Buger

6 speed transmissions allow closer spacing of the lower gears for better acceleration and a taller ratio in top gear for better fuel economy.

Mazda did not gear the RX-8 for maximium acceleration. They spaced the gears so that top speed is in 5th and 6th is just taller for better fuel economy. The approx speeds @ redline for each of the gears are below:
1st gear ... 42
2nd gear ... 69
3rd gear ... 96
4th gear ... 132
5th gear ... 157 but drag limited at approx 155?
6th gear ... 186 but drag limited at approx 150?

If you compare this with some other 6-speed performance cars, you can see that they are geared much shorter so that they can have better 0--60 acceleration #s. Usually 2nd gear ratios are designed so that redline is very close to 60 mph for best acceleration.

350z
1st gear ... 38
2nd gear ... 63
3rd gear ... 90

G35
1st gear ... 37
2nd gear ... 60
3rd gear ... 85

Even older RX-7s with 5-speeds were geared so that redline in 2nd gear was around 60 mph for faster magazine times.

2nd gen RX-7
1st gear ... 35
2nd gear ... 61
3rd gear ... 89

3rd gen
1st gear ... 35
2nd gear ... 61
3rd gear ... 88

The only way the RX-8 could make it to production was if concerns about rotary emissions and fuel economy were addressed. The 6 speed is not just for the "more is better" idea. The only way the RX-8 could exist is with at least one version of the 6-speed transmission so that the fuel economy figures can disprove the notion that rotaries are grossly inefficient.

and for MikeW,

Do you actually believe what you write? :D Let's go with your definition of the powerband and numbers.

You say that the RX-8 has 75% of peak power from (roughly) 6250 - 10000 rpms. Taking your numbers, this is 3750 rpms at 75% or greater peak power.

Of course the relevent question is how many rpms of their powerband do the BMW M3 and the Corvette Z06 have from 75% of peak power? The wideness of the powerband is what we were discussing yes? It appears that you got started on the right track than got confused again and tried relating % of peak hp under 5000 rpms for some odd reason? What does 5000 have to do with anything?

Of course you did pick two very good candidates that are known for having a very wide powerband. I can explain why their powerband is so wide in another discussion but it is off-topic for this thread about 6-speed transmissions. The simple fact is that the RX-8 does not have a narrow powerband.

The last RX-7 has a torsens with TBRs of 2.6:1 to 3.0:1. They were made to turn and not to just "go straight".

Brian

ironqqq

I made those numbers up. The numbers were to prove a point. Diesel engines are very low revving and low hp but veryhigh on torque. I should have pointed out that I made those number up. if you wish, I can dig up data I took about 2 years when I did a dyno pull from a Catapillar engine. The point is diesel truck are basically, very high torque and very low hp. It was to illustrate disparity between hp and torque and to show why they accell so slowly, because they have such a low hp output. I apologize if I made up numbers and did not clarify.


This is where you are wrong my friend.

I will attempt to explain why torque determines how you "pull a car" with why torque has no bearing on accelleration. I will also show why HP determines how fast you can accellerate.

I believe you have misinterpreted the basic definitions of torque. you may have mis-interpreted the effects of F=ma term.


Torque

By definition torque is "force applied at a distance", what is called a moment or couple. This is defined by the equation

T = r * F,

where r is the distance F is the force applied. Well, tehn you might say, F = ma, can be applied. Sure.. it can.

T = r * m * a

well, then shouldn't it be moving? yes and no. Let's say you are trying to turn a rusted bolt with a wrench. You are applying a force to the wrench causing resulting in a moment(bolt) at the bolt. You are applying a force and by newtons law your wrench should be turning, but it is not.

Let me give another example, A diver stands on the edges of a diving board. The diver is basically a weight,

T = r * m * a = r * m * g

where g is the accelleration due to gravity. Is the diver moving? No. Is there a torque being applied to the diver board? Yes.

No where does torque imply finite movement. Torque does applied at a cars wheels can push/pull the car, but does not determine how fast the car will move or whether or not the car will accellerate. Let's say your whells can apply 100 ft-lbf of torque at the road/tire interface but the car is pushed up against a solid brick wall. Does the car accellerate? No. Is there torque applied? yes.

Look at the units for Torque, T = ft-lbf

A foot pound only implies a force applied at a given distance. No movement is implied.


Work

This will show the difference between torque and work. Torque and work have the same units most often in automotive terms we use ft-lbf (lbf means "pound force", in engineering there is a clear distinction of "pound force" and "pound mass, lbm"... for all purposes, pound force is adequate for this discussion and usage. In the regular world, we blatantly use the term pound, making no distinction between the two. In 99% of cases, the distinction is a non-issue.

Work is derived again from the F=ma equation, but the key characteristic of work is it must have finite movement. meaning, something must move for work to be done.

