vehicular

iTrader: (3)

It strikes me that all of this masturbation about the NISMO/ S15/ FR-S/ CUSCO/ RX8 six speeds is isn't really beneficial to the discussion. None of us are going to spend the kind of cheddar that it would take to put this hypothetical, still-marginal gearbox together.

It seems to me that what we need as a community is a reasonably priced gearset to put in the Miata 5 speed that will survive 400whp.

It then strikes me that this discussion really should include a rear end swap kit to go with any trans solution because when we stop smashing transmissions we're just going to start breaking rear ends. My first instinct would be to choose a strong, cheap rear end with many readily available gear options, particularly the Ford 8.8, but an 8.8 is heavy and it's so big that the V8 guys consider it impossible to sneak an exhaust between it and the subframe. However, if the trans ratios are chosen to match a particular ring gear, then any single-gear-ratio-option rear end could be used. Maybe a 4.086 S13 R200? They're cheap, common, and have many good differential options. That would also mean that a stock rear end could be used until you inevitably destroy it, speading the cost of the parts out over at least half a track event, lol.

JasonC SBB

You want to hold peak-power-RPM forever, not torque-peak-RPM, 5252, 2112, nor 8675309 RPM.

Leafy

iTrader: (1)

I think the 7.5 ford is the best all arounder. Easy to find, cheap, tons of cheap rear gear choices from super low to wicked high, smallish, moderately light, still fairly low inertia. I mean its no Speedway Mini Quickchange IRS but there's an order of magnitude difference in price. The downside to the 7.5 is the LSD options, you're either getting a POSI style clutch type (think phantom grip but higher quality), some sort of gear type (torsen, et. al.), or a Detroit locker (lololol, this isnt a real option for a car that turns). I like torsen's so I dont have a problem with it, but there wont be a giken for it at least within a few years. In the next year there will be a giken for the ford 8.8, there also is one for the quickchange irs... But those are like 2 grand for just the housing and 1 set of gears, where as a a ford 7.5 from the JY is like 250. Of course the upside to the QC IRS is that you can swap your rear end ratio in 5 minutes to suite the track, and its lighter than the 7.5 rear. IMO I dont care about how much the diff weighs within reason, I have a minimum weight that I wont have a problem achieving and wouldn't mind putting more weight rear and low.

*also the aluminum housing 8.8's seem to weight more or less the same as the iron housing 7.5's. However, they're going to have more inertia.

Savington

iTrader: (31)

Yes. Power continues to rise to redline until you have a torque loss of ~1ft.lb for every ~35rpm. The only cars that lose torque that fast are cars that are designed for power-weight classes (i.e. PT/ST). The higher the revs stay, the more power you put to the rear wheels. Torque does not matter - what matters is how much horsepower is applied to the wheels and how often it is applied. 6-speeds apply more power more often because they maintain higher average RPM, and that's why they're faster on track.

Almost, but the gear drops are too large, especially 2nd to 3rd (30%). My biggest gripe with the 5-speed in Rover was the huge 2-3 drop - you end up with lots of places where you're between those two gears and either falling off the powerband in 3rd or trying to shift mid-exit. T4 and T7 at Sonoma, T2 and T11 at Laguna, Star Mazda and Buttonhook at Buttonwillow, and two corners at Streets of Willow. I solved it by giving up on 2nd gear entirely and going to 4.77 gears, which gave me a low ratio (3rd) of 6.35 and ratio drops of 24.9% (3rd to 4th) and 18.6% (4th to 5th). I get away with only having three ratios in that car because it almost never exceeds ~115mph - a high horsepower car has the same minimum speed requirements but needs to have 150+mph of top end, which requires a fourth high-speed ratio.

Savington

iTrader: (31)

I disagree for a few reasons.

• Most people have either 3.909 or 3.636 diffs in their high-horsepower cars, so it's foolish to design a ratio set that doesn't take that into account
• I doubt we're going to start breaking diffs suddenly. I've seen broken trannies, broken CV joints, fractured hubs, and autocross guys break axles on launch sometimes, but nobody breaks the Torsen unless you wheelhop it at the strip (and even then, it just breaks the LSD, not the R&P). Occasionally someone will break the housing, but the Rx7 housing is stronger.
• Even if we do start breaking diffs, you can get our ratios (or nearly ours) in Getrag and Ford diffs. The Getrag has ratios available in 3.42, 3.73, and 3.91. The Ford 8.8 has ratios available in 3.27, 3.33, 3.55, 3.73, and 3.90.
• The biggest issue with the 7.5/8.8 diffs is the lack of a decent LSD. I hope Leafy is right when he says that OSG will have an LSD for the 8.8 within 12 months, but when I talked to them at PRI last year they said their current design wouldn't fit into the case. They're on my list of people to talk to again this year.

