Thoughts on diff ratios

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Dave Woodward
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Thoughts on diff ratios

Post by Dave Woodward » Fri Apr 19, 2019 1:27 pm

So for my next trick, I’m going to replace my MGC/Quaife rear with the Jag IRS option.

I’m currently running a close ratio Toploader with a 3.07 diff. I’ve been given the option of a Jag 2.88 powerlock diff but worry it will be too long. Great for cruising but harder work for the box and clutch and a long way from what most seem to run.

Anybody have any experience of that ratio with a toploader?

Thanks.

Dave.
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Re: Thoughts on diff ratios

Post by peterc » Fri Apr 19, 2019 5:50 pm

Dave,
What sort of mph/ thousand do you get now as that would guide you to what the engine was happy with plus your preference on whether you want more cruise capability or more acceleration,
You also need to take into account the tyre size which I thought you were considering going wider/ larger.

I am mostly happy with 30 mph/ 1000 which I think gives a fair balance between the two requirements.
Peter C

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Dave Woodward
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Re: Thoughts on diff ratios

Post by Dave Woodward » Fri Apr 19, 2019 6:19 pm

Thanks Peter.

I’ve done some maths and I think the 2.88 with the straight through 4th gear will give about 30mph per 1000. What are you running with?

This is going to sound silly but I was wondering if that long a diff would put unnecessary load on the box and clutch etc under heavy acceleration (I know this old tech is more robust than I give it credit 😉).

Thanks again.

Dave.
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Re: Thoughts on diff ratios

Post by clive » Fri Apr 19, 2019 7:08 pm

I'm not sure of my ratios, but I have a t5 gearbox and Jag diff. I have found that fifth gear is only of use if travelling at 70 mph plus, as it is more economic in fourth gear in normal use. I have a Holley carb and it would appear that the secondaries aren't open when travelling at low revs.
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Re: Thoughts on diff ratios

Post by peterc » Fri Apr 19, 2019 10:47 pm

Dave,
I run a Rover with a LT77 box which I believe has a 0.79 ratio for 5th.
My Jag diff is 3.31 but my tyres are the larger Avon 245x 60’s used with the FIA alloys so have a 83.88” rolling radius which must be larger than your 205’s for wire wheels.
I understand your thoughts of over gearing but doubt the transmission would have an issue merely a blunted acceleration.
As previously queried what gearing have you got now as that will indicate which way to go.
Not forgetting that the original Cobra ran a 3.45 axle ratio.
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Re: Thoughts on diff ratios

Post by andyg » Sat Apr 20, 2019 3:03 am

Dave
I am running the 3.54 Jag powerlock diff and get 27mph per thousand rpm in top. But I am using a 5 speed Tremec box which has a 0.82 5th gear ratio. 4th gear is 1.0 but not sure of mph per 1000 off the top of my head.
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Re: Thoughts on diff ratios

Post by KevinW » Sat Apr 20, 2019 8:51 am

I am running a 2.88 (de-rated) LSD diff and a toploader, coupled to an FE428 Big Block via a CenterForce clutch, which is quite heavy.
This was the Crendon recommended set up for toploaders and FEs to help squeeze a bit more mpg out of the engine.
I have no concerns at all about strain on the drive components, as plenty of cars are running around with this combination, without issues. On a small block car I would guess that you would have to rev it a bit more in each gear to get the same performance at take-off.

Also, if its a LSD, I think you need to make sure its de-rated prior to fitting. (Also requires different oil to standard diffs).

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Re: Thoughts on diff ratios

Post by StewbieC » Sun Apr 21, 2019 7:39 pm

Hi Dave,
My Hawk now has a Jag IRS fitted.Its a bit of effort and certainly not a cheap upgrade but initial indications suggest it is well worth it.
Outboard disks remove the need for having the skills of a gynaecologist to work on the brakes.
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Re: Thoughts on diff ratios

Post by peterc » Mon Apr 22, 2019 4:29 pm

I’ll have to remember that saying Stewart when I’m next under the back axle. Not sure I would change mine for outboard brakes though. More authentic yes but increasing the un sprung load. I have read that actually the prototype Cobra used inboard rear brakes so that’s my excuse.
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Re: Thoughts on diff ratios

Post by simonjrwinter » Mon Apr 22, 2019 6:39 pm

My Hawk has a removable panel at the back of the cockpit. This enabled me to bleed the rear brakes while sitting in the drivers seat, pumping the brakes with a spanner on the bleed nipple.
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