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Car Forum / Subaru Cars / September 2008engine RPM vs MPG experiment
Impreza 1993, EJ18 engine, manual. Exclusively city driving, average trip about 5-7 miles. Was trying to drive keeping RPMs close to 2,000 or to 3,000. For the most part in practice it meant choice between 4th and 3rd gear. Same pump, fuel contains "up to" 10% ethanol, same weather, two full tanks for each "condition". The milage was consistent between two tanks in each case within 0.3 MPG. Results: ~ 1,800-2,300 RPM = 26.7 MPG ~ 2,500-3,200 RPM = 28.1 MPG (No, EJ18 is not a marvel of fuel economy :-)) Based on what I read before, I expected the opposite result. In any case, the difference is no more than 5%, so it's not really worth it to keep attention to. Lower RPM is less noise and less power. What's better for the engine in the long run? DK > Impreza 1993, EJ18 engine, manual. > [quoted text clipped - 16 lines] > > DK Not surprising as a result, smaller engines are set up to make better power in the higher revs, the cylinder filling is just so much better at those revs it offsets the pumping/friction losses. I'd have to guess an engine wears out after part X rubs past part y for the umpteen-billionth time and it just wears out. As long as you're not lugging the engine, having the car travel in the highest gear possible will minimize the number of 'rubs' per mile driven. Dave > Not surprising as a result, smaller engines are set up to make better > power in the higher revs, the cylinder filling is just so much better > at those revs it offsets the pumping/friction losses. I recall some years ago BMW did some tests with one of their cars and found that the best gas mileage was achieved by accelerating at wide open throttle (WOT) to cruise speed, then holding constant speed. That would apply only to that model of car, whose engine was designed to run most efficiently at WOT, and similar cars by BMW and others. > I'd have to guess an engine wears out after part X rubs past part y > for the umpteen-billionth time and it just wears out. > > As long as you're not lugging the engine, having the car travel in the > highest gear possible will minimize the number of 'rubs' per mile > driven. In 1955 I spent six months as a co-op student working at the Ford Motor Co. proving grounds, then located in Dearborn, Michigan. One of the things I helped do was to run fuel efficiency tests on Ford and competitors' cars, all at constant speeds (that is, not from a standing start like the BMW test above). The highest efficiency was invariably obtained at the lowest possible speed in the highest possible gear. Things may have changed in 50 years, but I expect that relationship still holds.  SignatureJohn Varela Trade NEW lamps for OLD for email. > In 1955 I spent six months as a co-op student working at the Ford Motor > Co. proving grounds, then located in Dearborn, Michigan. One of the [quoted text clipped - 5 lines] > Things may have changed in 50 years, but I expect that relationship > still holds. Nah, the laws of thermodynamics are still the same in Dearborn, last I checked. They have a bit more eclectic food these days, but that track is still there. I'll be breaking bread with one of many current Ford engineers this weekend and I'll ask if they still use that track for such stuff. -John O >> Not surprising as a result, smaller engines are set up to make better >> power in the higher revs, the cylinder filling is just so much better [quoted text clipped - 22 lines] > Things may have changed in 50 years, but I expect that relationship > still holds. if the fuel/air mixture were held constant, then the least number of revolutions would result in the least fuel per distance. (that's just simple math.) but modern cars change the mixture, and even the valve timing with speed and load, so it's quite feasible to have an engine that runs much more efficiently at higher revs (within reason). the bmw test also jives with my experiences with a nissan axxess, which seemed quite happy to accelerate hard up to whatever speed was needed, and then just cruise along. this wasn't what i had been taught, but it does make some sense, sort of, to spend less time accelerating. and it's exactly what cruise control does. (it can be a little scary to see/hear how hard the cruise control kicks it to get back to speed.) i haven't tested it thoroughly, but my '02 legacy *seems* to run better at 3-4k rpm than at 2-3k - both in terms of smoother