RB25 Gearbox Swap

[neil d]

But it all cames down to the gearbox flex.

I assume from that oil/oil temperature was not an issue?

I have used shock proof and tried running a cooler.

You need to understand we are running these cars with 350hp+ on 235 rear slicks.

[neil d]

The FC/FD box is not known as being super strong....albeit still stronger than the stock mx5 6 speed but probably not a big enough amount. The RB25 box is the same in the 300ZX TT's, R32 and R33 skylines and is considered pretty bullet proof. It's what the high powered Silvia's usually get modified to so it's a pretty well-proven solution.

The guy who helped with the conversion mods these boxes for drift cars with 600+hp with stock RB25 gearboxes hence the choice to go with the RB25 and stop farking around trying to get a mazda box to work.

[Hellmun]

The FC box is MUCH stronger than the six speed. Like at least 100HP worth and its a very straightforward swap.

Dann

Don't forget how much more damage there will be with Slicks using the same horsepower. I still remember the mania car ripped the rear passenger wheel hub in half during the 300 back in like 08 with lateral force. I've never seen a street car manage such a feat. 100hp more may seem like a lot but I'd rather do it once over the top...than keep slowly working my way up. I'd be keen to check out how far back the gear stick is pushed back in the console for the RB25, it's not a small gearbox.

[NitroDann]

I dont know, Im not at that point, but its weight also.
Guys like Bbundy and Savington are running regular 6 speeds on 275 Hoosiers and over 350whp and they dont have issues. In fact its not something that really spoken about, and rare few who do have GB issues usually just go to an FC GB and diff swap, which isnt hard, and Ive never heard of anyone breaking one of those in a mx5.

Dann

[neil d]

I dont know, Im not at that point, but its weight also.
Guys like Bbundy and Savington are running regular 6 speeds on 275 Hoosiers and over 350whp and they dont have issues. In fact its not something that really spoken about, and rare few who do have GB issues usually just go to an FC GB and diff swap, which isnt hard, and Ive never heard of anyone breaking one of those in a mx5.

Dann

Are these cars racing or street cars?

Brian Ferrabie has blown up his dog box in his turbo so I have no idea how these guys run that power and not have any trouble?

What rims do these guys run the 275's on?

[NitroDann]

Race cars.

15x9 or 15x10 6ULs.

I dont know either, its very conflicting info.

In oz theres magic deckers boxes and heaps of rubbish about se/sp boxes being also magical. But in the states its just 5 speed, 6 speed, or FC box and rare few have needed FC boxes.

Dann

[flycasta]

What is the problem with the 6 speeds and power? What breaks?

I know I read on a silvia forum that they do a mod that adds an extra circlip or thicker circlip somewhere in the box that then fixes the problems of breakages. Their 6 speed is the same as the MX5 6 speed, well the Aisin box anyway, I think the NC 6 speed is different.

[skypilot]

Although I know diddly squat about mx5 boxes I am pretty sure the issue is torque and not hp.

Subaru rally cars suffered the same fate and no amount of money really fixed the problem until they went to the six speed. The issue was that torque would twist the gearbox case and allow a bees dick of clearance between the gear mesh and bang it was all over. Subaru basically put a huge steel metal carrier section that held the gear section in place. They then bolted the alloy case onto that. i think it added something like 20 kgs to the box weight. Having said that 1000nm of Torque will break most stuff, thats why drag cars use auto;s

If that's the case with the mx5 then its replacement time. $$$$. or bulk strip every meeting $$$$

[deviant]

What is the problem with the 6 speeds and power? What breaks?

I know I read on a silvia forum that they do a mod that adds an extra circlip or thicker circlip somewhere in the box that then fixes the problems of breakages. Their 6 speed is the same as the MX5 6 speed, well the Aisin box anyway, I think the NC 6 speed is different.

I think this was disproven in a thread on here some time back>

[greenMachine]

But it all cames down to the gearbox flex.

I assume from that oil/oil temperature was not an issue?

I have used shock proof and tried running a cooler.

You need to understand we are running these cars with 350hp+ on 235 rear slicks.

Yes, I understand that bit Neil. Do you know what max gearbox oil temps you are running on the MX box and the new one? Just curious, not suggesting that the Nissan box is not the answer.

Thanks

[mx52nv]

Entering this thread late but any reason no one is opting for the Quaife 5 speed synchro gearset?

BOYRCR has one in his 200rwkw+ NA turbo and I have one on order direct from Quaife UK for a customer with a built SE motor.

'Time Attack' Matt used one successfully in his 2 litre stroker 340rwhp NA8 turbo in the Redline Time Attack Series which he eventually won for modified RWD class in 2008 and came second in 2009. All the Flyin' Miata team automatically recommends the Quaife gearset for serious turbo race cars.

Choice of gear ratios (depending on tracks and usage) and priced at approx $2500 delivered plus GST/duty or approximately $3000 all up worse case scenario to your door.

Les @ MX5 Shop

[deviant]

Neil d destroyed his in one meeting!!

[wazman]

I have 200 rwkw and blew mt 6 speed last weekend. Its started popping out of 3rd under hard load (drifting....so prob no where near the load you guys see) ....so i started holding it in gear (3rd). A couple of laps later - yay ! free reving in 3rd

I'm a budget racer so ill be doing the cheapest strongest option. In the drift community its common for RB25 boxs to blow these days.....lets face it they are old and prob all done 300,000 klm's or more. Most guys using them are pushing around 380 rwkw's or so.

If the nissan s15 6 speed is the same guts as the mazda why dont people just buy them and swap the guts over ? I've seen them sell for $350- $450. Most people going for power pull them out pretty quick because they are known to fail with over 250 rwkw through them. apparently this "Aisin" box is also in some bravo or ford ute/truck thing ?? ( i was stand by my car sulking when some guy rattled of a bunch of info then disappeared)....

Anyway. If i Cut n shut ill be sure to do a thread. Rx7 box's are cheap but im sure also high klm's :/

[sailaholic]

I thought the rx7 gearbox was the same as the mx5 5 speed but with some bigger bearings?


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[corners]

This is the fault with the S15 boxes, explained to me by my mechanic:
Issue is with thrust washer are that when you shift in to 4th or 3rd pressure is put on the grove and reverse gear on the shaft causing the washer to climb out of the groove and chip the reverse gear. This allows the washer to separate and move around in the groove. When this happens it fails completely and just moves around which is not great.
Options to repair that he was against was welding to the shaft as it could weaken the material but would stop the thrust washer moving around. You'd have to weld on the reverse side. He also said if you don't want reverse he could have a solution.....
He said straight cut gears would put bugger all load on the thrust washer and that the nismo stuff could possibly help as the gear angles reduced. Also food for thought would be getting a better circlip setup so there's less movement under load. Another option that would help significantly would be machine a couple of mm off the shaft and getting a custom thrust washer made so it sits deeper. This is not recommended as it weakens the shaft and the coating is damage.