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Old 01-13-2010, 11:12 AM   #1
sen7inel
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Default cruising rpm and gas mileage question

I just test drove a 2010 Legacy CVT and noticed that it happily (and quietly) does ~ 1900 rpm at 65mph while cruising steadily. My 07 MT WRX is at about 600 rpm more at that speed. Seeing that the CVT Legacy gets better rated highway mileage than the much smaller Impreza, one would surmise that the short 5th gear is at least partially responsible for this deficiency. I realize this is not an apples to apples comparison, but it's difficult to deny this correlation. Is there a reason why the 5th gear is so short? The Legacy proves that even the N/A motor has enough grunt to operate at much lower rpm at highway speeds. Help me understand.
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Old 01-13-2010, 11:40 AM   #2
FuJi K
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It doesn't take much throttle for the car to keep a steady speed on a flat hiway. Gear you have 5 gears and to make it have a lower rpm in 5th, you need to widen the gap from the lower gears. This does mean...longer 1st gear if you want, and longer 2-3-4 depending. Sometimes gearing is made just for the daily driver and it looks ugly to the performance driver.

The quick you get up to speed, the less you are on the throttle the rest of the way to your destination.
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Old 01-13-2010, 11:28 PM   #3
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its a CVT, it won't maintain 1900 rpm in all load conditions. it is entirely possible to end up running more rpm going up a decent hill than a regular 5mt would. So no, a regular low compression low displacement engine has nowhere near enough power to cruise at 1900 at highway speeds without a CVT to help it.
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Old 01-15-2010, 09:58 AM   #4
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I wouldn't believe it if I hadn't experienced it myself. Granted, the test drive was on a mostly flat piece of highway, and I agree that the rpm would change with increased load. But why then does the MT Legacy give up 4mpg highway? What else other than the transmissions could be responsible for the difference? 14% is significant. According to Edmunds, the CVT is over 100lbs heavier.
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Old 01-15-2010, 01:26 PM   #5
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There is a lot more to it than JUST the transmission.... You have to look at everything as a whole. The sum is almost always greater than its parts...

I highly doubt that 100lbs heavier is due to heavier rotational mass inside the transmission. Manuals and Automatics have many more gears to spin compared with a CVT. Less spinning mass in the drive train means less drive train loss... that 100 lbs my be due to increased case reinforcement or more electronics and pumps, but the rotating mass may be less. Who knows...

Beyond that a 5mt has SET gear ratios, where the engine has to spin X rpm to keep at Y speed regardless of load... CVT does not, engine can run a A through D rpm to keep Y speed. It will run as slow as it possibly can and produce as little power as is required to maintain speed, while the 5mt has to spin at a set RPM to keep the speed.

More RPM = more air flow = more fuel flow.

Its how the engine interacts WITH the transmission, NOT the transmission alone...
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Old 01-15-2010, 06:54 PM   #6
FuJi K
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do a real test on the street...thats where you'll get REAL results.
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Old 01-16-2010, 01:49 PM   #7
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Quote:
Originally Posted by rkramer View Post
So no, a regular low compression low displacement engine has nowhere near enough power to cruise at 1900 at highway speeds without a CVT to help it.
Yes it does. If the 5th gear were set to 1900rpm @ 75mph, and not 3000 rpm, your acceleration would be sluggish, but for fuel economy it would be great. The engine IS more efficient as it gets closer to WOT, so at the exact same speed, full throttle at 1900rpm will yield better economy than partial throttle at 3000 rpm.

My former car was an 88hp escort - exactly half of my OBS. Yet it had the very same rpm on 5th gear than the impreza, and I had no trouble on the freeway.
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Old 01-16-2010, 09:55 PM   #8
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In a manual, they compromise a bit on 5th gear to allow you some acceleration in gear, otherwise you'd be forced to downshift up almost every hill and to pass any car or accelerate. This compromise is a slightly shorter gear and perhaps 1-2mpg, but it makes the manual car infinitely more driveable.
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Old 01-16-2010, 10:04 PM   #9
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Default Re: cruising rpm and gas mileage question

VVT-i probably seals the deal with that cvt trans.


