Improving MPG

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hawkgoalie31

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Hey Guys,

So i have a 1974 Plymouth Duster 318 3 speed 904 8 3/4 3:23 gears that my dad and I completely restored. I was debating getting a new car but I dont want to spend the money. What are some ways that I can improve my MPG. I was thinking changing to a 4 speed auto trans and maybe the fuel injector kit that edelbrock sells. I m open to all suggestions and comments.

Thanks
 
How many threads do you need for this?
 
the fuel injection kits from Edelbrock are $3600 pretty hard to re coup that kind of outlay , if it's a stock 318 you can go to less gear for highway mileage , what kind of mileage are you getting and what sort of driving do you do ? in the long run if it's a city daily car you could probably get a slant 6 for free or next to it and cut your city costs that way .Remember getting better mileage won't save you anything if you have to spend thousands to do it .
 
Well the simplest and one of the cheapest is a multispark ignition. Makes start ups much easier as well. I had a mallory hifire on my 318,and that could start in the dead of winter without a choke! Mileage improved as well as bottom end torque..
 
Nothing in the way of EFI or ignition is going to pay for itself.

Get "stuff" in at least factory shape. That is, check over the advance on the distributor, make sure both mechanical and vacuum advance are working, and that the engine is in tune. Investigate the carb. Few aftermarket carbs, Ed, Holley, etc, are jetted for what you might call optimum mileage,............... bigger DIAMETER tires. If you are using very wide tires that hurts mileage

"What" is the setup now? What carburetor, what ignition, cam? exhaust? WHAT is your present mileage?

Some of those 318s in good shape could pull over 20 mpg. And "What" do you expect?
 
Hey Guys,

Thanks for the help. Ya I think the Fuel injector kit is too much. I mostly do highway driving and I dont drive a ton of miles.

The Current set up i have is 650 performer series carb, mild rv hyd cam, TTI long headers, and I currently get about 12 MPG.

I dont expect to get great miliage but I would like you say do small things to get better MPG.

What about changing to a 4 speed
 
That is, check over the advance on the distributor, make sure both mechanical and vacuum advance are working, and that the engine is in tune. Investigate the carb. Few aftermarket carbs, Ed, Holley, etc, are jetted for what you might call optimum mileage,
Click to expand...
Agree on all of these. Is the 12mpg mixed use or highway? Local driving is very hard to calculate because a lot of time can be spent idling. Highway you should 20 mpg if everything is in good shape. We got 20 mpg on our first trip to Florida in my folks then new '69 Belvidere. 318 automatic, points distributor and A/C.

The things to focus on are the timing curve and the idle and transition circuits. A sensitve AFR logger (like the Innovate) with RPM and preferably with throttle position and/or MAP will help.
You may find the easy way out is to install a factory cam, and use all factory specs for timing and carb (assuming you use a factory carb).
Headers should help milage, or at least not hurt.
Agree that wide sticky tires tend to have more drag. Radials tend to have a little less drag than their bias ply equivalents.
 
ok well i have been tuning my slant for best possible mpg for my driving and geo location (altitude and DA). Here are some things to check for

Ignition
Coil getting a high voltage coil that requires a ballast bypass.
Upgrade the stock spark box for a chrome/FBO/Rev-n-ator box
Plug wires and ngk plugs
Curve the distributor 16 initial, 34 total by 2500rpm, and about 50* vacuum advance with stock can

Carb
Back off the accel pump
Tune low speed air bleeds, adjusting the air bleed will richen or lean the transfer curcuit
Then adjust idle mix screw with vacuum gauge
Pull jet until it surges, and add 2 jet size back
Adjust choke to come off a little sooner
Lower vacuum of PV (6.5" down to a 4.5")
Raise idle up a little like 750 in P

Car
Tire pressure
Suspension is new and lubed
Get the car as low as possible (1.5" rake forward)
Align the car with you in the drivers seat and half a tank of gas, have them set it even so it wont compensate for road crown
Check brakes for drag

just a start
 
FORGET the 4 speed. "If you have money to burn" you could consider an OVERDRIVE 4 speed, but again, you won't pay for the mpg with the money you spend converting.

Those cars ROUTINELY used to get "touching" 20mpg. 12 is dismally LOW. You should EASILY get 15-17, anyway, out of a 318

The factories ALL used to use the same strategy with carbs. Very small primaries, and big secondaries. Quadrajet for most GM, Thermoquad for Mopars and I don't remember WTF the Ferd Autolite carb used to be -- I didn't like 'em.

That carb may not be "the best," and it might be jetted fat.

Numba Wone:

Run a compression / leakdown test and determine if the engine is actually in good shape. If possible, check cam timing.

Numba Teu:

CHECK carefully TDC accuracy, timing, mechanical and vacuum advance. GET the ignition "up to" factory standards

Numba Tharee:

Try to find a carb with small secondaries. Holley used to make "economy" carbs, but if you can find one used that isn't all hacked up, that's great. You might be able to gain a lot by reworking the one you have

Numba Fow-war:

Check tire pressures, critically look at what you have for tires, and things of that nature. Take a GOOD look at your own driving habits. "Feather" the throttle when leaving a stop. The suggestion earlier about reducing the accelerator pump shot is good, but learn to drive without moving the pedal very much.

