Mileage with a Motor Home

[TWright669]

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
>