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Old 01-16-2010, 10:09 PM   #10
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Default Re: cruising rpm and gas mileage question

What I mean is that the variable valve timing can shift the working power band to the lower rpms, which may explain some efficiency while maintaining ample power.


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Old 01-17-2010, 08:45 AM   #11
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Quote:
Originally Posted by Bluefoton View Post
Yes it does. If the 5th gear were set to 1900rpm @ 75mph, and not 3000 rpm, your acceleration would be sluggish, but for fuel economy it would be great. The engine IS more efficient as it gets closer to WOT, so at the exact same speed, full throttle at 1900rpm will yield better economy than partial throttle at 3000 rpm.

My former car was an 88hp escort - exactly half of my OBS. Yet it had the very same rpm on 5th gear than the impreza, and I had no trouble on the freeway.
This is correct, though confusing when you take into the effects of forced induction. For forced induction, you want the maximum throttle opening without entering open-loop fueling. There is a flip side, that head design is optimized for performance at specifc RPM's, but the benefit of that doesn't really overcome the pumping losses associated with partial throttle.

You can see this directly if you have a scan-gauge, or some other method of monitoring engine vitals.

Put it in, I don't know, 3rd gear at 50mph and maintain speed (use cruise control). Monitor injector pulse width. Injectors are on/off, so the shorter the pulse width, the greater the fuel delivery, the worse the fuel economy. Now, shift to 5th and watch the same thing. Cruise will have the throttle open more, and I bet you'll see a longer IPW. For Turbo cars, this is a little tough because you CAN get into open loop on cruise control, but below that limit, you'll see that it holds true.

As for an N/A having the power to turn 1900RPM at 60mph, well if you remember High School physics, you can calculate power required to overcome rolling resistance and aero drag, and you'll find that for an Impreza it is roughly 6hp.

There are a few things to note before getting unruly about this. The first is that this ONLY accounts for overcoming aero and rolling losses. It does NOT factor in things like wind, varying air temperatures, road surface conditions, tire life, status or temperature, rotational mass, drivetrain loss, and things of that nature.

This is the bare minimum net horsepower required to move an impreza at 60mph. In reality, I would imagine the actual power required is somewhere around 6 to 8 times that. Things like drivetrain loss and road surface condition are not inconsequential when considering moving a vehicle. But, a naturally aspirated Subaru should have no great difficulty maintaining 60mph at 1900RPM.

If you want to do a more effective test of power required to maintain speed, you can do a drag-down test where you find very flat ground and coast down from 60mph (or some other high speed) and note the time it takes for every 5mph drop in vehicle speed. From that you can calculate with a moderate degree of accuracy the power required to maintain speed. I imagine, again, that the power required to maintain 60mph in a not-super-areo-friendly Impreza is around 40 to 50hp.

If anyone wants to actually do a drag down test, I can provide the "standard" procedure.

Phil
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Old 01-17-2010, 06:53 PM   #12
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^ Thanks, Phil! I'll actually do the test at a few different speeds and report back.

My original post came in from a graph I saw - and that I try to find in vain - , which depicted the generated HP per fuel flow for different throttle openings. The best ratio was always at WOT. It kind of makes sense - say you need (arbitrary values) 40 whp. For that you can either be at 1) 2000 rpm at WOT or 4000 rpm at partial throttle. Given that all the rotational energy losses (per time) increase with the rpm, the overall efficiency is higher at low rpm. I don't know though about combustion efficiency for different A/F ratios and vacuum values. As you said, it's valid for NA engines.

Eyedoc is right too. With my 88hp escort, 5th gear 75mph at 3000rpm was very fuel friendly, but any overtake required downshifting. My NA, twice as powerful (don't laugh, FI owners ) requires barely any downshifting on a freeway.

Last comment - and here I have no clue - for the sake of engine maintenace, longevity, induced stress, etc. What's better: low throttle at 3000 rpm, or WOT at 1500 rpm?

Thanks,
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