Now, listen up. I REALIZE gas "ain't what it used to be," and I realize that alcohol diluted gas does not deliver. "Back then" I put a low miles 71 340 in my 70 sixpack Roadrunner. This was MUCH heavier than our Darts, plus it had a Dana 60, a heavy, power-eating rear. This car only had 3.54 gear. That 340 would deliver over 17 and many times over 17.5

It had "kneee knocker" add on AC, Thermoquad, and I routinely (was in the Navy then) had 200 lbs of crap or more in the trunk.

81sruds.jpg
 
Meage getters are;

Numerical low gear ratios like 2.76
MSD or equal
Dial exhaust, lower restriction mufflers
Small Primary carb

As long as the engine is in good shape. I was able to get 20 mpg's from a 360 in a '79 Dodge Magnum with these tricks.
 
Mileage improves with efficiency.
Make your engine more efficient in the rpm-range it spends it's time in often (800-3000) and you will gain mileage and power at the same time.

Along the lines what Mattax mentioned I can also add a few myself;

- Keep the car and drivetrain as lightweight as possible. (904, 7-1/4 or 8-1/4" axle). Use aluminium wheels and small/narrow spare tire.
- Install an overdrive-transmission (A500 or GM OD-transmission)
- The correct gearratio in the rearaxle to keep the motor in it's good/torque rpm-range.
- Use the lightest (synthetic) oil the engine (oilpressure) will need. (min. 20psi at 650rpm HOT in Drive.
- Install a new timingchain set with a multiple keyway cranksprocket so the cam can be advanced a bit (= more low-end torque).
- Good flowing header and exhaustsystem with H or X-pipe.
- Lower car stance.
- Front spoiler to counter air turbulence under the car.
- Good working AFR gauge to tune the carb with.
- Good ignition spark with properly tuned ignition curve.
- Proper wheel alignment setup for modern tires.
 
Only thing I can add to this is give that engine cool fresh air, we all know that if you are running an open breather and sucking 190 to 200 degree air from under your hood into the mix 80 degree air will help. simple and it works, get your fuel line away from any heat.. well thats all I have to add
 
Vacuum gage, highest vacuum at cruising speed, don't move the throttle unless you need to. 2.76 gears. Tune, Tune, Fine Tune.
 
While some of the suggestions above are good, in this case he is starting with such incredibly POOR mileage that I feel I must discount some of them

"Hi end" MSD ignition is NOT going to add much at an already PROPERLY running engine. That is, if we take a PROPERLY running, tuned, in good condition, that SHOULD be getting 17--20 mpg, and add an MSD, I doubt you'll see 1/2 mpg

12 mpg is POOR with this car. Something is WRONG, and I don't mean adding a bunch of aftermarket.
 
After you get the ignition sorted out, Tune what you have, first, then on to the Performer Carb. First, make sure the choke is full open when hot. In tuning a Carter / Edelbrock, all you have to do is remove the air cleaner and gasket, then remove the 2 screws and caps that hold the metering rod pistons in place. Then change the metering rods to thicker to lean the mixture or thinner to richen the mixture.
 
your best bang for the $ would be changing rear to 2.76 /2.96 range. but my suburban gets better mileage at 72mph then at 55/60 mph range it has a small camshaft and the engine is more efficient at that spot. also what converter did use ?higher stall speed?
 
The main question I would ask is how much money are you willing to spend to save a few dollars in MPG? 4 speed? FI kit? You are going to spend thousands to save 3 MPG? Not to sound like an asshole, but spend your thousands on a Honda Civic and call it even. Then you can prolong the life of a beautiful Duster by taking away it's daily driving chores and making it a pleasure driving car.
 
Just my $.02, do the math, mpg, miles you drive, how much you'd spend on changes and the effort it would take, and see if you'd really be saving much. Like others said, make sure the engine is in proper tune.
 
The combo you have should be getting 18-20 mpg as is. What cam? What intake? What is the compression ratio? What torque converter?

Start with the basics. Make sure your vacuum advance is working correctly, and your timing is optimized. Then take a look at your carb. The manual has some really helpful charts. BTW, Edelbrock does not make a 650 Performer.
 
The best improvement I got was to park my duster and buy a used 5-speed neon. got 36 mpg on the highway, and the car only cost me 800, drove it for four years. 200 a year isn't bad!
 
Hey guys thanks for all the help

I agree with you guys its not worth it dumping 1-3 thousand dollars on after market parts.

I think my best bet for now is ti tune it better. I know the timing is dead on and the vacuum advance works good because we just had to replace the distributor because mine went bad.

I think what is killing me most in my gears in the rear end 3:23s and my carb being so out of tune. I m thinking about re jetting the carb to make it a little more lean.

When I was a bit younger I wanted my car to go fast and now that I m a littler older and more mature I thinking I want to cruise more and possibly get better MPG.

That 13 MPG is a combo of city and Highway but now I do alot more highway and I really dont city drive any more.

I m going to check some things and I will keep you posted. I understand there are many topics on this matter but I wanted specific information with my car and I really appreciate all the help

Thanks
 
a couple of follow up questions , what heads are on your motor , did you do the 360 heads swap when you added the cam , what intake ? if you did do the swap then change them back to the stock 318 heads and look for someone to trade you for a smaller 4bbl carb , also what is the power band of your cam and what stall convertor ? maybe your car isn't making any power down low enough to push it and you may have to much stall , a lot of cam swaps require a 3000 stall minimum convertor when put in a 318 and your cruising RPM is probably down around 2200-2500 so your convertor would be flashing still at this RPM wasting power and fuel .
 
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