fuel economy discussion for 2gr-fe - MR2 Owners Club Message Board
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post #1 of 27 (permalink) Old August 15th, 2011, 20:59 Thread Starter
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fuel economy discussion for 2gr-fe

so i came across this graph and thought it was rather interesting:



the initial takeaway from that is that you should try to keep your highway RPMs at a max of 3400RPMs.

if we're talking about the MR2 swap with the likely E153 transmission and stock height tires (225/50r15) you're looking at 75mph before getting outside of the high efficiency band.

that said, with the possible 3.625 gears it takes the efficiency band to 89mph

so theoretically, fuel economy should be almost just as good with the stock gearset as the 3.625 gears as long as you don't pass over 75mph. but that's not quite what my experiments have shown.
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post #2 of 27 (permalink) Old August 15th, 2011, 23:14
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Don't forget that the chart you see there is only BSFC at WOT, which is not indicative of the BSFC you'll see at part-throttle and low-load.

Let me see if I can find a 3-D bsfc map with contours to give you a better idea. Generally speaking you want 60-80% throttle at the absolute lowest rpm for the best BSFC.
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post #3 of 27 (permalink) Old August 15th, 2011, 23:21
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I just got a new best of 30.279mpg, all highway in my 2gr mk2. (stock e153, stock tires, 87 octane)
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post #4 of 27 (permalink) Old August 15th, 2011, 23:22
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Aha, I forgot I had one already posted from my 1MZ tech posts long ago.



Notice that you want to target 2000-2200rpm with large throttle openings for best BSFC.

One thing to remember is that at any given road speed, the amount of horsepower required to balance out the wind/rolling resistance and maintain that speed stays constant, regardless of engine rpm.

Hypothetically speaking that means if a given road speed, say 60mph, requires 20hp to maintain (and those numbers are in the right ballpark for the purposes of this discussion), you'll need to be generating 30lb/ft of torque if you gear your transmission to have the engine turning 3500rpm at that road speed, but 60lb/ft of torque if you gear everything such that the engine spins at 1750rpm at 60mph.

Finding the optimal RPM for a given road speed is pretty straightforward as you only need to find your needed horsepower (lots of free calculators on the internet for this, ask if you need help finding one), plot two points on the graph that provide that horsepower level, and draw a straight diagonal line between those two points and see where it intercepts the lowest number on the BSFC contour map.

Looking at the 1MZ map shown above, you can see that you almost always are better off turning lower RPM and demanding more torque and wider throttle angles from the engine to reduce the BSFC. Technically a V6 in an MR2 would probably get the best gas mileage on the highway geared to spin about 1200rpm at 60mph, and running at 80% throttle the whole time.

This is why we want 6spd transmissions. Less highway rpm is almost always better unless you have a tiny engine. Once your engine is small enough that you actually need 50-80% of your torque output to maintain your road speed, then in *some* cases it can be advantageous to actually cruise at a higher RPM. This is rarely true though unless you're driving a 1.0L 3-cylinder Geo Metro or something equivalent.

In most cases, gear the car as tall as you dare for optimum highway economy.

Last edited by ThingyNess; August 15th, 2011 at 23:31.
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post #5 of 27 (permalink) Old August 15th, 2011, 23:26
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Incidentally, this is also why large engines give horrible fuel economy.

BSFC is largely independent of engine size. A 7.0L LS7 from a Corvette Z06 actually will have similar wide-open throttle BSFC numbers to a 1MZ-FE or even a 5S-FE.

However, since the 7.0L engine is so over-sized for the amount of power required to cruise on the highway in the slippery Corvette, even with ridiculously tall gearing it still spends all of its time at the very bottom of the BSFC map where throttling/pumping losses dominate and the BSFC shoots through the roof.

Amusingly, when cruising at 140mph, a Corvette might even get better mileage than a 1MZ-FE MR2, since it'll be happily in the 'sweet spot' of its BSFC map.

Last edited by ThingyNess; August 15th, 2011 at 23:33.
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post #6 of 27 (permalink) Old August 15th, 2011, 23:40
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Awesome technical data! lots to digest..

