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Any car equipped with a distributor and carburetor can get better mileage with some adjustments, changes and modifications. Some are more cost-effective or easier to install than others, but all will reduce operating expense per mile. There are many misconceptions and misunderstandings concerning mileage; I will address some of these here.
Carburetor choice
This is important, but the mileage (and maximum power) is not directly proportionate to the size of the carburetor, the throttle disc(s), or even the venturis.
A 2 bbl. will not necessarily get better mileage than a 4 bbl., nor will a small (500 CFM) 4 bbl. always improve mileage over a larger (750 CFM) carburetor.
To be valid, the comparison should be made only between the primary barrels of the 4 bbl., and where it’s smaller the mileage can be better. Needless to say, this will only be true if the secondaries are not used frequently.
The size of the secondary barrels in a 4 bbl., generally speaking, will not affect mileage unless, again, they are used frequently. However, any American car can reach and sustain any legal cruising speed on only the primary barrels of a 4 bbl.
“Spread-bore” (asymmetrical) carburetors, such as the QuadraJet, ThermoQuad, Holley 4175 & 4011, etc. have much larger secondary than primary barrels, even larger than many “square-bore” (symmetrical) carburetors such as Holley 4150/4165, Carter/Edelbrock AFB/AVS, etc. When properly selected, installed and adjusted, they give better mileage than any but the smallest 2 bbls. with no loss of power.
Jetting
In addition, carburetor jetting should be adjusted, especially if any changes have been made to the car (as described below), remember that this also leans the primary full throttle mixture.
If using a Holley, the power valve should be set to as close to cruise vacuum - 1” as possible (cruise vacuum = 10”, PV = 9”) so that it doesn’t open unless you need it. Drop the primary main jets 1 size and test.
If using a Carter WCFB, AFB, AVS, ThermoQuad, Edelbrock, Rochester 2G, 4G, QuadraJet etc. the cruise mixture cut-out point is regulated by the power piston spring. A weaker spring will allow running on the lean metering rod step at higher throttle openings. Don’t cut or modify the spring, replace both springs. Try 1 size smaller primary main jets and test.
Intake manifold exhaust cross-over passage
High temperature is better for mileage, but (according to Vizard) the cross-over is too hot unless most of your driving is short trips. Use a blank gasket, and some stuffing in the passages. With a water-heated manifold such as SBC just use a 195 thermostat. With dry manifolds such as B/RB you can close off the passage with weld on both sides, drill & tap the runner to FNPT on both sides, and put hot water through it. Use a T off your heater line in both directions so as not to restrict it.
Choke
The choke is always needed for reliable starting and stable idle under any conditions, except when the motor is already at or near operating temperature, even in warm climates. Even Texas does not reach 180° during the summer. If you “don’t need a choke”, your primary mixture is too rich.
Other modifications
Here are some other methods, with notes as to complexity, cost, etc. |
Modification |
Effect |
Complexity |
Expense |
Notes |
Reduce maximum oil pressure to lowest safe level (40 psi?) |
Reduces
internal
power
losses |
Easy to do in B/RB, hard in LA |
Gaskets and new relief spring |
Watch your gauge carefully |
Lightest safe oil viscosity (5W30?) |
Varies with air temperature and load |
Do with oil change |
Oil level 1 pint down from “full” mark |
Make a new mark on the stick (drill small hole?) |
Reduce line pressure in 904/727 to 90 psi |
Read your service manual |
Free |
Watch for slow shifts |
Dual exhaust |
Complex |
Expensive |
Large pipes no back-pressure best |
Increase alternator, PS & water pump pulley sizes |
Reduces
external
drag |
Service manual; 1 day’s work |
March, etc.; some JY parts will work |
Watch for low charging rate |
Use A/C water pump impeller |
Read your service manual |
Cheap |
Watch for overheating |
Wheel alignment |
Reduces
rolling
resistance |
Professional job |
Front end parts must be good |
High tire pressure |
Easy |
Free |
32 psi or highest your comfort level will stand. Varies with air temperature |
Weight reduction |
Back seat, etc. Also improves acceleration |
Lower stall speed |
Reduces
engine
speed |
Service manual; 1 day’s work |
Cheap used stock converter |
Does not work well with big cams |
Low number axle ratio
(2.93 instead of 3.55) |
Complex |
Major expense |
A833 OD (Feather Duster) 4 speed trans |
Very wide ratios. Reduces acceleration |
500/518; AOD, 700R OD trans |
Very complex |
Wide ratios. |
Lock-up converter |
Easiest to get whole transmission. |
Tall rear tires |
Easy |
Borrow a set for testing |
Watch for fender clearance |
Lower nose of car 1” |
Reduces
wind
resistance |
Torsion bar adjustment |
Requires wheel alignment |
Watch for bottoming |
Remove spoilers, roof rack, wings etc. |
|
Free |
Saves weight |
Slightly more initial ignition timing (2-4°) |
Increases
low-speed
power |
Service manual |
Watch for pinging |
More initial ignition timing (5-10°) |
May require distributor re-curving |
Cheap |
Maximum vacuum advance |
Allows
leaner
mixture |
Distributor may have to come out |
195° thermostat |
Service manual, 1 hour |
Hot air duct to air cleaner |
Use original air cleaner |
Other models and brands may work |
Does not affect power |
Correct choke adjustment |
Several hours |
Free |
Manual choke good substitute |
Dual-plane intake manifold |
Expensive |
Increases power at low to cruising speed. Moves torque curve into cruising range. May reduce peak power. Watch for pinging. |
Milder cam |
Increases
cylinder
pressure |
Very complex |
Very expensive |
High compression |
Increases power. Gets more energy out of every gas molecule. Watch for pinging. |
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