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Author: Subject: LSD Choice

posted on 26/3/24 at 10:58 AM Reply With Quote
LSD Choice

I have a Honda Blackbird Locost that I have been tracking for the last season. I am happy with the state of the car and I'm now moving onto making performance upgrades.

The first hurdle I have chosen to tackle is to fit an LSD.

I currently have an English Axle with an Open Differential. I am unsure of the current ratio but I have free rein when choosing a new diff.

My knowledge on the differential topic is limited so I'm looking for some advice.

Is there a "Best suited" Ratio for Bike engined cars given they rev so high? I've seen you can get 7" 3.14 ratio differentials but are they the best?

I have contacted a local company and they have quoted me 450 + vat for the install costs. Is this reasonable and to be expected? I would need to supply the LSD and the bearings.

Do I need to be careful with which LSD I choose given the car is a light weight 7 style car? I've been reading about Plated and ATB. Struggling to understand the difference and pro's / cons?

In terms of getting the LSD Setup, where do you start regarding what Ramp angles to specify, Pre-load etc. I feel very out of my depth trying to spec these for the builder. Or are these taken from dimensions and weights of the car?

Given I have an English Axle, can I just buy any appropriate 7" Differential with the correct spline no. and I can get it fitted to the axle I have?

As I have a solid axle, does this change anything about the differential I buy? I understand there are downsides to a solid axle but does this change the spec and style of diff I need to buy?

Any knowledge or advice would be great, thanks in advance.

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Mr Whippy

posted on 26/3/24 at 12:30 PM Reply With Quote
Could you convert the rear to IRS so you can use the lighter diffs from something more modern? and get better suspension to boot.

450 + vat for the install cost seems not bad but I'd be using a gearbox/diff specialist who is able to shim it up correctly. As for the other stuff, no idea as I went to a gearbox/diff specialist as I didn't have the tools to it myself.

Fame is when your old car is plastered all over the internet

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posted on 26/3/24 at 01:44 PM Reply With Quote
I didn't really even consider changing to an IRS setup. I thought this would be hugely intrusive and at that point I might as well change to a more suitable chassis overall?

Ok thank you, good to know I'm looking in the right region for fitting. Yes this was quoted by a local Gearbox company so should have the SQEP. Did they advice you for your car setup then?

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posted on 26/3/24 at 03:01 PM Reply With Quote
A 7" diff will not fit an English axle, you need an English crown wheel and pinion (CWP). What ratio have you currently got? I seem to remember 3.54:1 is as high as you could go from the ford parts bin although retro ford so a 3.3:1 for about 500 (parts only). I am not sure what the gearing on the blackbird is and if 3.3:1 is high enough for you or even any better than you already have.

Changing to IRS is possible if you have deep enough pockets or are a very competent engineer, I have only seen it done once or twice before.

The guys at Road and Race Transmissions are very helpful when it comes to choosing a limited slip diff type. Have a look at this and then give them a call.

They will give honest advice about what is best for your car and the use you want to get out of it

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posted on 26/3/24 at 11:38 PM Reply With Quote
As someone who changed from a live axle to IRS I can wholeheartedly say that I would never do it again. I literally had to cut the whole rear frame off and put atleast 6 months on my build time (albeit I don't get all that much time to work on it).

Changing to IRS you need to think, rear frame, diff mounting, more than likely custom rear wishbones, space for coilovers, available fuel tank space, weight, cost etc etc. I really would avoid it if at all possible.


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posted on 27/3/24 at 08:24 AM Reply With Quote

Theoretically.. a plated diff is the way to go for track use, but the pre-load settings on them are critical, given how light our cars are it's easy to end up a little bit out on the diff settings and that'll effect the way the car handles (this is all a bit 'generally speaking'!)
Most people, and I'd be included in that, go for a Quaife ATB - they simply do the job, and do it - IMHO - very well indeed, unless you're going for ultimate race performance (and even then it'd be a pub table discussion ) i'd recommend this route, it's very well tried and tested and your local mechanic will be able to fit it. It won't effect the chassis dynamics much so the cars setting won't have to change too much either.

If the car gets Any road use, then the above, imho, becomes a no-brainer - ATB every time..

Re the Ratio, it's as Peter mentioned, change the C.W.P. for a 3.3, again a simple fit for your local mechanic.

If the car is predominantly road use then an IRS gives, again generally speaking, a better ride and copes with the crap roads we have better, get to a track car and the differences are much smaller, to the point of their being very little difference between each, the swings and roundabouts pretty much balance out - certainly a change I wouldn't even consider for performance reasons, but if one was building from scratch that would, probably, be a different matter.


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posted on 27/3/24 at 04:16 PM Reply With Quote
Thanks for the input all, definitely a few things to think about. I had a discussion with Road and Race Transmission and they gave me some great advice also.
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