shifting, better torque, and (seemingly) slightly better gas mileage. i also think that it's the smoothest running engine that i've owned, because it doesn't have the "winding up" sound that you get from most little japanese engines. > Impreza 1993, EJ18 engine, manual. > [quoted text clipped - 16 lines] > > DK I would believe this if you could average over many more, not just the 2 tanks. >> Impreza 1993, EJ18 engine, manual. >> [quoted text clipped - 19 lines] >I would believe this if you could average over many more, not just the >2 tanks. I just got back from a 6231km road trip with my 2.4 liter PT Cruiser. Averaged 29MPG. When I was running over 3000 RPM (and 117-120KPH) I got 34MPG. When running under 2500RPM I got up to 28MPG. A lot of hills throughout the trip (Waterloo Ontario through quebec (north shore of the St Laurence) , through New Brunswick, PEI, Nova Scotia, (including the Cabot Trail), Vermont (including Smuggler's Notch and Stowe area) Main, New Hampshire, andLake Champlain and Finger Lakes districts, back to Waterloo Ontario. The car ran best, and gave the best mileage, when running over 2800 RPM - hills or no hills. Even on flat runs, like Waterloo to Windsor, at 100KPH (under 2500RPM) I can not do any better than 28MPG and I have to have my foot into it deeper than when running at 2800+RPM and 117KPH (72-73MPH) ** Posted from http://www.teranews.com ** >Impreza 1993, EJ18 engine, manual. >Exclusively city driving, average trip about 5-7 miles. Was trying to >drive keeping RPMs close to 2,000 or to 3,000. For the most part >in practice it meant choice between 4th and 3rd gear. >Same pump, fuel contains "up to" 10% ethanol, same weather, two >full tanks for each "condition". The milage was consistent between >two tanks in each case within 0.3 MPG. Results: >~ 1,800-2,300 RPM = 26.7 MPG >~ 2,500-3,200 RPM = 28.1 MPG >(No, EJ18 is not a marvel of fuel economy :-)) >Based on what I read before, I expected the opposite result. In any >case, the difference is no more than 5%, so it's not really worth >it to keep attention to. Lower RPM is less noise and less power. >What's better for the engine in the long run? >DK Longer trips will do the most to increase your engine life and minimis wear! Most engine damage occurs in the first few miles/Kms when the engin is not properly warmed up. If you run a longer distance for your test, yo may notice more variety in your figures. Cheers - Message posted using http://www.talkaboutautos.com/group/alt.autos.subaru More information at http://www.talkaboutautos.com/faq.htm >> Impreza 1993, EJ18 engine, manual. > [quoted text clipped - 25 lines] > Message posted using http://www.talkaboutautos.com/group/alt.autos.subaru/ > More information at http://www.talkaboutautos.com/faq.html Hi, If that is his usual driving pattern, at least the car needs more frequent oil change for sure. >> Longer trips will do the most to increase your engine life and minimise >> wear! Most engine damage occurs in the first few miles/Kms when the engine >> is not properly warmed up. If you run a longer distance for your test, you >> may notice more variety in your figures. Cheers I drove the way I use the car for, that's all. >If that is his usual driving pattern, at least the car needs more >frequent oil change for sure. The oil change has been done regularly at 5,000 miles for the entire life of the car, which is 15 years and 110,000 miles on it. Frankly, if it dies on me soon, I wouldn't care that much - I am starting to get tired of it. On the other hand, its maintanence so far averaged perhaps $50/month, so for as long as it runs and gets me from point A to point B, I can't justify buying another one. DK > to get tired of it. On the other hand, its maintanence so far averaged > perhaps $50/month, so for as long as it runs and gets me from point 15y*12m*$50=$9000 - wow >> to get tired of it. On the other hand, its maintanence so far averaged >> perhaps $50/month, so for as long as it runs and gets me from point > > 15y*12m*$50=$9000 - wow Well, that ballpark figure is perhaps for the past 5 years when the car started to "need" things: clutch, timing belt, half-axel, water pump, rotors/pads. So it's more like $3,000 (including oil changes and regular maintenance) - far cheaper than buying a new car, which would have higher insurance rates and carry about 1/3 of the same maintenance costs. DK > Impreza 1993, EJ18 engine, manual. > [quoted text clipped - 16 lines] > > DK Hmm, 2 tankful? Have you looked at