I have a 4.1 open diff right now in my car, and i didnt want to put the 3.9 LSD becuase i heard it kills acceleration, but is better for top end. Is there really a big difference.?



others have told me its barely noticeable. Now i get this gear ratio from Rotary Performance and from wha ti looks like. the 3.9 has a higher speed for each gear than the 4.1 or 4.3.

I am really debating just throwing on my s4 3.9 lsd in my car, this open diff stuff suckz lol

Suggestions?

Jason NYC

 

can u use lsd axles on open diffs and vice versa? Also i need a new front diff mount. It will fit both the open and lsd unitts?

Jason NYC

 

as far as I know the 3.9 was only on the auto (and maybe the vert, but I dont think so)

 

The numbers you posted are theoretical numbers, but they are accurate enough for your comparisons.

Yes a 3.9 rear end will have a theroetical higher top speed than a 4.1 or 4.3 rear end. However in the real world; the problem is having sufficent power to propell the car to such speeds even with the areodynamic drag.

As far as swapping the differential, the non turbos all use the same half shafts/axles regardless of if it is a LSD unit or not. However Turbo rear ends and non turbo rear ends do not use the same halfshafts or pinions.

All FC diffs use the same mounts regardless of if it is a Turbo with LSD or a non turbo from an automatic car without an LSD (and everything in between).

 

The 3.9 rear end was standard on all non turbo automatic tranmission cars (except the 89-92 verts) and standard on all 88 verts (regardless of transmission).

 

yeah my car was automatic. 87 GXL, i converted it and now i have a 5speed in there. The diff im using was from my 89 GTU, and it open diff (4.1).

I dont really care about the TOP SPEED per say i just heard there was a difference in acceleration thats all. I want to know if its very noticeable. If not ill just throw in the LSD =).

Jason NYC

 

As far as if it is noticeable, that would depend on how much expereince you have with your car.

I notice a difference between a 3.909 and a 4.10 rear end, but many people may not.

There is also the speedo issue. With a 3.9 rear end, the speedo will read different than a 4.1 rear end, and the tranny speedo gear will need to be changed.

 

**** i just changed my speedo. I had the auto speedo in there running with my 4.1 rear. I just bought a speedo cable for manual transmision , the only difference is that its shorter than the auto speedo cable, but i called mazdatrix and they said the speedo cable is the same for all Non turbo FC's.

So wouldnt it go for either 3.9,4.1 and 4.3

Jason NYC

 

Not the cable... the gear.

Speedo gear mounts to the tranny.

Speedo cable goes between the gear and the speedometer.

 

How much of a difference is there? Got pics of this gear, dont think ive seen it.

Jason NYC

 

3.909 rear ends will have a 19 tooth speedo gear, while the 4.1 rear ends will use the 20 tooth speedo gear (the GTUs used the 21 tooth gear).

 

oh thats kooll, good loooking out thank you.

Whats the difference in speed from 3.9 to auto?

Jason NYC

 

In an NA the 3.9 will ultimately slow you down - our top speed is reached in the top of 4th, and while with the 3.9 you MIGHT be able to stay in the power band you will get there much more quickly with a 4.1 or a 4.3 (you might could even use 5th with the 4.3 or above)

The RX-8 has a 4.44 and that is part of the reason why it is relatively quick.

 

ima talk to my boy james (spotskater) he took his 4.1 geariing and threw it into a 3.9 LSD diff. All he had to do was adjust teh backlash.

JasonNYC

 

I have the 3.909 rear lsd my car used to be auto and i kept the rear when i put the 5-sp in. I favor it becuase i have to shift less and i take 2nd gear to 80mph easy which most car's have to shift to third to get there.For me thats a good advantage especially when using nitrous! Ialso noticed that i have less wheel spin off the line with the 3.909.One thing that you will also notice and the only problem i have is top end hurt's becuase lack of tq. Ive recorded the top speed's in each gear when i shift .

1st-43mph
2nd-80mph
3rd-110mph
4th-148mph
5th-n/a

And yes i have swapped out speedo gear's so my mph shouldnt be any more off than it usually is.