Miata rear end

So, im coming up empty handed for inexpensive rear diff solutions.  However, foud this.

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F282732588583

It looks like a cheap Chinese knock off of the phantom grip. Any ideas if it'll work?

Also, other option i am considering is welded diff.

Thoughts?

The Phantom Grip has always seemed like a dumb idea to me, I wouldn't trust one to work for very long if it even works at all.  I've run several welded differentials and not had one fail yet, and they can handle acceptably if you set the suspension up to unload the inside rear tire, or are racing on a loose surface.

 did you look at Mazda Truck Stuff?

¯\_(ツ)_/¯ said:

The Phantom Grip has always seemed like a dumb idea to me, I wouldn't trust one to work for very long if it even works at all.  I've run several welded differentials and not had one fail yet, and they can handle acceptably if you set the suspension up to unload the inside rear tire, or are racing on a loose surface.

7th place autocross car from $2017 has one.  It turns fine at anything above 10mph on 245 hoosiers.  Below that it’s a bear.  

In reply to Patrick :

One what?  Phantom Grip or welded?

That ad says that that particular piece fits 1989-2017 Miatas. What? There are 4 completely different differentials in that timespan

In reply to NickD :

It also says "vehicle fitment: front"

What about a cut down 8.8? Or am I missing something?

The 8.8 or 7.5 are still in research as well. However, cusom built cv axles will be neede, so im looking for all alternatives

I ran a phantom grip. It works- just better than the viscous. You need to watch eBay for an rx7 clutch LSD. If you can find a first gen  GSL-SE (13b) lsd, you will likely get a deal since most Miata owners are looking at the FC rears. I have picked up two like this for $100- one had like new clutch disk. This lsd is rebuildable, lock-up adjustable with shimming, tame in daily driving and (in a sense) robust as it will burn up the clutch discs before grenading like a torsen.

¯\_(ツ)_/¯ said:

In reply to Patrick :

One what?  Phantom Grip or welded?

Sorry, welded 

FE3tMX5 said:

I ran a phantom grip. It works- just better than the viscous. You need to watch eBay for an rx7 clutch LSD. If you can find a first gen  GSL-SE (13b) lsd, you will likely get a deal since most Miata owners are looking at the FC rears. I have picked up two like this for $100- one had like new clutch disk. This lsd is rebuildable, lock-up adjustable with shimming, tame in daily driving and (in a sense) robust as it will burn up the clutch discs before grenading like a torsen.

Iwas under the impression that 85-88 rx7 was the one to get. Which one (year wise) are you talking about? Any difference in the swap to the miata vs 85-88?

The other thing is gearing.  Im not sure that the 3.90-4.30 is the way to go with the engine and trans were using. 

The gearing is good with a 3.90 or 4.10.

Give me your ideal top speed and tire size.  I'll run it through a couple of calculators.

Rx7 years (us cars):  79 through 85 is 1st gen.

86 through 91 is 2nd gen.

93 and up is 3rd gen.

The 85 gsl se is a 13b powered first gen with a different rear end and brakes.  It is sort of the ultimate 1st gen available in the US.

Just get an 86 through 88 gxl, sport, or gtu rear end.  All clutch type.  All 4.10 if manual car and 3.90 if automatic.

You can rebuild with beer cans...

Give me your tire size and desired top speed.  I can send you a traction force vs gear and speed chart that will spit out which ratio you want.

Oh yea, I need max desired engine speed.

6k rpm. 205/50/15

Top speed is kinda irrelevant. Its about kedping in power curve for drag racing and autocross. 

I have a 4.30 VLSD from my '90 sitting around. That would toooottallllly hold up to the nonsense you have planned 

NickD said:

I have a 4.30 VLSD from my '90 sitting around. That would toooottallllly hold up to the nonsense you have planned 

I will happily blow E36 M3 up as long as it's almost free

While I wholeheartedly agree that real LSD's are the best way to fly the math made the Phantom grip a good choice.  At the time it was 175 vs 1,000 for a quaiffe.  I ran it for 50k miles and towards the end it was just starting to lose it's pep.  I autox'd and drag raced it with much improved results.  If on a budget I would do it again in a heartbeat.

Dusterbd13 said:

6k rpm. 205/50/15

 

Top speed is kinda irrelevant. Its about kedping in power curve for drag racing and autocross. 

Not if you run out of gear at the 1/8 mile mark.....

I had good luck trolling back country junkyards.  I found a 4.3 Torsen diff (actually 2) out of '99 NBs for $125 each.  I ran one in my '97 and a buddy put one in his 93 turbo build.

6K rpm

23.07 " tall tire

4.10 rear gear / 3.90 rear gear

1st gear) 26.78 / 28.16

2nd Gear) 45.86 / 48.21

3rd gear) 71.23 / 74.89

4th gear) 100.44 / 105.59

5th gear) 139.50 / 146.65

I will whip up those other graphs in a minute.  A true Dyno graph is required to do it right, but I will use the stock hp and dyno chart I have to make the chart.  Once we get an estimate of the power curve for the engine you are going to have, I can redo the chart...

In reply to wvumtnbkr :

So tell me if I'm wrong here but a ~12 second car should be trapping around 115 ish MPH.  Having to shift to 5th 100 yards from the finish is gonna hurt as acceleration is pretty feeble at that point.  So the goal here would be to stay in 4th through the lights, yes?  Taller tires, longer diff gear, etc?

3.90 with 6300 rpm rev limit