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Old 01-24-2010, 10:27 PM   #1
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Proper RPM

What would be the proper RPM for a typical 2 Liter (approximately) four cylinder for the following condtions, at near sea level:

A. minimal power (eg. simply maintaing speed on level ground, under 35 mph ).

B. light power (eg. easy moderate or slight acceleration on level ground while going under 35 mph, or maintaing speed at 45 mph on level ground)

C. normal power (eg. accelerating in traffic or having penty of standby power to do so while in traffic)


I think I tend to lug my engine (don't give it enough rpm's):

A. 1300 to 1800 RPM
B. 1600 to 2200 RPM
C. 2400 to 3400 RPM

(maximal power, above 3500 and above is reserved only for emergencies or when feeling nutty).

I've heard that in some people's opinions, there is not much of a point in running the RPM's too low, because hydrodynamic lubrication in the engine breaks down more often at lower RPM's during WOT, leading to more friction.

My view though, less spinning, less distance travelled by pistons, and thus less work done by friction. Plus, mainly, I want to avoid engine operating at minimal load under higher rpms.

My particular car, is an older subie imp 5 speed, with a small 1.8 liter H4 engine, that is slightly oversquare and gets less torque at lower rpms than a longer stroke engine would.
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Old 01-24-2010, 11:08 PM   #2
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Your rpm ranges fit my experience over the years. My current 1,5L and 2.5L engines bracket the 2.0 L range, but I've owned five other cars with 2.0L engines.

35 mph best rpm varies with transmission and final drive ratio gearing. Generally, the lowest you can go without lugging is your best rpm. 1500 rpm hits the bottom end of most 2.0L engines' Brake Specific Fuel Consumption curves.

Best rpm for light acceleration is 1500-2200 rpm, again using the BSFC for best power/economy.

Your heavy acceleration numbers are probably spot-on, too, but I never go there if I can help it. You're tossing away fuel by going to those higher rpms.

My goal is to avoid running at higher rpms as much as possible. I'll accelerate to 60 or 65 mph, put the engine in neutral (750-800 rpm), and coast back down to 50 or 55 mph before accelerating again.

At 50 mph and 2500 rpm, I burn about 1 gph fuel, and get about 50 mpg. At 70 mph and 4000 rpm, I burn over 2 gph, and get about 32 mpg (in my xB).
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Old 01-25-2010, 12:26 AM   #3
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Very interesting Darrell. I had to look up BSFC, and found this good article explaing the concept and relevant info. http://autospeed.com/cms/title_Brake...6/article.html They also really say it's horrible efficiency to rev high under very low load-- you're basically using your engine as a vaccum pump at that point, something that happens to be good for only two things I can think of: 1. standby power, and 2., as the article points out, getting rid of crank cases gases and fuel through the EGR.

BTW, does the scion rev that high? (or is this 4th gear at 70?)
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Old 01-25-2010, 07:00 AM   #4
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You're probably not lugging at all, no matter how low your RPM. Engines with computerized ignition and a knock sensor can't lug. They run or they stall. There's a link in my sig about it.

In my experience with my 2008 VW, there is no limit to the fuel economy benefits of lower RPM. The lower RPM I'm willing to drive, the better my fuel economy. Unfortunately, the car is geared for people who don't agree with me...its highest gear is still very low, so once I'm in 5th gear I no longer have a choice about my RPM.
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Old 01-25-2010, 01:08 PM   #5
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eee, that was a typo.

My Scion is geared low, but not that low. It turns 3000 rpm to go 60 mph, which is 3500 rpm at 70 mph, and 4000 rpm at 80 mph. The fuel consumption figures are correct. It does drink ~2 gph at 3500 rpm (70 mph) v. 1 gph at 2500 rpm (50 mph).

You want to keep your engine in the red area of the BSFC graph, so at light load, it's centered around 2000 rpm, which is why I accelerate from 1500-2200 rpm.
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Old 01-25-2010, 02:01 PM   #6
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My scoobie ran at about the same RPM's as your scion, maybe just a tad lower. It now runs a little lower even after i replaced the set of tires with approx 5% larger 185 r14 75 or some such. it now revs more like 3200 rather than 3400ish at 70 mph. i have to adjust for the larger circumference in reading my speedometer, as well as in mpg calculations; I think it helped a little to reduce the hwy mpgs but not a vast amount).

