Re: Rear Differential Ratios

[tigercatkid]

Hi,

 

Just a quick check that I'm not barking up the wrong tree with rear diff ratios... I'm currently running a 7" 3.62 ratio rear diff from a Sierra. I would like to improve the acceleration of the car (not worried about top speed) so am considering buying and reconditioning a 3.92 ratio rear diff. I have an ST170 zetec engine.

 

I'm just checking that I am going in the right direction (ratio speaking) - higher ratio, better acceleration (lower ratio, higher top speed).

 

Is this correct thinking ?

 

Any thigh else to be considered when changing diff ratios?

 

Any downsides to doing this ?

[richyb66]

Correct, bigger number for diff is better acceleration. Apart from lower top speed, more revs and noise when cruising there's no downside.

[tigercatkid]

Thanks Richyb - just thought I'd check that my thinking was correct...

[Bob Tucker]

There is one big downside.

1st gear becomes even more useless, unless you have a gearbox with an uprated 1st gear.

[tigercatkid]

Aaaahh, yes you are indeed right Bob...that'll need to be considered...always fancied the longer 1st gear...might be a two stage update

[MarkBzero]

I have a st170 and 3.92 diff, but with MT75 gearbox the first gear is very short and noise is OK at 70mphish. Everything already mentioned above is correct

[speedtripledan]

I have a V6 spec type 9 on a 3.54 diff (English axle) net result is 60 in 2nd in the low 4s when I had the 3.89 diff it was 60 in the 5,s as that extra gear change into 3rd added time, the taller diff reduced wheel spin through the gears as well. Obviously you need the power and torque to pull the taller gears. Google Westfield gearcalc there is a great little page that works out gear ratio and speed in each gear.

[Longboarder]

I also run an ST170 and find acceleration pretty brisk. What size wheels are you running? I wonder if smaller wheels might not be an easier and cheaper way to go?

[Snapperpaul]

I use the V6 type 9 with 3.92 diff on 14 wheels now with 195-60

Revs gives me the edge with this setup as I can exceed 7,000rpm with one engine and near 8,000rpm with the other.

Peak bhp is an issue as I wanted a street car

I still want a low first gear

[Bob Tucker]

http://locost7.info/gearcalc.php

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

 

BGH is worth a call they can supply parts to uprate your own box

[Snapperpaul]

I am thinking about that but also want their case and oil modifications

[tigercatkid]

Thank you for your feedback and comments. Looking at a 3,92 diff later this week...

[dandan62]

I'm also thinking of changing to a 3.92. Mine has been making noises for a while. When I got the car it was dry of oil! I noticed some on ebay from Poland. Anybody got any experience of buying from him?

[tigercatkid]

I'm also thinking of changing to a 3.92. Mine has been making noises for a while. When I got the car it was dry of oil! I noticed some on ebay from Poland. Anybody got any experience of buying from him?

 

Yes saw those too... the guy has a lot of diffs on Ebay but a very low number of transactions... I was thinking it was worth a punt...? Worst case loose £135