power band curve of your eingine? And frequency of brake use. My '98 Honda CRV alwasy produce 10L/100Km gas mileage regardless of season or where I drive in the city. If you drive your car most of time like that in the city better go out onto turnpike and burn off all the carbon build up inside the engine. Two things, every engine has optimum cruising rpm and speed by desing which will give best efficiency. Of course this is based on sea level driving. I live near Rockies at altitude of ~2700 feet. > Impreza 1993, EJ18 engine, manual. > [quoted text clipped - 16 lines] > > DK Where are you? In Mass the pumps say, "Contains 10% Ethanol", but when I go to Vermont the pumps say "Contains Up To 10% Ethanol". Now, I thought the premium grades would have the less amounts of Ethanol, but I may be wrong. It seems to me I recall hearing that Ethanol is an octane booster, so I'd be willing to bet the 93 octane has the most. Maybe someone can clarify... >> Impreza 1993, EJ18 engine, manual. >> [quoted text clipped - 25 lines] > > Maybe someone can clarify... There are several ways to boost octane; ethanol is certainly one. Not sure if 93 in the US is boosted solely by ethanol, but I would doubt it. The more ethanol you put in, the less energy density in the fuel. Given that premium is more expensive, while ethanol is cheaper, I suspect that they are using another hydrocarbon to boost octane. On Sat, 30 Aug 2008 22:19:27 -0400, Hachiroku ???? <Trueno@e86.GTS> wrote: >> Impreza 1993, EJ18 engine, manual. >> [quoted text clipped - 25 lines] > >Maybe someone can clarify... Don't know about other companies in other markets, but Shell in Ontario puts 10% in regular and none in Premium, making the mid grade E5 (5%). I know a lot of other companies are reported to be doing the same. ** Posted from http://www.teranews.com ** 2001 Forester 2.5 AT Gas mileage measured by trip computer on the highway driving with constant speed for at least 15 miles. The best gas meleage was achieved on the lowest RPM. See table: RPM MPH MPG 1500 37 39 2000 50 34 2500 62 28 2750 68 26 3520 81 21 4200 105 13 > 2001 Forester 2.5 AT > Gas mileage measured by trip computer on the highway driving with Hi, Ya gotta love trip computers! A buddy has a Ford F-250 Super Duty, V-10 gas engine, four wheel drive, blah, blah--in other words, NOT a fuel economy champ. But he can get 99 mpg according to his trip computer. All he has to do is coast down one particular hill that's about seven miles long at whatever speed fifth gear and foot off the throttle allows... however, if he turns around and pushes it UP the same hill, the trip computer can register as low as 4 mpg at one point! But when he starts w/ a full tank, goes 170 miles (up and back down this particular hill) and it takes 10 gal to fill the tank at the other end of the "trip" he's ecstatic cuz that's the best it ever does. No matter WHAT the trip computer said, he ACTUALLY got 17 mpg for the trip. Fuel economy's a function of averages... over lots and lots of miles, IME. Rick >>2001 Forester 2.5 AT >>Gas mileage measured by trip computer on the highway driving with [quoted text clipped - 16 lines] > > Rick Another issue is whether chronic use of the car in high gears at low RPMs will overheat/or wear engine internals faster. Carl Signatureto reply, change ( .not) to ( .net) > > 2001 Forester 2.5 AT > > Gas mileage measured by trip computer on the highway driving with [quoted text clipped - 16 lines] > > Rick Trip computer is installed as additional feature. (can be installed on any vehicle by connecting to injector and speed sensor). I have calibrated it by measuring of actual consumed fuel. It always shows correct values. For example: Trip - 10 miles Consumed gas - 1.1 litres >2001 Forester 2.5 AT >Gas mileage measured by trip computer on the highway driving with [quoted text clipped - 8 lines] >3520 81 21 >4200 105 13 That's not fair comparison at all! You varied not just RPM but also speed. Of course the milage is lower at lowers speeds. When I compared ~ 2 vs ~ 3K RPM, I was driving at about the same speeds, determined by speed limits along my regular routes. DK > In article > <63a57ad1-50b2-4ad8-9b1b-be1d8754646b@k37g2000hsf.googlegroups.com>, Butch [quoted text clipped - 19 lines] > > DK Does that mean your gas mileage went down when driving in 5th gear, and went up in 4th, while driving at the same speed? -John >> In article >> <63a57ad1-50b2-4ad8-9b1b-be1d8754646b@k37g2000hsf.googlegroups.com>, Butch [quoted text