350 Qjet Motorhome

[lefty o]

worth trying 3.73's or 4.10's .

[pamotorman]

the way i did boats with Q jets during the gasoline crunch was to tighten up the secondary air flap so it did not open, Q jets are calibrated to run wide open throttle just on the primaries. the boat would still get up on plane just took longer.

[DaveMcLain]

The first thing to check on an old Quadra jet is to make sure that it isn't getting a heavy float which will make it run too rich a lot of the time and burn tons of extra fuel. A bad choke pulloff was also common.

[novadude]

You can play around with the APT feature to change cruise AFR as well.

[rfoll]

In that year 350 the compression ratio is likely 8:1 or less. Smaller chamber heads could make a significant improvement. I had a low compression 350 that got 14 mpg freeway. I put 1.72 valve 64 cc heads and an Edelbrock Performer 2101 on the motor and it went up to 18 mpg. There's 1 or 2 mpg to be had playing with the apt. in the 80s most of the Quadrajets were the 800 cfm version. They found the economy didn't suffer from the larger primaries.

[F-BIRD'88]

You can test to see if the engine running rpm
at hyw speed is the big factor by changing the diameter
of the rear tires. Can be either taller or even shorter tires
for the test purpose.. Does a short tire (more engine rpm)
make the fuel mileage worse? Does a taller tire improve
the mileage? Either way can be informative..

On a motor home I bet the BIG factor is body areodynamics
at speed... Reduce body wind drag.

Front spoiler below bumper.. Rear duck tail..(look at the shape of a teardrop. Make it cut thru the air better. Stop air from going under the chassis etc.

[SupStk]

The gears are probably hurting the mileage. Going to 3.73 would probably be a step backward. You would then have a gutless motorhome with poor mileage. Let it spin up and keep that barn door under 65.

[tenxal]

The gears are probably hurting the mileage. Going to 3.73 would probably be a step backward. You would then have a gutless motorhome with poor mileage. Let it spin up and keep that barn door under 65.

Good advice.

[gnicholson]

Vortec heads 256 260 adv on 108 373 or 410 gears . Optimize the cruise afr and timing

[Roundybout]

I'd bet the difference going 65mph compared to 55mph is pretty significant so doing everything to make it efficient at those speeds will be beneficial. I think 3200/3300 at 55 is excessive but I wouldnt change gears. From how you describe it has good tip in and throttle response at those speeds you're probably at or over the torque peak. I'd look at an overdrive transmission to bring it down around 500 rpm. Should still be near the torque peak for good throttle response.

Anything you do though will pale in comparison to helping areodynamics as much as you can. Get to road HP down needed to maintain say 60mph and you'll see real gains.

[MadBill]

It would be worthwhile hooking up a vacuum gauge to find out how hard the engine's working under typical cruise conditions. If it's really low, say <6-7" Hg., there's probably not much you can do; I don't think shorter gears would help, given the current RPM. If it's reasonably high, say over 8-9" Hg., I'd pull the APT (adjustable part throttle) plug at the top front of the carb and turn down the adjuster 1/2 turn at a time until a slight surge is felt, then back off 1/2 turn. If this results in a turn or more leaner setting, the effect should be noticeable.

[gnicholson]

Im sure you could optimise the tuning and pick up a little performance and economy but the engine rpm is to high to do much and im sure with that mid 80s stock 350 it needs that gear. Thats why i suggested the head and cam change to really pick up the part throttle torque . Maybe a good 2.5 into 3 inch single exhaust. That way you could drop the gear and rpm down .

[tresi]

Not quite the same but I had a large van that made 10.5 mpg hwy. I pulled the heads, gave it a good valve job, installed a new cam( 204* @ .050 int by 214 @.050 ex 110 lobe sep.) a performer intake and a 600 Edelbrock carb. Then it 11.0 mpg. I played with the jets, rods and power piston springs. My finally adjust was like 3 steps leaner at cruise, and 1 1/2 steps lean at full power according to Edelbrock's jetting charts. I went 1 step stiffer on the springs. Mileage increased 13 - 13.5 mpg hwy while picking up power at all ranges.

[Geoff2]

Double check that you are not getting nozzle drip at idle, usually only happens with long-er duration cams. Float level should be 1/4".

[mag2555]

Not only a sad level of comp with those era heads, but the chambers burn rate leaves a lot on the table also!
I would also be looking to ditch those Exh Manifolds.