Diff Ratio Change 3.46:1

speedo will be out and possibly hit the rev limiter before change
Cuckoo

 

It wont hit the rev limiter. Do it! Its the best money you'll spend!

 

yeh do it, ive got a 3:7 diff in my vt and i luv it.

 

What is the benefit to using a lower geared diff to the standard 3.08? Wouldn't the economy be worst?

 

yes economy will be worse slightly due to the engine revving higher, but the car will dramatically launch harder and there will be less stress on the drive train from the engine to the diff. DO IT!!!!!!!!!

 

How much does this cost to do ??

 

My fuel economy actually got better and that was going to 3.73!! The car doesnt need to labour quite so much as it gains speed so it gets there easier.

3.45 gears shouldnt cause it to hit the limiter but your speedo will be out slightly. Local dealer can adjust speedo with tech2. Gears cost approx $500-550 plus fitting.

 

Yeah I never thought off that! A higher ratio diff (lower geared) could be the way to go. What do you mean by hitting the rev limiter? Wouldn't the engine rev out the same regardless of what the diff ratio was?

 

The 4L60E trans fitted to VS onwards is Electronically controlled by the ECU. The trans changes gears at predetermined road speeds NOT by revs. Also WOT and part throttle setting influence when its changes gear relative to road speed also. Problems can start to arise when the diff gears are changed and the ECU is controlling shifts for the trans by road speed BUT also the ECU is safe guarding the engine for over revving. At WOT the engine will get to the revlimit before it gets to the correct or normal (factory) road speed so therefore it hasnt been commanded to change gears and it either keeps bouncing off the revlimiter or what most drivers do is back off the throttle to force a gear change due to the ECU then seeing a part throttle condition.

 

Thanks SUPERV6 for that great explaination. So therefore even under partial throttle, with a lower diff, the engine will rev higher through the gears? Can the ECU be programmed to change at a lower road speed?

 

Yes all road speeds with a lower diff will rev higher no matter what throttle setting.

Yes you can reprogram the transmission to whatever speed suits your needs. This is normally done with the Kalmaker software.

Often when mods are done to increase HP and things like diff ratio is changed its the sloppy factory gear change that causes the revlimiter to just be touched. Its the flare at gear change that allow the revs to rise too much. It can typically take .8-1.2 secs for the trans (longer on a worn trans) to complete its gear change and the clutches to grab fully. In this time the revs might of climbed another 300rpm from the point of when the trans started to change. Its that rev rise or flare that causes the problems in a stock transmission. One option to help control the problem is a shift kit to promote firmer shifts. A shift kit for example might of completed its shift within 150-200 revs so could avoid hitting the revlimiter. I had this problem when I first started to make good power and changed to 3.7 gears....the problem was solved by fitting a shift kit and promoting firmer transmission shifting. I have since had a full Kalmaker tune to reprogram the whole car...engine tune and transmission tune

 

Cheers for the info SUPERV6, your'e a mine of information! When you talk about shift kits I assume your talking about the "proper" ones not the cheap electronic ones.
My car is totally stock at the moment but I do plan some radical mods in the future and all this info is a big help! :thumbsup:

BTW, what type of shift kit do you have eg brand, and how do you rate it?

 

Yes I refer to shift kits as the type that need fitting internally to the trans with various springs, shims and some passages are drilled larger.

I am using a Transgo HD2 reprogramming kit which has everything you need in the kit. You dont need Corvette servos or anything else for that kit. Its been great for my setup and hasn't let me down. It came recomemnded from a highly respected trans rebuilder. That kit is refered to as a stage 2 kit however it has 3 stages within the kit to further tweak it more.

 

Thanks for your help, it's much appreciated

 

Dont suppose anyone knows what the correct PPK value should be with 3.45 gears? Most of the holden dealers around here wanna charge an absolute fortune for what most people tell me should only be a 5 minute job. any ideas?

 

Im lead to belvieve that std PPK is around 6250.
To work it out I'm told you dived your new ratio from your old then multiply it by your standard ppk value. ie (3.73/3.07) x 6250 = 7595 or (3.45/3.07) x 6250 = 7024

Hope it helps.

Jezz

 

hi i'm just about to do the same mode on the same type of vehicle on the weekend at work. some of the problems i encounted was the diff because its out of a VU SS 6 speed and the input for the drive shaft. the yoke is different but it can be replaced by the vt one, and there is a pin that has to be cut off

 

Im also in the same boat, im looking at upgrading the diff ration on my L67, i dont think 3:46 would impress me much, so i TRYING to find 3:7's. But now am concerned about this "limitier" issue. So your saying it will hit the limiter before being able to shift into the next gear??
When getting the speedo re-calibrated (trip comp ect) wouldnt that prevent the 'limiter' issue??
PLease email me at pluijmers@aapt.net.au
Cheers, Paul