I've been thinking a lot about the fourth gear ratio in the RX-8, and I thought I would consult the local brain trust.

If you could pick any ratio (obviously between the 3rd and 5th) what would it be?

The stock 1.187 seems a little tall to me (though not terribly so).

The 6spd NB Miata transmission has a 1.257 ratio 4th gear - which seems to make a lot more sense to me. All the other forward gears are the same to the US RX-8. This combination creates a more even spacing across the gears.

Example data:

MPH @ 9000 rpms

Stock RX-8:
1st 41.6 2nd 69 3rd 95 4th 132 5th 156.5 6th 185.6

theoretical RX-8 with Miata transmission:
1st 41.6 2nd 69 3rd 95 4th 124.5 5th 156.5 6th 185.6

stock 'top speed' spacing:
1st to 2nd - 27mph
2nd to 3rd - 26mph
3rd to 4th - 36.7mph
4th to 5th - 24.6mph
5th to 6th - 29mph

miata trans. spacing:
1st to 2nd - 27mph
2nd to 3rd - 26mph
3rd to 4th - 29.3mph
4th to 5th - 32mph
5th to 6th - 29mph

I'm sure the above illustrates my point.

The only reason I can think for Mazda picking this ratio is fuel economy (is that the consensus?). In that capacity it seems to make sense - I sometimes find myself cruising city streets in 4th (for 35 to 40 mph zones).

This also got me thinking about the JDM type-s rx-8 with it's third gear selected specifically to improve 1/4 mile times. I wondered if mazda also considered putting in the 'miata 4th' (say for improved road course performance in high speed straights).

Finally I should link to: https://www.rx8club.com/showthread.p...37#post1848637
Which sort've discusses the same thing but never really got anywhere (other than several posts about the RX-8 being the 'funniest' car to drive)

 

You really notice the tall 4th gear when you're trying to accelerate up the back straight at Mosport. Is the Miata 4th gear a drop in for the RX8 tranny?

 

I don't know but I'm going to look into it.
If anyone here knows, I'm curious.

 

*bump* (looking for more comments)

 

probably fuel economy, seeing as most people do their city driving in 4th gear. With the feed 4.77 I already find myself being in 5th gear where I would have normally been in 4th. The extra shift is really noticeable and probably would be too much for a person who wasn't building a track car.

 

well i have noticed that if you are in third at say 3k rpm, and you hit the throttle, it pulls pretty good. but if you were to do the same in 4th, it is pretty much nothing. first time i did it i was like WTF? it the miata 4th gear was a drop in, that would be sweet, but you would need both the the mainshaft gear and the auxiliary shaft gears and maybe the shifter fork for it. and it would be a a pretty hard install IMO.

 

^^^ thanks for the new comments!

I wasn't so much thinking of swapping a couple cogs as seeing if the whole box fits (since all the other gears are the same - but I haven't confirmed the reverse ratio yet)

Then the question is limited to how do durability and other 'performance factors' compare to ours... I would think that torque capacity is similar or better (should check this against the correct miata engine), but i wonder if it was designed to rev as high.... probably not. (maybe that's an even trade...)

That may not even be an issue if I don't cruise at 9K or choose to upgrade the unit's bearings, balance, etc at the same time.

I don't know if its a drop in (if the bellhousing would mount up and the prop shaft or whatever) I was just playing around with some excel sheets I made and observed the uneven spacing. We've all heard that the miata (and s2k) trannies are basically the same box as ours, so I became curious when i saw that it had a much more even distribution (basically the 4th gear ratio I picked out of my head).

Even if its a drop in as far as the drive line's concerned the shifter location could be off (isn't that the problem with the s2k box?), so i really wouldn't know unless i get some measurements.

 

RG should get in here, or charles, and MM

 

I don't know where you're getting your information that RX8 and Miata gear boxes share the exact same ratios.

