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Gear ratios and what they mean to me...


Xnke

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Been working out the exact gearbox ratios that I have in the new scrambled gearbox here. I have no idea what kind of frankenstein box this was to start with, but it DEFINTELY is NOT a standard 240SX gearbox that I've ever seen documented. 

 

Here's how the 71-series boxes are done:

Input shaft/Countershaft ratio sets the gear spread...in this case it's 22/31, or 1.409

1st gear is 33/14, or 2.357

So, 2.357*1.409= 3.321

1st gear ratio is 3.321

You can swap input sets around with the different counter/mainshaft sets and get a wide ratio, a W I D E ratio, or a close ratio box. There are three different sets of counter/mainshaft gears for the 71C transmissions, and AFAICT there are three or possibly four different input ratios out there too...so nine or 12 possible gearboxes, with 5th gear independently changeable and 4th gear fixed at 1:1.

This box will come out with the following ratios:

1st: 3.321, 2nd: 1.928, 3rd: 1.517, 4th 1.000, 5th: 0.758, Rev: 3.657

Not too bad! I wonder about the 3rd gear ratio though, because I can't find a gearbox in the listings even close. It is a possible combination, but I have no idea if it was ever a factory option, because every other gearbox with the same 1st and second ratios as this one, give a 1.308 3rd gear. In this case, if shifting at 6500, I won't fall out of the current powerband at all...and only the 1-2 shift comes close at 3750RPM, which is just above the bottom of the power band (3000RPM with the current N/A cam)

I don't think I'll have the money to get the blower cam swapped in before end of May, but we'll see. It'll cost me about 150$ plus one hellacious aggravation of an afternoon. I am going to be really strapped for cash if I can even make it out to Branson at all...But I digress.

 

I am not sure how to decide if I like the new ratios or not...I'll be running this box with a 3.545 R200 rear end, on 225/50/15 tires that are about 23" tall.

 

Currently, the gearbox in the car has the following ratios:

 

1st: 3.321, 2nd: 1.902, 3rd: 1.308, 4th 1.000, 5th: 0.833, Rev: 3.657

 

Anyone able to help me figure out the pros and cons of this for a street driven Z?

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Xnke,

I've analyzed gear ratio's on this spreadsheet I created.  I use it for evaluating crusing speeds, rpms and mph. 

 

https://docs.google.com/spreadsheet/ccc?key=0Ato3KCDX86aedFBpTkI0X0o5R2pQaHlrQzgzbGVrQXc&authkey=CPTj89IN&authkey=CPTj89IN#gid=0

 

Let me know if you need help help with it.

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