Changing diff ratio - worth it?

[domV8]

My Chim 450 currently runs a BTR diff with a 3.2something ratio.

Having tuned the engine to produce more BHP further up the RPM range, and gained another 1000rpm over the older engine set-up - conventional wisdom would be to swap the diff ratio out for a lower ratio (ie. the 3.4something) to give more torque at the wheels now I have more rev's to play with.

So the question is: if it costs aroundabout £600 to swap the diff ratio's from the 3.2x to the 3.4x - will it be worth it?

Will I notice any difference between the 3.2x and the 3.4x ratios...?

(Don't want to go any lower, as the car is a daily driver, so gets used on motorways etc too, and want it to remain driveable and not annoying/expensive on longer journeys...)

Thanks,

Dom

[A900ss]

I thought going from a 3.2x to a 3.4x would increase your revs at any given road speed, not decrease them...

Or have I missed the oint of the post.

(Worried I don't understand now...)

[K4TRV]

A900ss said:

I thought going from a 3.2x to a 3.4x would increase your revs at any given road speed, not decrease them...

Or have I missed the oint of the post.

(Worried I don't understand now...)

^^ ^^

[Laser Sag]

A900ss said:

I thought going from a 3.2x to a 3.4x would increase your revs at any given road speed, not decrease them...

Or have I missed the oint of the post.

(Worried I don't understand now...)

You would be right the revs would be higher at any given speed, think the original question is would there be enough of a difference by changing to the 3.4x ratio to notice in acceleration. The ratio could be changed to go even higher but then the increased revs when cruising can become annoying if the car is used as the daily driver as is the case here.
Can't answer the question reference the diff change as have never tried the 2 diffs, although those have gone one stage on the speed six cars think the difference is noticable and that 2 steps is even better.
I suppose the OP could try driving everywhere for a day in a gear lower than normal and that might give some indication what probably 2 steps on the ratio would be like in the terms of revs and noise.

[Graham]

I thought the stock btr diff in a chim was 3.45? Mine was and ive put that im my tuscan in place of the 3.3 gkn and have put the 3.7 spare challenge btr in the chim for more acceleration

G

[domV8]

Perhaps I wasn't clear...

I have an "option" diff ratio in my BTR according to John Reid - offered at point of purchase to help increase MPG by making the gears longer ie. less revs at a given speed = better fuel economy.

Now, longer gearing means less torque at the wheels - so for quicker acceleration you want lower gearing, not the "optional" higher gearing that mine has.

Higher gearing is great when you have a smaller rev band - as it gives you a higher top speed in each gear (at the expense of torque at the wheels).

I have now tuned my engine to the point that I have an extra 1000rpm over my old engine - so I am not worried about having a higher top speed in each gear (as I can achieve that with my extra revs).

What I want is the extra torque at the wheels for quicker acceleration - hence going DOWN from the 3.2x to the standard 3.4x diff ratio.

So the question is - will there be a noticable difference in acceleration between 3.2x and 3.4x diff ratios...?

I see the other thread about the guys going the other way from 3.4x to 3.08 - and thats obviously quite a jump - over 10% from what I can see, so I would expect there to be a difference in the driving characteristics for them.

But the difference between 3.2x and 3.4x is only half of that - so if it is going to cost me £600ish, will there be a noticeable increase in acceleration from the lower diff gearing..?

ie. will the change in acceleration be worth the £600-odd outlay..?

Hope this is more clear....

Dom

[db484bhpv8]

I honestly believe you wouldnt gain any significant acceleration. It would only 'feel' slightly quicker as you would be needing to change gear earlier.

I went from 3.45 to 3.08 and i love the way the car drives now. I can cruise off cam on the motorway at 90 mph at under 3000 rpm.
As for acceleration i have ran a few 12.5's on the quarter mile and previously had a best of 12.4 with better conditions on the day so as i said..... no difference in acceleration.

Dont bother Dom. Spend the money on a nitrous kit

[jamieduff1981]

If you can light up the back tyres in 1st and 2nd already I can't see any benefit in doing this. I should think that traction would be the limiting factor for ultimate acceleration at any roadish speeds.

[K4TRV]

I would respectfully suggest with a 450 uplifted power output that you have two options:

Drop the rear axle to 3.08 - as db484bhpv8 and Jamieduff1981 have reflected, you've enough oomph to run a 3.08, but you are already part way there with your 3.2x??

You may wish to consider: http://www.gearboxman.co.uk/infot5.html and stay with what you have in diff ratio and go for an uprated box with a close ratio set of gears which will get you up to higher top speed quicker??

Its all a question of which compromise suits your pocket and enhances your additional power - that's your call, I'm afraid 8-/

Trev