rpm vs. speed question

In article >, Lanny Chambers > wrote:

>I'd like to see some data that a taller top gear actually saves fuel,
>given the Miata's utter lack of aerodynamic efficiency. I suspect the
>only advantage would be a small reduction in noise. And not even that
>with the top down.

Probably hard to come up with the data unless someone is willing to
do a swap. But the physics backs up the expectation. The aero drag
stays the same, thus the same load on the wheels. But at lower rpm,
there will be less drivetrain losses, and less engine friction, thus
less power needed at the pistons. Plus at lower rpm at near the
same power, the throttle would be more wide open meaning less
pumping losses, and thus a more efficient engine. All would suggest
better fuel economy with taller gearing.

At 60, 70, or 80 mph, the Miata is still far from wide open
throttle. Lower rpm and more throttle would almost certainly lead
to better mpg.

In article >, "Dana Rohleder" > wrote:
>When I bought my
>2001 recently, I really wanted the AT, but couldn't locate one in decent
>shape because so few of them are available up here. I ended up with the 6
>speed because I wanted a LS, and so far have regretted it. Coupled with the
>clutch shudder that the previous owner never fixed under warranty, and the
>high cruising RPM which really surprized me, I thing I would have been
>better off with a 5 speed or the AT

Just FYI, the overall ratio of the 01's were such that the 6-speed
does rev a bit lower in top gear than the 5-speed (due to the FD of
3.909 vs 4.3). But I hear ya on the rest.

In article >,
(Dave) wrote:

> Plus at lower rpm at near the
> same power, the throttle would be more wide open meaning less
> pumping losses, and thus a more efficient engine.

That's assuming a flat torque curve; you know better than that. If the
taller top gear drops the tach off the cam, you'll be using a LOT more
throttle, at a lot lower efficiency. You'll be downshifting for hills,
too.

--
Lanny Chambers
'94C, St. Louis
http://www.hummingbirds.net/alignment.html

Well, only the 10AE has 3.909 : 1 in the rear. The NB8B 6 speeds have
3.636 : 1 in the rear. My 2003 LE 6 speed is 400 RPM lower at 60 MPH
than my '91 1.6 L was.
Please remember, RPM "IS" fuel comsumption !!!
I have spent over 5,000 hours behind large piston engines, and my job
was fuel management ! The lower the RPM's I could run and keep the air
speed that I wanted, the longer I could stay in the air. (yes, the
aero-drag was the same for any power setting !) Our Miata FI system
will only let the proper amount of fuel into the engine even at full
open. (*)

If you want gas milage, go drive in a flat freeway at 45 ot 55 MPH and
don't change the power setting. I am guessing not too many of you want
to drive that way,

(*) If you could control the RPM "and" the Manifold Pressure (boost),
you could do even better.


Bruce Bing '03 LE
(10,000 + hours of piston engine flying time)

In article >, Lanny Chambers > wrote:
>In article >,
> (Dave) wrote:
>
>> Plus at lower rpm at near the
>> same power, the throttle would be more wide open meaning less
>> pumping losses, and thus a more efficient engine.
>
>That's assuming a flat torque curve; you know better than that. If the
>taller top gear drops the tach off the cam, you'll be using a LOT more
>throttle, at a lot lower efficiency. You'll be downshifting for hills,
>too.

Thanks for the vote of confidence :-)

First, let's make it clear we are talking about small FE
differences. I think you'd see a 28 mpg car become a 29 or maybe 30
mpg.

Now, most brake specific fuel consumption maps for SI engines have
their minimum (most efficient) point at very high throttle - about
70-80%. And at lower than peak rpm. And it is significantly less
sensitive to rpm than throttle. I'm staring at a bunch of maps as I
type this. So a LOT more throttle is a GOOD thing, as far as fuel
efficiency is concerned.

To be sure, the Miata engine could be so peaky that you go too low
in the rpm and slip back into a low efficiency region, but I think
this is highly, highly unlikely. Most curves I've seen, even for
small high-revvers, don't have eff drop before <2000 rpm.

