94 1.8 rev limiter.

The ECU cuts the injectors at 7200 rpm. It may appear to vary from one Miata to the next, but that's because the tach isn't especially accurate. 7000 is a good shift point.

Was that the same for the 1.6? What Mal describes is exactly the way my 92

1.6 was. My 99 1.8 cuts out almost exactly at an indicated 7000, just as it is beginning to really 'SING'............Now even I am thinking about how to bump the rev limiter up a bit ;-)

Chris

99BBB

Before you do that, you might want to look at a power curve and see if there's actually any point...

I think 7000 is about the optimal point to change on my '94. A 7200 limiter would allow a little slack, but 7000 would be a pain.

But it's not one of those things that *requires* thought. Simply find a power curve for the Miata and *see* if that is the correct shift point.

Alan, I think there's a bit more to it than that but my butt dyno tells me the engine is still pulling strong and the sound is really coming into it's own right about there. In my old 92 I'd almost never hit the rev limiter (indicated 7200) because the engine felt like it had given all she had by an indicated 7000 but, in the 99, I hit it fairly often as it feels like it wants to keep winding out.

Chris

99BBB

Actually, there is almost nothing more to it.

Maximizing acceleration consists almost entirely of maximizing the area under the horsepower curve.

And I'm not saying that the rev limit *itself* constitutes a measure of where that happens; your feelings may very well be correct. But looking at a horsepower curve will tell you that, too.

Not quite; I think the torque curve is more relevant. Torque multiplication via gearing is a factor as well, and the optimum shift point may be different in each gear. The goal is to shift at the point the next gear will provide better acceleration. If you plot rpm versus acceleration in each gear, it's where the curve for one gear crosses that of the next. Practically speaking, many cars' gear spacing and redlines will not permit reaching the crossing point, and so shifting at redline is the best one can do.

No. Torque curve is *not* more relevant.

Horsepower is torque multiplied by rpm (multiplied by a constant, of course) and hence torque at the rear wheels, is horsepower divided by RPM at the rear wheels. Are you with me so far? So if you want maximum torque at the rear wheels, at any given moment you must be in the gear that delivers maximum horsepower, *not* maximum torque.

Well, for the 2006, I see no indication that you would want to shift at

7,000 if there was no rev limiter.

Leon

Actually, here is a better one

They think the difference between the curves is inertial, but I would think it should be an effect of mixture tuning?

Anyway, clearly silly to shift at 7000.

Leon

Does not look like you want to shift at 7000.

Leon

That is true. However, maximizing area below the horsepower curve does not have any meaning that I know off. To compare engines that are to operate on the same car, by approximation the best engine has the largest area below the torque curve in the used rpm range.

Leon

Your conclusion does not follow your statement. Torque can be changed by gearing, horsepower cannot. As you stated there is an rpm quotient in horsepower, so horsepower out of a gear box will always equal horsepower in (less the inefficiencies) no matter what gear is used. For example a 2:1 reduction will increase torque by a factor of 2 but also reduce rpm by that factor, so horsepower remains the same.

But that's just saying the engine with more horsepower, isn't it? At any given RPM, if you have more torque, you have more horsepower.

But horsepower combines the effect of engine torque plus the ability to multiply it by gearing ratios to achieve the maximum torque at the rear wheels.

Do a thought experiment. Imagine the engine in your Miata could not only operate in its normal range, but also in a range of three times normal. For some strange reason, it doesn't blow up and although it breathes worse, it still produces half the torque that it does in the conventional range.

So...

In which range to you want your gearbox set to allow you to use?

Go away and do some research. You'll see I'm right.

Alan,

In theory you are right.

In a practical application, you want to output the maximum average horsepower as you shift through the gears. The factory transmission and rear-end gear ratios may not be designed to give maximum accelaration. Allowing the engine to run past torque peak before shifting often produces higher average horsepower from the next gear's operating range. Take two stock 94 Miatas to the drag strip. Instruct one driver to shift at torque peak in each gear. Instruct the other driver to shift at 7000 rpm. There is no doubt which Miata will win.

"lgadbois" wrote in news:IFpcg.522$ snipped-for-privacy@newsread1.news.pas.earthlink.net:

The white one, of course...

No. There is a difference between peak horsepower, as reported, and area below the torque curve. Area below the torque curve is a valid measure for engine design and improvements. Unfortunately, popular books like Norm Garrett III make blanket statements about this which apparently fool a lot of people thinking that they somehow have to maximize area below the torque curve, or just torque, while *driving* instead of *designing* engines. There have been many discussions like this here, with countless people arguing that torque should be maximized instead of hp. Many painting very graphical pictures of what torque is versus hp, but showing little knowledge of dynamics and conservation of energy.

True but irrelevant. The point would be relevant if you had more torque at *every* rpm, but that is not necessarily how things work.

True, but still irrelevant.

You are missing the point. The area under the torque curve has to do with you not having full control over where you can operate. Unless you have CVT, you have to operate over ranges of rpm, even if you correctly select the gear that operates at the largest possible hp at each rpm. While in such a range, if you replace your engine with one that has a lower peak hp but more area under the torque curve, you will accelerate faster.

Of course, the swap has to be done very quickly.

Leon

Correct.

Correct, but it is the *time-average* maximum horsepower. Which is equivalent to saying that you must at each *instant* use the gear that gives the highest hp at that rpm.

Which is exactly what Alan said.

Actually, the only thing is losses. This is a secondary effect that will slightly favor the higher gear if the higher-gear-rpm hp and the lower-gear- rpm hp are the same. Let's not worry about those small unpredictable effects. And the rear-end ratio is irrelevant regardless of losses.

True. But note that what Alan is saying is equivalent to shift after *peak horsepower*. He is in no way telling you to shift at peak torque, which would be ludicrous. Peak horsepower is after peak torque.

No. Shifting at peak torque is ludicrous. However, shifting, (after peak hp), at the time that the *hp* at the rpm you are shifting into has become equal to the *hp* at the rpm you are shifting out of is exactly what you need to do for best performance, and that is what Alan told you to do. And I myself over the many years in this group. Let me join Alan in sighing. :)

Leon