DeerHunter

Aah, memories...

bbundy

In theory the S15 box is slitghtly stronger and is pretty much the same price as a miata 6 speed cirtainly less than a Mazdaspeed box which I have found to not be any stronger. Also in theory the RX8 box is the same as a miata box but with a stronger 4th gear only. I don't like the odd gear spacing for 4th they went to to make it stronger however but it also pretty much the same cost as a miata 6 speed. As a bonus the S15 box has slightly better gearing for my purposes especially if you put a miata 6th gear in it. I have 3 broken 6 speed miata boxes. out of those I have enough parts to put together a working one with about $50 worth of additional parts plus I have enough parts left over to make an S15 box into a miata box with a Miata 6th gear just need to come up with the money to buy an S15 box.

sixshooter

iTrader: (12)

Did that happen back when you were ignoring us and enjoying a glass of Chardonnay and some Brie somewhere else?

DeerHunter

¿Que?

Seefo

iTrader: (5)

basically, french Canadians and square tires suck.

sixshooter

iTrader: (12)

Deerhunter spent a lot more time posting on the wine and cheese Miata forum than the Miataturbo forum in the last year or so. Therefore I didn't know about the diff part failures until now.

Look! Here are dozens more transmission options to consider. I wonder if 300zx Z32 transmissions are any good? I need to check the math on the ratios.

240sxONE Tech » Blog Archive » Gear ratios and drivetrain info

There are also some favorable rear diff ratios in some of those cars with IRS.

vehicular

iTrader: (3)

I'm with you on most of this, but we've spent 8 pages discussing a $10,000 Cusco/ NISMO/ FR-S/ Miata hybrid thing that none of us are going to build.

z31maniac

80s BMW "Medium Case" diff's are plentiful and STRONG. Available ratios from 2.73 to 4.44 and possibly higher.........just as another data point.

Most commonly found in the 84-91 3 series.

Leafy

iTrader: (1)

I guess commonly available is a relative term. I might find 2 or 3 e30's with 6 cylinders in a junk yard within 100 miles, I can find at least 4-5 7.5" fords in each junk yard. Same with the unbreakable nissan Diff (R30?), non-existent.

vehicular

iTrader: (3)

The S13 R200 diff is nigh on indestructible up to 500whp in a 2600lb S13 drift car. You'll never break one in a 400whp 2300lb Miata. Never. Plus they're fairly compact, within 20% of the weight of a Miata R180 (probably less), and they have a 2 stud pattern on the diff case that would be cake to adapt to the Miata mounts with a simple wing bracket.

Plus, there are a few gear options. Pretty much all US market R200s are 4.08s, but there were some Q45s and J30s with 3.538s, 3.692s, and 3.91s. There is minor stub shaft/ axle funny business with being able to use them, and I don't know if the good diffs swap into them directly.

Leafy

iTrader: (1)

R200's have pretty good diff options, you can get a giken for them or a torsen (or OBX). You will have to talk very slowly and in small words when ordering the diff directly from OSG so you get one that isnt setup for sideways though. But they fall under the drift tax and hard to find, and dont have easy to get low ratios. Everywhere I look, its hard to get low ratios for import rear ends.

thenuge26

iTrader: (2)

Most of the MTs look like they have 1.9xx 2nd gears, which is too short for everyone I thought. Savington said he can't get traction in anything lower than a 1.6xx

bbundy

I might be willing to do a $2500 cusco 1-2 gear swap when I can find the money. hell I have almost that much in the diff. And I have ~$10,000 in my built motor that likes to be fed expensive drive line components.

It would allow me to have better gearing for both Autocross and road race applications at the same time. I could go back to a 3.909 instead of a 3.636 final.

z31maniac

There are a few good yards that will ship. The E30 diffs are strong and there are plenty of options available IIRC as far sa the LSD portion is concerned.

DeerHunter

Not willing to debate the first statement but, for the record, I'm a Prairie boy who has trouble pronouncing "poutine".

I post when I have something to say (my post count on the other forum isn't that high either, relatively speaking). Also for the record, a quick search revealed that I have posted here about my Torsen travails.

Back on topic: I currently run a billet Guru diff. Sadly, it isn't available anymore, but it's a nice piece and should be significantly stronger than the OEM bits.

I'll also reiterate my fondness for the Quaife gear set. It's not cheap but it's also not $10K. The gearing is good for both autocross and track use and it hasn't protested about all the hard use I've given it over the last 5-6 years.