Why is 6th gear so low (2006 Sport)

Reply to
Chuck
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highway because in 6th gear,

Reply to
Chuck

I admit no such thing.

If you want to believe that a car going 60 MPH will get better MPG at

5000 RPM than at 2500 then feel free to drive 60 MPH in third gear. You'll waste resources, generate a lot of useless noise and accomplish nothing.

Just like your posts.

Reply to
Natman

Fact: it takes a certain amount of horsepower to drive a car at 60 mph.

Fact: given a certain engine efficiency it takes the same air and fuel to do that at any RPM with the exception of frictional losses in the engine.

Fact: engine efficiency is at a maximum at approx. peak torque RPM.

All you have to do is show that frictional losses are sufficiently less at 2500 to make up for the loss in engine efficiency.

Ball's in your court....

Reply to
Alan Baker

True, but actually very little compared to the amount of HP to get it up to

60mph. Just enough to overcome air and mechanical friction.

Wrong. It will take the same amount of fuel per revolution, EVERY revolution. If it takes X amount of fuel to make one revolution, it will take 1000X at 1000rpm over a minute's time - 2000X at 2000rpm over a minute's time. The amount of fuel and air used/minute will increase with the number of engine revs /minute (RPM). That's why we don't drive around at highway speeds in lower gears - at any given speed on a level highway, the higher the rpms, the higher the fuel consumption. Every revolution of the engine uses fuel. If I use X amount of fuel to drive 1 mile at running at

2000rpm, I will use roughly twice as much fuel (2X) over the same mile at 4000rpm (because the engine will have fired twice as many times burning twice as much fuel/air), and I will get through the mile twice as fast.

Depends how you define efficiency. This is untrue for fuel efficiency. Thought experiment: transmission in neutral - will you run out of gas sooner at idle or at 6000 rpm??

Wha??

Reply to
Dana Rohleder

That's only true if the intake manifold pressure and throttle position is the same in all cases (and hence the mass flow rate is the same). At a given rpm, the amount of fuel changes depending on throttle position, atmospheric pressure, and air temperature.

Reply to
Grant Edwards

I've been trying to find real test data, and haven't found much other than this:

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Reply to
Grant Edwards

Not really germane.

Once again, not germane. You can't run at a given speed with the throttle closed.

Yeah.... ...that's what I thought.

Reply to
Alan Baker

Well Bud, get out from behind your desk, get in your car and see for yourself. Since logic isn't your strong point, you'll have to learn it by trial and error. If you drive at 5000k all day, your gasoline budget will soon overtake your internet access budget.

Reply to
Dana Rohleder

I believe it was to maximize the top end speed, get the motor up in the power band to achieve the 130 mph top end speed. With an aftermarket exhaust ours will do 134, on the level, top down. Coming down out of the mountains in New Mexico, 148, at which point the rev limiter sets off and scares the bejesus out of you.

There is only a slight difference in final ratio between the 5 and 6 speed manuals btw. The 6AT on the other hand has a much lower final drive with lower rpms at a given speed. Surprisingly, it gets worse gas mileage.

Driving 60 mph on the highway, you should be seeing somewhere around 32 mpg.

Jim

2006 MX-5 Sport
Reply to
George Jetson

Even by approximation, not true in terms of fuel consumption at partial throttle.

Untrue. Lower rpm will increase throttle.

You can, if you have any practical knowledge about how engines work.

Untrue at equal power.

Untrue.

Mysterious, is it not?

Untrue. There are many factors. But the primary one in the cited examples is pumping losses, the friction in pulling the air past the partially closed throttle.

Stop pulling peoples legs and go do something useful.

Leon

Reply to
Leon van Dommelen

Not sure if this is the same person but I remember someone in one of the car groups that really doesn't understand this subject. For the others, one of the fallacies in the argument is that the efficiency is not constant at a given RPM. It also depends on the throttle position.

Less than wide-open there are losses due to the air restriction (pumping losses). At full throttle there might also be losses in efficiency due to the engine management chosing maximum power instead of maximum efficiency (I think in modern (catalysed) engines this no longer applies, but in old times the mixture would probably be richer).

