Diffs, et cetera

[4A-GE]

Mods please move this if it's in the wrong place

I am completely new and scouring for an mx5 - My 7th car since 2009.

Now, what irks me about smaller displacement, manual cars is that they usually rev quite high at 100ish km/h. That is, 3,000 and up.
(Examples: 2003 Astra, 1.8: 2,900; 1981 924, 2.0: 2,800; AE92 corolla, 4A-F 1600: 3,000.)
Some bloke on tardme had a picture of his mx doing 100 at about 3,500 and that... Well, I'm not completely put off but I'm not over the moon about it.

Is there a different diff I could [theoretically] dump in to up the gearing and have it cruise at less revs? Most of my driving is motorway; 30 odd kms each way to school and back. Not quite brave enough to tear open a gearbox and swap all the little coggies around.

Thanks guys.

[dylann]

I do similar k's to you to and from work every day and spend 40/50 a week on gas.

But I believe the NB 1.8's have a lower ratio diff compared to the rest of them, 3.6 compared to 4.3-4.1 depending on the car
I think with the 1.8 diff you need the driveshaft and half shafts aswell (correct me if i'm wrong) You could run a higher profile tire/bigger rim but that might look stupid or ruin handling etc.

EDIT: now that I research this, i might look into it myself

[siren676]

Welcome!

The usual ratio diff in na's are 4.3's(so 100k's @ 3400ish), you can swap in a 4.1, 3.9 or 3.6 out of a newer mx5 to bring it down but acceleration will take a hit.

By swapping the diff though you will mess up your speedo's accuracy e.g indicating 100kph but actually doing a bit faster

You can use this calculator to help work everything out
http://www.flyinmiata.com/tech/gearing.php

[marcellarius]

The MX5 has pretty short gears. If you have the 4.3:1 diff, you will be doing about 3500 at 100ks. It's not that bad, but it's a lot better suited to twisty roads than a motorway.

First gen 1.6L (1989 - 1993):
4.3 : 1 ratio. (~3500 @ 100kmh)
Open or viscous LSD (depending on trim package)
6" diff

First gen 1.8L (mid 1993 - 1997):
4.1 : 1 ratio; Mazda may have changed back to the 4.3:1 gearing at some point? (~3200 @ 100kmh)
Open or Torsen LSD
7" diff (stronger)

LSD depends on trim package, market etc. In the Japanese market, the Torsen was a standard feature in all manual transmission cars, but starting in the 95 model year it was made an option. ~96 Mazda changed to using Type 2 torsen

Second gen (1999-2005)
The 1.8 models with a 6 speed transmission came with longer diff gearing -- 3.909 (3000rpm @ 100, 6th) and 3.636 (2800rpm @ 100), otherwise they had the 4.1 gearing.

The 1.6 models retained the 4.3:1 gearing.

Open diff, Type 2 Torsen LSD, Tochigi Fuji Super LSD (2003+)

---
Diff gearing info, based on Aussie market
http://www.miata.net/garage/KnowYourCar/S8_Gears.html

[4A-GE]

You could run a higher profile tire/bigger rim but that might look stupid or ruin handling etc.

That's what I've done with the current car. 17" wheels suit a Carib fine so it's ok. 17's on an mx will look a bit toilet. It's auto though so it does 100 @ 2600 anyway.

Second gen (1999-2005)
The 1.8 models with a 6 speed transmission came with longer diff gearing -- 3.909 (3000rpm @ 100, 6th) and 3.636 (2800rpm @ 100)

So, this sounds like the one I want if I get sick of cruising so high! Next thing to worry about will be rich fuel mix but there's tons of crap on here about that. #economy, etc.

More diff suggestions completely welcome - Inexpensive-r is better.

[Timmo]

What's your budget?...instead of arsing around with swapping diffs and/or gearboxes, it just be easier/cheaper to get a 6 speed NB

Then you also get a more solid car, airbags, more power etc. I've owned both an NA8 and now a NB8 and prefer the sturdiness of the NB. 100 KpH = 3000 RPM

[2old4this]

Really think about it. Does or car blow through the RED line in TOP ? If it does, u need higher gearing. Or can use higher gearing, not the same thing. If u can't PULL the red line in top...definitely not ! Nothing kills the Fun 2 Drive factor in any vehicle car or bike faster than having 2 longer legs !!! Ur using about 1/2 ish ur revs and ur going about 1/2 ish max speed. There r a lot more 90mph straights in the world than 100mph 1s ... And how much time do u spend at warp 6 ? As 4 ur revs at 100kph - get used 2 it ! They LOVE it :-)

[Angreal]

What's your budget?...instead of arsing around with swapping diffs and/or gearboxes, it just be easier/cheaper to get a 6 speed NB

Then you also get a more solid car, airbags, more power etc. I've owned both an NA8 and now a NB8 and prefer the sturdiness of the NB. 100 KpH = 3000 RPM

6th gear is shorter than the 5th gear on a 5 speed.

Personally the best option is a 6 speed with a 3.6 diff but I couldn't be bothered reading everyone else's rely and should point out that the 3.6 is pretty rare. They do exist but be aware that you'll have to hunt around to find one unless you're lucky and potentially might have to pay quite a lot for one. Then again you can get lucky and get one for cheap. Gives you slightly shorter gears 1-5 then a 6th gear (vs the na8c with a 5 speed and 4.1)

[BRM]

These things run fine at 100kay / 3000 rpm
I do 50k a day mostly motorway driving and only uses $30/$40 a week

[4A-GE]

What's your budget?...instead of arsing around with swapping diffs and/or gearboxes, it just be easier/cheaper to get a 6 speed NB

Then you also get a more solid car, airbags, more power etc. I've owned both an NA8 and now a NB8 and prefer the sturdiness of the NB. 100 KpH = 3000 RPM

Budget will be what I get for the current car when I sell it (~$5000). I want an NA6. I don't want to be tearing round in the thing everywhere so I figure the 1600 is better suited to me than the 1800 (more top end?).