While i pretty much observed my car takes about 2 gph on the freeway, it kind of shocked me how little fuel the engine sips cruising at 50, I would've guessed a fair bit higher than 1 gpm.

Being able to cruise at 50 mph is one big advantage of taking country roads over freeway, but getting there at roughly 7/10ths the time often seems worth driving the 70 mph, even at twice the fuel rate, or the 56% extra trip fuel expense. On freeways driving 50 is not an option here (many are just 2 lane freeways, and you'd be leaving a trail of havoc).


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Originally Posted by SentraSE-R View Post
eee, that was a typo.

My Scion is geared low, but not that low. It turns 3000 rpm to go 60 mph, which is 3500 rpm at 70 mph, and 4000 rpm at 80 mph. The fuel consumption figures are correct. It does drink ~2 gph at 3500 rpm (70 mph) v. 1 gph at 2500 rpm (50 mph).

You want to keep your engine in the red area of the BSFC graph, so at light load, it's centered around 2000 rpm, which is why I accelerate from 1500-2200 rpm.
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Old 01-26-2010, 06:32 PM   #7
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nevermind the whole bit about manifolds and cams changeing the torque curve... every engine will be different depending largely on those.

my (1987 S10) 2.5 L4 idles at 950 and drives from about 500 in 1st, 600 2nd, 800 3rd and 1200 4th (and final). it'll carry loads uphill in 4th at 1500. compare that to my toyota (1990 cressida) 3l L6 which idles at 650, 1-3 can drive from there, 4th 1k, 5th 1300 but have less acceleration than the truck despite shorter gearing. you can feel both of them start to breathe well in the mid 2k range but the truck falls off over 4k whereas the cressy goes strong to 6. drove a 2k2 celica GT today, 3500 in that was smooth and comfortable and it winds out well past 7k
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Old 01-30-2010, 08:36 PM   #8
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I don't think there is a sensible answer to your question.
If you run an image search for 'BSFC' you'll get a number of different maps. Note that the 'islands' of maximum efficiency are all of different sizes and are located at different areas on the RPM / Throttle graph. Engines with variable valve timing, may even have two distinct, widely separated islands! In general, lower RPM and moderate throttle is best, but to try to come up with some sort of A, B, and C 'ideal' RPM values is silly without specific knowledge of your car.

If your car has a OBD2 port, you can build your own map by buying a Scangauge and plotting the data.

Poke around and see if you can find the official factory service manual for your car. Some manufacturers publish BSFC maps and other useful tech info in the book.

Good luck!
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Old 01-31-2010, 03:33 AM   #9
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Geonerd, can you elaborate on what data to collect from the ScanGauge? Just instant MPG, or something more?
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Old 02-01-2010, 08:56 AM   #10
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Quote:
Originally Posted by eee View Post
My scoobie ran at about the same RPM's as your scion, maybe just a tad lower. It now runs a little lower even after i replaced the set of tires with approx 5% larger 185 r14 75 or some such. it now revs more like 3200 rather than 3400ish at 70 mph. i have to adjust for the larger circumference in reading my speedometer, as well as in mpg calculations; I think it helped a little to reduce the hwy mpgs but not a vast amount).

While i pretty much observed my car takes about 2 gph on the freeway, it kind of shocked me how little fuel the engine sips cruising at 50, I would've guessed a fair bit higher than 1 gpm.

Being able to cruise at 50 mph is one big advantage of taking country roads over freeway, but getting there at roughly 7/10ths the time often seems worth driving the 70 mph, even at twice the fuel rate, or the 56% extra trip fuel expense. On freeways driving 50 is not an option here (many are just 2 lane freeways, and you'd be leaving a trail of havoc).
sitting for long periods of time can be dangerous for your health, thats why in the winter I try to go entire days without driving(i love being outside in the 10 degree weather, yea haha), this is even more fun in the summer when I can bike everywhere
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