I'm not sure you are saying this, but I'm unsure of a good
correlation between torque curve and bsfc. Isn't the cam more about
volumetric efficiency than fuel efficiency? So yeah, as you go off
the VE peak (ie "off the cam") you can't get as much air in so the
max torque suffers. But that doesn't mean that at a given torque,
the fuel consumption suffers. Looking at a bunch of curves, yes the
peak eff is often close to the torque peak rpm (often
considerably lower). But again, usually far broader.

Last, I agree you'll downshift more. Goes back to the drivabilty
discussion. And I thought shifting was a feature, not a bug with the
Miata :-)

In article >, (BRUCE HASKIN) wrote:
>Well, only the 10AE has 3.909 : 1 in the rear. The NB8B 6 speeds have
>3.636 : 1 in the rear. My 2003 LE 6 speed is 400 RPM lower at 60 MPH
>than my '91 1.6 L was.

My '01brochure lists 3.909 for all 6-speed FD's.

>Our Miata FI system
>will only let the proper amount of fuel into the engine even at full
>open. (*)

It doesn't do throttle enrichment at WOT?

>If you want gas milage, go drive in a flat freeway at 45 ot 55 MPH and
>don't change the power setting. I am guessing not too many of you want
>to drive that way,

There's an experiment to prove it in! Go steady 40 mph in 4th.
Repeat in 5th. Then 6th. Repeat at 50 mph. Repeat at 60 mph.
Repeat at 70 mph. Plot results. Anyone game? Anyone? ...

I recently bought a ScanGauge < http://www.scangage.com/ > that actually
allows instantaneous RPM/MPG readings, among many other things. I currently
have it hooked up to my new Ridgeline because of poor fuel economy issues I
am having with it. But when/if I get a chance, I may be able to do some
testing on my 6 spd and come up with the plots you suggest. I'll post to the
group if I ever get around to doing it.

Boreal
2001 LS 6 speed Silver/tan



>>If you want gas milage, go drive in a flat freeway at 45 ot 55 MPH and
>>don't change the power setting. I am guessing not too many of you want
>>to drive that way,
>
> There's an experiment to prove it in! Go steady 40 mph in 4th.
> Repeat in 5th. Then 6th. Repeat at 50 mph. Repeat at 60 mph.
> Repeat at 70 mph. Plot results. Anyone game? Anyone? ...

Lanny Chambers > wrote:

>In article >,
> (Leon van Dommelen) wrote:
>
>> >6th is not a cruising gear in the Mazdaspeed Miata.
>>
>> In none of the Miatas. Which is just a waste since the highest gears are
>> useless anyway from a performance standpoint in a Miata.
>
>That depends on your concept of performance. When I was trying to keep
>up with the turbo crowd at the final Western Colorado Strafing Run,
>fifth came in pretty handy when I ran out of rpm in fourth. At 6000
>feet, a taller fifth would not have extended my speed past the limiter
>in fourth, nor pulled the slightest uphill grade. Same thing at track
>days.

So few (US) Miata owners run out of rpm in 4th except in rare circumstances.
And keeping up with turbos when you are running out of rpm in 4th seems
like an exercise in futility even in those rare circumstances. You are
spending most of your power on drag, having little left for acceleration;
they have plenty left.

>I'd like to see some data that a taller top gear actually saves fuel,
>given the Miata's utter lack of aerodynamic efficiency.

I do not have the data, but the claim that it has anything to do with
aerodynamic efficiency is ridiculous. It has to do with pumping losses.
The only way for an engine to be efficient when it is operating at less
than its maximum work per stroke is to shut down a few cylinders. Dragging
air past a valve to reduce its density by a significant percentage is
a major loss of power.

> I suspect the
>only advantage would be a small reduction in noise. And not even that
>with the top down.

I suspect you have no clue.

Leon
--
Leon van Dommelen Bess, the Miata Bozo, the Miata
http://www.dommelen.net/miata
The only thing better than a white Miata is two white Miatas

"BRUCE HASKIN" > wrote in message
...

> Please remember, RPM "IS" fuel comsumption !!!


No it's not!

Imagine you are in your Miata, doing 60Mph in 5th, on a flat road, throttle
at say 60%, cruise control engaged. You change upon a slight hill, whereby
you remain in 5th, but the CC increases throttle to 95%. RPM remains
constant. Are you saying that fuel consumption would not increase?

No ! That is not what I said. Go back and read the part about a "flat
freeway" !!

Bruce Bing '03 LE