios, Using a 1.419 with a 4.1 rear,would be the equivalent of a 1.366 with a 4.26 rear end ratio. A 4.3 rear ratio with a 1.419 becomes the equivalent of a 1.366 with a 4.47 rear.Unfortunately, this is an inexpensive, convenient but illegal way of improving the performance (somewhat)of a car without the 4.88 or 5.12(mazda truck) ratios that have become necessary to run up front!
BB

[Steve Ostrovitz]

Thanks for the great info Brian. I really appreciate it.

Steve