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Ls1 sedan diff gearing ratio

matqld

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Hey all, Got vt/vx sedan, fresh rebuilt ls1, (fresh strong auto and shift kitted) ramjet otr, mafless tune, cam 224-228 112lsa, 918 springs, comp rods, 1:8 yt r/r, chain, oil pump ect, hand port heads (mild-mod) not cnc, catback, pacemaker Headers n ballistic cats (coming now) ,,,, i want to change my diff gearing from standard, was thinking might be cheapest option to look for maybe a diff out of 6sp man ute from a wreckers or something?
I have done this on small budget... i realise may not be best setup to suit all liking.. goin for retune once headers n cats on.
When i do 100-110kph the car seems to really be at a sorta flat spot in revs, (just under 2000 rpm) this is when im cruising along, its like a flat spot just before the cam comes into operating range but like all cars it seems to have its "sweet spot" when it comes to revs and speed on the hwy like a happy cruisin speed lol, and when i do say 120-125 its seems much happier ;p crispier throttle response ect, the rpm's are a little over 2000rpm when im doing that speed rather then just under 2000rpm.

I want do be doing those revs that i do now when i do 120-125 kph but with different diff gearing to reduce my speed down to say around 100-110kph.

So really what im asking if anyone knows is what diff ratio am i after ?

Like a factory man ute lsd? or factory sedan manual lsd? atm i have the "factory auto diff". thinks its 3:46 or something lol i dont have a clue.

Im not after a drag ratio lol, Its my everyday driver hehe,, just a small difference in ratio to what i have now so the car cruises at a better revs for me, i think fuel would prob be better off as its cruising with less effort at that speed or wouldnt be much different.. although i realise the principal of usin higher ratios reduces your overall top speed which would/could result in more fuel use lol, but im not after 4:11's lol


Also is changing the diff ratio something that requires a tune touch up for performance reasons? (apart from speed recalibration)
As headers and cats will be goin on soon and getting tune tweaked but wondering if i should hold out for diff gears or just put new diff ratio
down the track... could a tech2 be used to recalibrate speed?
If there is a whole thread already on this sorry had no time to search through the jargon.
Thanks in advance anyone who offers advice/opinion/help ect thanks very much....
 

MrSirWantalot

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I changed my diff from a 3.07.1 to a 3.46.1 & it took the revs upto 2300 approx from 1800, but because I do alot of hwy driving I asked the tech that tuned my car to give me less revs in top gear (auto) witch he did & @ 100kph it sits on 2000. Ideally if I didnt do as much hwy driving I would of went & got a 3.7.1 diff out of a manual HSV. A 3.46 is good for standing starts & towing witch I dont think you will be doing with ur mods...unless you wanna pull the trailer apart :). I think the speed recal is all you should need to do!
 

TI3VOM

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I changed my diff from a 3.07.1 to a 3.46.1 & it took the revs upto 2300 approx from 1800, but because I do alot of hwy driving I asked the tech that tuned my car to give me less revs in top gear (auto) witch he did & @ 100kph it sits on 2000. Ideally if I didnt do as much hwy driving I would of went & got a 3.7.1 diff out of a manual HSV. A 3.46 is good for standing starts & towing witch I dont think you will be doing with ur mods...unless you wanna pull the trailer apart :). I think the speed recal is all you should need to do!

How did the guy that tuned your car do that? That's not possible to do!
 

MrSirWantalot

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oh yes it is...I think its all to do with the torque convertor.....you know when u get to top gear & the last drop in rev is to do with the convertor...well mine is a very noticable drop in rpm
 

goose202

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No, it's not. The torque converter locks up at about 80km/h with the standard tune, making it a 1:1 link between the crank and the trans input shaft. The only thing your tuner can change in software is when that lockup happens, and shift points. When it's not locked up, you will be doing more revs at the same speed since there's slippage. Maybe that's the higher revs you saw.

If the car was doing 1800rpm at 100km/h before with a 3.07:1 diff then it will be doing 2028rpm now at the same speed with a 3.46:1 diff.
3.7:1 diff would get you 2169rpm at the same speed.
 

MrSirWantalot

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Its a good thing you own my car & not me....otherwise I would'nt be able to give my opinon. Btw i didnt say how he did it speciffically ....but i know he ask me if i wanted it like that & he did it. So therefor Im guessing it is possible!
 

goose202

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I'm glad you're happy with your tuner and your car.
But the fact is if you went from 1800rpm to about 2000rpm with the 3.07 to 3.46 diff change, then that's just what it should have been, and that's what the OP should expect in the same situation. It's not possible to go back to the tuner and ask for less revs than that with no mechanical changes.
 

MrSirWantalot

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I went from around 1800 (3.07) to 2300 with the (3.46) diff in, if thats not right well...... i cant explain....... So at 2300 @100 kph The tech asked me if i would like less revs in top gear witch I said yes witch once he done brang the revs down to 2000 at 100kph (or 2023 to be exact) . Anyway we have drifted off away from the question matqld asked....& in my opinion Id use a 3.7 out of a man hsv. :). Btw this was all done whilst he was plugged into the ecu, & yes he did change trans pressure, for harder changes i suspect!?
 

Jesterarts

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OMG this forum is so full of fail today.

I thought I had heard it all and then I came across this chestnut. Tuning the car to rev lower in a certain gear. Brilliant.

I'm going to have to pop down to my tuner and get him to dial out a few few's at 100kmph to same some fuel.

To the OP, not sure what ratio you should run. I'd say the highest you would want to run in an auto would be 3.73 and still retain sensible fuel economy.

Most people will probably agree that 3.73 is plenty for an auto while the 6speeds work well with 3.91's.

A tech2 SHOULD be able to recalibrate the speedo to account for the new diff ratio but in the past I had an issue with holden calibrating a 3.73 diff when I put it on my old VX S.

Though I think this will be more to do with the fact they couldn't simply look up the pulse count in a book and would have had to work it out.

This is a none issue since any tuner can adjust this value so not to worry about.
 

MrSirWantalot

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Dont beleive i said i tuned the car to rev lower keyboard warrior. But i guess you just needed to be there to understand what i mean...or maybe you could of told the tech how to do his job. In My opinoin i already said i would go a 3.7....so nothing new there.
 
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