Diff Change ?

[Guest david_l_perry]

I have come to the conclusion that I want 'longer legs' on the mway as I tend to do a fair bit of longer runs in the hood. I don't do any racing/track days, I use the hood as a roadster / long distance touring.

 

I am looking at options of how to give me a better cruising on the longer runs.

 

I also find that first gear is over way to quickly so all this is leading me to think about a diff change as a first port of call rather than masses of work and money on an engine or even car change as I pretty much love the hood still for all that it is

 

Mitch who built the kit originally is pretty sure it's the original 3.62 diff as the original donor was a 1.6

 

Just trying to build info on best options for me

 

I could also change wheel/tyre size but want to weigh up what's going to give me the best result.

 

Essentially I want to work out how much increase I will get for certain diff ratio's

 

Anybody know of any spreadsheets or how to work out what sort of increase at certain engine revs I can expect....

 

 

 

Dave

Edited July 2, 2014 by david_l_perry

[Mitch]

Thinking about it the dif that's fitted is the touring rather than exceleration one . Can't remember ratios but I think 3,9 is preference for acceleration

 

Hth

Mitch

[Guest Ian & Carole]

You don't want to go any longer on a diff than 3.6 or your car won't pull 5th gear.

3.9 will get you screaming at motorway speeds in 5th.

 

http://www.willtheyfit.com/ might be worth looking at tyre sizing.

[RedTrev]

Have a butcher's at this:

 

https://dl.dropboxus...675/gearing.xls

 

I can't remember where the original data came from so can't verify its accuracy but I manipulated the data to draw the graph.

Just insert your wheel/tyre size into cells H3 to J3. Insert diff ratio into cell B4. Insert gear ratios into cells I9 to M9 and it should work.

Copy the file if you want to keep it before I delete the file from dropbox.

Anybody spot anything fundamentally wrong with it please let me know.

T

Edited July 2, 2014 by RedTrev

[Guest whiz]

What about a 3.38

[Guest Ian & Carole]

What about a 3.38

 

A pinto wouldn't pull a 3.38 it would make 5th gear redundant.

[Bob Tucker]

Not cheap, but I reckon the only way to solve all these problems is a different (rebuilt) gearbox.

You can get a higher 1st gear, closer ratios for 2,3 & 4, & a higher 5th.

My box came from these guys...

http://www.bghgeartech.co.uk/

 

this is also useful....

http://locost7.info/gearcalc.php

Edited July 2, 2014 by Bob Tucker

[Guest david_l_perry]

Thanks for those spreadsheet and web links - very very helpful

 

Not convinced that changing the Diff will have such an impact making 5th gear redundant.....

The Pinto in mine is a 2.1 with high lift cam and has a fair bit of grunt - more so than the original sierra I would suggest

 

The total wheel diameter on my hood is a little less than than the sierra so its down straight away, changing the dif to a 3.38 puts it almost back to the original sierra final drive ratio

 

Having said that it does not appear to make a vast difference to just change the Diff

 

It appears to be just matter of 80mph becomes 85mph at 3.5k revs

 

I was expecting a little more difference or am I missing something

 

Changing the wheel size makes less of a difference, and would probably be far more expensive than a diff change...

 

What wheel size do people tend to be running ??

 

Mine is 195x50x15 Total Dia is 576mm - are people running larger Dia as the norm

[Longboarder]

3.62 diff and 185 x 60 x 13 gives me 70 mph at about 3000 rpm. You can identify your diff. It might just still have the ally tag held by one of the cover bolts. The numbers on that would tell you. Usually gone but if you remove the diff cover the ratio is written on the edge of the crown wheel in white paint.

 

Nigel

[stevedohc2b]

Can you not do a check by jacking back up and spining wheel looking at prop shaft to do 1 turn?

 

Stephen

[Snapperpaul]

The gear ratios you need are a much longer first and 5th, it is the wide saloon car gearbox that is the issue

By changing the diff you only shift the issue to the other end of the box, 1st gets better 5th worse

Your ideal would be a 2.7 1st and 0.75 5th

If you use the gear calculator and try 205/60/15 it may show a better result

[Guest mower man]

I run 205 /50 x15 tyres on 6.5 inch rims with a 3.62 diff ,used to have a 3.38 in it, the std ish 2.1 would pull 5th but for any thing like accel it was down to 3 rd now with the 3.62 and alittle more power [145ish ] its only 1st that gets to me being too short so its a quick change to 2ndand lifes great , ideal situation as Bob says isthe BGH gear box but have spent my limit this year after the gass cutting episode a couple of weeks ago mick

[agent_zed]

might be talking rubbish but didn't one of the sierras have a different 1st gear? the V6 or diesel?

[Mitch]

V6 box has dif ratios but you need an adapter spacer to make it fit .

[Snapperpaul]

I've fitted the V6 box

Better 1st better 2nd

 

[Type 9 (1.3-2.3)

 

1st 3.65

2nd 1.97

3rd 1.37

4th 1

5th 0.82

 

Type 9 (2.8 inc 4x4 )

 

1st 3.36

2nd 1.81 (this is the ratio that lets the 2.8 go over 60 in 2nd )

3rd 1.26

4th 1

5th 0.825