What diff ratio have I got?

ED_MX5 · Postby **ED_MX5** » Fri Nov 21, 2014 10:09 pm

Having done a few long drives now, say last time I drove to wakefield on freeway.
I notice that the car revs pretty high, even in 6th gear.
I find it's doing almost 3000k at 110kph.
Has some previous owner changed the ratio? or is it normal on an NB8B?
The following question is, I know it may be down on power a tad when starting, but what ratios can I choose to give it longer legs? I could always just pull it apart to have a look, but I just thought I'd ask.
I am aware it is supposed to have a 3.6 or thereabouts.

Cheers for the info.

Apu · Postby **Apu** » Fri Nov 21, 2014 10:15 pm

If you are doing 100kmh or thereabouts with 3000rpm, highly likely you're on a 3.63 FD

Magpie · Postby **Magpie** » Fri Nov 21, 2014 11:02 pm

Longer legs as in less RPM at 100 kmh? I'm at about 3,900 in 6th at 100 with a 4.3 diff.

ED_MX5 · Postby **ED_MX5** » Fri Nov 21, 2014 11:23 pm

So maybe Im stuck then? do they make a better ratio?

Black_Penguin · Postby **Black_Penguin** » Fri Nov 21, 2014 11:57 pm

3.6 is the tallest ratio the mx5 came with, the others being a 3.9, a 4.1 and a 4.3.

hks_kansei · Postby **hks_kansei** » Sat Nov 22, 2014 1:18 am

3.6, the standard NB8B ratio, was the tallest factory ratio.

I recall Mazda Competition had a lot more ratios available, but I seem to recall them all being shorter (ie: down to 4.9:1)

I'll look up the old catalogue and see what they had available in the day, they may still make them

hks_kansei · Postby **hks_kansei** » Sat Nov 22, 2014 1:23 am

ok, checked the old Mazdaspeed catalogue.

Mazdaspeed made the following ratios:

4.875:1
5.125:1

So yeah, a lot shorter than what you want lol

Apu · Postby **Apu** » Sat Nov 22, 2014 9:20 am

I should add...about 3000rpm at 100kmh (thereabouts) with a 3.63 but when I switched to a 4.1 the engine revved about 300rpm higher at the same speed.

ED_MX5 · Postby **ED_MX5** » Sat Nov 22, 2014 9:28 am

That sucks. Has anyone had any problems running out of revs on the track with this ratio then? If it revs that high on the freeway, surely a good straight like eastern creek or wakefield you could read a book while waiting for the straight to end due to being on limiter half way along.
Is anyone aware or know of somewhere good that may do a custom set if need be? Is it worth it?

Caffeine · Postby **Caffeine** » Sat Nov 22, 2014 9:34 am

ED_MX5 wrote:That sucks. Has anyone had any problems running out of revs on the track with this ratio then? If it revs that high on the freeway, surely a good straight like eastern creek or wakefield you could read a book while waiting for the straight to end due to being on limiter half way along.
Is anyone aware or know of somewhere good that may do a custom set if need be? Is it worth it?

You won't run out of revs, 6th is good for at least 240, but you wont have the power to get there on Wakefield's straight...

Black_Penguin · Postby **Black_Penguin** » Sat Nov 22, 2014 9:42 am

Most mx5s dont have enough power to trouble the limiter in top gear. They hit their aero imposed top speed before then.

ED_MX5 · Postby **ED_MX5** » Sat Nov 22, 2014 10:03 am

That's good to know. I guess when I do take it to the track at some point when I'm not driving our other car, I'll just see how it goes. I must admit, for a 1.8 mazda motor of that era (which are not know for massive power) it does accelerate well. And it's good fun through corners the way it is now, just frustrating ringing it's neck on the freeway.

Magpie · Postby **Magpie** » Sat Nov 22, 2014 10:09 am

For my build, based on its power and drag there is a physical limit of 225 kmh but its gearing will allow 240.

I either need to reduce drag or increase power. Increasing power is not an option so it now down to drag, however to create downforce you need to create drag (simple explanation as it is possible to make downforce with no real increase in drag like a gurney flap). What a 3.9 dif does not do well is acceleration.

Acceleration
3.9 - baseline
4.1 - +5%
4.3 - +10%
5.1 - +31%

But one of the biggest impacts on acceleration is weight!
a 3.9 dif, in a car that weighs 1000kg will accelerate 10% faster than one that weighs 1100kg. Loose 200kg and acceleration is 22% faster.

If the OP wants the maths I'm happy to repost it.

ED_MX5 · Postby **ED_MX5** » Sat Nov 22, 2014 10:23 am

^ sorry, should clarify. Comparitively all things considered to another car with same power output.
I was being very vague there, I was comparing in a Jeremy Clarkson way.... just car to car not considering weight, drag, gearing etc. I have had two 89 model 323 astinas, a Toyota celica and those were my hatches. dríven many more hatches of the era, and just comparing car to car for power output, it's not such a bad little car when you flattin it. You still need to ring it's neck though. Lol.

But a good point on weight, I'll see how it goes. I may need to add some lightness.
Just don't want to go too far, I'm intending it to be mostly a nice weekend car, very occasional track use. So comfort is still wanted in some way or form.

forcedfive · Postby **forcedfive** » Sat Nov 22, 2014 10:24 am

Ed,

You wont even get top gear in your car with a 3.63 at Eastern Creek unless you have double the wheel HP of a stock car. You may just need to get yourself a louder stereo for highway trips and dont look at the tacho.