Diff measurements

[leroybrown911]

Hi all

Just a quick question. I have been trying to source a LSD for my roadster that I plan to run with a R1 exup engine. From my searching so far cosworth diffs have varying dimensions from 7 to 9''. Is it just a case of making the prop the correct length to suit? or will the hole pattern and diff mount be different? Sierra diffs in general seem to vary between 7-9 inch.

Thanks
Leigh

[snapper]

You will only find 2 sizes available, 7 & 7.5" although there was a 6.5" and a 9" motorsport one.
The Haynes will take either 7 or 7.5
You will only find a 3.6 or 3.9 LSD.
For your R1 engine a 3.38 would be better
You can fit an LSD unit in to the 3.38 or take the crown wheel and pinion from the 3.38 and put that in the LSD diff

[mdasseville]

Hi

I run a 3.5 LSD with an R1 engine and it runs Great,,,,i had a 3.3 first of all it gave it a good top end abit slow though the gears.

I like the 3.5 it goes like hell but only to 125mph then it runs out of steam

Just my 2 pence worth

[leroybrown911]

Thats great, thanks gents

[snapper]

Don't forget the wheels and tyres, the rolling radius will also effect your gearing.
There are a number of gear ratio calculators, I used them for my initial build.
Diff ratios you are likely to find will be from the Sierra, 3.9, 3.6, 3.38 and 3.14 you can then fine tune with wheel size and tyre profile.
From my experience by this time either your wallet will collapse or your head will explode

[les g]

if you want to use the 7.5 diff dont forget to space the diff uprights further apart
when building the chassis its a cock to put right later
cheers les g

[leroybrown911]

Thanks les. Do you mean the diff mounts will be different between a 7 and 7.5 inch diff? I presumed the 7'' dimension referred to part of the length of the diff, hence why I was referring to the length of propshaft being altered to suit. I guess I might be best trying to source a diff so I can see what Im playing with asap

[snapper]

I believe the 7" or 7.5" refers to the diameter of the crown wheel.
The top mounting points on the diffs are indeed wider for the larger diff, if the top mount has a long single through bolt it's a 7", if it has 2 captive bolts it's a 7.5"

[leroybrown911]

Thanks for the info. I think I have reached the point with the chassis that I now need to buy some donor parts to make sure they fit when Im finished fabrication. Thanks guys

[les g]

ah nope not quite
the 7.5 is wider where the diff mounts attach
so the to uprights on the rear of transmission tunnel
where cp10 are welded too need to be further apart
cheers les g