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

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

Post by gt_james » Fri Jan 10, 2020 5:51 pm

I wouldn't bother trying to fit an MX5 rear end in an FB, especially if your only goal is to handle 350hp. Sounds like a lot of fabrication and custom work to fit something which isn't going to be amazing anyway. Stick with upgrading to a stronger live axle. In your comments about spring rates, you're not considering the wheel ratio difference when comparing spring rates from the MX5 double wishbone to the RX7 macpherson strut set up. The difference in spring lengths is because the car isn't set level from factory, I believe the passanger side is taller to account for the crown in most roads.

I reckon stick with live axle, but modify the upper links like the group b rally cars or go with a "g force SCCA IT7 type" 3 link and a panhard, this solves your binding and pinion angle geometry problem, but doesn't involve cutting up the car or loosing any interior space, like switching to IRS would. Then get a different axle, like a navara, pajero or ford atlas axle, which are all commonly available in the uk. There are companies who could do this for you, such as modception, who build pajero axles for AE86 conversions, which is a very similar rear suspension design to an RX7, just with a panhard.

I've looked at different live axle options that are available in the UK, if it were me I'd go with a ford "atlas" (capri axle) as there's lots of support and upgrades available which would be great for getting the best performance. I believe they're a 7.5" ring gear sized axle, which is a bit bigger than the mazda 7", but it is a very sturdy design, larger pinion shafts, bearings, side gears etc. They come in 48.5", 50", 52" and 54.5" widths, I suggest you look at a 52" width depending on what size wheels you want to run, and you wont need wide arches. LSDs, brake kits, billet axles, wide range of Ratios, floater hubs etc all available on the shelf here in the UK.

BTW, I am building a track car but with an NA 12A bridgy, and I've been looking at swapping to a Ford "English" (anglia, escort, capri 4cyl) axle. The mazda and Ford english are both 7" ring gear size, front loading axles and are very similar, infact the toyota (AE86) axle and mazda (RX7, B2200 etc) axle looks to be pretty much japanese copies of the english. when stock, the ford isn't any stronger than the mazda axle. But the ford english is more easily available, cheaper to build than an S3, I think with more easily available upgrades, so I could upgrade to stronger shafts, LSD, change ratio, get brake upgrades without going to an unnecessary heavy axle which increases unsprung weight. I think I could even save weight by using an aluminium diff carrier which isn't available for the mazda axle.
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rotarypower (Mon Jan 13, 2020 5:05 pm)

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

Post by rotarypower » Mon Jan 13, 2020 5:16 pm

Cheers James.

This is all very much pie in the sky at the moment.
I've a boat rebuild to finish that's becoming life consuming, and I have to (apparently) spend some time with the gf as well.

First goal will be to get it through the MOT (end of this month) and for sure it will need some welding on the rear suspension mounts for that, but while I'm in their I need to take care of the arch rust, & investigate the sills. Etc etc.

The main reasons for IRS aren't the power upgrade (although that's part of it), its more that my vision for the car is an old school thing that can be driven every day, having owned a couple of MX5s, I can say that the suspension in general is far and away better than the RX7s. Its not just the worn parts either as my first MX5 was pretty shafted.
I'm not set on MX5 stuff, its just cheap and available.

I've seen an RX8 suspension swapped onto the FB but that looks a huge amount of work and its far too wide as standard. Some one put in an S13 rear end which looked like a much better fit. With the design of the subframe you don't really lose any space in the boot.
I've also seen an MX5 one done with a bit too much chopping IMO. However on another site someone actually was able to shorten the MX5 rear subframe to fit into the axle area. (of a different but similar car)

Again all very much theoretical at this point, my 12a is on 130k right now, it still seems to run well, but I need to get a compression test done as I feel just from how it starts when warm that it may be a little low.
When it finally gives up I've got a 13b in my loft (2 in fact, in pieces) or I'll put something non rotary in. (sorry!)

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

Post by spirit r » Mon Jan 13, 2020 7:00 pm

rotarypower wrote:
Mon Jan 13, 2020 5:16 pm
I'll put something non rotary in. (sorry!)
You will change your name then! "Pistonpower" :lol:

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

Post by Hobbawobba » Mon Jan 13, 2020 7:07 pm

I don't find the suspension/live axle all that bad if I'm honest. I am running decent ACBRON coilovers now. But replacing the rear end with poly bushes made a world of difference to the comfort levels of the car aye!