By the sounds of things it seems I need to man up and do 100 wherever the damn thing pleases.

[4A-GE]

Really think about it. Does or car blow through the RED line in TOP ? If it does, u need higher gearing. Or can use higher gearing, not the same thing. If u can't PULL the red line in top...definitely not ! Nothing kills the Fun 2 Drive factor in any vehicle car or bike faster than having 2 longer legs !!! Ur using about 1/2 ish ur revs and ur going about 1/2 ish max speed. There r a lot more 90mph straights in the world than 100mph 1s ... And how much time do u spend at warp 6 ? As 4 ur revs at 100kph - get used 2 it ! They LOVE it

I see what you're saying, but by that logic, I should be doing 4,000 revs at 100 cuz my current redline is 8,000.
I may not need the longer legs, but I'd like them.

[marcellarius]

I'd say just drive one and decide whether it's for you.

If you're after something that's smooth, quiet and efficient on highway driving, the MX5 is probably the worst possible choice

The overdrive ratio in an slushbox is probably their best feature, but works because the torque converter allows a lot of 'slip' when you try to accelerate. A manual with equivalent ratios would really labour the engine.

[4A-GE]

I'd say just drive one and decide whether it's for you.

Yeahp, looking for a decent one without dicking anyone around.

If you're after something that's smooth, quiet and efficient on highway driving, the MX5 is probably the worst possible choice

I duno, the 924 wasn't great. Quiet didn't exist and smooth was subjective. Efficient enough, oddly.

The overdrive ratio in an slushbox is probably their best feature, but works because the torque converter allows a lot of 'slip' when you try to accelerate. A manual with equivalent ratios would really labour the engine.

Oh, so we can use that word here?
Ok, labouring isn't good. I guess the guys who designed the thing knew what they were doing, so it can't be all bad.

[GRUVBB]

Yeahp, looking for a decent one without dicking anyone around. [quote]
I reckon you should bid on Bluey The MX5 on Trade Me.....

[aidwano]

You will want a hard top if quiet is your thing

[SLYDIT]

rpm at 100k's does not equal economy! its about putting the engine in the efficiency sweet spot. for example you may drop the cruise rpm by 10% but make the efficiency worse by 15% so you get worse mileage.... theres nothing wrong with the mx gearing... get used to it.

[BRM]

^^^^^

[4A-GE]

I reckon you should bid on Bluey The MX5 on Trade Me.....

I see what you're doing there.

You will want a hard top if quiet is your thing

Maybe later, for winter. Not a bad idea.

rpm at 100k's does not equal economy! its about putting the engine in the efficiency sweet spot. for example you may drop the cruise rpm by 10% but make the efficiency worse by 15% so you get worse mileage.... theres nothing wrong with the mx gearing... get used to it.

Yeahp, ok, accepted fate: Manning up and not worrying about the diff.

[NicF]

I do 60km each way in mine, pretty much all open road with only a few k's town stuff each end. Yes it's buzzy but I like the fact that it's cruising at the sweet spot power wise. And I still get a good 12 km/l, unless I take the bendy road home.

If 100kph economy is really important to you, you might consider installing a megasquirt. MX's run rich at the top end, and being able to fine tune with an eftermarket ecu as well as remove the afm would mean fuel savings over a couple of years would pay for it. That's my next move anyway, before my hairdresser's car finally gets a hair drier under the bonnet

[Angreal]

I dunno, while cruising at 185kph I'm always wishing there was another gear I could go to Actually are mx5s restricted at 180kph? I guess that's no longer the case after replacing the ecu?

Never really been so keen on testing the stability of the car just in case anyone is wondering if I do actually cruise around at 180 ... But needless to say I would love 6th gear for the motorway since FI really aids the cruising on the motorway

[marcellarius]

With my stock 1.8 and a 4.3 diff, I'm sometimes driving along, go to upshift and realise I'm already in 5th.

The 180 cutout can be overcome by removing a screw from the instrument cluster PCB. Not that they go much faster without forced induction or aero improvements.

[SLYDIT]

if you want to eek out some cheap HP and economy, change the ECU clocking crystal from a 4MHZ one to a 4.4MHZ one and itll remove your rev limit too. noice!

[4A-GE]

Went out and drove a couple today. Probably not ideal diff thread material but I'm pretty sure I'll be able to live with the short gearbox. Even going up and down the road is fun in these things. No cup holders though. Also where's the fuel light?

[marcellarius]

Well, Mazda took the perhaps unconventional approach of using an analogue gauge to indicate fuel level Figuring out how far below E you can go is part of the fun.

[GRUVBB]

Went out and drove a couple today. Probably not ideal diff thread material but I'm pretty sure I'll be able to live with the short gearbox. Even going up and down the road is fun in these things. No cup holders though. Also where's the fuel light?

Bluey The MX5 has a cupholder.
I just wedge the stubby or can between the passenger seat and the rear of the middle console in GRUVBB. It sits there a treat if you leave the seats slid fully back.
Yp can remove the ashtray and insert a piece of wood with a couple of holes drilled in it to suit your favourite beverage. Dont think a 1125 Jim Beam will fit though so RTDs or beers suffice.