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Old 07-21-2009, 01:00 AM   #1
Bluefoton
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Default Improved gas mileage, or the “but i gained 2 mpg after my mod!” myth

Gas mileage improvements are amongst the most willingly argued and erroneous figures in common discussions. To start with, there are two widely used units: mpg (miles per gallon) in the US, and lph (litres per hundred kilometres) elsewhere. As a little cultural note, it’s interesting to see how the US metric cared less for the price and just aimed for distance, whereas the Europeans are more interested in knowing how many litres (or Euros) it costs to go over a certain distance. But that’s just a little diversion. The conversion is mpg = 1/(0.00425 * lph) or lph = 1/(0.00425 * mpg)

• The anally proper method to measure MPGs involves filling your tank, resetting your odometer, driving, refilling your tank at the same station at the same ambient temperature, and dividing the miles driven by the gallons in the tank. On board computers which have a trip function (such a ScanGauge) also provide a decent indication, but they too suffer from errors.

• A more normal method is as above, but not bothering with the same temperatures or station. However, even then one must account for possible variations in between pumps or volumetric expansion of gas. In Canada, volume at stations is required to be corrected to a reference temperature of 15C (65 F). Nevertheless, as gasoline expands by 1% for every increase of 15 F, even this regulation allows for a 2% error in between summer and winter. As far as the volume measurement go, NIST requires an error within 0.5;, yet for example an official Utah fuel pump report labelled the statewide accuracy at 97 %. There have been reports of pumps making as much as a 25% mistake – though typically faulty pumps deliver more than they claim, not less.

• Fuel gauges: they are NOT accurate enough to claim “I drove x miles on a tank, and y miles on the next tank, my mpg changed”. They are deliberately set up so as to remain on “F” for a certain time before even moving and the needle behaviour is geared towards making the driver appreciate the fuel economy of the car. You’ll be lucky enough if the ½ mark corresponds to half your tank, and there are well observed occurrences of driving the car even if the gauge goes down from E after turning the ignition on.

• Now the big paragraph. For each trip, the fuel consumption depends on, in no particular order: Mass of the vehicle, cargo load, nr of passengers, tyre inflation, tyres, alignment, aerodynamics (drag), headwind, humidity, terrain profile, overall altitude difference, AC use, all the electrical accessories (headlights, fan, defrost, etc), windows, traffic pattern, your mood, and your driving habits. I deliberately omitted the internals of the car which cause a consistent drag, and which normally aren’t modified on a trip-per-trip basis. This is where the often encountered scenario comes: You drove, then added a miracle pill/increased your octane/put a magnet/ did any other mpg mod, following which you noticed that your mpg went from 25.1 to 26.2. That’s a 4.3 % improvement. Here is the catch: Even assuming you pump twice at the same petrol station, can you claim with certitude that your second trip had a headwind, temperature, humidity and road inclination, along with your speed, acceleration, and breaking distribution within 5 % of your previous trip? NO. Unfortunately, such a number of variables contribute to a final mpg figure, that repeated and consisted measures are required for all the random variations to slowly cancel out and yield a convincing measure
Conclusion: It’s NOT enough to see an improvement over one or two trips to jump on the conclusion that whatever has changed had an effect on mpg. Unfortunately, as with any measurement of a variable heavily depending on a vast array of different contribution, some statistical thoroughness and a lot of common sense are required.
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Old 07-21-2009, 01:00 AM   #2
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When to shift?
There seems to be a lot of foggy discussions about how to drive for optimum fuel consumption. Everyone seems to agree (and rightly so) that the most efficient (power delivered per fuel consumed) point for engines is at the peak torque. It is - but it's also important to keep in mind that manufacturers give only the torque / hp vs rpm at WOT. The problem is that for driving at 70mph you CANNOT operate at WOT because you will accelerate. For a constant speed, partial throttle opening is required...and this is where things get messy, as it's more difficult to find any info about engine efficiency @ partial throttle. Say hi to Brake Specific Fuel Consumption (BSFC) - which measures fuel consumption as a function of RPM and engine load. The first plot shows BSFC as afunctio nof RPM for different throttle openings.



Three curves here, for WOT, 50%, 25%. There is a stunning difference on how larger the BSFC is for partial throttle - at some points, 2x as big as for WOT. Translation: you engine's overall efficiency, namely brake horsepower per fuel used, dropped by half. One intuitive cause for that is simply manifold vacuum. At WOT, you're getting all the air into the engine you can. At partial throttle under high vacuum, your cylinders have to fight that vacuum - a significant portion of the fuel is thus used just to overcome this resistance, leaving only a fraction to generate useful (brake)torque.

