What diff ratio?

[ClassicAU]

I have decided i want to do the LSD conversion to the classic, but am unsure of what ratio i should get, or if i should change it at all? i have been getting conflicting advice on how high the car should rev at 100km/h on the freeway. I was under the impression that the higher it reved, the more fuel i will use, but have also been told that it should be reving at about 2500 so it breaths better?
just wanted to get some feedback on experience?

[ClassicAU]

i think i am going to go with 3.45 ratio, seems like a bit of a balance between takeoff and economy on the freeway. Thanks anyway

[stiddy]

I would of gone with 3.7's or 3.9's personally..

theres next to no difference between the 3.45's and 3.7's from what ive felt in my car, 2/10ths if that.. and about 250rpm higher reving at speed.

I wish I went for the 3.9's to be honest.. at 110km/h now I sit on about 2350-2400rpm

[ClassicAU]

thanks for that stiddy, thats what i was after. i havent confirmed it yet, i might do it next week so it has time to wear in before the aff nats. what is your car like day to day wise? does it make much of a difference when driving in trafic to the way the trans shifts at all?
cheers

[stiddy]

Ive got a manual, but day to day driving you can barely notice it.

When you start playing silly buggers you can tell that the car is faster not a big difference though, mainly off the line.. 1st gear disappears on me in a blink, and 2nds not much slower..

and also, it seems to cruise better at lower speeds in top gear..

my car used to misbehave crusing at 80km/h in 5th, now I can cruise in 5th all day long from 70km/h onwards...

[ClassicAU]

awesome, i was originally reccomended 3.7's but was a bit scared it was too low for day to day driving. but if its not all that bad, i might go that way if it will make it feel more responsive. but then anything is going to feel better than the 3.08 i have now!! do you remember how much it cost you to get done, if you dont mind me asking?

[stiddy]

If you have an auto go for the 3.9's, I really recommend you do.
It will work so much better with the auto ratios. Huge improvement in acceleration, still be a very comfortable highway cruiser, and be a like a stock falcon in traffic.

It cost me $760.

[mik]

stiddy have you noticed much difference in your weekly fuel consumption???

[39ClevoUte]

Before you buy do some calcs.
Get your current gear box ratios and multipy each one by each of your possible diff ratios. This gives you the overall ratio of each combo in each gear.
What you will then notice is things like a 3.9 x 5th (say 0.8) may equal a 3.45 x 4th (say 1.0). This is why a 3.7 or a 3.9 will cruise happily in top at low speeds, because its the same a driving arround in 4th with a 3.45, except a 3.45 will still have one gear to go for ecconnomy.
The higher the ratio eg 3.9, the closer or less spread out the gear ratios become. So its a balance. Not much good having a first gear that is usless and an overdrive that is no longer doing its function.
Do the maths, ask more questions, no what you really are trying to achieve.

Also if you work out the rev difference for a given speed in top, take this number over your original revs at this speed and multiply this fraction plus 1.0, by your current fuel consumption, you will get very very close to what the end result will be.

[stiddy]

Originally Posted by mik

stiddy have you noticed much difference in your weekly fuel consumption???

Yeah, heaps man.. only because I love launching the damn thing and taking out all of 1st and 2nd lol..

Being serious though, im getting more k's per tank since doing the diff gears..
I used to sit at 80 in 4th(because it didnt like crusing in 5th), now I sit on 80 in 5th at lower revs. I get to sit in 5th more often over 4th.. so lower revs is more fuel economy, well thats how it works in my car.
In peak hour traffic 1hr 30mins every morning and arvo on the way to and from work 5 days a week, I get 400k from a tank.. I used to get about 320-350. If im not in peak hr I get 550k's, used to get about 480-500k.. and on the highway my biggest tank so far is 935k from about 860k.

If I sit on 105-110km/h my car sips the fuel, if I sit on 115-120km/h it uses considerably more fuel.. strange, but that 5-10km/h makes a huge difference in how much I get out of a tank

[Stav]

What Do You Want To Do With The Car?

