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#11
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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. |
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#12
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rpm vs. speed question
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. |
#14
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rpm vs. speed question
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) |
#15
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rpm vs. speed question
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 :-) |
#16
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rpm vs. speed question
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#17
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rpm vs. speed question
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? ... |
#18
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rpm vs. speed question
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 |
#19
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rpm vs. speed question
"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? |
#20
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rpm vs. speed question
No ! That is not what I said. Go back and read the part about a "flat
freeway" !! Bruce Bing '03 LE |
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