Now the next step: a full BSFC curve, this time for the EJ22 N/A subaru engine. A different layout, as the verticle axis is now engine torque.



The numbers here depict values of BSFC contour plots, plotted as a function of engine torque (vertical) and RPM (horizontal). The top curve depicts peak engine torque - or the same torque plot generated from a dyno/spec sheet, corresponding to a WOT operation. Anything below that curve represents a partial torque output...or partial throttle operation (as opposed to the first graph, there is no indication on the actual % throttle opening).

Now, the observations: the bottom region of the graph represents low torque (or partial throttle) operation. Again, the BSFC values are horrendous meaning that if you run under high vacuum and partial throttle, you're getting an awfuly bad fuel economy. Conversely, at WOT (high torque output), you're operating near the best efficiency point. Interestingly the best region for the EJ22 seems to be in between 1800-3500 rpm, almost at WOT. This allows to conclude that for acceleration purposes, the best fuel efficiency will occur - for this engine - shifting at 3500 and nearly WOTing at all times.

This tackles engine efficiency - but it's still not clearly related to fuel economy at a given speed. Constant speed requires constant power, which means that rpm x torque has to be constant. Thus, one can operate in 5th at 3000 rpm, 4th gear at 4000 rpm, or 3rd gear at 5000 rpm. (arbitrary values). Which is best? Difficult to know unless power curves (rpm x tq) are superimposed on the graph. Here, HP = torque (Kgf.m) x rpm x 7.2353 / 5252. Tweaking the above graph by picking 100, 60, and 20 hp and checking the RPM at which they occur for 18,16,12,14.... kgf.m of torque yields the following:



This is where we get complete info for fuel economy at a given constant speed (or for constant horsepower required). Let's assume for example one needs 20hp to cruise. Where does the highest efficiency point occur (and thus, lowest fuel consumption)? Lowest RPM and full throttle. Same thing if the desired output is 60hp - going uphill for example, it's more efficient to be at 3000 rpm and WOT as opposed to 5000 rpm and partial throttle. One thing missing from this graph is actual gearing - but it does nonetheless show that 70mph at 3000 rpm and nearly closed throttle (typical cruise control speed ) is FAR from acheiving a good fuel economy. It also points out why "hypermilling" methods such as pulse and glide are so effective. Pulse refers to accelerating at WOT (near the peak of the effective efficiency), slowing down due to drag, and repeating. Energetically, not only the engine is then operated always at peak efficiency, but because of the acceleration, the engine is then inoperative (or at idle) for periods of time, during which one coasts almost for free.

One additional thing to mention is that the BSPS curves are quite engine specific - below is that of a Saturn 1.9 DOHC, where the best efficiency doesn't occur at WOT, but rather at ~85% of torque delivery (i.e. almost full throttle).



I haven't found a chart depicting the BSFC curves for a gasoline turbo engine, but will update the post as soon as I do...

Last edited by Bluefoton; 04-13-2011 at 12:22 PM.
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Old 07-21-2009, 01:06 AM   #3
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Use a scangaugeII or some other scan tool like software coupled with a portable computer to get a more accurate reading of what the ECU thinks the MPG is.


Also, if you are serious about increasing mileage, research hydrogen injection, on demand hydrogen systems and HHO dry cells.

Good luck.
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Old 04-13-2011, 12:24 PM   #4
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Updated...
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Old 04-13-2011, 12:49 PM   #5
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Who talks about fuel economy around here?

C/N: There's variability in scientific studies, and you have to do them more than once to get good data.
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Old 04-13-2011, 01:42 PM   #6
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I personally use my Cobb AP to keep an eye on MPG. Is it the most accurate? Probably not, but better than going off the odometer between fills.

When I went stage II I did notice an increase in my MPG, can I really say what I had before than after? Not really, but I didn't buy this car for the fuel economy
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Old 04-13-2011, 01:43 PM   #7
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So.............what is the bottom line? Do you believe that it is unlikely that people will see a real world increase in fuel economy after any modifications? Or specific modifications? Funny I just received an email regarding fuel economy from Subaru? Just with the basics, weight, tire pressure (that seems like something that is ignored frequently) etc.
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Old 04-13-2011, 01:47 PM   #8
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Quote:
Originally Posted by subaru_steve View Post
I personally use my Cobb AP to keep an eye on MPG. Is it the most accurate? Probably not, but better than going off the odometer between fills.

