Miata Diff Housing Arm Reinforcement?

Hi guys. So the quest for 300-340whp in my Miata continues and I now have a Torsen 3.9 in my possession. One thing I'm considering now is getting weld in reinforcement plates for the differential housing arms due to that being a weakpoint. However, I'm not sure if its worth it for a 99% street driven Miata. What do you guys say? Is it worth getting the housing reinforced at the power level of 300-340, or should I leave it as is and use that money towards another piece of the project?

The plates aren't complex, they're just slabs of aluminum so anyone with an angle grinder or a hacksaw can make them. While we've never broken a diff arm at FM, it does happen. It's a good preventative measure if you want to make the car as strong as possible.

Also, set the car up so it doesn't wheelhop. That's the important thing for the health of the rear end.

In reply to Keith Tanner :

Got it, I appreciate the info! I don't have an aluminum welder myself, and I'm so so with the steel MIG. I've been getting quotes of $400 to $850 from local fabricators which is insanity to me. There is a company that makes the plates and will weld it to the housing for about $200, but after shipping it comes out closer to $400 as well unfortunately.

Any tips on reducing wheel hop?

Wheel hop is about shock settings and suspension bushings. I've also heard that a stiffer rear sway can help - but to me, the #1 thing is shocks.

If you break the diff arm, nothing terrible will happen but you'll have a dead car until you fix it. This may involve a tow, depending. So you decide if the insurance of doing the work is worth it. They don't all break, but some do.

Or you make friends with someone who can TIG :) 

In reply to Keith Tanner :

I drove a certain NA for a while, as did the car's owner, and discovered during a pre event nut and bolt check that the diff housing was broken.  I chained the PPF up to the rear subframe and we hit send.  Made the drive to and from the event (200? miles each way, towing a trailer) and won the class, no problem.

Just tigging up the notch in the right side arm will add significant strength. That's where they all break.

DeadSkunk (Warren) said:

Just tigging up the notch in the right side arm will add significant strength. That's where they all break.

That's why it's there. What breaks after that?

mr2peak said:

DeadSkunk (Warren) said:

Just tigging up the notch in the right side arm will add significant strength. That's where they all break.

That's why it's there. What breaks after that?

This was my other concern...if I reinforce it, is something more costly going to break instead?

In reply to Japanspec :

From inference, the Miatas have an engineered weak spot there so the engine can shift backwards in a crash, because the PPF directly connects the diff to the engine.  If the housing could not break, then collision forces could not be absorbed properly.  It's like the engineered bending locations in hoods and chassis rails.

RX-7s do not have the weak spot, because they do not have a PPF.  The engine and front subframe can move back without being restrained by the diff mounts.

Pete. (l33t FS) said:

From inference, the Miatas have an engineered weak spot there so the engine can shift backwards in a crash, because the PPF directly connects the diff to the engine.  If the housing could not break, then collision forces could not be absorbed properly.  It's like the engineered bending locations in hoods and chassis rails.

RX-7s do not have the weak spot, because they do not have a PPF.  The engine and front subframe can move back without being restrained by the diff mounts.

That's my understanding about the reason for the notch on the Miata as well.

SA/FB/FCs do not have a PPF, but FDs do.  The FD does not have a notch on the diff housing (it doesn't have the big long arms like the Miata one does), but it's certainly possible that there's a similar engineered weak point somewhere else.

Edit:  RX-8s also have a PPF and a similar (armless) diff to the FD.

In reply to codrus (Forum Supporter) :

I understand that RX-8s have carbon fiber driveshafts so they can collapse in a crash, maybe the weak spot is in the PPF?

Likewise, I know very little about FDs but PPF braces are a thing for them.

Pete. (l33t FS) said:

In reply to codrus (Forum Supporter) :

I understand that RX-8s have carbon fiber driveshafts so they can collapse in a crash, maybe the weak spot is in the PPF?

Likewise, I know very little about FDs but PPF braces are a thing for them.

Yeah, FD PPFs are welded stamped steel vs the cast aluminum on the Miata.  I haven't stared at an RX8 one, but google says it's steel as well.

We did salvage for a while. After seeing a few dozen wrecked Miatas, that notch is very clearly there to let the whole drivetrain move back and down in a crash.

In reply to Keith Tanner :

Assuming stiffer bushings are better? Any tips on shock settings?

Ummm, at 300 to 340 horsepower, the stock trans will be toast, right?

I'd kind of think the same for the rear end, even the 3.9.

wheelsmithy (Joe-with-an-L) said:

Ummm, at 300 to 340 horsepower, the stock trans will be toast, right?

I'd kind of think the same for the rear end, even the 3.9.

A 6-speed can live for a while behind a 340 rwhp Miata depending on how it's used.  Drag racing will be a short life, track days it'll go a while but probably wants a cooler.

Even big turbo Miatas rarely break the stock 7" rear end (the 6" that came with the 1.6 is a different story).  The transmission is weaker, and anyone who upgrades one of those is almost certainly upgrading the diff at the same time because of the PPF.

In reply to codrus (Forum Supporter) :

RX-7 guys run 600hp through the 7", the main strength issue is axles, not the rearend itself.  By the time one is making that much power, the available ratios leave something to be desired, so in a way the rears are strong enough for what you can do with them.

Thanks guys, maybe its smarter to just bypass the reinforcement of the housing. Sounds like its engineered in there for a reason, and I'd rather not my 6 speed or built engine take the brunt of a force if it so happens. I would guess my chances of breaking a housing on the street with that much power are possible but unlikely.

If it does break, you can get the fabricator to weld it back together and add some reinforcements at the same time if you want :)

In reply to Japanspec :

I broke one by getting bumped from behind. Just hard enough to do some light damage to the bumper cover.

In reply to DeadSkunk (Warren) :

That does mean there was enough of a shock that went through your drivetrain that it tried to move. You popped the fuse.

I'm second guessing myself again...maybe I should just do it...