would it be worth changing a '94B(torsen, 4.10?) to a different final drive with 200-225hp? comparing optimal output and bang-for-the-buck? the 3.909 doesnt seem like it would make that big a difference, but would the 3.6 weaken the acceleration too much?

 

Keep your 4.10 for that power level. No point in spending the extra dough. If you were going from 4.3 to 3.9 that is a different story though. I'd keep the 4.10s personally. Never done the swap myself though.

 

Zombie thread.

I just ordered a 3.9 Torsen from a primo 02 SE. My question is, does anybody have the party numbers for the speedo gear I would need to read correctly?

 

I'm not certain there is one for you.. the 3.9 only came in the 99-05 miata to my knowledge. the 99+ has an electronic speed sensor the 90-97 is mechanical.

The only route i could see is if the 1st gen RX-7 uses the same speed sensor as the NA Miata.

 

They do exist, you just need to find out which mechanical drive. (there are many)

For example the part number for a mechanical speedo drive for an NA with 3.636 gears is M502-17-441. It obiviously wasn't designed for that application, but it is accurate.

There has to be a calculator somewhere to calculate this.

 

http://www.bgsoflex.com/speedo1.html

How bout that? I just don't know what numbers to use.

 

Assuming the speedometer revs per mile is 1000 like it says it is on most modern cars, tire diameter should be 23.21, I want to figure out the number of teeth on the driven gear I'll need with a 3.909 differential ratio.

 

Here's what I might need. I assume that's the stock gear on the right, will pull out my gear and make sure. Got these pics from bill at miataroadster.

 

the speedo is optimistic to begin with. mine is dead on now with my 3.90 swap as compared to my GPS and the dyno. I swapped from a 94 though. Tire size affects speed calibration too so you gonna account for that in your gear selection?

 

I found my speedo nearly dead on with 185/60/14 yoko es100's (yes I know they are ****, they were free) and a garmin gps in my 94.

 

You'd like the 3.90.... trust me... BTDT... with less torque... It was really optimal with 225-250 RWHP and the 3.909/5 speed combo. I liked it, great auto-x setup.

Mark

 

Perhaps. But I'm not at all displeased with my 4.10s that I have now. I'm either going to keep the 4.10 and put an rx7 5th gear in the trans, or maybe even jump on a 6 speed with the 3.63s we've been selling at $320. Depends on the $$$ situation after the motor.

 

5-spd with 3.63!!! lol

 

Several people with 5 speeds have bought 3.63s from me.

 

There is a good chance I’m going to be running that set up for a short time while I source a 6sp.

I'm curious to see how it is at ~250 WHP. If it's not too annoying I may stick with it until the tranny blows..

Any one run this set up?

 

I'm glad there will be many more people with a 5 spd 3.63 setup. I want to drive one to see if I like it.

 

As soon as i'm done with the set up i'd like to meet up somewhere. Maybe a trip to Mammoth Cave is in order. I think that's close to half way between us.

 

personally, i think a ~300rwhp or more car would be pretty damn fast with a 5spd/3.63. Anyone done the calculations on what the "theoretical" top speed would be on that?

 

Gear Mph per 1000 RPM Mph @7200 RPM Mph @7200 RPM
--------------------------------------------------------
1 6.03 43 43
2 10.01 72 72
3 14.22 102 102
4 18.91 136 136
5 23.23 167 167

Mph RPM (in Gears)
-------------------------------------------------
1 2 3 4 5
-------------------------------------------------
5 829 499 352 264 215
10 1659 999 703 529 431
15 2488 1498 1055 793 646
20 3317 1997 1407 1058 861
25 4147 2496 1759 1322 1076
30 4976 2996 2110 1587 1292
35 5805 3495 2462 1851 1507
40 6635 3994 2814 2116 1722
45 4494 3165 2380 1937
50 4993 3517 2644 2153
55 5492 3869 2909 2368
60 5991 4221 3173 2583
65 6491 4572 3438 2798
70 6990 4924 3702 3014
75 5276 3967 3229
80 5627 4231 3444
85 5979 4496 3659
90 6331 4760 3875
95 6683 5025 4090
100 7034 5289 4305
105 5553 4521
110 5818 4736
115 6082 4951
120 6347 5166
125 6611 5382
130 6876 5597
135 7140 5812
140 6027
145 6243
150 6458
155 6673
160 6888
165 7104

Gear Change RPM drop (change @7200) RPM drop (change @7200)
-------------------------------------------------------------
1 -> 2 -2865 (to 4335) -2865 (to 4335)
2 -> 3 -2128 (to 5072) -2128 (to 5072)
3 -> 4 -1786 (to 5414) -1786 (to 5414)
4 -> 5 -1339 (to 5861) -1339 (to 5861)

 

That's just the gearing limits to redline.. It doesn't account for the ability of the engine to over come resistance caused aerodynamic drag.