Rear end options for more power? (With UK Parts)

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rotarypower
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Rear end options for more power? (With UK Parts)

Post by rotarypower » Wed Aug 28, 2019 3:51 pm

So the last few days I've been trawling though countless forum posts and articles of stronger rear axles for the car.

A lot of them seem to revolve around vehicles that were rare in the UK or we never got at all, does anyone know if there are any options that can be done sourcing parts in the UK to upgrade the rear end to take a bit more power.

Plans are afoot to get rid of the 12A and put something a bit more powerful & turbocharged into the car in time for next summer, so the rear end will need to withstand at least 350hp & mid 300s lb-ft of torque. I do intend to drive it fairly hard & its going to be a daily in summer so it needs to be able to take some abuse.

Are there any easy options in the UK or am I going to have to bite the bullet and find a different path?
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Rich

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Re: Rear end options for more power? (With UK Parts)

Post by TOOL » Mon Sep 02, 2019 3:27 pm

A S3 rear end fitted with a clutch type LSD or Torsen centre will take 350 bhp.

These are bolt in fitment. I have built and installed both types.
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KiwiDave (Tue Sep 03, 2019 3:26 am) • rotarypower (Wed Sep 04, 2019 6:41 pm)
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Re: Rear end options for more power? (With UK Parts)

Post by Hobbawobba » Mon Sep 02, 2019 8:27 pm

Forgot to reply on here.

I had been looking in to this. I've ended up ordering an OS Giken MX5 diff and plan on getting some billet axles made up. I think it should hold up just fine with 400 odd RWHP. Long as I don't do 7000rpm launches all day -haha-
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rotarypower (Wed Sep 04, 2019 6:41 pm)

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Re: Rear end options for more power? (With UK Parts)

Post by rotarypower » Wed Sep 04, 2019 7:03 pm

Cheers for the responses guys.

@Tool, the S3 I'd been looking at, but they're pretty rare now. There's a chap in Ireland selling one, but after sending him a few messages he didn't seem like a very communicative seller, plus getting them to the UK would be expensive. (Unless I persuaded my girlfriend that a weekend break to Ireland was a great idea haha)

Interesting that they can handle 350hp, for some reason I thought it would be less than that.

@Hobbawobba, I think if I couldn't get S3 gear to handle it then I'd take a totally different route to you, I've spent the last 2 weeks offshore studying all the measurements I can find of MX5 subframes, RX8 subframes, Rx7 chassis diagrams and reading tons of threads on the subject.

For the rear end it looks like the MX5 or RX8 rear subframes could be made to fit with a reasonable amount of fabrication, they have the advantage of being 'bolt on' units that contain all the suspension pick up points already. So if you can engineer them in correctly you just need to get the subframe location accurate and everything else will follow from that.

The RX8 rear is the better design, multi-link but apparently one of the best Mazda have every come up with, the diff is also contained in the sub-frame. However the wheel base is a bit wider which would most likely necessitate wide arches, also the minimum wheel size for RX8 brakes is 17", which I think would look wrong on our cars, so there would be more work in fitting different brakes. That said the rears may fit as they're smaller./

The Miata (NB is apparently best) one is very close to the width of our rear frame rails (before they widen right at the rear), comes with wheels and brakes more closely the Rx7 (although a 4x100 stud pattern), bolts on in one plane vertically which makes fitting easier, the wheel base is slightly wider but close enough that it could probably be fixed with the correct offset wheels. Again its fully self contained double wishbone IRS.
A diff mount would have to be fabricated as they PPF normally supports the front of the diff & rear of gearbox. However that's a minor thing compared to fitting it.
Other advantages are that for the MX5 you can get a ridiculous amount of aftermarket parts which is distinctly not the case for our cars anymore.

In either case new rear strut towers would have to be welded in (probably cut from the donor car), they could be located by using a jig made off the donor car.
I'm tempted to pick up a cheap MX5 (can get them for £500 or less around here) just to do a feasibility study, worst case it gives me something RWD to hoon about this winter while I'm fixing rust on the Rex.

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Re: Rear end options for more power? (With UK Parts)

Post by Hobbawobba » Sat Sep 07, 2019 4:49 pm

I'm sure there is a series 3 or two that are breaking for parts. If not - then in the near future.

Getting IRS in the car would be bloody awesome. There is a Kiwi guy here who has done it https://www.rx7club.com/build-threads-2 ... p-1101100/. But I don't want to be messing around with all of that stuff as it would take me forever on the driveway :lol:. If I had a unit with a lift on the other hand :lol:. Be aware with the MX5 diffs. Their diff housings are known for snapping once you start making power. I believe that a lot of them tend to put FC units in.

