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Hi all,

I'm finding that now I'm using an NB8B on the track I fluff the 4-5 change on the 6 speed. Have to slow the change and make it a very deliberate dog-leg move rather than just push quickly at an angle like I could in the NA 5 speed. Fresh Mazdaspeed hard rubber engine mounts and poly diff mounts (and a Beavis engine damper for those bling kW's ), so I don't think it's gearbox movement.

I've found a couple of references to the same problem on the web, but no solutions other than 'slow down' the shift.

It's like there's an empty slot for "7th" over above reverse that i keep hitting. I assume the shifter is moving partially into the reverse gate. Is this common? or a fault with this particular box?

Been contemplating maching up a "H" selector gate something like this - but a simpler solution would be good!



Thanks
Ian

What gear box oil?

You may find that the gearbox is just tired. How many KM's does it have on it?

After 10,000 track km's my 6 speed was doing the same, oil changes helped for a while but eventually it just became 'annoying'. Had the gearbox stripped and it was deemed uneconomical to repair. I replaced the 6 speed with a 5 speed and now changing the final drive to 4.7 which doing the math will be 3 seconds quicker over 600m than the 6 speed 4.3 combination.

Depending on your budget you could contact Neat Gearbox in SA and ask about a rebuild. I am talking to them at the moment about main shaft replacement and other parts.

Thanks Magpie

Car shows 182k, but unknown history. Seems to have been well cared for in the past though. I’ve only had it out 3 times so not a lot of track kms yet.

Gearbox oil is fresh Redline MT90 75w90 GL4

Budget is what I can sneak past the home budget controller Have got a spare 5spd I was thinking of using if I can’t either fix this one or get used to it.. how involved was the retrofit?

Btw - running a 4.44 T1 diff.

ianoz wrote: how involved was the retrofit?

Btw - running a 4.44 T1 diff.

I'm in the process of trying to build 1 x decent 6 speed out of the parts I have at home. However one has a completely stuffed main shaft and the other one is 'worn' and maybe not serviceable as well. The OEM main shaft is EXPENSIVE and not easy to get hold of

Straight swap for a five speed. The five speed ratios are not as close as the six speeds so you will change less, but may not be as useful on the track.

This may not help in your case Ian, but I've recently changed from MT-90 to the newish Penrite GL-4 (75W-90), & the shift of my 6 speed is the best its ever been. https://www.penriteoil.com.au/applications/car-4wd/gear-oils/pro-gear-gl-4-75w-90#/

what about those solid hot-side engine mounts?
I know they will not fix anything that's already broken.....if it is broken. but would they help getting the gears in? I know thw big HP cars are loving the solid mount.

Ive got one too but I dont race NA6. It feels fab.

StanTheMan wrote:what about those solid hot-side engine mounts?

I'm running the Blackbird 'stiff/lowering' engine mounts and in the end this did not help. As I mentioned frequent oil changes made it 'better' but it is just a band aid.

I would suggest having it striped and inspected. However you could try a different oil, but if the problem returns...

Magpie wrote: The five speed ratios are not as close as the six speeds so you will change less, but may not be as useful on the track.

Not keen on rebuilding the box if I can avoid it - rather spend that towards a full cage. If it's not unique to this box then I'll learn to drive around it until it goes bang... (when I hit reverse instead of 6th again ) or see if I can work out some gate lockout mechanism. Was hoping there might be a spring that I could just shim/replace to make the gate harder to pass without pulling the entire box apart. Nothing is ever that easy with cars though

Certainly changing gears a lot more than before - finding that the current diff is fun but maybe a bit short for a stock engine. 4.1 would probably have been a better choice right now - but aim was for more top-end in future where I could use the revs. Looking at your graphs you're running well over 8000 - I've got the shift lights down around 7000 to keep the spinny bits in the right places..

Is there any mechanical advantage either way of a [shorter gearbox ratio + taller diff] vs [taller gearbox ratio + shorter diff] if they both multiply out to the same?

