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transmission gear selection questions

Old 09-23-08, 09:47 AM
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Question transmission gear selection questions

since i am going to have to drop the fuel tank here soon and see why the pump is getting no fuel, lets talk about a future upgrade i am planning!

The transmission replacement!

The stock rx7 transmission has some **** poor gear selection. The stock 12a gearbox is shown below.

3.674 (1st) 2.217 (2nd) 1.432 (3rd) 1.00 (4th) 0.825 (5th)

The spread does not take advantage of what the engine can really do. For the sake of my build, the Mazda Comp Manual PDF (see the FAQ) has some good information on the Mazda Competition Transmission. If you want one...well that sucks because your looking for a surviving used one.

BUT the information i did get from the PDF is the gear selection of the comp. transmission.

2.350 (1st) 1.608 (2nd) 1.240 (3rd) 1.00 (4th) 0.881 or 0.839 (5th)

This information is priceless for the sack of want to do. As a bonus, that setup was used with the 4.88 final drive and a 12 PP. Ill be running a 4.78 with a 13b Bridgeport. So i am close to that the "perfect" setup would be.

So why does it matter that i know the ratios of a transmission i cant buy anymore? Well thanks to people with more time and money than i have, there are bell housings and other adapters to run other transmissions on the Mazda Rotary.

The transmission i want to use, if i can find an adapter for the GM 1-1/8" 26 spline, is the Richmond Gear Super T10 with the 2.43 (1st) 1.61 (2nd) 1.23 (3rd) 1.00 (4th) non overdrive and is only about 1500 dollars.

The ratios are very very close to the Mazda Comp.

But i dont know of a GM adapter, ill have to look, but if i cant find one, the option for the Ford T5 adapter exists.

The ratios for the Richmond Gear Super Street 5spd are:
2.89 (1st) 1.85 (2nd) 1.31 (3rd) 1.00 (4th) 0.77 (5th)

OR there is also the Richmond Gear 4+1 5pd with

3.27, 4.06, or 4.41 (1st) 2.13 (2nd) 1.57 (3rd) 1.23 (4th) 1.1 (5th)

I do not like that setup for one reason, first gear is useless with a 4.78.

I think i need to call Summit and see what other options not listed in the catalog are....
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Old 09-23-08, 09:48 AM
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as for the miata transmission here is what i found

Ratios:
1st 3.136
2nd 1.888
3rd 1.330
4th 1.000
5th 0.814
Final Drive 4.300

That is basically a split between the stock rx7 and the mazda comp transmission. I think i need to go talk to some people who have been racing with these ratios and get some advice.
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Old 09-23-08, 10:24 AM
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Old 09-23-08, 11:25 AM
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when you look at the gears, you also need to consider that the MFR box was being used with giant tires (16x12 wheels). that IS a factor
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Old 09-23-08, 11:37 AM
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hmm...that is a point i wasnt thinking of..

so what would be a recommended ratio if i am using 15"x8" wheels, a 4.78 final drive and a 13b n/a Bridgeport figuring maximum rpm of 9,500? At least in terms of what i can get from the summit catalog...
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Old 09-23-08, 03:15 PM
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Go here:

What you want to do is go here, download the QSpeedcalc and then start monkeying around. It isn't a great application but it works pretty well. Put in a new name, put in your data and then save. Your data is now at the end of the list.

http://www.autotech.com/quaife/index2.htm


Other things you need to know:
Circumference = 2xRxPi
1inch= roughly 25.4 mm

enjoy,

-Trent
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Old 09-23-08, 11:26 PM
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that is actually the best program applet ever.
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Old 09-23-08, 11:37 PM
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As I'm sure the Quaife applet points out, gear selection is track dependant. You don't want to be constantly having to shift in the middle of corners for instance.
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Old 09-24-08, 08:36 AM
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at the same time i also dont want to have the gear change bring me into too low an rpm. I could also play with my final drive a bit more easily once i figure out what i am looking for on the track.
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Old 09-24-08, 12:10 PM
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Originally Posted by Low Impedance View Post
hmm...that is a point i wasnt thinking of..

so what would be a recommended ratio if i am using 15"x8" wheels, a 4.78 final drive and a 13b n/a Bridgeport figuring maximum rpm of 9,500? At least in terms of what i can get from the summit catalog...
i was looking at this stuff a couple months ago. you need to look at the DIFFERENCE in rpm drops between the available gears (sometimes its like 100-200rpms) and then i also looked at total gearing, like gear + rear end.

then you can look at your dyno sheet, and where you wanna run the thing

and then put the stock trans together,because it was the one with the most new parts....
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Old 09-24-08, 01:58 PM
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the plan is to build the motor first and do an engine dyno to see what ill really be needing.

but in the meantime, learning about what ill be needing to know is a good use of downtime.
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Old 09-24-08, 04:50 PM
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Since none of us are crazy high dollar racers we all have to make trade-offs. During the Edmonton Indy event this past year I wound up running 3rd and fourth gear almost exclusively. I was racing aginst a few cars with five speed jerico gearboxes and in most cases they were running 2nd through 5th. I had around 150 hp (yay turbo) on those cars and we were very close in terms of acceleration. For the next year I'll be running the Geforce T5 kit, so I should be running 2nd, 3rd and fourth, with some fifth if I increase in speed like I think I will. The difference will be quite pronounced. The good thing with a rotary is you have a pretty wide rev band to play with, so if the gearing isn't perfect you can leg it out a bit for the odd corner or go into another corner a little low and lug it out a bit.

I'm a bit off topic, but the point I was trying to make is look at what the options are and try to make one work. It won't be perfect, but it will beat having a two speed transmission.

-Trent
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Old 09-24-08, 10:19 PM
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Several SCCA E Production racers use Mazda Miata 1.8L gear sets in a stock RX7 case. This yields 4 useable gears (2-5th) at a fairly economical price. Bob Neal finished 3rd at the 2006 Run Offs with one of these setups. Chuck Clark from the Midwest Division is also using one. Mazdatrix, Prather Racing and Pinneaple Rotary know how to set these up. The main modification is to the input shaft.

Another alternative is a RWD 626 transmission. I believe Tom Thrash - E Production Nat'l Champ - has used these boxes. Hard to find, however.

For a pure racing option, an economical alternative that two Midwest SCCA EP drivers have selected is a Jerico Y2K 4spd transmission. The Y2K box was used in Nextel Cup for road race quaifying when specific qualifying transmissions were allowed. These boxes feature gun drilled shafts and narrowed gears for weight reduction. They were just strong enough to survive 700HP V8s for a few qualifying laps. Behind a rotary they are bullet proof. Numerous gearing options are available and they can be setup so that all 4 gears can be used on track. For example:

1.8 1st
1.5 2nd
1.3 3rd
1.0 4th

To install one you need either an RX3 or FD bell housing. Adapter plates are available from Jerico. This setup uses a Chrysler Hemi input shaft and a clutch with a Hemi spline. Major trans tunnel clearancing is needed as well. Heorr Racing products can hook you up www.hrpworld.com. Charlie Clark at www.kcraceware.com can provide information as well - he has one in his E Production FB.
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