I understand the RX-8 reaches its max speed in 5th. So is 6th simply an overdrive not really meant for acceleration? i.e. If I were racing on a track, would I never bother with 6th?

And if so, why isn't 6th taller so that we can cruise at a lower RPM?

Thanks.

 

If you were racing on a track it's doubtful you'd ever go higher than 4th.
________
Lovely Wendie99

 

Lets just suppose there was a long straight that called for 5th gear. Jesus christ.

 

It's a trick question. Redline in 5th gear is 148 mph. The speed limiter in 6th gear is set at ~148 mph. Without the electronic limiter, 6th gear would be good for 155 mph, maybe 160.

But yes - 6th is an overdrive, not meant for acceleration. 6th isn't taller because then it would drop the engine too low on the power curve, and would have insufficient torque to accelerate at the levels Mazda desired at normal highway speeds.

Regards,
Gordon

 

This makes perfect sense, thanks Gordon. Now the other question... in terms of cost, could we see 7 speed manuals on cars with such close ratios soon?

 

Hey, you were the one who brought up using 5th gear on a track. I simply pointed out your question was invalid given the scenario you described. Don't blame me for your bad examples.

 

Regardless, the question is quite clear. Sorry I harmed you with my bad example.

 

I think this may be going down a rat hole, but why exactly do you want a 7th gear? From your original post: "why isn't 6th taller so that we can cruise at a lower RPM?"

Why do you want to cruise at a lower RPM? I assume your desire for a 7th gear is to accomplish that goal. Do you hope to achieve either a) lower RPMs for reduced wear on the engine, or b) improved fuel economy? You mention developing such a gearbox in the future but I'll assume you meant for the RX-8.

If you look at most manual trannys they typically have a 6k redline and are designed to cruise freeways around 3k RPM. Extend that to the RX-8 and a cruising RPM of 4k is actually below the halfway mark of the 9k redline.

Also keep in mind the 4k RPM is when you're travelling at 80 mph. That's well above the speed limit in most states. I don't know about FL but in CA the typical limit is 65 mph. Cruise in 6th gear in that situation and you're close to 3k RPM.

Meanwhile, what are the consequences of driving at such a relatively low RPM in such a high gear? The RX-8 doesn't have much torque and low RPMs compound the problem. End result? As Gord96BRG mentioned the available torque in that scenario is so low that you'd have lousy acceleration. Not what you want from any car--especially a sporty one.

Some cars either have (thought I heard that) or will have (new M5) a 7 gear manual tranny. However the primary intent is to make gears 1-6 closer in ratio to stay in the powerband. The 7th gear ratio is similar to current 6th gear ratios. So while 7 gear trannys may eventually become more common, they still won't satisfy your goal of freeway cruising at lower RPMs.
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UPSKIRT ***

 

That wasn't so hard was it? I regularly travel at 80mph on the Sawgrass Expressway in Florida as do most people in the morning commute. This is for a period of 30-40 mins everyday. A lower cruising RPM would mean better gas mileage. Because this car isn't very fuel efficient, I wondered why 6th couldn't be taller. Gord answered my question.

 

Some while back someone on the forum was trying to see if they could get any interest for a "group buy" to get a taller 6th gear fitted to their transmissions.

Personally I almost never use 5th gear. If I could do it by snapping my fingers (and not voiding the warranty) I'd be more than happy to have 5th be the ratio that 6th is now, and a truly tall gear for 6th. Something that would pull, say, 2400 RPM @ 65 mph (2950 rpm @ 80 mph).

 

Technically probably yes, because if there's 7 speed automatic, why not build a 7 speed manual. I guess the question is if enough people would actually buy it. Most people that drove my car so far were already struggling with the fact that is has six speeds. Unless six speed becomes the common standard, seven may be too much for a bigger market.

 

Does anyone know if this group buy ever occurred, and if so, who the vendor was? I'd be keen to see if they have any remaining stock, assuming such a product exists. I frequently commute on I287 in Westchester County, NY. I can assure you that the posted limit has little to no bearing on prevailing traffic speeds. Indeed, I would go so far as to suggest that attempting to drive at the posted limit would be tantamount to suicide. Consequently, I would very much like to have, at the very least, the 6th speed ratio in the manual transmission be the same as in the current automatic, or even the 4th speed ratio in the original automatic. I find the jump from 5th to 6th to be too tight and the engine does not wind up in the part of the rev band that I get with all the lower gears when upshifting to the next higher gear.

 

But probably no doubt. There is a Japanese company that makes a taller 6th for $200 . Of course you'll need to spend a lot of gas money getting installed too. So lose, lose.

 

^^^There's more to fuel economy than RPM obviously. It has yet to be seen if a taller 6th gear would actually improve fuel economy significantly.

On a closed course with no traffic where you could keep a perfectly steady speed it would probably help. I don't know about the highways you drive on, but where I am I rarely get to maintain a rock solid cruising speed, and I think the slight deceleration/accelerations with a taller 6th would come out as a wash on the fuel economy front. Of course I've been wrong before

Out of curiosity, what type of fuel economy are you seeing, and what do you expect to be able to get out of it? I recently went on a long road trip averaging around 80 MPH (again, not able to maintain a steady speed nearly as much as I would like) and I got 23 MPG.

 

So many misguided souls in here.

If you want real fuel economy, install a fuel pump that doesn't work. I just made 300 miles without even trying to get economy and I still have 1/4 tank left. Of course, I also couldn't accelerate or even maintain 60 mph... but that's another story.

 

I hear the bad fuel pump also gives increased throttle response.

 

yes, increased throttle response.

I also vacuumed the carpets this weekend, that increased throttle response as well. You would not believe how much throttle response I have.

 

I get into 5th at the track I go to - hardly worth it though because you are hitting the brakes straight after shifting

 

maybe cuz there's less weight haha