The other fallacy is that cylinder "filling" is not constant either. At some RPMs it might exceed 100% (due to cleverly using the inertia of the air in the intake) and near the maximum RPM it will drop. So, more torque doesn't necessarily mean more efficiency. It might just mean that engine is managing to burn more fuel in each cycle.

For instance, at 120 km/h the car will be using about 30 HP (~22 kW).

The current 2.0l engine in the Miata has the following specs: Max Power [kW (CV)/rpm]: 118 (160)/6700 Max Torque: (Nm/rpm) 188 / 5000

According to the original poster, the gearing in his Miata is about 3000 RPM for about 1000 km/h. So at 120 km/h, it would be 3600. At that RPM, the engine is capable of producing a bit less than 70 kW (W = 2 * pi *

3600 / 60 * Nm). So, less than 1/3 throttle (ok, it is probably not linear, but you get the idea) is enough to keep at that speed. If the gearing was twice as long (1800 RPM at the same speed), it would still be enough (~35 kW) with some reserve power (~2/3 throttle).

It is quite likely that less engine speed and more throttle would reduce the fuel consumption.

Anyway, these days you don't need to think about the theory. You just need to drive a car with a display of instant fuel consumption (most recent cars have that. Not sure about the Miata). Keeping the same speed, try 5th and 6th gear and see which gives better fuel consumption. I have tried that and there is no doubt, even at very slow speeds.

BTW, the MX-5 2.0 top speed is 210 km/h. 210 km/h / 6700 = 31.34 km/h at 1000 RPM. So, 3000 RPM = 94 km/h ~60 mph. So the gearing is well chosen to get to the top speed at about max power, which is ideal for a sports car (although in the real world a cruising gear is useful).

BTW2, even yesterday I was reading a magazine (French Sport-Auto, some issue of 2006) and they commented about the MX-5 that the lower gears were a bit long and very close, requiring a lot of gear changes.

Reply to
Rui Pedro Mendes Salgueiro

He was just being difficult.

My aftermarket ECU enriches the mixture at high power and I would think the OEM ECU would do the same. Increases power and reduces knock.

Probably really displays the amount of fuel the ECU thinks it is telling the injectors to spray.

No. It is supposed to be a sports car, not a econobox. You are supposed to worry about trailing throttle oversteer, not fuel consumption.

It is supposed to be a sports car, not a plushy transportation device. Do they like BMW's?

Leon

Reply to
Leon van Dommelen

Leon van Dommelen wrote: >

My intuition was always that the lower RPMs would translate into higher fuel economy. I could be wrong, though; there's one way to be sure.

I drive from Tampa to Gainesville or vice-versa pretty often. I could fill up at the first exit out of town, drive via Interstate 75 at, say, an indicated speed of 75 MPH, and take the last exit before the destination where I'd fill up again. According to my tach and speedo 75 MPH translates into about 3900 RPM in fifth and 4800 RPM in fourth. By doing this fixed-distance, fixed-speed trip a few times in fourth gear and a few times in fifth gear, I could get a statistically-significant comparison of the mileage in real-world conditions. I don't think a hour and a half at 4800 RPM will hurt a Miata engine.

A pretty boring experiment, but now you've got me curious, so I'll post the results in a few months when I've got them. I won't be able to stand doing it both ways on a round trip, though, because I-75 is so darn boring. If I could stand that I'd drive a Camry. On the way back I'll have to take the much, much nicer scenic route.

Yours WDK - snipped-for-privacy@ij.net

Reply to
W. Kiernan

Often, yes, but not always. It depends on the situation, and generalizations aren't necessarily predictive. The trip computer in my Mazda3 shows better instantaneous mileage (4 mpg better in one case) downshifting from fifth to fourth for steep hills below a certain speed. Variables probably include rpm, load, torque curve, and throttle opening.

Reply to
Lanny Chambers

Ok, but out of curiosity: is it lean at partial throttle and stoichiometric at full throttle or does it go to a rich mixture ?