I've had my MX-5 OS Giken diff arrive at work yesterday yay . Will try and get some pics up in my thread over the weekend
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rotarypower (Sat Sep 07, 2019 5:18 pm)

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Re: Rear end options for more power? (With UK Parts)

Post by rotarypower » Sun Sep 08, 2019 3:10 am

I've read the Kiwi IRS thread about a million times now, there's also another thread from the USA where the Rx8 one was fitted with a BMW engine, but I'm not so keen on how much the car was hacked up to make it all fit.

I think a full floorpan conversion would be over kill for the MX5 rear end.
This chap did it to an MGB.

https://pbase.com/speedmonkey/image/63220623

That's sort of what I have in mind with some hopefully neater welding and probably to try and integrate it into the existing factory rails with plenty of gussets & reinforcement.

Once I get the rear stripped out to see the rust situation I'll have a better idea, if its just a case of minor repairs I may go that route, otherwise I may take the nuclear option of rip out and IRS.

Long term I'm planning an engine swap, the new block will have around 350hp and will be (sorry) a piston engine, as much as I love the character and idea of the rotary, this will be my daily and I just can't justify 20mpg for 115hp.

I'm hoping to fit a Ford 2.3 ecoboost unit the same that normally goes in the mustang, 350hp standard, direct injection, 25-30mpg in a heavy mustang so I reckon 35 would be possible in the Rx7. That kind of power and torque for that economy would be immense.

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Re: Rear end options for more power? (With UK Parts)

Post by rotarypower » Tue Sep 10, 2019 12:03 am

So to bring some more figures to my ruminating.

The front subframe pick up points and the middle of the engine bay frame rails on the RX7 are 800mm apart.
The MX5 upper subframe mounts are 812mm apart, well within the width of the rail so width wise we're good.

However due to the double wishbone suspension, the MX5 subframe mounts much higher and has upper and lower mounts. Now the drawings that are available through scouring the web are not particularly great, the heights are given from a datum line, but how that line refers to the ride height of body of the car is not clear at all. The same is true of the RX7, no external reference points other than an unspecified datum line.
However at a rough guess the Rx7 rails would need to be between 10-13cm higher than they are now, but in actuality the best way would be to take the angle of the Mx5 lower arms at rest with the car sat normally on level ground and then set the height of the subframe so that at normal ride height the lower arms are making the same angle.

So a cut out would need to made in the rails and then have them joined in.

Image

As per the above schematic. Obviously this structure would need tying into the surrounding body to keep everything strong & rigid, and structure cut out to make room for the upper arm.

Strut towers are 991mm apart on the MX5, 1060mm on the RX7, so they would need to come inboard and of course be at a different height. Most likely the best way would be to make a jig that bolts onto the subframe mounts and sets the shock tower geometry from that.

Lower mounts would need to be made for where the subframe bolts at the rear of the engine bay.
So its definitely not a drop in solution, however it would leave you with double wishbone suspension & a proper steering rack plus far more aftermarket options for brakes and suspension.
Oh and a big headache sorting out engine mounts as well.

Regarding Spring rates and length:
Rx7 (s2)
Front 120.9lb/in - 12.8" free length
Rear 100lb/in - 12.74" fee length
Weirdly the left and right springs are listed as different lengths in the workshop manual. I went with the shortest.

MX5 (changed over the years so this is just a middle ground)
Front 153lb/in - 11.52" free length
Rear 96lb/in - 13.22" free length

The MX5 has stiffer springs up front, I thought this might be down to engine weight, but it seems the 12a and MX5 engines are pretty close. And in terms of construction I doubt the MX5 front end is much heavier, so I guess its down to wanting a more sporty feel.

And onto the rear end.
The MX5 rear subframe mounting holes are 790mm apart (front) and 800mm apart (rear).
The Rx7 narrows around the arches (from its original 800mm) and then widens to 890mm at the rear.
The height is hard to compare with the drawings I have, but it looks like the underbody floors are very similar in terms of the way the floor pan sweeps up at the rear under the boot area, most likely the spare wheel well would interfere and need to get chopped, but maybe only down to space saver size.
The fuel tank could possible cause issues as well, but really without one to offer up its impossible to say.
Again a new rail section would most likely have to be installed to carry the suspension & this tied into the body work.
Shock towers would either need fabricating or cutting out of the MX5.

MX5 shock tower width at the rear is 984mm, which could work out as being a little bit right in the rear of the RX7.

Sorry for the essay, nothing like typing it all out to realise how much work is involved!
Though when I see the time people spend fitting & engineering custom axles and tri-links etc its probably comparable.

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Re: Rear end options for more power? (With UK Parts)

Post by rotor_dude » Fri Sep 13, 2019 3:43 pm

mazda b1600/b1800/bt50 etc :D
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