I have thought myself in a circle ( aka am I overanalyzing?)

[Chris_Swearingen]

Time is drawing short to order a torsen differential for the miata and get it in by the next event. The current quandry is gear ratio. Stock is 4.30 to 1 but there are also 4.10 and 3.90 units out there. My intended use for the car is AutoX. I drive it around town some, but I don't see long trips or track events in it's future.

Lots of the miata forum discussions suggest going to the 3:90 with forced induction for lower cruise rpm and higher top speed. For autocross my thinking is that it can delay/prevent the 2/3 shift and perhaps provide a more tractable power delivery. Especially since I now have 13" wheels.

The 4:30 offers greater acceleration and the possibility to handle bigger ( read wider ) tires at some point. Picture 335/35R17s on a Miata. I think with a sawsall and the right offset it might work;) Okay so maybe a 26" tall tire is too big for a miata and weight would be an issue also. But there may be a happy medium.

Or do I split the difference and go with the 4:10.

Sorry if you are all as dizzy from the circles as I am now, but any input would be appreciated.

[BoulderG]

Hi Chris,

Cool questions, though I can't say the sawzall image sounds pretty...

Locally, see if you can get in touch with Dave Kern. He's been very fast in a normally aspirated and a big supercharged Miata, and I know he was running 13 inch wheels.

Also, try contacting Flyin Miata in Grand Junction at www.flyinmiata.com. If you don't already know these people, you should! They are a wealth of information.

Good luck.

[ianacole]

I would be interested in seeing where each gear ratio put the power band in reference to RPMs, versus where you usually drive in the RPM band. That's where I would make my decision. I would think that the lower in the RPM band power comes on the better, especially with forced induction.

[00 SS]

Stick with the 4.30:1. You will hate yourself if you ever drop out of boost with the taller gears. The FI will add power, but not right off idle and you know how little torque those cars have down low. Why would you even consider adding good power and then crippling it again with taller gears? Better to have the power and torque and learn to shift and modulate the throttle in my opinion.

[kirstein]

Remember that if you end up rowing back and forth from second to 3rd gear a lot, you will be costing a few tenths for every shift. If you can figure out what speed you will hit the rev limiter at you can probably have a good idea how much shifting you will do.

[Chris_Swearingen]

It's supercharged so whatever power there is, is pretty linear.

Gear 4.30 4.10 3.90
1st: 34 36 37
2nd: 55 58 61
3rd: 79 83 87
4th: 106 111 117
5th: 132 138 146
(borrowed chart, I am only assuming its accuracy)

Chart above assumes a 195/44/14 at 22.4" diameter.
So - 8% for a 225/45/13 20.7" diameter = 50.6 mph 2/3 shift with a 4.30
Or +16% for a 335/35/17 at 26" diamter = 63.8 mph 2/3 shift with a 4.30

So... the object of the game is to get into 3rd and never slow down. Or just accept that I am shifting somewhere

[ianacole]

Any chance you can drive a car with each setup your looking at? Not knowing anything about Miata's, do the gears go in a rear diff? How expensive/difficult/time consuming would it be to swap out the gears and play around a bit? It' may be just me, and that I never watch my speed, but 55-79 MPH on course seems really fast, but then I spend my entire time in second gear.

[Chris_Swearingen]

Thanks to all who provided input and to both of you who actually participated in the poll.

I ordered the 4:30. It should be in for the next event, barring shipping or install difficulties.

That brings me to the next problem, now that I will have the LSD, new wheels and hoosiers, will upgrade the springs while I have the diff out,

:D :D HOW DO I EXPLAIN BEING SO SLOW?:D :D

[00 SS]

That's a big probelm with making improvements to the car, your excuses get thinner and thinner.:cool:

[Chris_Swearingen]

WOW! :D

The event this weekend was fun!

I got it aligned and corner weighted in time for the event.

Did anyone happen to get a picture that shows body roll? The stiffer springs felt like they eliminated a bunch of it. I am not going to tinker with it too much more, but may consider even bigger front sway bar at some point, although the balance felt really good to me, transitions were very smooth and even ( as long as I was on the inputs ).

The torsen helped a lot, power went down smoothly. I am going to hold off judgement on the gear ratio for a couple more events. I was bumping the rev limiter lightly in one spot and for a couple of seconds going into the last big sweeper before the finish slalom. ( of course that could have been tailwind assisted )

I was getting some front brake lockup so I am swapping to a different proportioning valve. The stock one drops the rear line pressure 40%, one from a 94-97 car will only drop it 20%.

One big change left to go then: FIX THE LOOSE NUT BEHIND THE WHEEL.

[alby426]

Hi Chris, ok let meput my 2 cents: first, what Mike Minear said is quite agreable to me. Don' t waste the power you gain with FI and shifting in a Miata is the easiest of all cars.
As far as anything bigger that a 13 inch wheels, as long as you can get fat tires in that size, I wouldn't change!Think weight and moment savings.
As far as the car ballance, it felt quite good, but it takes time to squeeze the maximum out of a car like that, so, I would like to remind you what often we forget: work on improving the driving aspect!
You showed me that you have a lotof good characteristics in your driving, so don't focus too much on the hardware.

Good luck