Mileage with a Motor Home

[Eric Elliott]

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