[Mechan1k]

Would be interesting to see what it would be like on the ute as well ... i am sick of the auto being a dog down low ... it's rare for me to be out of the highway ... and a lot of my cruising is between 60-90km/h anyway.

That coupled with a few other things as well. hee hee
Also LSD might be handy too

[xr8ute]

Originally Posted by stiddy

If I sit on 105-110km/h my car sips the fuel, if I sit on 115-120km/h it uses considerably more fuel.. strange, but that 5-10km/h makes a huge difference in how much I get out of a tank

I've found exactly the same thing with my ute...fuel economy goes to crap when I cruise much above 110. Same before and after the gears.

I love my diff gears (3.9's), I wouldn't think twice about doing it again or recommending it to anyone else. Anyone want to swap some 4.11's for my 3.9's?

[Mechan1k]

So in other words it really does transform the ute ... even though it's a 99kw slug like mine????

I guess it will make it a hell of a lot more responsive in traffic than it is now.

[xr8ute]

Originally Posted by Mechan1k

I guess it will make it a hell of a lot more responsive in traffic than it is now.

I found the most noticable and impressive difference was cruising around in city traffic in 4th (auto gearbox)....car feels much more responsive.

The shorter gear means there is more torque at the wheels at the same road speed.

[Mechan1k]

Sounds good to me ... I hate having to wait for the sluggish auto to respond to downshifts when wanting to push away ... might actually be better for the auto in traffic as well ... less stress and load on it. And better for towing me thinks as well.

[I6corp]

Originally Posted by Mechan1k

Sounds good to me ... I hate having to wait for the sluggish auto to respond to downshifts when wanting to push away ... might actually be better for the auto in traffic as well ... less stress and load on it. And better for towing me thinks as well.

Thats what ill tell the missus ;)

[Mechan1k]

Originally Posted by I6corp

Thats what ill tell the missus ;)

Same here .... hee hee

[FRDGAL]

Originally Posted by xr8ute

I love my diff gears (3.9's), I wouldn't think twice about doing it again or recommending it to anyone else.

Exactly what I was going to say. Would not change my decision to go 3.9's at all. And it's made stuff all differnece to economy but then it all depends on how I drive it :Up_to_som

But saying that, I had my reasons for wanting that particular gearing - the track. Suppose it all depends on what your reason is for doing yours really. If you're going with a complete LSD conversion, then I wouldn't even bother with 3.45. You may aswell go the whole deal and throw in some "fully sickness" lol.

[Mechan1k]

I just want everyday better driveability in traffic and load pulling in the ute ... plus being underpowered and running on LPG with a tired auto transmission ... I think the 3.9s are sounding very good for me .... with a bit of highway work involved as well ... but mostly metro and outer metro driving .... and towing involved.

i don't need an exact gearset to get me over the 1/4 mile line at a certain amonut of revs, etc.

[ClassicAU]

Originally Posted by useless

What Do You Want To Do With The Car?

Thanks heaps for the feedback guys and gals!! all i am really after is a bit more pick-up down low. but my main reason for changing the diff is traction. i am getting well over spinning the inside tyre every time i try and go round a corner or want to punch it off the line! i was reccomended 3.7, and was initially scared by this, but its now starting to sound like the right way to go. as long as it doesnt change that car all that much day to day, and i get a bit more response, i will be happy! also, lsd is always fun every now and then!

[FRDGAL]

Originally Posted by ClassicAU

but my main reason for changing the diff is traction.

Traction? Ohhh traction. Yeah I think I remember what that is :

[mik]

Originally Posted by stiddy

Yeah, heaps man.. only because I love launching the damn thing and taking out all of 1st and 2nd lol..

