I can see the need to keep your engine revving within a certain rpm for a piston engine, hence the need for a 6 speed (or 5 speed) manual tranny.

What real use is there going to be with having a 6 speed hooked up to the Renesis? Won't 3 or 4 speeds be all you really need?

 

well, good question, but two reasons really... first (the one that Mazda or any manufacturer will feed you) is that there is still significant gain in fuel economy and performance when going from a 5 speed to a 6 speed, allowing greater gearing advantages, and smaller steps inbetween each gear to exploit the highest portions of the power band (this actually does help)... but the real reason is marketing... if buying purely on what's written about the car, who really wants a 5 speed RX-8 when there's a 6 speed RX-8 for about the same money?? more is better in the public eye, and it's difficult to market an "inferior" transmission with less speeds... people don't realize that a 5 speed could be made significantly smaller, lighter, cheaper, and more mechanically stout than any given 6 speed: for those reasons Mazda went with just a Porsche-derived 5 speed for the Le Mans winning 787B in 1991... almost no "respectable" race car has used anything less than 6 speeds for a while now...

 

The way trannies are going, im looking forward to my 22-speed 911 in the year 2030 :D

 

The "more is better mentality was disscused here about 18 inch vs. 17 inch wheels. The main reason was because there was a demand for them and they looked cooler. It is a decent read. Check it out.

 

For many lower horsepower cars, 6 speeds will be useless because the wheel torque of one gear will overlap with the wheel torque of the next if the gears are spaced too closely.

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

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.

Brian

 

This car has a wide "torque band", not a wide power band. Cars with wide power band have a big torque peak at low rpms. The 6 speed allows you to keep the engine rpms near the power peak.

 

I'm with Wakeech on this one. It seems trendy to have a 6 speed these days. People will explain the performance gains to be had with a properly spaced 6 speed transmission (vs. 5 speed tranny) but just the words " six speed transmission" are like instant buzz words to most americans. My take on it? *yawn* it's about time...

Darril

 

The Renesis has a narrow powerband.
It only make power when all three ports are online and cooking. 6250 revs the 3rd port opens up, and the renesis only really shines from 7250 to 8750

http://www.rx8forum.com/showthread.php?s=&threadid=2026

 

isnt it supposed to be like 90% torque from 3500 up? or something
________
INFANTS DEPAKOTE

 

sure it may produce 90% torque at 3500 rpm, but lets say at 45mph, and you're in 4th gear at 3500 rpm you are getting less torque at the wheels than if you were in 2nd gear at 8000 rpm because of gearing, sure the torque coming out of the engine is pretty similar you lose a lot more of that torque at the wheels when in 4th gear, than in 2nd gear.

btw those numbers are just a rough estimate for example

this is one of those cases, where horsepower, which is the energy output of the engine is a more applicable figure, as opposed to the torque the renesis puts out

-nate

 

Hi MikeW,

Another classic from MikeW. I didn't think you could outdo your statement that cars like the fd3s were only made to go straight (and not to turn) because of it's torsen LSD. :D

It would be interesting to know... What is your definition of a narrow powerband and what is your definition of a wide powerband? Examples?

Brian

 

Anytime the peak torque is near the peak horsepower, the car has a narrow powerband. My RX-8 will have a narrow powerband because it doesn't have a turbo or a large displacement engine. A 6 speed transmission will allow the skilled driver to select a gear that puts the engine near peak horsepower. Horsepower RULES!

 

Hi Randy,

My question was originally meant for MikeW but since you tried answering it...

You mention that a narrow powerband is "Anytime the peak torque is near the peak horsepower". What would you consider "near"? How would you numerically quantify "near"?

I was hoping for some examples of cars with a wide powerband. Would the Chevy Corvette Z06 be an example of a car with a wide powerband?

Thanks in advance, Brian

 

One example is a C4 L98 Corvette. Max torque about 300, max HP of 250. Here is a link to some dynos:
http://www.corvette.co.za/DynoDay2.htm

 

I'm no expert, but the Renesis peaks on torque at 5500 rpm and hp at 8500 rpm. Those are 3000 rpm apart. That doesn't seem "near" to me. That's not to mention the flat torque curve of this engine. There really isn't a huge difference between peak torque and the rest of the curve with the Renesis. I think Randy is assuming that if the peaks are near then they are both at high rpm. In that case you would only be getting power at high rpm indicating a narrow power band.