Looking at your graphs and the ratio's, if I was to drop the spare 5spd in with my 4.44 diff then it would be going back to close to a stock NB8B in 3/4/5 or even a bit taller - so not really worthwhile unless combined with changing to an even shorter diff like your 4.8. Looks like that's a backup plan only then and you've saved me a bunch of work

Gearbox ratio x Diff ratio

RS2000 wrote: ...Penrite GL-4 (75W-90)

shift feel is fine except for this business of missing 5th.. no catching or harsh snicking etc, just a big hole around 5th.. but I'll certainly give that a try when I change the oil next. Quite like the Penrite oils.

StanTheMan wrote:what about those solid hot-side engine mounts?

time over again I'd do that just for the peace of mind, but only have stock power so there doesn't seem to be much movement as it is now. Mind you even stock NB8B feels a lot more than a stock NA6!

Has anybody had experience with the Quaife c/r gearsets? The cost seems to be in the same ball park as a 6 speed rebuild.

https://shop.quaife.co.uk/mazda-mx5-mia ... fe-gearkit

I think this would be a far better solution.......

https://shop.quaife.co.uk/mazda-mx-5-na ... al-gearbox

Cost could be a factor though!

OK my research to date:
http://www.pms.net.au/03_gearbox.htm Old price but $6k for a 5 speed dog box from PPG. Swaps 1st into 5th. Whilst this seems a great choice a rebuild maybe cheaper depending on the condition of the box. PPG no longer have the MX5 gearset on their website however it may still be available.

Rebuild options and possibly parts. I am talking to them about various 6 speed parts. They may know what parts are interchangeable. I maybe sending the best of my parts to get them inspected and treated etc.
http://www.neatgearboxes.com.au/ They do not have a MX5 6 speed price but use the S2000 as a basis. Cost about $2.2k provide all gears can be reused. Other items such as synchroniser rings, sleeves, gears, final drive sets, shift forks and drain plugs. These items are all at additional cost.

I have other places I have spoke with but getting OEM parts for the 6 speed is so expensive it is not worth the effort.

Final drive ratios, there is a person with a 5.3 final drive ratio. However at 8,000 RPM based on my car with a 6 speed max speed will be 198, with a 5 speed 205. But the acceleration! in 3rd with a 6 speed 1.59m/sec² compared to 1.29 with a 4.3 final drive!

Not useful but in first it is 3.62m/sec² compared to 2.94m/sec².

ianoz wrote:Is there any mechanical advantage either way of a [shorter gearbox ratio + taller diff] vs [taller gearbox ratio + shorter diff] if they both multiply out to the same?

No. If you cannot change power then your choices are reduce drag, weight or gears to improve acceleration.

Here is the basic file I use, feel free to modify it. You will need to input your cars torque curve as well as weight, drag co-efficient, frontal area, wheel radius etc. If not sure look in the formulas.

RS2000 wrote:This may not help in your case Ian, but I've recently changed from MT-90 to the newish Penrite GL-4 (75W-90), & the shift of my 6 speed is the best its ever been. https://www.penriteoil.com.au/applications/car-4wd/gear-oils/pro-gear-gl-4-75w-90#/

Thanks for this suggestion RS2000, I've noticed this oil out on shelves now and considered giving it a go.

Danny wrote:Has anybody had experience with the Quaife c/r gearsets? The cost seems to be in the same ball park as a 6 speed rebuild.

https://shop.quaife.co.uk/mazda-mx5-mia ... fe-gearkit

I used their gear sets for over a decade racing cars in the UK. Pretty much the defacto club racing transmission supplier over there. They used to have a calculator online so you plug in your engine power band, diff ratio, wheel diameter etc and the match it up to track data to pick the right set for each circuit or a generic set depending on budget.

Engine mounts

Aisin 6-speed is shiite, they still have problems with it in the AE86.

Try a MiataRoadster tall-angled shifter, it stopped me accidentally going from 4th to grinding against reverse when driving on the road and changing gears on high speed curves - also made it a hell of a lot easier to engage reverse when I needed to.