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SP 250 Differential (original vs Austin Healey)

JSabah
Posts: 86
Joined: Mon Feb 15, 2016 9:27 pm
Location: Los Angeles

Re: SP 250 Differential (original vs Austin Healey)

Post by JSabah » Wed Jun 06, 2018 4:18 pm

Ian, thanks for the pointer on the serial number. Unfortunately that is one of the things I am trying to confirm. I have only found 1 post on the web confirming the serial number indicates the % reduction. What is more confusing is that I have found many J type ODs that have come from Volvo’s but the serial number starts with 27/ and the research I find on the Volvo model (for instance I recently found one 27/ that came from an 80’s 240) shows that the OD gives a reduction of 21%. Hopefully Overdrive Repair Service will reply to my e mail
Sp250, Sunbeam Tiger, MGTD, Jensen Interceptor conv, Jensen Interceptor Coupe', '49 Plymouth Woody and now a '65 Jensen CV8 Mk3 LHD

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Vortex O'Plinth
Posts: 771
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: SP 250 Differential (original vs Austin Healey)

Post by Vortex O'Plinth » Wed Jun 06, 2018 6:17 pm

JSabah wrote:
Wed Jun 06, 2018 4:18 pm
Ian, thanks for the pointer on the serial number. Unfortunately that is one of the things I am trying to confirm. I have only found 1 post on the web confirming the serial number indicates the % reduction. What is more confusing is that I have found many J type ODs that have come from Volvo’s but the serial number starts with 27/ and the research I find on the Volvo model (for instance I recently found one 27/ that came from an 80’s 240) shows that the OD gives a reduction of 21%. Hopefully Overdrive Repair Service will reply to my e mail
I think you'll find that the info from Ian is correct. The two digit prefix to the overdrive serial number indicates the percentage increase in roadspeed for a given engine speed. So all Laycock units, 'D', 'LH','A', 'J' or 'P' types will carry a prefix of a number typically between 20 to 28. To reiterate, this number is not the percentage reduction in engine speed but the percentage increase in roadspeed at a fixed engine speed.

The reduction in engine speed at a fixed roadspeed is given by the function [1 - 1/(1 + R/100)] x 100% where R is the percentage overdrive ratio.. So, in your example a 27% overdrive ratio would give a [1 - 1/(1 + 27/100)] x 100 = 21.25% engine speed reduction.

Volvo units - of varying types - were typically 27% or 28% ratio. A 'J' type 28% unit fitted to the SP give an engine speed reduction easily accommodated by the torquey V8 with standard gearing. This ratio would make the most significant reduction to your overgeared axle problem.
Nick

Pas d'elle yeux Rhône que nous.

markone
Posts: 2
Joined: Sun Oct 02, 2016 5:12 am
Location: Ruakaka

Re: SP 250 Differential (original vs Austin Healey)

Post by markone » Fri Apr 12, 2019 11:56 pm

My Canadian SP250 has a non original Dff in it with drum brakes. I was going to swap it out for a modern Diff during restoration but for expediancy, I am going to run with it. What I am not sure about is the origin of my Diff!, Is there an easy way to identify if it is a Jag, AH or TR diff???

Ian Slade
Posts: 421
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: SP 250 Differential (original vs Austin Healey)

Post by Ian Slade » Sun Apr 14, 2019 6:58 am

If it has drum brakes it's either Triumph, A/H or Morgan, Jaguar had disc brakes all round, not sure about other uses of the same axle, the gear ratio of the axle normally gives a clue, the basic differential housing is standard for many makes, the tube length and and hubs are different for each car maker, and ratio for each car model. It has always surprised me that on an axle failure owners throw away a complete axle and bodge in another make axle when the solution is to purchase new crown wheel and pinion for the SP and swap the tube or hub ends into one of the more common makes casing. The tubes are a shrink fit and tack welded to the differential casing.
Owner since the 70's, Genghis is slightly to my left.

markone
Posts: 2
Joined: Sun Oct 02, 2016 5:12 am
Location: Ruakaka

Re: SP 250 Differential (original vs Austin Healey)

Post by markone » Tue Apr 16, 2019 5:29 am

Thanks Ian,
the ratio is about 1:3.75, not sure if that helps. I read here that there is an option to fit discs to the AH diff, but doing that will also push back the completion date so I will just box on with what I have at least for know.

Ian Slade
Posts: 421
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: SP 250 Differential (original vs Austin Healey)

Post by Ian Slade » Tue Apr 16, 2019 7:17 am

The ratio is usually on a metal tag on the diff casing or on the crown wheel.
Owner since the 70's, Genghis is slightly to my left.

Warsash 2
Posts: 239
Joined: Sun Feb 28, 2016 1:53 pm

Re: SP 250 Differential (original vs Austin Healey)

Post by Warsash 2 » Tue Apr 16, 2019 11:07 am

Hi
I seem to remember that the treasure who proceed me hab an MGB back axle on his.
Regards

Colin

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