clipped - 22 lines] >Does that mean your gas mileage went down when driving in 5th gear, and went >up in 4th, while driving at the same speed? Here is my original post: Exclusively city driving, average trip about 5-7 miles. Was trying to drive keeping RPMs close to 2,000 or to 3,000. For the most part in practice it meant choice between 4th and 3rd gear. Same pump, fuel contains "up to" 10% ethanol, same weather, two full tanks for each "condition". The milage was consistent between two tanks in each case within 0.3 MPG. Results: ~ 1,800-2,300 RPM = 26.7 MPG ~ 2,500-3,200 RPM = 28.1 MPG DK >>> In article >>> <63a57ad1-50b2-4ad8-9b1b-be1d8754646b@k37g2000hsf.googlegroups.com>, Butch [quoted text clipped - 37 lines] > >DK Sounds right to me. The engine is rnning more efficiently at 2800 than at 1800, for sure. ** Posted from http://www.teranews.com ** >>>> In article >>>> <63a57ad1-50b2-4ad8-9b1b-be1d8754646b@k37g2000hsf.googlegroups.com>, [quoted text clipped - 42 lines] > Sounds right to me. The engine is rnning more efficiently at 2800 than > at 1800, for sure. At city speeds wind resistance is a non-factor, so you're right, and this is an interesting experiment. Once you add a wind load however (above ~45 mph), this engine efficiency benefit falls apart, rather dramatically. Efficiency goes up fractionally for a while, but wind loads increase by factors of two and four as speeds increase! -John O >>>>> In article >>>>> <63a57ad1-50b2-4ad8-9b1b-be1d8754646b@k37g2000hsf.googlegroups.com>, [quoted text clipped - 50 lines] > >-John O Not from my considerable experience I've found on several vehicles (my current PT Cruiser included) that running at low speeds, OTHER than on dead level terrain, the engine is not working efficiently - and burns MORE gas than at higher (reasonable) speeds. Untill the engine "gets on cam" a bit, you are using large throttle openings to produce small results - and burning more gas in the process. I can also consistently beat cruise control for mileage under normal driving conditions - again except,sibly, on dead level driving - which I see very little of. The wind load on most of todays vehicles only really starts to be a serious issue above somewhere around 120kph. (unless you have a roof rack, tow a trailer, etc) I've put a LOT of miles onto a lot of cars in over 40 years of driving - and as a mechanic I do understand the factors that come into play with RPM, gear ratios, torque curves, etc. ** Posted from http://www.teranews.com ** > I can also consistently beat cruise control for mileage under normal > driving conditions - again except,sibly, on dead level driving - which > I see very little of. The wind load on most of todays vehicles only > really starts to be a serious issue above somewhere around 120kph. > (unless you have a roof rack, tow a trailer, etc) That isn't totally true. The primary input into wind load (drag) at low speed is the frontal cross-section area; which is not that different today than it was 20 years ago. Consequently, the speed at which wind begins to be a factor is around 80 km/hr, and it goes up exponentially. Once you get to higher speeds, shape becomes an issue and can reduce the increase in drag somewhat. With performance cars 120 is probably a safe bet since a performance car burns a fair bit of gas just doing nothing. But with a normal passenger car, you would begin to notice the effects certainly by 100 km/hr, and a significant drop in gas mileage by 120. Air is a fluid and anything moving through it obeys the same set of rules. > In article <63a57ad1-50b2-4ad8-9b1b-be1d87546...@k37g2000hsf.googlegroups.com>, Butch <QXBu...@gmail.com> wrote: > [quoted text clipped - 19 lines] > > DK I also measured gas mileage during daily driving. 1-st mode: 1,2,3,4 gears city driving (1500 - 2000 RPM) 2-nd mode: 1,2,3 gear driving (2000-3000 RPM) fuel economy is actully better in the 1-st mode by about 2-4 mpg I have additionally measured gas mileage at same speed, same gear but different RPM. See results: RPM MPH MPG 1500 37 39 1800 37 34 Note: (cruise control mode) > I have additionally measured gas mileage at same speed, same gear but > different RPM. See results: > RPM MPH MPG > 1500 37 39 > 1800 37 34 Note: (cruise control mode) Hi, You have an automatic? IF it's a stick, speed/rpm ratios are FIXED, so you CAN'T go the same speed in the same gear at different engine RPMs (assuming your foot's OFF the clutch, of course!) OTOH, w/ an auto, there's "slippage" in there under various