From my inserted picture you will see that F=ma is used in the derivation of work, but you will see that the "dr" and "ds" terms imply a movement for work to be possible. In this case, accelleration must result in a movement for work to be accomplished.

Again, there is no term that determines the rate at which work is done. in this case let's say we were to displace an engine block 1 feet. There are no terms in the equations I posted to determine whether that displacement occurred in 1 second or over a period of 1 million years.

Again work has the same units, ft-lbf.

Power
This is where power comes in. Power by definition is a "rate change of work". There are derivations to obtain power, but we will forgo that and look at the popular equation

hp = T * RPM / 5252

This is a equation that relates hp to torque, not a equation that defines power or torque. That is very key.

From this equation we see the addition of the RPM term, this effects a "rate" at which torque is be applied. only from this equation do we introduce a rate, a rate of work is done. Here the equation says that the torque is applied at every cycle divided by a factor (5252). It is only when we introduce the RPM has a "rate change" been introduced.

You can apply all the torque you want but if there is no movement, there is no work and this there is no power delivered.

This is why I say that HP determines how fast a car accellerate. This is why I say that torque determines how a car pulls.

Torque alone has not bearing on how fast a car accellerates. Torque applied at a specified rate determines how fast you accellerate.

with the HP = T * RPM / 5252 equation if the RPM wa high enough you coudl accellerate like crazy even with very little torque.

Please don't say my physics isn't there.

RedRotaryRocket

It appears I have offended you. For that I appologize. My intent was not to berate you, but rather to correct a few points which were not correctly stated. I will submit that I did say your understanding was better than most...that was intended as a compliment.

You have written a nice summary of the concepts of force, work, and power. As I said before, it is clear you have a better understanding than most; however, I will re-assert that force (torque when rotational) is the motivation behind acceleration. To drive home this point, re-consider F=ma. The relationship between force and acceleration is DEFINED right there. There is no need to go through convoluted contortions to derive work or power. There is no need to know extra information like the time or the distance the force was applied.

There can be no acceleration without force. You're examples of a rusty bolt and a car against a wall were not quite complete as they did not take into consideration all the forces. The rusty bolt does not turn because the force you apply is directly opposed by two other forces: 1) friction and on a smaller scale, 2) the force of the electrical bonds you must break to break the rust. The same can be said of the car against the wall: the normal force of the wall on the car is exactly the opposite of the force generated by the wheels (actually, it is the road that imposes a force on the car.) The net force on the car is zero. This is the only reason why it does not accelerate.

I don't think I can make it more clear than that. If you still have your doubts, that's ok...but you'll just have to trust me. No flames intended.

maxcooper

The top speeds in gear for the 3rd gen RX-7 that were posted are wrong for 5-speed manual, 4.10-rear end versions (the ones sold in the U.S.). The correct speeds are roughly:

1st 40 MPH
2nd 70 MPH
3rd 105 MPH
4th 140 MPH
5th 200 MPH (drag limited to 160 or so in stock form)

-Max

rxeightr

Still don't see the need to have 6 speeds, other than to improve gas mileage some.

Unless someone can convince me otherwise, because of the extended torque range, I see me using selected gears routinely, such as 1st, 3rd, 4th (or 5th) & 6th.

bdclary

Using information that I've found in this thread and others, I've calculated that 3,000 rpms in 6th gear is about 60mph.

Is this correct? If it is, that seems awfully short. I'm doing at least 70mph at 3,000 rpms in 5th gear in my '86.

Why would I rev higher in a car with more gears?

Brian

justinm2

Go to a mazda dealer, and ask to drive an Ls miata with a 5 speed, and an Ls with a 6 speed back to back. the 6 speed seems to accelerate marginaly quicker. Plus it's easier to find just the right gear for each curve. on the 5 speed model, some curves are better suited to second gear, but your bouncing off the rev limiter coming out of the corner, put it in third gear, and you don't have the power to pull through the corner. in the 6 speed 3rd is just right, a little taller than the 2nd gear and not quite as tall as the 3rd gear in the 5sp. Personally I'm sold on 6 speed transmissions

Y&Y

Now when it comes to driving skills on the track, I have none.

But if bdclary is right about 60mph in the 6th gear hitting 3k rpm, then no wonder the mileage is so very low . I love to drive fast well cuz I can't stand driving at speed limit (65 mph in so cal). My average mileage is at 30 (70 mph or less) and I usually drive around 75-80 mph. and I know that I hit 3k rpm at about 78 mph (guestimate) driving a crappy 97 civ hatch. The only thing I like about the car is that is has really good mileage. I like to drive and I like to drive far. Man this is going to suck that means on average I'll get 6 less miles per gallon. Oh yeah does any know what fuel grade is recommended for the 8?