MX5 diff into RX8? Bear with me....

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RupturedDuck
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MX5 diff into RX8? Bear with me....

Post by RupturedDuck » Tue Aug 26, 2014 8:05 am

Trying to squeeze more go from my RX8 which is used solely for Sprints and Speed Hillclimb, so road manners/fuel consumption not important as the only time it is driven on the road is to and from events.

I've reached pretty much the limit of how fast the car and I can go in our current form, and have decided to look at the diff as my winter project. This is to assist with getting what torque there is to the ground better for the hills, and also to change the gearing around 1st and 2nd as they are too far apart, especially for one important bend on my regular hill.

Having researched parts, I almost fell off my chair when quoted £1500 for 5.125 crown wheel and pinion, so I need to find another way to do this. I don't want to just go to 4.77 FD as possible for half the cost, or even cheaper using Kia Sportage parts unless I absolutely have to. But then I stumbled across the 5.125 MX5 (94-01) crown wheel and pinion for under £700.

My thinking is this: build an MX5 diff with 5.125 FD and Quaiffe ATB for less than the price of the FD only on the standard unit.

Question: will the MX5 diff fit directly or will I need to modify subframe and use different driveshafts and props?
Last edited by RupturedDuck on Tue Aug 26, 2014 8:20 am, edited 1 time in total.
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Re: MX5 diff into RX8? Bear with me....

Post by Bulldog » Tue Aug 26, 2014 8:19 am

Clive has been building up diffs for his race car might be worth a word with him
.. The R3 model of the rx8 has the diff from the mx5 but it's not very robust
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Re: MX5 diff into RX8? Bear with me....

Post by RupturedDuck » Tue Aug 26, 2014 8:21 am

Thank you. What is a lives username?
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Re: MX5 diff into RX8? Bear with me....

Post by Arnie_O » Tue Aug 26, 2014 8:22 am

Clive's username is Clive
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Re: MX5 diff into RX8? Bear with me....

Post by RupturedDuck » Tue Aug 26, 2014 8:32 am

Arnie_O wrote:Clive's username is Clive
Hahahah! I should have guessed that. Thanks. :D
Hillclimb/Sprint car thread.
It's not a car, it's an incline assault vehicle

231: BHR coil conversion, Pettit Racing front ARB, updated drop links, Axial Flow Short Shifter, EBC Red Stuff pads. Eibach springs. K&N panel filter, Maxsport RB5 225/40R18 (track). Unknown brand ditchfinders (road). So far....


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Re: MX5 diff into RX8? Bear with me....

Post by Liam22 » Tue Aug 26, 2014 10:25 am

Have you considered smaller diameter wheels/tyres?

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Re: MX5 diff into RX8? Bear with me....

Post by RupturedDuck » Tue Aug 26, 2014 10:34 am

Liam22 wrote:Have you considered smaller diameter wheels/tyres?
Yes, 17s are on the cards, and already on 225/40R18 this season. Really want to go to 5.125 too.
Hillclimb/Sprint car thread.
It's not a car, it's an incline assault vehicle

231: BHR coil conversion, Pettit Racing front ARB, updated drop links, Axial Flow Short Shifter, EBC Red Stuff pads. Eibach springs. K&N panel filter, Maxsport RB5 225/40R18 (track). Unknown brand ditchfinders (road). So far....


VIDEOS HERE OF EVENTS

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Re: MX5 diff into RX8? Bear with me....

Post by Clive » Tue Aug 26, 2014 9:16 pm

Responding to your PM, I would stick to the RX-8 "super diff" as it is a very good diff, if not use the RX-7 FD torson diff in the RX-8 case as this will definitely not induce any understeer along with the 4.7 and 17 inch wheels as the best way cost wise.

Talking of understeer I see in your signature you have a big front roll bar but no rear bar and if so this will bring understeer and particularly in slow corners, from what I gather, hillclimbing has lots of slow speed tight corners and if this is the case I would look at the either the stock bar on the front and a better bar at the back or a cheap trick I have used on other cars, clamp another stock bar to the existing one.


Back to ratios, personally I hear the Kia Sportage ratio thing banded about but I don't know of anyone who has actually performed it, so be careful that it fits without any lathe work needed

Cant remember which one but there was also an early Mazda B150 Truck ratio that will fit but it needed a spacer machined up to get the pinion to fit.

Hope this helps a little.

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Re: MX5 diff into RX8? Bear with me....

Post by RupturedDuck » Tue Aug 26, 2014 9:40 pm

Thanks Clive, that is some very useful info there. I've got a spare stock diff on the way, so I can play about with that. I may try the Kia solution as it won't cost much to try.

The ARB hasn't induced understeer, but I have had to play a little with tyre pressures to increase the feel a little on turn in, but nothing major, more of a tweek. I like a pointy car, so the rear ARB will be next to get sorted after the diff.

Thanks again for the help, and I'll let you know how I get on with the Kia diff.
Hillclimb/Sprint car thread.
It's not a car, it's an incline assault vehicle

231: BHR coil conversion, Pettit Racing front ARB, updated drop links, Axial Flow Short Shifter, EBC Red Stuff pads. Eibach springs. K&N panel filter, Maxsport RB5 225/40R18 (track). Unknown brand ditchfinders (road). So far....


VIDEOS HERE OF EVENTS

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Re: MX5 diff into RX8? Bear with me....

Post by macgtech » Tue Sep 30, 2014 3:23 pm

Clive wrote:Talking of understeer I see in your signature you have a big front roll bar but no rear bar and if so this will bring understeer and particularly in slow corners,
This is only true if your are disregarding the effect of getting into +ve camber on the outside wheels due to body roll. With negligible body roll, the above statement is true due to increase weight transfer (which is true at, dare I say it, 'road speeds'!), but if you are getting lots of roll, you will be better off with a stiff ARB to control camber better in the turns.
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Re: MX5 diff into RX8? Bear with me....

Post by RenesisRaceBuggy » Tue Oct 21, 2014 10:07 pm

Camber change shouldn't be too much of an issue with the RX-8's geometery up front however.

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