Got my Axle all POR15'd up! Going to order diff/pinion bearings and seals. Then hopefully get back together and test how this OS Giken beauty.
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rotarypower (Mon Jan 13, 2020 8:06 pm)

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

Post by rotarypower » Mon Jan 13, 2020 8:08 pm

Haha possibly so spirit-r!

I want to daily drive this car when its complete & unfortunately I've been spoiled by 40 years of automotive advances, but if I do change it I'll be sure to fit an engine that suits the 'character' of the car. High revving most likely.

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

Post by TOOL » Tue Jan 14, 2020 2:59 pm

The Mazda 7” LSD is roughly twice as strong as the stock open diff.

The 26 spline (S3) LSD side gears can be machined to take a 28 spline axle.
Then get some axles made up.
Stronger again.

This will handle any NA rotor motor you install.
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Re: Rear end options for more power? (With UK Parts)

Post by KYPREO » Wed Jan 15, 2020 10:44 pm

As noted above, a Torsen will be OK with 350hp. Depending on how you drive, the axles might be the first to go however. 26 spline series 3 axles are superior to the earlier 24 spline kind. Stronger axles are definitely worth considering.

If you push past 350hp, it would be worthwhile consider upgrading to a different altogether. The go to option in Australia at this power level is a Hilux conversion. Hiluxes are strong and widely available. Drag cars will often go for a Ford 9" - also very strong and widely available, but overkill for a street car making under 500hp and heavier than a Hilux.
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KiwiDave (Thu Jan 16, 2020 1:14 am) • spirit r (Thu Jan 16, 2020 9:28 pm) • Hobbawobba (Thu Jan 16, 2020 9:30 pm) • rotarypower (Thu Jan 23, 2020 3:47 am)

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

Post by rotarypower » Thu Jan 23, 2020 3:47 am

Thanks for all the advice guys, more options is always good.

I think the hardest part of the S3 axle is getting hold of one, there was an S3 being broken a while back in Ireland & I did enquire about the rear suspension bits, and I had to send the guy several back & forths to just get a price.

I can't remember exactly the amount quoted, but I think it was the better part of a grand which seemed excessive.
Maybe that's the standard going rate?

Engine wise I've mulled over lots of options, turbo, non turbo, old, new etc.
My main consideration as well as more power, better economy & modern EFI reliability was to try & have something in keeping with the car.

So 90% sure it will be 4-cylinder, weight wise I should be able to end up with something similar to what's already there, plus its more likely to actually fit in the bay. So after looking at Saab Turbo engines, 2.0 Ecoboosts, SR20DET, even MX5 engines, my conclusion is that provided it will fit, a good match for the car would be a Honda K20A2.

8K+ Redline, a reasonable amount over 200hp by the time you've fitted basic mods and that's in NA mode, and of course no cat required on our cars. Plus lots of options for boosting if I later wanted to and a very strong engine that can take a lot of power just as standard.

I realise to some this is sacrilege, so I apologise in advance, it may not happen. The 12a could have many miles in her yet!

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

Post by gt_james » Thu Jan 23, 2020 9:31 am

I think that's more than the going rate, but finding an S3 axle is going to be hard, that's why I suggested an Atlas, or maybe pajero or navara. Plentiful in the UK and much cheaper. I'm sure you could have an Atlas housing modified with new brackets cut and welded on and uprated shafts and LSD for around a grand, which is going to take more than 350hp and LSD too! for what they're asking for the S3 rear end.

If you want "in keeping with the car" stick a 13b in it as an upgrade. EFI doesn't necessarily mean more reliable, in fact electronics are what normally cause cars to fail in modern cars, carbs are simple and normally if something does go wrong you can fix with simple tools at the side of the road.

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

Post by rotarypower » Fri Jan 24, 2020 2:44 am

Well its a decision for the future.

I have two 13b NA engines that I disassembled sat in my loft.
Between them I have a set of good housings, rotors, plates etc. But then I need a rebuild kit and they're pretty dang expensive now, but at the end of it I'm still only at 150hp and rubbish economy. Just to be clear economy isn't my prime consideration, but if sacrificing economy it is nice to get some power back in return.
Porting just seems to make a car that's even worse on fuel and that will make me want to kill myself driving it regularly. Maybe I'm just super fussy.

Could get a 13b turbo, but they aren't exactly lying around these days and will cost plenty + rebuild kits if I want to make sure its reliable.
Anyway I've been torturing myself with this decision for a while now & am no closer. First of all I need to get the chassis & suspension in order and then see where I can go from that.

Its a shame the renesis is so hard to run in another car & not so reliable because they'd be great in the lightweight FB.

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