EDIT:

It just occured to me that Randy might be talking about "near" in number. I assumed that he meant "near" the same rpm. If that is the case then I am not sure how the two being "near" proves anything. I would think that a low torque and high hp would show that the engine has to go to high rpm to make power. A high torque number and low hp would seem to indicate and engine that either has a low redline or torque that falls off at a low rpm. The only conclusion I can make on an engine where the hp and torque peaks are near in number is that the peaks are both close to 5252 where hp always equals torque. I don't see where that is a bad thing though.

Can anybody clear this up?

 

Hi again Randy,

I was hoping that you would define how you quantify "near" and tell us whether you thought the Corvette Z06 had a wide powerband.

You did mention the C4 L98 Corvette as an example of a wide powerband. The stock car has it's peak torque @ 3200 and peak hp @ 4300.

Brian

 

Hi Fëakhelek,

I am hoping that Randy will try to clarify exactly what he meant.

Most people get a little confused because they use torque and peak torque and hp and peak hp interchangeably. The peak torque and peak hp figures do not explicitly tell you anything about the shape of the torque curve below the 2 peaks.

Mazda currently says that the renesis' peak torque occurs @ 5500 rpms and the peak hp occurs @ 8500 rpm. For anybody that hasn't already seen the renesis torque curve, this gives 2 points on the curve but doesn't give any information on what the torque curve looks like before 5500. Knowing nothing else, it is possible that the torque at 2000 rpms could be 158 ft-lbs (approx 60 hp) because 158 < 159. Of course this isn't the case (renesis makes approx 50 hp @ 2000 rpms) but this shows that the "nearness" of the peak figures is not a definition of a wide powerband (some people like to call it a "torqueband").

Brian

 

Here's my perspective on this issue:

The FD (93-95 U.S. RX-7) has a 5 speed. On this car, 4th gear tops out at about 140 MPH. 5th gear tops out at about 200 MPH (though of course you don't have the power to redline in 5th). There are some tracks where you can go 140+ on the straights. 5th is too tall, so unless you have a bunch more straight away, you just stay in 4th to the corner. With 6 speeds, you could have 5 gears for accelerating and one highway gear to cruise in. That would be nice; 6 speeds sounds good to me.

With the powerband as it is (note: I don't want to argue about what I mean by that), a six speed in the RX-8 is a good idea to keep the revs up.

-Max

 

An objective definition of powerband used to be the revs between peak torque and peak horsepower. But that is old way, and it neglects the power over the horsepower peak.

I would say that the powerband is the rev range that the engine makes 75% or better of maximum horsepower. So 75% of 247=185.25, and that occurs between (roughly) 6250 and in excess of 10,000 revs. The 3rd port opens @ 6250, and the variable resonance switches @ 7250, and the variable air filtration opens also at 7250

The BMW (euro) M3 makes 338 hp@7900, 269 ft-lbs @4900, So 75% occurs (just before) 5000 revs.

The Corvette Z06, makes 405 hp@6K, 400 ft-lbs@4800, horsepower@ 4800 is 365.5 =90%

Hey I NEVER said: the fd3s were only made to go straight (and not to turn) because of it's torsen LSD
stop putting words into my mouth.

 

Back to the original post, there is a need for a 6 speed transmission. While the torque curve may be flat, the power curve is not. Saying this you ask, what is the difference between torque and power?

In simplest terms, torque is a measure to pull the car. Horsepower is ability accellerate the car. Torque is the measure of work. Horsepower is the measure of work per given time. Reading a torque value by itself will not tell you how fast a car will accellerate.

A flat torque curve will be adequate for you to spin your tires at any speed simply by mashing the throttle, but that does not neccearrily mean you will accelerate at a high rate. Horsepower is what lets you accellerate.

Let's take a look at a diesel big rig engine. many of those engines are only 150-200 horsepower or so. Well, you say, my RX-8 has more horsepower but can the RX-8 pull a 20,000 lb trailer? Hell no. The difference is that a big rig is 200 hp, but the torque is 2000 ft-lb where your RX-8 is maybe.. 158 ft-lb. See how slow big rigs accellerate, they are low horsepower. Torque is the ability to pull, not the ability to accelerate.

Now let's look at the Honda J series engine found in the S2000. The engine makes almost no torque at low rpm but there is loads of power above 8000 rpm. It's probably making 200+ hp from 7500-10000 rpm redline. Well, to keep the car accellerating, you need the the engine to be keep in the high horsepower band. By having closely spaced gears, everytime you shift, you will still stay withing the narrow power band and you will keep on accelerating.

The intent of having the 6 speed is to keep the car within the narrow high powerband to keep the car accellerating.

whatever... i just cause ppl more confusion.