When I went stage II I did notice an increase in my MPG, can I really say what I had before than after? Not really, but I didn't buy this car for the fuel economy
No it's not. The most accurate measurement of fuel consumption is to fill up at the same gas station and pump every time and use the odometer to do the math.
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Old 04-13-2011, 02:03 PM   #9
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http://hp2g.com/

400HP V8 + 110MPG = fun
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Old 04-13-2011, 02:07 PM   #10
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Quote:
Originally Posted by drummerboy827 View Post
No it's not. The most accurate measurement of fuel consumption is to fill up at the same gas station and pump every time and use the odometer to do the math.
How would that be more precise?

You fill up at the same gas station etc, however both the vehicle and pump can change how much fuel is actually put into the vehicle.

The AP is reading from the ECU which gives a fair understanding of how much estimated fuel is being injected into the cylinders.
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Old 04-13-2011, 02:54 PM   #11
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The ScanGauge "can" be very precise if set up properly... I have mine set up so when it says it will take 56.5L to fill up the tank, it will take 56.6L or 56.4L or even 56.5L exactly, that has happened quite often... As for the cars fuel mileage as shown by the ECU it's consistently optimistic by about 1.5 - 2 MPG.

EDIT; as for increasing fuel mileage I have found that 1 1/2 OZ. of Acetone per tank does help... JMO
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Old 04-13-2011, 02:58 PM   #12
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You spelled tires wrong.
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Old 04-13-2011, 03:58 PM   #13
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I don't want to seem like I understand this....because I don't. haha.

So, is there anyway to convert this into laymen's terms?

Cruising the speed limit on cruise control is bad because it's not using an effective amount of horsepower from the engine for constant throttle? Eh?
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Old 04-13-2011, 04:13 PM   #14
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Point 1: Claiming that a modification improved/decreased MPG just basedon the mileage over a few fill-ups is pointless, as too many variables influence each reading. It doesn't mean there is no impact, but that it's a pain to determine one ( the effect of tyre pressure, or ethanol content on mpgs has been actually documented). But it's hard to prove anything without serious measures or datalogs..

Point 2: Lower RPM + higher gear + more throttle gives better mileage than higher rpm, lower gear, less throttle

Point 3: Floor it and shift <3000 rpm for best fuel economy (for N/As)

Point 4: Hypermiling (pulse and coast) works. Am I going to do it? Hell no. I just wish the last gear were taller for freeway cruising.

Point 5: Not really trying to prove anything here, just thought it could be fun technical info.
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Old 04-13-2011, 04:33 PM   #15
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Quote:
Originally Posted by Bluefoton View Post
Point 1: Claiming that a modification improved/decreased MPG just basedon the mileage over a few fill-ups is pointless, as too many variables influence each reading. It doesn't mean there is no impact, but that it's a pain to determine one ( the effect of tyre pressure, or ethanol content on mpgs has been actually documented). But it's hard to prove anything without serious measures or datalogs..

Point 2: Lower RPM + higher gear + more throttle gives better mileage than higher rpm, lower gear, less throttle

Point 3: Floor it and shift <3000 rpm for best fuel economy (for N/As)

Point 4: Hypermiling (pulse and coast) works. Am I going to do it? Hell no. I just wish the last gear were taller for freeway cruising.

Point 5: Not really trying to prove anything here, just thought it could be fun technical info.
Awesome thanks! I guess I've been doing things right all along.
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Old 04-13-2011, 04:57 PM   #16
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Here is some info I got with my ScanGauge on my '02 WRX stage 2, while testing things...

KMs/MPH ....L/100.....MPG
70 - 44...........6.2 - 38.0
80 - 50...........6.9 - 33.8
90 - 56...........7.5 - 31.5
100- 62...........8.8 - 26.6
110- 69...........9.7 - 24.3
120- 75..........10.5 - 22.4
130- 81..........11.9 - 19.7
140- 87..........13.5 - 17.5
150- 94..........16.7 - 14.0
160-100.........18.2 - 12.9
170-106.........20.6 - 11.4
Oh and if the math isn't right by your calculator that's cause I left off some decimal points, just wanted an average number...
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Old 04-13-2011, 05:22 PM   #17
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Does this whole analysis get thrown off considering that in e-throttle the ECU is programmed to think that if you are pushing the pedal for WOT then you "want to accelerate a lot" and it enrichens the mix?

BTW, I really appreciate obtaining graph/info like this. I'm just wondering how you obtained them (to confirm their value).

So according to your n/a EJ22 data/analysis, you're saying flip it into the highest gear that lets it rev at ~1200 rpm and don't be afraid to hold the pedal down to maintain your speed? Why then (when I use a scangauge or AP) does the fuel consumption shoot up? The load at this point is 100% which IIRC means rich mix.
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Old 04-13-2011, 06:08 PM   #18
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Chimchimm - I googled for BSFCs curves until I stumbled on the EJ22 one . Just do image search on BSFC EJ22...