Being serious though, im getting more k's per tank since doing the diff gears..
I used to sit at 80 in 4th(because it didnt like crusing in 5th), now I sit on 80 in 5th at lower revs. I get to sit in 5th more often over 4th.. so lower revs is more fuel economy, well thats how it works in my car.
In peak hour traffic 1hr 30mins every morning and arvo on the way to and from work 5 days a week, I get 400k from a tank.. I used to get about 320-350. If im not in peak hr I get 550k's, used to get about 480-500k.. and on the highway my biggest tank so far is 935k from about 860k.

If I sit on 105-110km/h my car sips the fuel, if I sit on 115-120km/h it uses considerably more fuel.. strange, but that 5-10km/h makes a huge difference in how much I get out of a tank

thanks stiddy thats the info i was after, mine is auto but cruisin around the burbs at 60 is too slow for overdrive i reckon, and apart from lugging with minimal revs it cant be good for the autos or the engine im thinking, cant be much line pressure for the auto at 1100/1200 revs in top gear i would imagine. cheers mik

[FS5]

i have recently changed to 3.9's in mine (5spd) .it sits on 2450 rpm @ 100kph ,with 225 /50 16inch tyres.on the hwy the econemy has gone from 750+ to around 650,around town i have picked up an extra 50 k's to the tank .i don't do much hwy driving so it's good for me. but when i'm out there and need to overtake i don't have to change back to 4th just put the foot down and go roun them .another bonus is, if you tow things it makes a huge difference with the shorter gears. so much easier to pull things.
if you dont do much freeway stuff i wouldn't put changing to 4.11's off the list either.

[ClassicAU]

Originally Posted by FRDGAL_AU6

Exactly what I was going to say. Would not change my decision to go 3.9's at all. And it's made stuff all differnece to economy but then it all depends on how I drive it :Up_to_som

But saying that, I had my reasons for wanting that particular gearing - the track. Suppose it all depends on what your reason is for doing yours really. If you're going with a complete LSD conversion, then I wouldn't even bother with 3.45. You may aswell go the whole deal and throw in some "fully sickness" lol.

Hi Krissy, i think your right when you mentioned what you wanted to do with the car helping you decide on the ratio. i am not looking to turn it into a track monster, so i would like to keep it civilised. can i ask, with the 3.9, does your car break traction too easily? i have pretty much decided to go 3.7's but i was told that with 235/45/17 tyres that i could get away with 3.9's. i am just not sure, it sounds a bit low to me for every day use.

[xr8ute]

Originally Posted by ClassicAU

can i ask, with the 3.9, does your car break traction too easily?

I hardly break traction with 3.9's in my V8.

If you want that urge off the line, from a standing start, then you want a higher stall converter not shorter diff gears. A higher stall converter will make traction "tricky" from a standing start with too much throttle ;)

[FRDGAL]

Originally Posted by ClassicAU

can i ask, with the 3.9, does your car break traction too easily?

It does BUT bear in mind the mods to my car though compared to your car. I got a bit of power being put to the ground. And I have a 2800 stall in mine. Plus it all depends on how I play the throttle too. Some days I take off slower than a 180B lol, other days it's outta control.

So basically, yes. But that's my car. I've never driven a stocko with 3.9's in it so I don't know what effect it would have. Since you want to keep it civilised, then go with the 3.7's. It'll give you a decent punch off the mark and you'll still retain fairly good economy, especially if you do freeway driving.

[ClassicAU]

Originally Posted by FRDGAL_AU6

So basically, yes. But that's my car. I've never driven a stocko with 3.9's in it so I don't know what effect it would have. Since you want to keep it civilised, then go with the 3.7's. It'll give you a decent punch off the mark and you'll still retain fairly good economy, especially if you do freeway driving.