 

yup.

nope. big rigs accelerate slowly becuase they're really, really heavy. torque is a measure of the force the engine is producing. force is the only thing that can accelerate a mass: power (energy) doesn't do this, but you can do some messy converting to find acceleration if you know the time over which there is a change in energy... bah!! i'm making less and less sense all the time.

the reason we use power to measure performance of an engine is because it brings the speed the engine is going into the torque equation... like on an electric motor, it "makes" only as much power as you put into it at all rpm, so in theory it has maximum torque at zero rpm (which would be infinite... torque, that is). the higher you rev it, the slower it will accelerate the mass, becuase its torque is deminishing as rpms rise if power stays constant (which would only make sense to use max power at the lowest rpms).

in application to IC engines, we can use power to account for gearing advantages 'cause the traditional empirical way for converting ft*lbs into hp is POWER * RPM / 5252... simple equation, and kinda sorta accounts for rpm...
to really get a fix on it, what you'd ideal want is some fucky unit where you've just got the torque * rpm, which would have some screwy name or something, and that would give a way, way better indication of how "well" that engine would accelerate a vehicle as you could easily factor in gearing and stuff like that.
(Rich used to explain this way way better, but he's not been around for a while...)

 

Something just seems really screwy to me with these numbers. To make 2000 lb-ft of torque and only make 200 HP, the engine would have to be spinning at 525 RPM. That sounds unrealistically low to me. Admittedly, I don't know much about big rig engines, so could you tell me what is their operating RPM range?

You're not quite there on the physics, but you do have a better grasp of what's going on than a lot of people do. At least you realize that how fast a car goes is more subtle than reading the peak torque figure.

"Pulling" the car and "accelerating" the car are the same thing. Torque is not a measure of work as you state, but is a measure of rotational force. If you want to measure the acceleration of a car, the torque value is exactly what you need to calculate it.

Force and acceleration are related by F=ma (remember Sir Isaac Newton ) If you want to know how fast the car will accelerate, you need to know the torque at the wheels, the diameter of the wheels (so you can convert rotational force into linear force) and the mass of the vehicle. That's it. Horsepower is nowhere in the equation.

But horsepower is important. Notice I said torque at the wheels and not at the engine? That's because there is gear reduction between the engine and the wheels. The effect of the gear reduction is to multiply the torque but reduce the rotational speed. While torque at the engine may only be 150 lb-ft, torque at the wheels will be as high as 2500 lb-ft due to the gear reduction. But it is possible to have too much gear reduction since it also reduces rotational speed. You could be geared so short that you are making ridiculous amounts of torque at the wheels, but the wheels are spinning so slowly that you are not going anywhere. Because of this, you can see that it is advantageous to have an engine which revs higher...if it revs higher, you can get away with more gear reduction, increasing your torque at the wheels. In a sense, you could say that revs are worth torque. This is where horsepower fits into the picture.

Horsepower is the relationship between torque and revs. HP = T *RPM / 5252. If you've ever noticed that engine dyno curves always have HP = Torque at 5252 RPMs, this is the reason. Although HP doesn't directly accelerate the vehicle, it is very useful in comparing two dissimilar engines. Take a high revving engine that makes 250 HP at 8500 RPM, and another low revving engine that makes 250 HP at 4000 RPM, and you'll find that they'll provide roughly equal acceleration given the cars they are in are properly geared, are of the same mass, and the shape of their power curves are similar. The horsepower takes into account the revs so you don't have to worry about how the torque multiplies through the gear box.

Anyway, that's how it works. I hope that helps shed a little light on what accelerates a car and how torque and hp fit into the picture.

 

RIGHT!! :D what he said.

well, he's talkin' about the big *** turbo deisels, which have redlines at 3k rpm, which is why you see them switch gears like a million times, with all those plumes of black smoke billowing as they mash the throttle after switchin' cogs. the first ones barely move it, so it can have some amount of momentum, and they just keep workin' their way through the box up to the gears that are massively overdriven for cruising on the freeways.

(only 99 more to go to a grand!! :D)

 

Three grand huh? Then the numbers don't make sense to me. Unless the power curve is really screwy, they either have to make a lot less torque than 2000 lb-ft, or a lot more HP than 200. I guess it's time for me to do a little google search :D

 

Ok, I found some info. The lowest power big rig engine I could find makes 315 HP and 1000 lb-ft of torque with a max engine speed of 2100 RPM. There are some engine models that make closer to 2000 lb-ft of torque, but their horsepower ratings are much higher...in the 500 to 600 HP range.

To keep this somewhat on topic...wouldn't it be nice to have 500 to 600 HP in an RX-8? :D

Or how about:

I wonder if anyone has ever made a wankel engine for commercial big rigs? :D