speeddemon7

Ok so im curious about upgrading my 3.9 differential gears to 4.3's in hopes of gaining better low end speed..I need everyones help on this subject.My first question would be can this be done to the auto model.
What exactly is involved in this moddification? for example will I need to take this to some kind of shop to get it done.Will the 4.33 gears help me accelarate like a 5 speed or better?
Last but not least is this.Will upgrading to 4.3 diff gears damage the auto tranny.Will this modification have any negative effects on the tranny? I know top speed will be affected slightly.

turbojeff

iTrader: (30)

It won't have any negative effects on the trans.

The 4.33 gears will have a dramatic effect on low end acceleration!

speeddemon7

kick ***.I really hate that 4 to 5 second delay before the car starts to really get moving.Hopefully the 4.33 gears will help out in that department.
When you say dramatic effect.You mean it greatly increases the low end power right?

speeddemon7

one other question.Will swapping in the 4.3 gear affect my speedometer and how?
If it does affect the speedometer how exactly?
will i be going faster than what the speedometer says or slower that what the speedometer says?

maxpesce

The Trans OUTPUT Shaft will be turning FASTER for a given road speed (by the ratio 4.33/3.9) so the Speedometer will read FASTER than your true road speed.
The Ring & Pinion swap is fairly straight forword but requires some care and special tools to set the proper clearance/mesh between the gears. The job is probably best left to a qualified shop unless you have experience doing the work.

speeddemon7

so basically the speedometer will read about 6 to 7 mph faster than what the car is going.Heres something to screw things up for ya.Now what if I change rim sizes to say 17 or 18 inches.How will that affect the speedo?
As for the ring and pinon job.I have no clue how to do it so a professional shop will have to do it for me.

dclin

iTrader: (4)

It shouldn't make a difference if you choose a tire size that equals the stock diameter of 24.8".

Keep in mind that that your rpms will be higher for the same cruising speed as well (everything else being the same), though within reason for a daily driver (depending on how fussy you are). It's been a couple years, and I can't remember what the difference was for a given speed.

Going from a 3.9 to a 4.3 is noticable once you get going. However, it will still pretty much be a dog off the line unless you start making use of the Hold button and manually shift it. I shifted mine manually 90% of the time before I finally swapped it out for a manual.

speeddemon7

im the same way.I use hold like 99 percent of the time.I like not having to use both feet.Especially when I get off from working the graveyard shift.I can barely stay awake at the wheel.

speeddemon7

So if I use the hold feature which I do all the time.Will it not be as laggy like the 3.9 gears? Thats what im looking for right now.To reduce the lag of the tranny on takeoff.I dont neccesarily need to have a racecar 5 speed.As long as the car accelarates better that it does right now I would be happy.
I guess im just one of those guys that wants to go against the grain and try to make a auto fd really fast.

BMike

I kinda get the feeling that torque converter response may be more of an issue here than gearing. If the car is sequential still especially, the car should make more than enough power to yank itself off the line with a swift kick to the throttle, it may be that the converter isnt locking up quickly, or the clutch bands could be slipping too. Obviously not driving the car its hard to say, but i cant see how the ATX would be that slow to respond well unless there was a problem somewhere.

speeddemon7

So then the converter could be faulty then?
The car isnt dead from a start.But it certainly doesnt put you into the back of the seat untill after you press on the gas and slowly accelarate for a bout 4 to 5 seconds untill the boost kicks in.Then the car flies.what would you suggest I do Mike?

BMike

So rolling if you PLANT the gas the car takes 4-5 seconds to really start pulling? Seems kinda off on the response, you have a boost gauge in the car? If the motor revs arent shooting up the problem is probably tuning related. If you have a boost gauge you can make sure the turbos are working correctly.

speeddemon7

I actually happen to have a boost gauge installed.I see good boost.especially now.Since I replaced the check valve in the brake booster line that was missing.Ok I lie its more like a two to three second delay.would having a slight fuel pressure leak cause this problem?

speeddemon7

as for how much psi im seeing.The turbos make up to 1 bar if I really stomp on it.but m also at a high altitude.So the engine is really seeing 10 to 11 psi at the most.
I just hope my turbos arent in danger from this.

BMike

Not very likely that a fuel pressure leak would cause it, if your rolling at say 5 mph and you put the pedal to the floor, what does the boost gauge do? If your boost pattern isnt good it may be that your primary boost isnt comming up right, it should be 10 psi, quick drop to 8 at 4800 or so, and then quickly build back to 10 when the second turbo comes on. This assumes you have a more or less stock sequential system.

speeddemon7

I would say that if im rolling at 5 mph if I floor it it will hesitate for a second before it starts to pick uo serious speed.I cant exactly check it right now since we have about a half foot of snow on the ground here in colorado.
Well.Ive also talked to my father in law and after some serious talking I think he may help me swap the car to a 5 speed.He is a mechanic by the way.The only thing that hes worried about is that the whole thing wont work or if it does work it will not be perfect like a regular 5 speed.Have you driven a stock 5 speed and a modded 5 speed that was an auto? If so were there any differences? I wouldnt think so if it was done correctly but then again you never know.

BMike

I forget its March, its 73 with light rain here in ohio today for some strange reason. Anyways, if you boot it hard, normally it'll just go hard, but I havent driven an automatic model to compare. As for the swap, its alot of work, but done correctly the car should run fine.