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JamesMcMc - 4/7/14 at 08:20 PM

Hi - I'm new so please be nice!

Tomorrow I pick up an MK Indy that has been fitted with a Fireblade engine (originally had a 1.8 pinto).

The guy says it needs a new diff as the current one is noisy (how noisy I'm not sure - I know they all can be!).

I'm not too worried about replacing it as I assume it will be an open diff and I prefer a LSD.

I was after some advice as to what to go for. There's an XR4i LSD on ebay fairly cheap, or I've head good things about 3J diffs (but they are about £700). I'm planning on using the car on track days and drive pretty hard.

Your thoughts and suggestions are welcome.

Ugg10 - 4/7/14 at 08:38 PM

You probably need to check the ratios as most of the bec boys look for a long diff to give a reasonable top speed and cruising revs. For sierra based ones is the 3.38, the lsd is typically a 3.62. Any reason you need a lsd, the lower torque of bike engines does not normally warrant one.

[Edited on 4/7/14 by Ugg10]

[Edited on 4/7/14 by Ugg10]

JamesMcMc - 4/7/14 at 08:44 PM

Thanks for the post.

Just always had lsd cars - find them better to drive. But saying that not had a BEC so, yes might not warrant one.

Think I remember a forum where a guy had a turbo'd BEC MK (about 250bhp) and he had swapped to a diff from 3J and said it made all the difference on the track.

matt_gsxr - 4/7/14 at 08:49 PM

Worth finding out what version of diff it has before you buy the replacement.

Push-in shafts
or
Bolt on shafts

6.5inch (rare), 7inch or 7.5inch (most likely 7 but not always the case with DIY cars)

Otherwise the driveshafts won't fit

Then you need to think about the ratio. If you are going to drive on the motorway then a longer ratio (i.e. 3.38:1 or 3.14:1, instead of 3.92:1) is desirable. If you just want acceleration and tyre smoke then perhaps the other way around. Worth looking at the spreadsheet on this which works out RPM at different speeds for different tyre sizes and engines and such.



Buying a new diff is the easy bit, there are loads out there, so no need to rush that until you are sure its the right one.





Welcome to the forum,

Matt

JamesMcMc - 4/7/14 at 08:56 PM

Thanks for the advise - just what I needed! Guess I'm just a little overexcited/impatient about getting the car and getting it all working correctly.

Where might I find one of these mystical spreadsheets? Sounds interesting!

CosKev3 - 4/7/14 at 09:38 PM

If your going for a LSD with bolt on shafts the most common is from the XR4X4 Sierra or the Sierra or Escort Cosworth 4x4,they are 3.62.
My m8 has built three BEC MK's and used the 3.62 in all of them,plenty of top speed unless your doing tracks with long straights?

JamesMcMc - 4/7/14 at 09:51 PM

Not too worried about a massive top speed - maybe around 120.

Found a XR4x4i diff 3.62 but until I get the car won't know what it has in it regarding bolt on or push in.

Interestingly doing some research I am led to believe that the XR4i (RWD) actually has an open 3.62 diff, and it is the 4x4 that is LSD


Great posts guys - love learning new stuff!

JamesMcMc - 4/7/14 at 09:53 PM

Forgot to mention the car has 18in wheels - guess that would massively effect the gearing too? Will probably go for something smaller myself...

CosKev3 - 4/7/14 at 10:05 PM

My m8 ran his on 17's with 205/40 tyres and they would top out circa 125mph on speedo.

Is the xr4i diff a 7.5 inch wide diff?As RWD Cosworths are 7.5inch wide LSD's.

Your chassis is prob built for a 7 inch wide diff,like the XR4X4 and 4X4 Cosworth rear diffs.

snapper - 5/7/14 at 06:01 AM

Just google gear speed calculator there are loads

GreigM - 5/7/14 at 08:38 AM

I ran a fireblade engine with a 3.62 diff and the LSD was definitely needed (particularly in the wet!!) - nice combination, fired out of corners with plenty gusto. Yes, it wil be revving high, even at 70mph in top, but if its for track days the 3.62 isn't a bad ratio - it will just touch 120 in top.

hkp57 - 5/7/14 at 11:35 AM

quote:
Originally posted by JamesMcMc
Thanks for the advise - just what I needed! Guess I'm just a little overexcited/impatient about getting the car and getting it all working correctly.

Where might I find one of these mystical spreadsheets? Sounds interesting!



Use the link from the BEC section thread.

Linky

A buddy of mine uses a 3.62 open diff in his indy blade, he believes this ratio is best for his 919 between acceleration and cruising. A 919 will struggle to pull a 3.14 me thinks.

[Edited on 5/7/14 by hkp57]

Matt21 - 5/7/14 at 12:35 PM

I have an mk with a 919 blade engine

I got it with a 3,62 open diff, it drove fine tbh
swapped it for a 3,9 lsd, I've never thought 'oh blimey this lsd is amazing' it drives pretty much the same, a bit easier to control once it slides I guess but thats all

top end is around 130 bouncing off the limiter

after 80 you loose poke quite alot

but on track Ive not had any issues with topping it out so far

motorway cruising, 7k rpm at 60-70, I tend to stick around 60 to keep the revs down a bit

back roads, awesome

fimi7 - 8/7/14 at 02:59 AM

I have a Hayabusa in my MK, with a 3.6 Sierra. Was originally a standard cosworth LSD, but swapped that out for a Trans-x disk type LSD. Worth every penny. Some will say the BEC does not have enough power to warrant a LSD. But it depends on the use. If your plan is to putt around town on the streets, then yes, no need.

