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Cost v Benefits of diff change compared to manual conversion in v8 NC

3K views 13 replies 7 participants last post by  mongrelEB 
#1 ·
Hey all,

I was wondering what your views are on costs and benefits (well mainly benfits) of either

1. Changing diff gears to 3.7s or 3.9s

or

2. Manual conversion.

Car is a v8 NC fairlane.

Some problems i think could be a problem with the manual conversion would prob be tailshaft length due to LWB, and ofcourse finding a V8 box...

To be honest I reckon the diff gears would give me manual like performance, and probably be a bit cheaper; Down side, would rev maybe a bit too hard on the highway (currently have 3.27 LSD and revs around 1900ish @ 100kph) plus I dont have the control I would wit the stick shift....

What do you think?

hot_eb
 
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#4 ·
WBT56 said:
You use the same tailshaft for auto or manual. V8 Boxes are easy to find.
you got one for me then!?!!? How hard are the v8 conversions compared to 6 cyl?? You deal a lot with diffs dont you (judging by your other posts anyway) what would you recommend?

qwikcorty said:
once rolling say past 40-50 kays, the two might be similar, but u could never match the 3000rpm clutch dump and squat on take off the manual will perform.
hmmmm, V8, 3.9's and a manual hehe fun.
3.9s and manual would be the way to go, but unfortunately the bling bling dont allow it! A 3000rpm clutch dump though and i will spin my sorry arse half way to redline...I dragged a supercharged commy and the 3000rpm lost me the race in my old 6 pot eb! 2200rpm had us dead even to a few kms over 100....
 
#5 ·
hot_eb said:
you got one for me then!?!!? How hard are the v8 conversions compared to 6 cyl?? You deal a lot with diffs dont you (judging by your other posts anyway) what would you recommend?
I have one, PM me if you are serious. V8 are no harder than I6 to convert, just an extra pipe to dodge. Most windsor/T5 combinations perform outstanding with 3.45, and don't perform better by going lower without mods to help it spin up quicker. I have no experience with autos in this department and won't comment.
Surprising as it may sound, you should be less likely to light up the rear with a lower diff ratio.
 
#6 ·
WBT56 said:
I have one, PM me if you are serious. V8 are no harder than I6 to convert, just an extra pipe to dodge. Most windsor/T5 combinations perform outstanding with 3.45, and don't perform better by going lower without mods to help it spin up quicker. I have no experience with autos in this department and won't comment.
Surprising as it may sound, you should be less likely to light up the rear with a lower diff ratio.
Interesting stuff, you will have a PM soon!


why would you spin less with a lower ratio....you would have thought much more....
 
#7 ·
hot_eb said:
why would you spin less with a lower ratio....you would have thought much more....
This is what everyone seems to think. Try this: Same car, same gearbox, same engine. Go from 3.08 to 3.23 to 3.45 to 3.91 and find the tendancy to actually light up the rears lessens. It did go much better though.
 
#9 ·
I have just done a conversion on a I6 and the front yoke has a different spline in the auto and manual. The auto has a keyway that will only line up in one spot on the gearbox output shaft, the manual seems to have the same spline just no keyway
 
#11 ·
EATURBO said:
I have just done a conversion on a I6 and the front yoke has a different spline in the auto and manual. The auto has a keyway that will only line up in one spot on the gearbox output shaft, the manual seems to have the same spline just no keyway
Some slipyokes/shafts have a master spline, but they still interchange.
 
#12 ·
WBT56 said:
This is what everyone seems to think. Try this: Same car, same gearbox, same engine. Go from 3.08 to 3.23 to 3.45 to 3.91 and find the tendancy to actually light up the rears lessens. It did go much better though.
This is the opposite to everything I know and have experienced. Going from 3.08 to 3.91 will mean the engine will be reving higher at any given speed, therfore producing more power at that speed, therfore will spin the rears much easier. I don't see how it can be the other way around. I know for a fact that a stock auto xr8 with 3.9 gears will light the rears really easily, where with the stock 3.23 gears, it is very hard to do.
 
#13 ·
EDXR8 said:
This is the opposite to everything I know and have experienced. Going from 3.08 to 3.91 will mean the engine will be reving higher at any given speed, therfore producing more power at that speed, therfore will spin the rears much easier. I don't see how it can be the other way around. I know for a fact that a stock auto xr8 with 3.9 gears will light the rears really easily, where with the stock 3.23 gears, it is very hard to do.
Interesting point..can anyone else confirm or not?
 
#14 ·
I have just done a conversion on a I6 and the front yoke has a different spline in the auto and manual. The auto has a keyway that will only line up in one spot on the gearbox output shaft, the manual seems to have the same spline just no keyway
Some do, some don't. It's easy to find a tailshaft to suit down at the wreckers, if it's too short for the Fairlane swap the front yokes over.
 
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