mx5-kiwi

This a bit of a strange post and being MT.net may very well go sideways but we have looked everywhere and can't find it....

After suffering another driveline failure (likely gearbox) today, I was reminded by our race mechanic of an article I forwarded him approx 2 years ago about a guy in the states who races an 1800 turbo (we think) with a T56 conversion. It was a metallic blue car and we cant recall if the article was a facebook one or some other car forum / blog (Grassroots Motorsport etc.). It delved in to how much racing he had done and a bit about the t56 conversion. mid wesk USA springs to mind but could easily be wrong....(I know needle in a haystack!)

We are very keen to read this again and possibly touch base with the owner to answer a few "real world" questions re ratios, diff ratio etc.

themonkeyman

Savington

iTrader: (31)

The T56 is an exceptionally poor option. It is brutally heavy (140+lbs), expensive, and the ratios are very poor. Wait for the KMiata E46 6sp setup.

mx5-kiwi

Thanks for the replies.

Savington, is there something about the zf 5 speed option you don't like?

Savington

iTrader: (31)

The gearing on the 6-speed is vastly superior. Geared for the same top speed and a 7000rpm redline, the 5-speed loses roughly 500RPM on every shift to the 6-speed. The gears are so much wider that with the same top speed, 2nd gear in the 5-speed becomes unusable on track (too short). It might end up being better to give up entirely on 2nd and just use 3rd, 4th, and 5th, but then you give up top end speed unless you can rev the car much, much harder.

As a point of reference, the BMW 5sp has a similar gearing spread to the OEM Miata 5-speed, except with a shorter 2nd gear. The 6-speed allows you to use your existing R&P, ignore 2nd, and use 3-4-5-6, which has a similar spread to the OEM 6-speed.

themonkeyman

Savington

iTrader: (31)

Nope. Simply put, there is no way to set up a 5-speed car to match the performance of a 6-speed car. For a road race car, longer gears and larger gaps between gears are never, ever faster than short gears and small drops.

Scaxx

Can confirm. Take it from a guy who has one, you don't want one lol. Been trying to figure out a decent solution to get rid of mine for a while and there's not a whole lot that isn't $$$.

themonkeyman

KMiata

Keep in mind guys, the ZF 5 and 6-speeds have virtually identical ratios, the 6-speeds just have an overdrive gear. The 1:1 gear is 5th in both transmissions. And the 3-4-5-6 ratios are nearly identical to the Miata 6-speed.

So if you don't use 6th gear on track, the 5-speed gets you the same performance with about a 35 lb weight savings and lower cost. If having that 6th gear is essential for your setup, then obviously go with the 6-speed.

In our K24 cars, our local tracks asked for 3-4-5 shifting, so pulling out the Miata 6-speed and replacing it with a ZF 5-speed gave us identical performance but in a bomb proof package.

E46 6-speed parts are almost all in stock and very close to being released. Just picked up another E46 6-speed trans to play with here for $425 delivered. Nice and plentiful.

Edit: Remember, we're also talking about two different BMW 5-speeds. The Getrag 260 5-speed is more similar to the Miata 5-speed, to Andrew's points below. But the ZF 5-speed is geared similarly to the Miata 6-speed and ZF 6-speed.

ThePass

iTrader: (3)

ZF 5-speed is quite the box. 77 lbs vs 90-something for the ZF 6 speed. Pretty incredible how strong it is for the weight. It comes down to if you need that overdrive for the tracks you intend to run with the rear ratios available. If not, I'd choose the lighter trans. If you're using a Getrag rear end, 3.2, 3.4 and 3.7 rear ratios are you options. With 3.4 rear, the 5 speed gives you a top speed of 141mph. A 3.2 rear bumps that up by 10mph. If you're using a Miata diff with 3.9 or 4.10 options, you probably need the 6 speed for most road courses.

themonkeyman

mx5-kiwi

Thanks for all the info guys.

After a lot of soul searching, scratching around old wallets and coins under the couch we have had really interesting discussions with many people but especially David at K-Miata and committed to the ZF 5 speed kit.

I would especially like to thank David for his time and advice.

Our top speed at most tracks with 300/300 ATW is approx 210- 215 Km/h which is approx 7200 rpm with a 4.1 Mazda diff (approx 6800 with the 3.9). We "think" we will be okay with using our spare 3.9 diff and a 7500 - 8000 rpm rev limit but have access to a 3.6 if necessary.

In relation to Savingtons comments about the 6 speed, we decided against for these reasons (valid to us with current knowledge, in New Zealand).

- Weight, zf 5 speed same as Mazda, 6 speed approx 25 lbs more.
- cost, $2k up versus approx $800 for 5 speed
- Availability, 5 speeds are considerably more available than 6 speeds.
- Size, they are apparently considerably larger and tighten up the available exhaust gap to possibly compromise routing..
- with 320 hp/300 torque ATW (and more to come once we resolve trannie failures) we "believe" we can pull the gears if there is a large gap between.
- I have a track setup e36 Euro M3 with 300/300 atw and a 3.7 worked diff. It has no problems with gear ratios on at least 2 of the 4 circuits we run. Not conclusive but "should" be similar or better given we are probably 250kg's lighter than the m3.

welcome suggestions or comments otherwise.

themonkeyman

Savington

iTrader: (31)

ZF5/3.23 is the same ratios as an OEM 5sp/3.909, ZF5/3.47 is the same as OEM 5sp/4.30.

The only way I'd do a 5sp is if I did it as Mark is doing it, with a 3.909 and lots of RPM. Gearing it down is a big mistake IMO. If you need to gear it down for the top end speed, you really need a 6sp box.

mx5-kiwi

edit. unnecessary.

KMiata

It was great chatting with you yesterday Mark. We're looking forward to helping you end trans failures and win more races.

ZF5 + 3.9 is exactly what's in my car and a great starting point. As you keep turning up the boost, the 3.6 may be a happy medium. At your power level, I don't expect the slightly longer gearing will hurt you much when it boils down to actual lap times, but I'll be interested to see any data you gather.

Plus with no PPF it's quite fast to swap diffs, so you could always have the 3.6 and 3.9 ready to go and install the best diff for any given track.

Gee Emm

ADM NB8Bs, including the SE(MSM), came with the 3.6. NB8A 4.1.

mx5-kiwi

Thanks David, I sat down for 2 hours last night wrote down all the variations of diff, gearbox and rev limits top speed for each gear, then converted each of those to Km for my own reference point (be nice if your conversion table had that option though I appreciate your metric customer base may be too small to justify).Then drew up circuit maps of the 4 circuits we run and wrote down current gear change points.

From there I could approximate the projected ZF gear change points.

It "looks" like the taller 3rd will be an advantage, as I am currently changing 3rd to 4th exiting a number of corners before I even get off the rumble strip....I'd never "swear" these estimates are right but I have high hopes it will be even quicker....or at least gain some lose some to be similar. 3.6 and 3.9 is still a bit up in the air (for longevity i'd prefer to not have to rev to 8k each gear change) and at this point I suspect only real world testing will provide the answer.

A regular 8k rev limit (apart from increased stress and decreased part life) might also mean having to change the turbo setup as the torque curve drops quite a lot from 6/6500 rpm...once again, only real world testing will show if it is a problem or not.

But that is the fun of what we do.