If you plan on doing track days, its worth it.

As for the ratios, again depends on use. Track days, top speed is limited by aero, so the 3.6 right now is okay. For street driving, if you plan on going above 70 MPH, then the 3.6 is too much, meaning you need something closer to the 3.3 or 3.1.

Right now, mine does about 90-100 MPH in 6 gear at about 8.5-9k RPM (with the 3.6). Sounds like its screaming, but you still have another 1500-2000 RPM to go before you hit the limiter.

My suggestion, if you have a 3.6, run it for a while and see how the acceleration feels, then make a choice...

mcerd1 - 8/7/14 at 07:55 AM

try this gear calc:

http://locost7.info/gearcalc.php

there are lots of spread sheet based ones, but this one is my favourite standalone one

quote:
Originally posted by JamesMcMc
Found a XR4x4i diff 3.62 but until I get the car won't know what it has in it regarding bolt on or push in.

Interestingly doing some research I am led to believe that the XR4i (RWD) actually has an open 3.62 diff, and it is the 4x4 that is LSD


an XR4i might have had a LSD, but it was an option not a standard

all the 4x4 cars had an LSD in the back and centre as standard, that includes XR4x4 (both 2.0 and V6), Ghia 4x4 (even the estates) and Granada 4x4's (very rare beastie)
and all the cossie's had LSD's as standard too, except for the later Granada 24v cosies that had traction control instead



all ford LSD's must use bolt-on shafts - the push in ones will not work
(some open diffs are bolt-on too - these were the ones that had rear discs rather than drums - e.g. normal sierras with ABS / later XR's etc...)



all 4x4's should have 7" diffs - these are a 3.62 ratio for the V6 and cossie cars or a 3.92 ratio in the 2.0 XR4x4's
a small number of the 4x4's seem to have used 7.5" diffs but these are uncommon
(basically ford raiding the parts bin when the ran out of 7" ones)


XR4i's and other cars were the LSD was optional can have either a 7" or 7.5" diff


sierra/saphire cossie 2wd's use 7.5" diffs (3.64 ratio) but with bigger 108mm CV joints, driveshafts and hubs - these are only on this model

Granada 24v cossie's have a 7.5" LSD (3.64 ratio) but with the smaller 100mm CV joints and shafts - the same size as all the other sierra's and Granada's

jwhatley - 10/7/14 at 08:24 PM

quote:
Originally posted by JamesMcMc
Thanks for the post.

Just always had lsd cars - find them better to drive. But saying that not had a BEC so, yes might not warrant one.

Think I remember a forum where a guy had a turbo'd BEC MK (about 250bhp) and he had swapped to a diff from 3J and said it made all the difference on the track.


Aye that was me, not 250 but 450bhp.

The 3J unit is very good, it is fully adjustable and can be set up to how you want it unlike other ATB diffs on the market.

If your BEC you ideally want to find a 3.14:1 ratio. If you can find it out of a diesel it might already have the ford viscous LSD in it. That will be fine until you go big power, might just need a refresh.

The bolt in shafts on are different between the LSD and Open Ford diffs. If you are going to fit a 3J diff for example make sure you buy an open diff, otherwise you will be looking for the driveshaft hubs to fit it.

John

[Edited on 10/7/14 by jwhatley]

mcerd1 - 11/7/14 at 07:20 AM

^^ as he said all the aftermarket LSD's are designed to be fitted into open diffs - not ford LSD's

quote:
Originally posted by jwhatley
If your BEC you ideally want to find a 3.14:1 ratio. If you can find it out of a diesel it might already have the ford viscous LSD in it. That will be fine until you go big power, might just need a refresh


3.14 diffs were like rocking horse poo 10 years ago - so these days I've not seen one sell for less than £500

also I don't think they ever sold any with an LSD as standard (at least no one has ever found on as far as I know) - these diffs were mostly used on just a handful of the 2.3 diesels, the big heavy, low powered Peugeot lump so not exactly the cars that are going to have the expensive LSD option

the next nearest ratio is a 3.36 or 3.38 (7" or 7.5" ) these are a little bit more common, found on some of the 2.3 diesels, small V6's and other odd cars, but again not the cars that had LSD's

but it is fairly easy to swap over the bits of diffs the same size (i.e. 7" or 7.5" to make an LSD of any ratio you can get your hands on
if you find one that's already be re-built as a 3.14 / 3.36 LSD it'll likely be selling for big money


btw - the later 1.8 diesels mostly have the very common 3.62 / 3.64 ratio diffs, just the same as the V6's

[Edited on 11/7/2014 by mcerd1]

jwhatley - 13/7/14 at 08:57 AM

3.14 LSD was fitted to the 2.3 deisel with the factory towbar option. I was lucky enough to get one of those units. Then the diff ate itself!

Agreed, they are certainly more rare than the usualls but if you look in the right places then they are still available. Iirc yellowcab had 2 st in his garage for a while.