My 2 bits: You don't need six gears.. you just need a tall 5th!
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post #7 of 27 (permalink) Old August 16th, 2011, 01:17
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nah.. tall 5th and taller final drive
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post #8 of 27 (permalink) Old August 16th, 2011, 06:38 Thread Starter
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Quote:
Originally Posted by ThingyNess
Don't forget that the chart you see there is only BSFC at WOT, which is not indicative of the BSFC you'll see at part-throttle and low-load.

Let me see if I can find a 3-D bsfc map with contours to give you a better idea. Generally speaking you want 60-80% throttle at the absolute lowest rpm for the best BSFC.

that's a good point. that explains why i'm getting better fuel economy with the 3.625's


Quote:
Originally Posted by immorality
I just got a new best of 30.279mpg, all highway in my 2gr mk2. (stock e153, stock tires, 87 octane)
with the 3.625's and slightly taller rear tires than stock i'm getting about 39mpg on 75mph highway only driving with my MR2. but that is with premium fuel also. it does give a difference in the power not sure how it affects economy.
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post #9 of 27 (permalink) Old August 16th, 2011, 06:42 Thread Starter
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Quote:
Originally Posted by ThingyNess
Aha, I forgot I had one already posted from my 1MZ tech posts long ago.



Notice that you want to target 2000-2200rpm with large throttle openings for best BSFC.

One thing to remember is that at any given road speed, the amount of horsepower required to balance out the wind/rolling resistance and maintain that speed stays constant, regardless of engine rpm.

Hypothetically speaking that means if a given road speed, say 60mph, requires 20hp to maintain (and those numbers are in the right ballpark for the purposes of this discussion), you'll need to be generating 30lb/ft of torque if you gear your transmission to have the engine turning 3500rpm at that road speed, but 60lb/ft of torque if you gear everything such that the engine spins at 1750rpm at 60mph.

Finding the optimal RPM for a given road speed is pretty straightforward as you only need to find your needed horsepower (lots of free calculators on the internet for this, ask if you need help finding one), plot two points on the graph that provide that horsepower level, and draw a straight diagonal line between those two points and see where it intercepts the lowest number on the BSFC contour map.

Looking at the 1MZ map shown above, you can see that you almost always are better off turning lower RPM and demanding more torque and wider throttle angles from the engine to reduce the BSFC. Technically a V6 in an MR2 would probably get the best gas mileage on the highway geared to spin about 1200rpm at 60mph, and running at 80% throttle the whole time.

This is why we want 6spd transmissions. Less highway rpm is almost always better unless you have a tiny engine. Once your engine is small enough that you actually need 50-80% of your torque output to maintain your road speed, then in *some* cases it can be advantageous to actually cruise at a higher RPM. This is rarely true though unless you're driving a 1.0L 3-cylinder Geo Metro or something equivalent.

In most cases, gear the car as tall as you dare for optimum highway economy.
we need that chart for the 2gr-fe
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post #10 of 27 (permalink) Old August 16th, 2011, 09:24
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I wish I had it. BSFC contour maps are super useful, but often closely guarded by the manufacturers. It's not something they routinely publish outside of their organization. I've only managed to gather a handful for other engines, and never one for the 2gr-fe, sadly.

Quote:
Originally Posted by Gouky
we need that chart for the 2gr-fe
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post #11 of 27 (permalink) Old August 16th, 2011, 10:09
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Interestingly, did anyone notice that the 1MZ-FE actually has better BSFC numbers than the 2GR-FE at wide open throttle, if you compare the data that I posted (which is directly from a Toyota Engineering paper) and the data that Gouky posted?
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post #12 of 27 (permalink) Old August 16th, 2011, 11:52 Thread Starter
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Quote:
Originally Posted by ThingyNess
Interestingly, did anyone notice that the 1MZ-FE actually has better BSFC numbers than the 2GR-FE at wide open throttle, if you compare the data that I posted (which is directly from a Toyota Engineering paper) and the data that Gouky posted?
interesting, at full throttle at low RPMs the 1mz is more fuel efficient.
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post #13 of 27 (permalink) Old August 16th, 2011, 12:52
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Quote:
Originally Posted by Gouky
with the 3.625's and slightly taller rear tires than stock i'm getting about 39mpg on 75mph highway...
That's amazing. But of course in Calif. you're only allowed to use a 1MZ + S/C to legally achieve approx the same performance, but you only get about 26mpg highway. :sigh:
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post #14 of 27 (permalink) Old August 16th, 2011, 14:46
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Damn 3.944!!

