cruzing at 130 kmh is a lofty 4000 rpm
any other ratios out there ?

 

It's only "lofty" for a boinger. For a Mazda Wankel, it's just off idle.

Piston engines are best-operated in their near-zero rev range. Wankels are best-operated near red line. That's why they rock.

I thought 130 Km's was car-seizing illegal up there?

 

only if it's on a public road... on the highway (while it's not right) it's pretty much the cruising speed at some points along the 401 (the actual limit is 100).

The impound point comes at +50km over the speed limit.

 

Instead of rear end, how about a different 6th Gear?

 

I think that people have the misconception that reving high means poor fuel economy. There's a post almost every few weeks on the Mx-3 boards because we're reving at 4000 @ 120km/h. Fuel economy is the total of many factors, beside for the given rpms, it's also about engine load/fuel consumed, aerodynamics & weight and the torque required to sustain that given speed while pusing against the air. These aren't torque monsters and I'm sure the manufacturers have done the math to figure out the optimal cruising speed. Kinda the reason why a v10 Viper can get 30mpg, because it's making so much low end torque that it's basically idling down the highway...

I find it hard to explain, but look at is this way, by putting in a longer gear/rear end, you could and would drop the rpm's you're revving on the highway, but when you do that, you also have less torque because there's less multiplication, so if there's less torque making it to the rear wheels, you need to push more on the throttle to make up the difference, more throttle means more air, and with more air means more fuel.

A good example is my buddy when he had a k20 Civic for a while. He would merge on the highway in 6th gear to conserve fuel, but press the throttle all the way to the end; so even though his revs are at their lowest, he's also at WOT. I recommended to use the lower gears, you can sometimes skip a few but find the balance of throttle input and let the revs do the work rather than raw power. A good way to learn about engine load which is directly relevant to fuel economy would be to get a vacuum gauge.

 

I find the car gets better fuel economy on country roads where the speed is 80-90km/h even with stops/starts at the odd intersection, than on the major highways where the speed is higher and more constant. (at speed limits or with minor speeding) Anyone else notice that? I think it would have made sense for Mazda to make a lower 6th gear. I think the reason they didn't is to preserve a sense of decent performance without downshifting. The car doesn't pull in 6th at 2k RPM and the average person doesn't use more than 1/3rd throttle in any situation, and doesn't use the available revs appropriately.

 

Last weekend, trip to Roebling, played with Cobb fuel economy. At about 70 mph, it was registering 25 to 26 over 10 mile increments. At 72 it was down to 24 or so over 10 miles. At 75 it was down further to 22, again over 10 miles. This, to me, is a pretty severe roll-off. This is also about the time that closed loop turns into open loop. I have an MM tune coming that might improve open look mileage, but best I can tell is that the faster the engine turns, the faster it burns fuel. I cannot image drag increasing that fast.

But, there as to be smarter people out there. Thoughts?

 

the whole point of a transmisson with overdrive is to reach your cruising speed then shift
to the final gear (over drive) to maintain it at a lower rpm... saving wear and fuel.

the ratio is calculated using weight, cd, available torque and the nationaly maxumum
posted speed limit lol
in the early days you would reach top speed in the second last gear then shift to overdrive
to maintain it.. usually not being able to accelerate any more.

the current more politicaly correct ratios are heavily influenced by the speed limit.

 

So, the goal is to match the most efficient part of the plot ( http://en.wikipedia.org/wiki/Brake_s...el_consumption ) with the speed you want to drive. Right?

There is a really old rotary plot at http://www.rotaryeng.net/RX8-TC-eng3.html that points to a sweet spot around 3.4K, but it is really old, and not for this engine.

If the torque peak is where you want to be, in our case, that is up in the 5K to 6K range, and we are screwed, right? If the plot is close to right, we are in the best spot already, and we are screwed, right?

Option N is to make the most of what we inefficiently burn, and that may be with fewer revolutions per mile, and that is gears and tire height, but that will only do so much, maybe get the sweet spot, assuming the old plot is even remotely relevant, up around 80 mph.

Then again, all cars get poor mileage over 100 so we fit right in....

 

And, to close out any happy thought, here is almost no way to get 3 to 5 mpg to pay for the cost of new 6th gear or rear gear within the lifetime of the car, all we would do is slow it down...

 

has anybody got a hp/torque graph ?

 

Please correct me if I am wrong but above 70mph the RPM'sare getting into the 4000 range and second set of injectors are starting to dump fuel. MM was discussing the fueling maps as far as injector staging awhile back and at 3200rpm the secondaries are on.
This is a good reason for the fuel econ. to drop after say 65.

 

Are you trying to change the speed at which you shift or trying for a higher top end?

 

Thanks, I am presently hurting at that speed and almost ran out coming back from Roebling (280 mile trip) at 80. It was not an issue going up at 70-75. I am also due for a MM AP tune (in the queue). I expect that will make open loop more efficient as well as more powerful.