Thanks heaps Krissy! I think i will go the 3.7 as you sugested. I dont want to go as far as stall converters ir anything like that, i would like to keep its all unopened. Hopefully i will be getting the diff in this week, depending on how i go for time with work and all. I want to get it as soon as i can so i have a few weeks to run it in before the aff nationals! :sm_headba

[GK]

Originally Posted by 54Ghia

Before you buy do some calcs.
Get your current gear box ratios and multipy each one by each of your possible diff ratios. This gives you the overall ratio of each combo in each gear.
What you will then notice is things like a 3.9 x 5th (say 0.8) may equal a 3.45 x 4th (say 1.0). This is why a 3.7 or a 3.9 will cruise happily in top at low speeds, because its the same a driving arround in 4th with a 3.45, except a 3.45 will still have one gear to go for ecconnomy.
The higher the ratio eg 3.9, the closer or less spread out the gear ratios become. So its a balance. Not much good having a first gear that is usless and an overdrive that is no longer doing its function.
Do the maths, ask more questions, no what you really are trying to achieve.

Also if you work out the rev difference for a given speed in top, take this number over your original revs at this speed and multiply this fraction plus 1.0, by your current fuel consumption, you will get very very close to what the end result will be.

Good advice!

GK

[39ClevoUte]

Take FS5's example.
750 reduced to 650 on the highway. This is a 13% reduction in economy. Because his engine is now turning 13% more revs than before. However at low low speed, in the burbs he claims an extra 50ks per tank. Let's assume in the burbs he normally gets get's 400ks per tank, with the new gears he gets 450ks, this relates to a 12.5% increase in economy.
You can see by this it all depends where you are doing your K's .

In FS5's example (highway driving) we can assume he had a 3.45 ratio diff initially, by (assuming his 750 to 650k's is accurate):
At 100k his engine was spinning at 2450 with a 3.9 diff, therefore the wheels were spinning at 628rpm (2450 divided by 3.9)
Since his total k's went down by 13%, with a 3.9, his original diff must have caused the motor to spin 13% slower, so: The wheels still need to spin at 628 but the motor needs to spin 13% slower at 2168, 2168 divided by 628 = 3.45. Therefore, if the facts given by FS5 were accurate and assuming highway driving is relatively efficient the original diff ratio was 3.45.

So why does this same change give better Ks in the burbs??? Why does the maths not work for the burbs???
Simple....labouring.....read all the above and (apart from burnouts) everyone talks about not labouring in top gear or top gear now being sporty, same thing.

Again using maths (and assuming your not after burn outs or towing a Mac truck) you can work out the following:

Assume FS5 has a 5 speed box with the following ratios maultiplied by 3.45 and 3.9 and 4.11:

5 speed 3.45 3.9 4.11
Overall ratios 1st 3.35 11.56 13.07 13.77
2nd 1.92 6.62 7.49 7.89
3rd 1.29 4.45 5.03 5.30
4th 1 3.45 3.90 4.11
OD 0.72 2.48 2.81 2.96
Speed at 2168 1st 3.35 21 19 18
2nd 1.92 38 33 31
3rd 1.29 56 49 47
4th 1 72 64 60
OD 0.72 100 88 84
Speed at 2000 1st 3.35 20 17 17
2nd 1.92 34 30 29
3rd 1.29 51 45 43
4th 1 66 58 55
OD 0.72 92 82 77
Max revs 5500 1st 3.35 55 48 46
2nd 1.92 95 84 80
3rd 1.29 142 125 119
4th 1 183 162 153
OD 0.72 254 224 213
Revs at 60 1st 3.35 >5500 >5500 >5500
2nd 1.92 3469 3921 4132
3rd 1.29 2331 2635 2776
4th 1 1807 2042 2152
OD 0.72 1301 1470 1550

(bugger I can't get this table to display right)

Studying this data is interesting. Note that in 4th with a 4.11 diff at 60ks is like driving in 3rd with 3.45's. Note also that none of the diff ratios really suits driving in OD at 60ks.

Moral is for fuel economy in the burbs drive in lower gears and don't labour motor. Unless you want to do burn outs and tow Mac trucks in which case a 4.11 is the only way to go.