Mileage with a Motor Home

[Clare Snyder]

Eric Elliott wrote:
> 
> Fuel mileage is not an inverse function of vacuum. The later GMD 4.3L
> engines use cam duration to decrease vacuum at low engine speeds for better
> economy. Less vacuum = less pumping loss. What other engines use this I
> don't know. I think it was Volvo that reportedly came near production with
> and engine that cut off fuel to cylinders to reduce power, causing near WOT
> operation at all times. Clean burn & less pumping loss were the goals.
> Uncomfortable uneven power pulsations were disadvantages. Remember the
> Cadillac 4-6-8 engine? Bet it didn't do much vacuum
> I run lean (stoich) @ 1000 to 2500 RPM idle to WOT in sustained operation.
> Engine doesn't mind, 700R4 doesn't last. Edelbrock 3500 EFI, ZZ3 roller cam,
> World products Sportsman heads, manual torque converter clutch switch. 350
> HP engine does about 1 MPG less than stock engine, but it doesn't do much
> vacuum.
> 
> Eric
> -----Original Message-----
> From: JRE CPA <JRECPA 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 10:03 AM
> Subject: Re: Mileage with a Motor Home
> 
> >In a message dated 98-03-30 09:05:37 EST, you write:
> >
> ><< Will it not take more than headers to pull the max torque down by
> > nearly 30%?? Even a 454 may tend to lug at 1800RPM on a machine the size
> > of your MH. If it lugs, the vacuum drops, and with it fuel economy. The
> > reason MAX torque RPM gives best fuel economy is because it allows
> > highest sustained cruise vacuum. Anything above or below tends to drop
> > off vacuum if running anywhere close to rated load. >>
> >
> >Your theory may be correct for full throttle torque, but I don't know too
> >many people that drive at full throttle. You need to know the part throttle
> >torque curve of the engine to know what rpm you can cruise at without
> >booging down the engine. The flatter the torque curve of the engine is
> >the lower you can run the engine at cruise speed.
> >My two cents worth.
> >
> >James
Fuel injection does change the possibilities. I assumed (possibly in
error) the MH was running a carbed 454. With carburetion, you will find
there are "rules" that hold almost universally - and low vacuum means
poor efficiency. When vac drops, power enrichment happens - the "vacuum
computer" that is a carb does not adaptively reprogram very well. When
too lean, power and vacuum drop.
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