Final Drive ratios and LSD

[Nuddy]

From another sub-forum but probably more appropriate for Motor Sport:

I have a 1998 NB8A, completely stock AFAIK until yesterday when I had the final drive (diff) swapped.
I am told the standard diff was an open diff with 4.1:1 ratio.
Before the swap I checked the speedo against GPS and found it to be 'spot on'.
After the swap I rechecked and found the speedo is now 8% 'optimistic'.
I was told the second hand unit from japan is a Torsen Type 2 with 4.3:1 ratio and it has NB8C painted on the housing.

Another check I did (on a private road) was max speed in 3rd gear (5 speed box).
The rev limiter limits to 7200 rpm (according to the stock tacho) and at that speed the gps was reading 130.00 km/h.
This is very useful for me if I compete in Targa Tasmania in the GT Sports Trophy class.
In that class the competitors are penalised if they exceed 130 km/h. The speed is monitored by the rallysafe unit.

My MX-5 is primarily for motorsport, tarmac rallying and maybe MX-5 Cup

In order to qualify for entry to MX-5 Cup I need to run lap times at Wakefield Park below 1:16

I imagine that changing from the open 4.1:1 to the Torsen 4.3:1 will help there too.

[david_syd_au]

In order to qualify for entry to MX-5 Cup I need to run lap times at Wakefield Park below 1:16
I imagine that changing from the open 4.1:1 to the Torsen 4.3:1 will help there too.

At the last MX-5 Club track day at Wakefield Park, the fastest 3 drivers in class 6 (ie modified road-reg NBs) were all under 1:16, with the fastest going under 1:12, and this is in road-registered cars that are not highly modified.
My advice: Leave the car as is for now, forget over-analysing the various mods available, and just drive it. Bum in seat time is more important than any of these mods. Better to achieve sub 1:16 times by driving a slow car well, than by driving a fast car poorly.

[Nuddy]

In order to qualify for entry to MX-5 Cup I need to run lap times at Wakefield Park below 1:16
I imagine that changing from the open 4.1:1 to the Torsen 4.3:1 will help there too.

At the last MX-5 Club track day at Wakefield Park, the fastest 3 drivers in class 6 (ie modified road-reg NBs) were all under 1:16, with the fastest going under 1:12, and this is in road-registered cars that are not highly modified.
My advice: Leave the car as is for now, forget over-analysing the various mods available, and just drive it. Bum in seat time is more important than any of these mods. Better to achieve sub 1:16 times by driving a slow car well, than by driving a fast car poorly.

Yes that's the plan. I won't be chasing more power from the engine. I will use up the tyres that are on it then decide what tyres to replace them with.
I will run with the suspension as it is and see how that goes. If the shocks are worn out I will have to do something there.
Spending my money on track event fees rather than on the car (except for safety gear).

[Nuddy]

My first Supersprint was at SMSP North on 2nd April.
Tyres were quite slippery but predictable and controllable.
There is a lot to be gained with better tyres.
The torsen was definitely a big help.
4.3:1 final drive was not ideal on this circuit as i was hitting the rev limiter in 4th (5 speed) 3/4 down the main straight.
With a 4.1:1 final drive I think I could have stayed in 4th and entered turn 1 with more confidence.
By the end of the day i was entering turn 1 in 5th but was not happy with the low rpm in that gear.
However i do think the 4.3:1 ratio will be better at the South Circuit and at Wakefield Park.
My first run at Wakefield Park will be on the 23rd April and wil, be on the road tyres.
I learned a lot about the car and how to drive it on the 2nd.
Next step is to learn a bit about Wakefield Park.
Then it is Ringwood Hillclimb on the 30th
Back to SMSP North on 13th May and Wakefield Park on 14th and 15th
I'm hoping to be on Rspec tyres by then.

[david_syd_au]

How many revs were you carrying into turn 1 in 5th? My car is at about 6500 rpm at turn 1 entry.
It is possible that going into turn 1 in 4th with more revs and perhaps more torque available could unload the front a bit and lead to understeer, which is something that can be unpleasant at SMSP turn 1.

[Nuddy]

I'm still building confidence for turn 1. Initially I was just backing off when hitting the rev limiter in 4th and running at 7000rpm then a dab on the brake and back on throttle heading for turn 2. Then I forced myself to change up to 5th at 6500 and back to 4th as I dabbed the brake. By the 3rd session I was staying in 5th but still dabbing the brake, then back onto throttle. Still a long way off sliding in turn 1. Turn 2 I am sliding all the way through. I'm quite happy to be sideways at 90 km/h. I'm not yet brave enough for sideways at 180 km/h.
David, what speed are you doing at entry into turn 1? I am getting an indicated 180 on the straight which is actually 167 km/h then I dab the brake taking me down to, I guess about 140. Perhaps with Rspec tyres I will carry a little more speed onto the straight and get maybe 175 by the end of the straight. Wondering if I should expect to be able to enter turn 1 at 175. An old mate from the 70's runs a Renault Megane on Rspecs and gets a genuine 190 down the straight. I'm not sure if he brakes or just backs off a little to enter turn 1.
A Lotus driver put it into the wall on turn 1 on the day I was there.

[david_syd_au]

Turn 1 at SMSP is a serious corner. The best advice i can give is to gradually improve speed through the corner, and if the car starts getting out of shape dont try to correct it, but jump on the brakes hard and bail out. Too many drivers end up over correcting and hit the wall on the inside of the corner. If you miss the apex, ease off very slowly, as you will be running wide on the exit.
It took me 3 years of supersprints at SMSP GP to slowly go faster through turn 1 until i could hold it flat through there on a good day, entering the corner at about 6500 rpm in 6th. I have no idea how fast, as the speedo finishes at 180.
Eg

[Nuddy]

I would certainly get 'out of shape' on my current tyres.
What's your final drive ratio?

[david_syd_au]

Diff was 4.3, plus the car had race suspension and some aero.

[Nuddy]

According to the Flyin' Miata calculator, if using 205/50R15 tyres or tyres of the same circumference, with 4.3:1 final drive ratio, 6th gear 6500 rpm gives 197 km/h.
Using 195/50R15 tyres and 5 speed 5th gear 6500 rpm gives 200.5 km/h and 6400 rpm gives 197 km/h.
Using 195/50R15 tyres and 5 speed 4th gear 7200 (rev limiter) gives 181km/h.
So I was getting to 181 3/4 down the main straight and more than that by the end of the straight before dabbing the brakes, probably about 190 km/h.
Next time at SMSP North, 13th May, I should be on R-spec tyres by then so maybe a bit more speed at the end of the straight.
I will be strting off with a bit more than a dab on the brakes and have a target of just a back off the throttle and on again.

[StillIC]

Of possible interest, I find my standard tachometer to be optimistic by about 300rpm at 8000rpm, at least when compared to the Adaptronic readout.

[Nuddy]

Yes my rev limiter cuts the ignition at an indicated 7200rpm and I'm sure it would be set to a true 7000
Probably a factory 'safety feature' like speedo's being around 5%optimistic.