Since the top trace is for WOT and corresponds to the peak torque vs delivery curve of an engine, I would presume it takes into account everything that happends in an ECU. Mind you, the trace is rough, so maybe the "best" point occurs somewhere in the top patch, not at WOT per se.

Your point is very valid though insofar as for every other BSFC trace (like the Saturn's), the lowest value NEVER occurs at WOT, but a bit underneath (85% or so torque delivery). This is the "250" patch on Saturn's curve for example. It could very well be due to what you said.

Fuel consumption DOES go up with throttle (more air + same A/F ratio = more fuel). If you floor it though, you accelerate (usually), which means you generate more torque than you need. Ideally to corroborate these graphs, one would need either a steep, and constant inclination hill, or a hell of a drag - say, towing a boat without the trailer. Basically something that would force you to be at 85% WOT in 5th gear. Then set cruise control, monitor the instantaneous MPG, downshift, cruise control again, measure, etc. My claim is that you'd still get the best mpg for the highest gear...


larkdemon - keep in mind you've got a WRX. Forum wisdom suggest to keep OFF boost to improve mileage, I just wasn't able to dig as to why operating under boost actually decreases a bit power-per-fuel efficiency (which doesn't negate the fact that boost allows much more power per displacement).
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Old 04-13-2011, 06:26 PM   #19
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Quote:
Originally Posted by Bluefoton View Post

larkdemon - keep in mind you've got a WRX. Forum wisdom suggest to keep OFF boost to improve mileage, I just wasn't able to dig as to why operating under boost actually decreases a bit power-per-fuel efficiency (which doesn't negate the fact that boost allows much more power per displacement).
Oh I know. It's funny how just going the speed limit gets you over EPA estimates.
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Old 04-13-2011, 07:28 PM   #20
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Interesting read. I see now why manufacturers push smaller motors into cars despite some finding many of these motor options a tad...underpowered.

It might also be nice to note how significant engine efficiency is versus bulk car weight and aerodynamics. For example, if we had the same car with two engine choices, one a small 4 making 100hp and a big V8 making 400hp, assuming end weight remained the same, how big of an influence would motor alone affect the end mileage? I bring this up only because I've seen some big motors shoved into small cars and mileage remained rather high. I am curious if it is more advantageous to focus heavily on the motor as many companies do or focus back to light, small, and aerodynamic cars like in the 80s. It's more of the idea of where is the time better spent or are we simply accepting bigger and heavier cars because that's what people want and simply focusing on engines so strongly because that's sort of all we have left?
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Old 04-13-2011, 07:51 PM   #21
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If you want a 100MPG car I'm sure it's doable... A 0.5L engine with a CVT designed to keep the engine in the 90% throttle range all the way to 60MPH and keep it there while cruising and presto, You have a 100MPG 3000Lb car... It will take over a minute to get to 60MPH and the top speed would be 70MPH (another minute to get there) but so what... JMO not an engineer just made it up as to what I think a 100MPG car will be/would like.

Last edited by wrxdrvr; 04-13-2011 at 08:16 PM.
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Old 04-13-2011, 08:54 PM   #22
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wrxdrvr, I would be interested to know what gear you were in and or rpm range for the mpg values/speeds that you posted if you have them. Thanks.
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Old 04-13-2011, 09:43 PM   #23
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Quote:
Originally Posted by oscarfish View Post
wrxdrvr, I would be interested to know what gear you were in and or rpm range for the mpg values/speeds that you posted if you have them. Thanks.
I got those numbers by getting up to speed, (usually getting about 1 to 1.5 MPL) then resetting when I reached the speed I was testing at and drive for 2 KMs... Write down the Scangauge average L/100KMs turn around and do the same thing going back the other-way, divide by 2 and thus I got the average L/100KMs my car could/would cruse at, at that speed (in "D", as it was a 4EAT) .
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Old 04-14-2011, 01:55 PM   #24
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The way you drive will have more of an effect on your MPG than any mod will.
If you have SI drive, try driving all the time in I mode and shift when indicated. You'll hate yourself for driving like that, but you'll love your MPG numbers.
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Old 04-14-2011, 02:15 PM   #25
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I'll just say this and be on my merry ****ing way

I was getting on average 28-31 mpg per full tank in my bone stock 2004 wrx

after the mods and tune my 302 awhp 2004 wrx gets about 28-31 mpg with mixed city, highway driving, normal and small spurts of spirited driving.

sorry, but I don't have fancy numbers/graphs/theories, just observations.
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