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Old 02-25-2009, 01:34 PM   #11 (permalink)
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I'd tell you, but I'd have to move my MPGuino, since it sits in front of the tach...

I seem to recall 2000 RPM being just under 60 MPH.

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Old 02-25-2009, 01:36 PM   #12 (permalink)
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Fifth gear at 2,000 rpm = 51 mph in my Pontiac Vibe.
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Old 02-25-2009, 03:55 PM   #13 (permalink)
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Cruising at peak torque RPM for best fuel efficiency is a "rule of thumb" myth that gets regurgitated regularly. Not sure why.

Granted, different motors behave differently and there may be some where it may be true, but it's just not that cut & dried.

Graph a speed vs. MPG, or RPM vs MPG in top gear for your vehicle and I bet you a donut you'll find your best efficiency is far lower down the tach than peak torque (and also below the best BSFC zone).
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Old 02-25-2009, 03:55 PM   #14 (permalink)
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2007 Honda Civic EX, 5 speed manual @ 2000 rpm = about 45-46 mph.

When I try to go for maximum mpg (and I won't block/endanger other traffic), I try to keep the engine between 1750 and 2100 rpm. Nothing nicer than cruising around Mount Rainier going 40-45 mph.

Wasn't there a post on calculating the "ideal piston speed"? You entered bore and stroke and the formula produced a rpm range where the engine isn't lugging or working against the momentum of the piston motion. I immediately got an analogy of riding a bike - there is a range where your legs will feel most comfortable spinning. Slower, and 'yer thighs burn up, faster, and you start bouncing on the seat.
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Old 02-25-2009, 06:16 PM   #15 (permalink)
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the two rules of thumb I have heard are that the point of lowest brake specific fuel consumption is usually near the torque peak, or where average piston speed is 1200 feet per second.
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Old 02-25-2009, 06:47 PM   #16 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Graph a speed vs. MPG, or RPM vs MPG in top gear for your vehicle and I bet you a donut you'll find your best efficiency is far lower down the tach than peak torque (and also below the best BSFC zone).
Amen!!

My CRX Si has short-short gearing, and is only going 40 MPH at 2000 RPM in top gear. At 60 MPH, it's spinning 3000 RPM (yes, it's a linear relationship--it has to be).

The torque peak for my motor is in the 4000-4500 RPM range. I guarantee you that the mileage is lower at 80 MPH than it is at 40 MPH. In fact, I gained about 10% fuel economy by backing off my top RPMs from 2500 down to 2000. This is measured over several full tanks of fuel, since I don't have instrumentation to give me instantaneous fuel usage.

I do know that a steady-state 80 MPH cruise will net me right around 33 MPG on the SF-LA roundtrip. I lack the patience (and the opportunity!) to try it at 40 MPH, but I am pulling right around 40 MPG in my daily commute these days. And since it takes four times the energy to push my car through the air at 80 MPH as it does at 40 MPH, I'd be really surprised if a steady 40 MPH didn't pull down MPGs in the mid-40s or higher.


...BTW, I'm surprised at the OP's Taco takes 40 HP to run at 55 MPH; I would have SWAGged (Scientific Wild-*ss Guess) it to be half of that.

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Old 02-25-2009, 06:52 PM   #17 (permalink)
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Quote:
Originally Posted by shovel View Post
the rule of thumb has always been to gear (axles) and/or shift (manual trans) so your most frequent travel speed coincides with the torque peak RPM of your engine. .
Quote:
Originally Posted by MetroMPG View Post
Graph a speed vs. MPG, or RPM vs MPG in top gear for your vehicle and I bet you a donut you'll find your best efficiency is far lower down the tach than peak torque (and also below the best BSFC zone).
I don't know the ideal answer to this quandry, but I want to point out that there's a difference between each of the scenarios laid out here.

The expedition rule of thumb implies that you predict your desired travel speed and then install suitable differential gears which place your engine speed at peak torque RPM when you are traveling at that road speed.

It doesn't mean you are getting the best mileage the vehicle is capable of, but instead you are getting the best mileage at that speed for that vehicle (presumably) - and it is still only a rule of thumb.

I'd wager you're right about the efficiency scenario you described, where you take an existing vehicle with existing/unchanged gear ratios. It's also likely that the expedition vehicle rule of thumb applies far more closely to the types of engines used in heavy overland trucks, than zippy town cars.
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Old 02-25-2009, 11:48 PM   #18 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Cruising at peak torque RPM for best fuel efficiency is a "rule of thumb" myth that gets regurgitated regularly. Not sure why.
I notice that as well and I think the mix up is mistaking the engine BSFC islands for the vehicle optimum FE.

There is one general rule of thumb which states:

"As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s).
Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720"

As with any "rule of thumb" there are as many exceptions to it as cases supporting it.

In my car GM 3.8 V6 with auto 4L60E trans and four door sedan body 2 000 RPM = 110 kph (65 mph approx) and the most economical steady cruising speed is around 1750 RPM (95 kph = 55 mph aprox.)

Pete.
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Old 02-26-2009, 07:11 AM   #19 (permalink)
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Accelerating near peak torque will usually get you the best FE. Cruising at peak torque will definitely NOT get you the best FE. To look at this we need to take speed out of the question (obviously, if you slow down your load decreases, and thus the amount of fuel needed). We need to look at a specific load and graph it on a BSFC chart. I did just this a while back in this thread.

http://ecomodder.com/forum/showthrea...html#post42773

If you take a look at a bunch of BSFC charts, you'll see that in 99% of the situations you could gain quite a bit of efficiency by lowering rpm and increasing load. I'm sure there is an exception out there somewhere, but I have yet to see it.
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Old 02-26-2009, 04:07 PM   #20 (permalink)
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A lot of interesting stuff here
At 55 I doubled checked it takes 35hp in 32 degree weather to do 55. About 1.9gph. In warm 60+ 1.7gph ?hp to go 55 mph.


177ft/lbs at 4000 rpm
150hp at 4800 rpm

Stroke 95mm bore 95mm

According to. The equation. 2000rpm= 21.1
1750 rpm = 18.4. I think about 45 mph. But I get the same usually from 35 till about 50-55

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