Final Drive Swap

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Final Drive Swap

Post  Jared Mk3 on Sat Jun 16, 2012 1:44 pm

I would just like to know the potential gains in cruising speed and overall driving experience if my car's diff were to be swapped from 3.765:1 to 3.44:1 . It's a 998cc that runs really well. To my knowledge the motor was fully rebuilt by the previous owner. It will have a Maniflow manifold fitted and an RC40 exhaust system, with a Weber to assist the fueling.

The gear ratios are as follows :

1st - 3.525:1
2nd - 2.218:1
3rd - 1.412:1
4th - direct (naturally)

Any suggestions would be great.
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Re: Final Drive Swap

Post  Elf Service on Sat Jun 16, 2012 6:10 pm

Jared Mk3 wrote:I would just like to know the potential gains in cruising speed and overall driving experience if my car's diff were to be swapped from 3.765:1 to 3.44:1 . It's a 998cc that runs really well. To my knowledge the motor was fully rebuilt by the previous owner. It will have a Maniflow manifold fitted and an RC40 exhaust system, with a Weber to assist the fueling.

The gear ratios are as follows :

1st - 3.525:1
2nd - 2.218:1
3rd - 1.412:1
4th - direct (naturally)

Any suggestions would be great.

With a "stage one kit" you should be OK. Top speed will increase to 80+ mph and cruising at 60 to 70 will be easier on the engine and driver. I personally would opt for an 1,1/2" SU with a K&N filter as a Webber will tend to use more fuel and is better suited to bigger engines, just my personal views and experience.
You will need to get the car set-up correctly on a rolling road, re mixture across the rev range. The speedo will read slow and either need re calibrating or you will need to change the speedo drive gears to suit. Or just be aware of the inaccuracy and drive accordingly.

I have a 3.1 diff in my red Elf and although this produces 80bhp+, it would easily pull a taller diff (2.9 or even a 2.76) but with the power and rev range, it will top 100+ with ease, so I'm leaving it as it is.

If you can get your hands on a 295 Cooper head and have it skimmed to bring the compression ratio up to at least 10:1 then you will be looking at around 50bhp+. Another consideration is to look at some 1.5 rockers as these will improve the engine breathing by opening the valves further. Changing the cam to give a longer duration will really hot things up but the 998cc engine isn't too tolerant of a sporty cam and will tend to run rough at low revs, with power kicking in around 1800 to 2000 rpm.

Loads of options available to suit your pocket and what you wish to achieve.

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Re: Final Drive Swap

Post  asahartz on Sun Jun 17, 2012 2:08 am

The quickest 998 I had used a 12G940 head with a pocketed block (though you can do the same by recessing the valves) and kent 266 cam. It was a good general drive, nippy off the line, and my daughter got clocked by a speed camera at 93mph. And that was with a bog standard exhaust and HS4 carb.
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Re: Final Drive Swap

Post  TONYELF on Sun Jun 17, 2012 1:14 pm

Thats a expensive way of proving your car does over 30mph! Very Happy

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Re: Final Drive Swap

Post  asahartz on Mon Jun 18, 2012 1:41 am

True, but I thought she'd done quite well to get that out of a 998!

And she wonders why I'm only fitting the 998 version of the Micra engine to her estate project!
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Re: Final Drive Swap

Post  Jared Mk3 on Mon Jun 18, 2012 1:35 pm

Thanks for that advice. So as a general drive with the ability to do some highway cruising, the 3.765:1 diff is better suited, with those mods in mind?
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