V8 Miata Conversion Axles

Hello Everyone,

A piece of my current project involves running a 28 spline thunderbird 8.8 in a miata rear subframe (in a pinto). After researching the high dollar swap axles available I have come to the conclusion that I can make the center bars and have them heat treated for a reasonable cost. I am particularly interested in the "direct fit" axles that use a miata outer cv and a ford inner. These axles are rated to 400hp. My question is this: does anyone have any experience with these axles behind a mild V8? My quick math for the splined end of the miata cv bar puts the torque capacity at an optimistic 325ish ftlb for case hardened steel. I'd love to know if the outers on these axles resemble the stock outers or if they are a different design. Really any information I can gain before ordering the chromoly shafts is much appreciated.

Thanks,

Shavarsh

I’m fairly certain that CV failures on high power turbo miatas are not a common problem.  Broken transmissions and differentials, yes… but not axles.

Has anyone seen the direct fit conversion axles? I have found lots of questions about them across the internet but haven't found any first hand experience.

What exactly is a direct fit conversion axle?

I got my axles from Monster Miata and it seems to be a ford inner stub to a ford CV to a custom middle shaft to a Ford outer CV with a cord outer stub going into a "broached" Miata hub. So really, just a standard Lincoln rear axle with a shorter mid-shaft.

Can your tires stick 325ish lb/ft to the pavement?

If they can't the axles will be fine.

NOHOME,

The axles I am referring to are sold by DSS and probably some other companies. They have an 8.8 stub to 930 CV to custom bar to a miata outer cv joint. I plan to make a 4340 center bar to connect a stock 8.8 inner plunge joint to the stock miata outer CV. This will allow me to keep everything outboard of the CV joint stock miata (hubs, brakes, wheel pilot, etc.) The only part I am unsure of is if the outer "miata" cv joint they are using for the conversion axles is in fact a stock style joint, or if it has been beefed up/heat treated/etc.

ShawnG,

You make a good point. I'm only running a 225 but at the drag strip they hook pretty nicely. The 325lb/ft sounds like enough until transmission/ring gear multiplication is considered. I did use some very conservative values to take shock loading into account so hopefully with the added strength of the chromoly i won't have a problem.

A few more questions...

Has anyone here compared the internals of a protege outer cv joint to a na/nb outer cv joint? I'd like to know if there is any reason the protege joint might be stronger. I'm still oscillating between building bars to accept a mazda outer cv or just building bars/spacers to use the thunderbird outer and paying for the broached hubs/audi boot/possible shock relocation. For context here is the inner/outer thunderbird joints and the miata outer.

And here is the comparison of the inner races (outer joints). The DSS rx7 axles include a chromoly inner race upgrade to use a higher spline count bar. I'm thinking they also use a higher spline count inner race for their direct fit miata axles.

I have a set of Thunderbird hubs (to use instead of broached miata hubs) but the amount of machine work needed to fit stock miata rear brakes/brake pilot/wheel bearings doesn't seem to be worth it at my power level. The benefit however would be a much longer spline engagement length.

The Mongrel Miata had a Ford aluminum 8.8 IRS in a Miata subframe with a few custom mounts.

The car originally had a set of "sleeved" axles using the stock Miata outer CV and TBird inner.

In reply to lotusseven7 (Forum Supporter) :

How well did the sleeved stuff hold up? Im looking at an 8.8 swap in my miata for gear ratio difference. It really needs 3.23 or 3.55....

These homemade sleeved axles worked for a while, but one broke at the 2003 GRM Challenge and was replaced by a Monster Miata center axle. When I was selling the car, I did a BIG burnout on 275 HOOSIER R7's and snapped the other one. Another call to Martin and a new set arrived. I replaced the broken one and gave the new owner the spare.

How much power were you putting down? 

Shakedown pass

Probably just shy of 400.

lotusseven7,

where did the axle break? If it broke the centerbar and not the cv maybe I'll just use the mazda outers. How many miles were on it? I'll have to find your build thread...

Also when you replaced the axles with monster axles, did they provide a bar that would go between ford/mazda cvs? I thought they only sold a ford/ford bar.

Thanks for the information!

From my research it looks like the protege inner race spline is 30 compared to the miata 26. Can anyone confirm that a protege axle will fit in a miata knuckle? I believe the splines are compatible with the hub but unsure if the sealing surface lines up. 

Also after speaking with the driveshaft shop about the components in their axles it is clear nothing in their package is oem based or compatible in regards to the miata outer joints.

If my shocks would arrive I could decide how much of a clearance issue the outer thunderbird CV will pose. Buying broached hubs and short boots and cutting down the beefy ford axle seems to be the cheapest and possibly strongest option at the moment.

I figured I should wrap this up. After lots of consideration including quoting a few sets of center bars, the tooling and material to build them myself, and looking at the aftermarket options I ended up going with the Monster Miata kit. Although it cost some $$ I don't think a comparable option can be built one off for considerably less. 

I would also like to make people aware that the aftermarket axles for the ford 7.5" diff cars have changed their internal design (spline count and snap ring placement).  They cannot be used with the Monster center bars. The junkyard is now the source for compatible axles.

Thanks for following up, that's good info.

Halfshafts are surprisingly difficult and always a bit of a shock price-wise. But they have to transmit a lot of torque.

I should also mention my main consideration was reliability as I am building a street car that could leave me stranded. If I was building a challenge or track car it would be more tempting to try some of the cheaper options (sleeved) in a setting where spares are not far away.

A coworker bought a Locost that had a bike engine.  The halfshafts were sleeved Miata pieces. He broke them and broke them and broke them until he finally managed to come up with something strong enough. I don't recall the final solution but it involved money :)

That's a sub-1000 lb car with a sub-1000cc engine. Halfshafts are more difficult than people think.

In reply to Keith Tanner :

Can you share with the class why you guys went with the GM diff instead of the Ford 8.8?  I think Dusterdb13 might be interested also.

Weight, size and the fact that it was available new from GM. The latter isn't important for the junkyard builder but it is if you're building for customers. The size definitely made it easier to package an exhaust for the CTS rear. I think we used an 8.8 in Atomic Betty if memory serves.

For the ND, we went with a Camero/G8 diff because of better availability by that time.

To date, I haven't heard of a failed CTS diff in a Miata. Took GM a while to get the bugs sorted out, but they did manage. 

Awesome. Thanks!

And i will probably wind up with sleeved axles unless the bonus and scratchers are good this year.