Because if it is richer than stoichiometric, then there will be significant amounts of carbon monoxyde and unburnt hidrocarbons on the exhaust. The catalyser can't get rid of it without some extra oxygen.

Is that still allowed ? Maybe it is, since pollution tests (at least in the anual inspection. Homologation is probably different) are not done at full throttle.

Yes, the value will not be 100% exact, but it should be comparable for the same car in several situations.

In that case, should it not have a powerful engine ? (just kidding, see below :)

In my car (VW group, previous generation onboard computer) the display shows only one info at each time, and you can select the one you want. You could select the average speed, for instance. OTOH, the information about fuel consumption can be useful. For instance the estimated range before you need to refuel.

I should correct/clarify this. The very slow speeds don't include when in 6th the engine would be below idle speed, which means that in my car in 50 km/h zones 5th gear is more appropriate than 6th.

June 2006 issue.

I misremembered a bit:

- the car they tested had the 5-speed gearbox, not the 6-speed, so the comment about the gearing should be about the 5-speed.

- what they wrote exactly was (translated): "Its very long gearing (101 km/h in 2nd, 144 km/h in 3rd) forces one to play with the precision handling, short travel, small "joystick". If one must !"

Rereading it, I think they meant it as a positive comment.

Again, in that case should it not have a better engine ? :) But in fact this article is quite positive about the engine. Although on paper the power is low (only 160 HP, from a 2.0l, that would have been nice some 20 years ago), the low weight means good performance, and in fact they got better values than the official ones.

In general yes, but they have some recurrent critiques. In this case, for the same article they tested a Z4 3.0si (but this was not a comparative. The two cars are not in the same market segment).

About the BMW Z4 3.0si, they wrote the following:

good points: - great engine - great handling - nice gearbox (although 6th gear is a bit long)

bad points: - electric power steering instead of the hydraulic power steering - no limited slip differential - brakes that fade much too fast - run flat tyres are unconfortable

BMW seem to have some fixed ideas about some subjects (it doesn't seem to be only about cost), with which French journalists disagree.

The open diff is one of those. The other are the brakes. BMW thinks that a single piston sliding caliper is enough, even for the 500 HP M5/M6, and replies to the journalists that the only people complaining about that are French journalists, not costumers. Mercedes uses 4-piston calipers in the same type of cars.

Reply to
Rui Pedro Mendes Salgueiro

Definitely rich. 1:12.. IIRC.

Well there is definitely a lot of soot on and around the exhaust. Though I am unsure whether it is oil or partially burnt fuel. ;)

I allow it. :)

Well, the new Miata has. Actually, it has been inching up for quite a long time.

It would be nice if you are running low, but how about the accuracy of the sensor that says how much is left?

Sounds reasonable, given the title of the magazine.

That is true. BMW does not get its money from sports car enthusiasts and knows it.

Frankly, I think of a BMW as a comfort car that has a sporty, expensive image, but that may be just me.

Leon

Reply to
Leon van Dommelen

It is low because it is a sports car. You pay for torque and power and a fancy convertible, form 5,000 to 10,000 dollars more than other small cars. So why worry about gas mileage.

highway because in 6th gear,

Reply to
Robert E. Ford

Mark, you're going to hear this "it's a sports car" and ""if you wanted gas mileage you should have bought a corolla" crap all day long from some people. They simply can't admit that there's anything about their precious miata that could be better. I think that's a bunch of crap to be honest. I've long held (here) that all miatas would be better cars with a taller top gear. It's simply my opinion and I believe given the chance to play with the gear ratios, I could convince allot of other people. They might even enjoy the quieter cruising and the added fuel mileage (heaven forbid). Mazda's had their collective mind made up on this forever and I guess not enough people have the opinion or not enough have voiced their opinion that this would be a welcome change.

On we go, cruising at 4k and up :-) Chris

99BBB

Reply to
Chris D'Agnolo

I think it should be taller, too.

Reply to
Dana Rohleder

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