At least the S51 has decently tall gears 2-5 (tall gear is 0.731)

I want the S54 1st gear installed in my S51 .... then I get this:
http://mr2.phpwerx.net/turbocalc/gea...ixHgEAAA%3D%3D
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post #15 of 27 (permalink) Old August 16th, 2011, 14:48 Thread Starter
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Quote:
Originally Posted by uspspro
Damn 3.944!!

At least the S51 has decently tall gears 2-5 (tall gear is 0.731)

I want the S54 1st gear installed in my S51 .... then I get this:
http://mr2.phpwerx.net/turbocalc/gea...ixHgEAAA%3D%3D
keep in mind, on the e-series you have to use the 1st and 2nd gear from the same transmission as both are machined into the input shaft. you can't pick 1st from one and 2nd from a different one.
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post #16 of 27 (permalink) Old August 16th, 2011, 15:51
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Quote:
Originally Posted by Gouky
keep in mind, on the e-series you have to use the 1st and 2nd gear from the same transmission as both are machined into the input shaft. you can't pick 1st from one and 2nd from a different one.
interesting point. I seem to recall one of our v6 AutoXers using a S54/S51 hybrid.

It turns out the S51 has the same 1st and 2nd as the S54... I guess I would grab the entire input shaft from the S54 with 1st and 2nd, if the construction is like the e-series.

Right now, with stock redline and stock S51 gears the 1-2 shift has a big gap and puts me down near 3,500 rpm into 2nd gear. The S51 1st would get me 3,800 with stock redline. What I really want is a 7000 RPM redline, and the S51 1st gear! Even if my 2-3, 3-4, 4-5 shifts were shy of 7000, it would be nice for the 1-2 shift to not drop under 4,000.
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post #17 of 27 (permalink) Old August 16th, 2011, 15:57 Thread Starter
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Quote:
Originally Posted by uspspro
interesting point. I seem to recall one of our v6 AutoXers using a S54/S51 hybrid.

It turns out the S51 has the same 1st and 2nd as the S54... I guess I would grab the entire input shaft from the S54 with 1st and 2nd, if the construction is like the e-series.

Right now, with stock redline and stock S51 gears the 1-2 shift has a big gap and puts me down near 3,500 rpm into 2nd gear. The S51 1st would get me 3,800 with stock redline. What I really want is a 7000 RPM redline, and the S51 1st gear! Even if my 2-3, 3-4, 4-5 shifts were shy of 7000, it would be nice for the 1-2 shift to not drop under 4,000.
I'm only referring to the E series, the S series may not have the same limitation
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post #18 of 27 (permalink) Old August 16th, 2011, 18:19
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I believe the S54 is the same. Virtually all transverse manual transmissions have 1st and 2nd gear cast/forged into the input shaft as the gear diameters are so small for them by necessity of their high ratio.

It's not a great sample set, but of the dozen or so various manual transmissions that I have torn down, 1-2 are always built into the input shaft.

Last edited by ThingyNess; August 16th, 2011 at 20:40.
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post #19 of 27 (permalink) Old August 16th, 2011, 18:22 Thread Starter
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the only way i'd buy that statement is with the following amendement:
Quote:
Originally Posted by ThingyNess
Virtually all transverse manual transmissions
and then, i'd certainly beleive it.
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post #20 of 27 (permalink) Old August 16th, 2011, 18:49
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very interesting read. I look forward to more data.
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