2004 Mazda RX-8
Transmission: 6-speed manual
Gear Ratio Overall RPM Mph
1st 3.76:1 16.71:1 (9000) 39
2nd 2.27:1 10.08:1 (9000) 65
3rd 1.65:1 7.31:1 (9000) 89
4th 1.19:1 5.28:1 (9000) 124
5th 1.00:1 4.44:1 (9000) 147
6th 0.84:1 3.75:1 (7650) 148
Final drive ratio 4.44:1
Engine rpm @ 60 mph in top gear 3100

2006 Miata MX-5
Transmission: 6-speed manual
Gear Ratio Overall RPM Mph
1st 3.82:1 15.64:1 (6750) 31
2nd 2.26:1 9.27:1 (6750) 52
3rd 1.64:1 6.72:1 (6750) 72
4th 1.18:1 4.83:1 (6750) 100
5th 1.00:1 4.10:1 (6750) 118
6th 0.83:1 3.41:1 (6200) 130*
Final drive ratio 4.10:1
Engine rpm @ 60 mph in top gear 2800

 

In fact, the big difference between the ratios is only on 1st gear.

Edit: My bad. Just saw you were talking about an NB and not an NC. Nothing to see here... move along.

 

no prob YaXMaNGTO, we all make mistakes.

Charles I've considered this (that the priorities for 1-3 are performance and 4-6 are efficiency - since you really shouldn't need power above 3rd on the street - you'd be way past legal speeds)
but as DarkBrew observed at mosport there are racing conditions where this is an issue - maybe 5th but definitely 6th are outside of the 'usable' range in anything except a long drag race (top speed test), right?

Edit: added a power per gear chart with a fake (but close) rx-8 dyno - the two light blue lines are the two 4th's.

 

correct. i know we wouldnt be using 4-6 on the street except for cruising, but on tracks with longer straights, we do get to upwards of 120 mph, so a more useable 4th gear would really help with that. plus, i cruise around town in 5th or 6th.

 

true, but changing the 4th gear ratio wouldnt effect street drivablility that much

 

lucky i dont think porting would be a simple power adder

 

I think that the RX8 was built for a big turbo, 1-3 need no help, then 4-6 are made for the power.

 

very true.

 

heh, i knew that power adder's would be a conclusion that would arise...
but then wouldn't 1-3 be too short (at least 1st anyway)? Some PPL comment that the FD tranny would be better for a turbo rx-8 (i don't know the gearing off hand).

also i agree with CRH's comments about a race team swapping gears for a specific environment, but I'm looking (in theory) for a general improvement in all scenarios not one for a specific environment like those race teams that have that luxury/need to swap all the time
(ATTN nitpickers: I'm sure some roadcourses with just enough straightaway to get up 130 instead of 120 would benefit from the taller 4th we have now - so my proposed swap would not be better in all circumstances either)

the affect to street driving would be more acceleration available in 4th with a lower top speed, and slightly worse mileage when cruising in 4th at the same speed VS the old gear.

As i mentioned before, mazda deemed it worthy to have a special 3rd for the JDM typeS for the 1/4 mile, but not to change the 4th - it seems like they had their eye on the common performance #'s and then anything beyond that cruise efficency is more of a priority.

CRH did you get your car to a dyno yet? (I could've missed it) I'm really curious...

I agree that adding power solves the problem of 4th being weak @ speed - but it still would seem like a significant drop in power from third, and then a small drop going into 5th - since the spacing would be unchanged.

i'm going to try to come up with an acceleration graph that combines dyno and gearing data to better reflect these intricacies. (it won't be sophisticated enough at the beginning to reflect the real world - of course)

also when i get home i'll post up an overlay graph comparing the stock box with the stock rear VS feed rear for the curious (and for my kicks)

 

just for fun...

(feed vs stock)

 

The s2k and 8 do not share the same trannies or rear ends (sounds like perverted sex talk). The 8 and MS miata do have similars. A shorter 4th would be great and all, but I'd kill for a taller 6th. There is a Japanese company that makes one. $$$

 

taller 6th would be great for mpg

 

I'm sure it wouldn't hurt. If I could be at 2500 rpms at 60 I'd be a happy camper.

 

it would be bad if it were at 1k, or 1.5k, but as savedsol said, 2.5k would be good

 

I'd be happy if curising at 80mph I wasn't at 4.7K rpms

 

well you do have the feed gear. but even for us the rpms are at 4k at 80

 

The 4th from the NB Miata would work, but it would require the complete mainshaft to be replaced (the counter gear for 4th is built into the shaft.) provided the output shaft is the same, it would work - but I'm not sure that the length or diameter of the shaft is the same.