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Author Topic: 3.875 / 4.125 / 4.375 ?  (Read 5192 times)
RUSS74GHIA
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« on: January 26, 2010, 04:21:19 am »

Has anyone run different boxes/final drives with the same engine & suspension set up? What was the effect on your ET/Terminal Speed?
I'm curious to know what difference the gearing makes on performance figures
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benssp
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« Reply #1 on: January 26, 2010, 15:45:45 pm »

Johns gear calculator is pretty good

http://www.johnmaherracing.co.uk/gearcalc.htm
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RUSS74GHIA
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« Reply #2 on: January 26, 2010, 22:25:55 pm »

Thanks Ben,

I'm pretty much down with the maths though. What I was after was how this translated performance-wise.

For example, for someone running standard ratios and a 4.12 final drive, what difference would you expext on the strip by running the shorter 4.38 or the longer 3.88?

I realise a lot will depend on the torque/power characteristics of any given motor though, but I was after some ball-park figures.
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Fiatdude
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« Reply #3 on: January 26, 2010, 23:16:15 pm »

The bigger gear (4.38) will make your car quicker accelerating and a better ET in the quarter -- but you have to have the combo so that you can drive out the back door -- ie enough engine RPM or a taller tire

A certain ProMod gentleman swithched rear tires from a 28" to a 26" and ended up going onto his limiter at 1000' oops

A Higher HP car like a turbo'd car can run a smaller gear but they will run out the back door at 135-150 mph

It is all about the combo you have (max rpm of motor, power band of motor, gear ratios in trans, and height of rear tire)
 
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RUSS74GHIA
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« Reply #4 on: January 26, 2010, 23:41:27 pm »

Thanks.
I've picked up an AT box which has the long final drive. I'm hoping new motor will be putting out about 15bhp more, say 110/115bhp.
So, what I'm getting at is, if I swap the box out, will the car become a dog, or will the extra power allow me drive through it?
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Fiatdude
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« Reply #5 on: January 27, 2010, 02:59:51 am »

what size tires do you have?Huh
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Bruce
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« Reply #6 on: January 27, 2010, 05:02:41 am »

Thanks.
I've picked up an AT box which has the long final drive. I'm hoping new motor will be putting out about 15bhp more, say 110/115bhp.
So, what I'm getting at is, if I swap the box out, will the car become a dog, or will the extra power allow me drive through it?
I recommend a 4.12 up to about 180ish hp.  If you change from a 4.12 to a 3.88 with less hp, it'll be a dog by comparison. 
IMO you're making a mistake with the AT.
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Fiatdude
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« Reply #7 on: January 27, 2010, 13:47:19 pm »

agree -- AT only if you are not worried about performance
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RUSS74GHIA
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« Reply #8 on: January 27, 2010, 15:36:12 pm »

agree -- AT only if you are not worried about performance

I'm worried, very worried!

Tyres are 205/60 Federals, so slightly shorter than stock.

The box came up fairly cheaply, so I bought it without knowing what it was. I've no plans to retire the car to daily use for a few years yet, perhaps I'll hang onto it for when I start wearing slippers.
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AlexB
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« Reply #9 on: January 27, 2010, 17:15:36 pm »

Before you decide against the box - check it does have the R&P you think, many boxes have been rebuilt over the years and dont necessarily have
the R&P in that the code on the box says, your 4.125 box may be a 4.375 or a 3.875 - best check is to take out the drain bung on the diff, put a spot of tiipex on one of the ring gear teeth and count how many teeth you have - a 4.125 should have 33 teeth on the ring gear - more teeth and its probably a 4.375, less and its probably a 3.875 Cheesy
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nicolas
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« Reply #10 on: January 27, 2010, 19:18:09 pm »

Before you decide against the box - check it does have the R&P you think, many boxes have been rebuilt over the years and dont necessarily have
the R&P in that the code on the box says, your 4.125 box may be a 4.375 or a 3.875 - best check is to take out the drain bung on the diff, put a spot of tiipex on one of the ring gear teeth and count how many teeth you have - a 4.125 should have 33 teeth on the ring gear - more teeth and its probably a 4.375, less and its probably a 3.875 Cheesy

can't this be done with a spot  on the outputshaft and axle??? i remember seeing someone do this years ago to see what kind of box it was. not sure how it works out.
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RUSS74GHIA
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« Reply #11 on: January 27, 2010, 21:22:08 pm »

Before you decide against the box - check it does have the R&P you think, many boxes have been rebuilt over the years and dont necessarily have
the R&P in that the code on the box says, your 4.125 box may be a 4.375 or a 3.875 - best check is to take out the drain bung on the diff, put a spot of tiipex on one of the ring gear teeth and count how many teeth you have - a 4.125 should have 33 teeth on the ring gear - more teeth and its probably a 4.375, less and its probably a 3.875 Cheesy

can't this be done with a spot  on the outputshaft and axle??? i remember seeing someone do this years ago to see what kind of box it was. not sure how it works out.
Would be fairly straightforward, stick it in a given gear and count the number of times the flange turns in relation to the input shaft, that will give the net output ratio.

As it happens, I cross the line at a smidge under 82mph @ a smidge over 6000rpm. Given my tyres, I'm pretty sure my DC box is 4.125
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Billyisgr8
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« Reply #12 on: January 27, 2010, 21:45:04 pm »

This is what Bruce T told me about finding out what ratio the tranny has  I copied this from his e-mail.

Kevin

screw a bolt in one output flange.  Engage 2nd gear.
Clamp a pair of vise grips on the input shaft, away from the splines.  Count
the number of input shaft turns necessary for one output flange revolution.
If you get 8 turns, you have a 3.88.  8.5 turns means 4.12.  Less than 4
turns means you are in 4th gear.
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nicolas
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« Reply #13 on: January 28, 2010, 07:20:56 am »

yep that's it. couldn't remember the number of turns anymore. thanks
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Neil Davies
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« Reply #14 on: January 28, 2010, 12:47:00 pm »

Before you decide against the box - check it does have the R&P you think, many boxes have been rebuilt over the years and dont necessarily have
the R&P in that the code on the box says, your 4.125 box may be a 4.375 or a 3.875 - best check is to take out the drain bung on the diff, put a spot of tiipex on one of the ring gear teeth and count how many teeth you have - a 4.125 should have 33 teeth on the ring gear - more teeth and its probably a 4.375, less and its probably a 3.875 Cheesy

can't this be done with a spot  on the outputshaft and axle??? i remember seeing someone do this years ago to see what kind of box it was. not sure how it works out.
Would be fairly straightforward, stick it in a given gear and count the number of times the flange turns in relation to the input shaft, that will give the net output ratio.

As it happens, I cross the line at a smidge under 82mph @ a smidge over 6000rpm. Given my tyres, I'm pretty sure my DC box is 4.125

When I ran my Ghia in the VWDRC back in 1999, I tended to cross the line around the same sort of speeds, no tacho, in third gear. That's a little bit over 5000rpm according to the JMR gear calulator. Ran a basically stock 1600 with 36Dells on an AO or AS box (3.88R&P - same ratios as a GT s/a box) and 165/15 tyres, running 18.1 and 18.0 quarters. Not quick, by any means, but I could drive to work up the motorway every day in the fast lane, cruising along with modern traffic, and drive the 250 mile round trip to and from the Pod. Work out your gearing for a three gear quarter mile too - it can work!
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