Mileage with a Motor Home

[Eric Elliott]

Hello,
My truck makes best MPG @ about 41 MPH @ about 1000 RPM. Peak torque is
about 3400 RPM, it is not stock. Original stock engine did same @ same
speed. 41 MPH  is where the 700R4 will do 4th gear & locked torque converter
clutch. At 41 MPH &16 MPG, the fuel required to idle the engine, 0.6 GPH is
a substantial part of the total. Granted the engine is most efficient @ 3400
RPM given WOT, but we don't measure fuel economy @ WOT. The truck likes the
minimum speed that can get minimum engine turns per foot travel.

All fuel, HP, torque and BSFC curves that I have been able to find specify
operation @ WOT. No curves ever describe part throttle operation. WHY?

Cummins sales sheets show their 5.9 L engines from 160 to 300 HP all get
best BSFC near 1100 RPM. Even a short drive in the ram will show the turbo &
torque happen at 1900. Go figure.
-----Original Message-----
From: TWright669 <TWright669 at aol.com>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Monday, March 30, 1998 6:59 PM
Subject: Re: Mileage with a Motor Home


>OK, I've been peeking in on this thread on and off for a while now, and I
feel
>compelled to put in my $.02 worth.
>You'll have to excuse me if I'm wrong, cuz I'm not an engineer (I'm an
>archaeologist), but there are a few things going around that I dont believe
>are correct.
>First is that the most efficient cruising speed is at peak torque.  I had a
>3spd auto in my car, and on the highway I would cruise at about 3000rpm
(which
>is about where peak torque is on my car) and got 17mpg.  Then, I installed
a
>4spd auto and my cruising rpm dropped to about 2100.  My milage went UP to
>22mpg (as I would expect.)  There is something you guys are missing when
you
>say an engine is most efficient at peak torque.  I dont know what it is,
but
>it probably has something to do with the fact that the engine is most
>efficient at peak torque, in that it makes the most power per stroke or
most
>power per unit of gas or something.  BUT I think there are other  (more?)
>factors at play when talking about MILAGE.  (like all the extra energy
>required to turn the engine at a highr rpm,  and transmission gears, etc.?)
>Let me try and pull something out of my ass, here:  Power = work/time.
but
>miles/gallon does not have the factor of TIME anywhere in it (no mention of
>how long it takes you to get there!). THERFORE all this talk about where an
>engine produces the most power per gallon is irrelevent.
>In other words, what good is peak torque, if your not using it to go down
the
>road?  My guess is that it is being wasted somewhere.
>My guess: fewer strokes per mile = fewer injectons per mile = less gas
being
>consumed.
>My conclusion, Lower RPMs are better, period.  You need, maybe, to look at
>where your engine produces just enough torque to move you down the road at
the
>speed you want. Any more torque=wasted gas.
>Dont flame me to hard!  I'm doing my best!
>Tom
>
>
><Andris,
>
><There is one HUGE variable that's hard to account for: POWER.   how much
><power does it take to propel your buggy down the road at 60 or 70 mph.
>
><Now you can look at driving the same speed in different gears. This would
><presumably require the same HP, so it would cancel out.   If you had an
><nfinite number of gears, you could travel a constant speed and select the
><most efficient RPM.
>
><On paper you can do this only if you have a fuel consumption curve [bsfc]
><for your engine (although if you've modified the cam, heads... this may
not
><be true anymore). in that case, you can look for the lowest point on the
><curve for your operating conditions.
>
><Where can you find this curve?  I don't know.  I've only seen and used
them
><for diesels. I have been looking for one for a gas engine, but I'm still
><empty handed.
>
><^Bernie
>
>
>
>-----Original Message-----
>From: Andris Skulte <askulte at emerald.tufts.edu>
>To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
>Date: Monday, March 30, 1998 4:42 PM
>Subject: Re: Mileage with a Motor Home
>
>
>>Bernie Jacobsen wrote:
>>>
>>> I want to get more reliable data!
>>> My plan as of now,  is to run the GB headers cruising at about 2700-2800
>>> and check mileage. The next step is to lower the cruising rpm with OD
>(found
>>> one used for $400) and torque converter.
>>
>>Can you figure out mileage if you know the vehicle speed, rpms, fuel
>>pressure and injector pulsewidths? This way I could figure out the best
>>cruise gear/rpms for my Camaro... Everyone's mentioned best cruise at
>>max torque, so should I be cruising at 3,000 rpm (off dynojet runs)? Do
>>I want the most vaccuum? If I go WOT at 3,000 rpm, my torque peaks at
>>475 ft-lbs, which propels me up to ludicrous speed in no time... I get
>>nervous cruising at more than 90 anyways... TIA!
>>
>>Andris
>>
>
>
>