conditions, so it IS possible to go the same speed at different RPMs. Hence your 1800 rpm/34 mpg measurement makes sense if you're going the same 37 mph you were doing at 1500 rpm/39 mpg part of the test: you're "wasting" 300 rpm thru slippage, creating heat and burning extra gas. This is the main reason behind the development of "lock up" torque converters some years back: if the t/c and trans can be made to act like a stick, w/ minimal to zero slippage at certain speeds, MPG increases. In the case of autos, there are times that what we "think" we see can be deceptive... Rick > > I have additionally measured gas mileage at same speed, same gear but > > different RPM. See results: [quoted text clipped - 21 lines] > > Rick you are right. I have an auto. 37 mph/1500 rpm - 4-th gear with torque converter "lock up", cruise control mode is OFF 37 mph/1800 rpm - 4-th gear w/o torque converter "lock up", cruise control mode is ON at higher speed with gear selector in "D" position, I have the only "torque converter "lock up" mode at highest 4-th gear position. So I could perform measurements at same speed but different rpm only if I set gear selector into "3" or "D" position. For example: 1-st mode: 4-th gear, 50 mph, 2000 rpm, with "torque converter "lock up" mode 2-nd mode: 3-rd gear, 50 mph, ???? rpm, with "torque converter "lock up" mode is it fair comparison? I will do such comparison soon and post the results. I'm sure that 1-st mode has better gas mileage. My statement is: lower RPM -> better gas mileage. There is no any extremum in mpg / rpm chart. > I will do such comparison soon and post the results. 4-th gear, 50 mph, 2000 rpm - 34mpg 3-rd gear, 50 mph, 3000 rpm - 27mpg > 4-th gear, 50 mph, 2000 rpm - 34mpg > 3-rd gear, 50 mph, 3000 rpm - 27mpg Hi, Are those "measured" or "calculated" figures? I ask not to challenge, but because I've often experienced situations where the actual "measured" mileage ran contrary to the "lower rpm=higher fuel economy" dictum. In other words, measured results were in opposition to what the trip computer was telling us (somewhat in line w/ other parts of this thread where the discussion's focusing on engine efficiency at a given rpm.) Just curious... Rick > > 4-th gear, 50 mph, 2000 rpm - 34mpg > > 3-rd gear, 50 mph, 3000 rpm - 27mpg [quoted text clipped - 13 lines] > > Rick 1. "calculated", but it always equal to "measured" after fuel the tank on gas station. +- 1% 2. originally topic was engine RPM vs MPG. What is "efficiency at a given rpm", MPG, "engine wear" or somthing else? > Impreza 1993, EJ18 engine, manual. > [quoted text clipped - 16 lines] > > DK Hi DK this is about another issue, was it you who was having trouble with a p172 code on your subaru? my subaru is behaving the same with the same code, what was the problem. thanks GM >this is about another issue, was it you who was having trouble with a >p172 code on your subaru? Yes, but that was wife's Forester. >my subaru is behaving the same with the same code, what was the >problem. In this case, the problem was definitely front oxygen sensor. The moment it was replaced, the troubles disappeared (and milage improved somewhat). DK > In article <5ba25b8a-ad42-4407-8cce-9b03d782d...@p10g2000prf.googlegroups.com>, sexsmithguit...@telus.net wrote: > [quoted text clipped - 11 lines] > > DK Thanks DK sorry to hijack your discussion, I was at a bit of a loss, i went through the whole check list, and the front o2 was just replaced about 8 months ago, do you think the rear o2 would have that much influence over the way the car runs? or does it just serve to monitor the catalitic converter? Thanks GM >> In article <5ba25b8a-ad42-4407-8cce-9b03d782d...@p10g2000prf.googlegroups= >.com>, sexsmithguit...@telus.net wrote: [quoted text clipped - 19 lines] >the car runs? or does it just serve to monitor the catalitic >converter? It certainly did in my case. And it makes sense. You might be thinking of the rear O2 sensor, which indeed just monitors the cat. converter and is unimportant for driving. When I was digging probable causes for P0172, here is one fix I found that turned out well for at least couple guys: make sure the ground cable attached to the intake manifold is clean and tightly screwed. It's a ground for a bunch of sensors and when it is not connected, many things can go wrong. Or so I read. DK Thanks again, I'll let you know how i go, will take a while for parts to arrive though. GM |
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