Guys,
I have finally got the 56 Dodge 4 door runnning for my wife's everyday driver. Problem is, it is only getting 10-12 mpg!!

I know it is a heavy car, but I thought it would do better. Here is the rundown on it.

383 with projection fuel injection. 727 transmission with 3.23 rear axle ratio. I am running P215/65 R 15 tires.

I built the motor around 1998 and it doesn't have many miles on it, so I threw it in the coronet. This car was supposed to be a budget car, so I didn't go the route of buying a newer model 318 or 360 with overdrive, even though that is what I intended to do at first.

I am thinking of putting a 2.76 rear in the car and seeing what that will do first. I would ideally like to get around 18 mpg with it, which a lot of guys in this area said it would do with the fuel injection.

My other thought is a new cam. I don't remember the specs on the cam that went into the motor since it was built so long ago, but it seems like it made power in the 3500 to 5000 range and was for a mild drag car (built it for a 1970 Coronet but never put it in there).

Do you guys have any ideas for cams that will give good on ramp power, yet also deliver on the fuel economy at 65-70? With 3.23 I am running around 2800 rpms at 65.

Thanks

I'll post pics of the car as soon as I get a chance to take some.

Have you modified the advance curve in the distributor? Maximum advance without detonation or pinging should deliver best fuel economy. Are you running vacuum advance? It's a good thing for fuel economy. It may be that the engine will need less vacuum advance after resetting the mechanical advance.

Does the fuel injection system allow setting the air / fuel ratio at cruise? If so, leaning the mixture can help economy.

The cam and axle gears need to be matched. The cam determines, to a large degree, the RPM range where the engine will make power and deliver fuel economy. The gear ratio needs to be such that the engine is running in the power band of the cam at cruise.

I have not modified the advance curve on the distributor. I really don't know how to do that. If there is a tech article on it and someone can post a link to it, I would apprciate it.

I am running vacuum advance, and we have set the advance on the distributor at 34degrees.

I can lean the air/fuel ratio out on the fuel injection system, and so far I have it leaned down. I still have pretty strong exhaust fumes at idle, even with it leaned down. I have also adjusted the fuel pressure down to 18 psi which is the minimum for the big blocks on this system. The car seems to run better at 20 or 21 psi, but I am afraid fuel economy will really suffer.

I wish I knew what cam was in the car, and I am sure the machine shop would not keep records to show what was in it either.

That Projection system does NOT have a very good REP on being a good combo. You would be muchO bester off with a good carb and a MPG intake.

And you know what I would suggest !

Doc Fiberglass-

What combo would you suggest? I don't spend a whole lot of time on here, so don't know what you would suggest. Am I opening a can of worms?

What is an MPG intake? I will have to look and see what kind of intake I have on it. It is an aluminum one from Mopar Performance I think, and it only has one line in the middle where the carb sits (dual plane maybe?).

I think your cam is killing your MPG. If it makes power from 3500-6500, your probably never in its power band. Going from 3:23s to 2:76 will probably make it worse because you getting farther from your power band.

Doc Fiberglass is a Thermoquad fan. Personally, I think you would just as well with an AFB. I have not heard a Projection was any harder on gas then anything else, I simply think they are a bit less tunable then the newer efi stuff is, which gives them a bad rep to some. You already have the projection, I'd keep using it.

Swap the cam for something more street friendly (talk to a cam grinder) and change to a 2:76 gear, and see where your at. I think 14-15 mpg with city and country driving, on todays gas, would be about as good as I would expect with a 383. When its all done, the driver has as much effect on fuel mileage as any other part of the car. You have to constantly drive for fuel mileage if you expect to get it. Gene

I don't know much about tuning the projection system for economy or power. It seems like that is one of the older aftermarket fuel injection systems. I suspect that if the air/fuel ratio can be leaned out enough, it can be made to deliver decent fuel economy. I believe the older lean burn systems Chrysler made took air/fuel ratios to around 16:1 - 18:1 during cruising conditions. That might be a good initial setting.

As far as fuel pressure goes, lower pressure will lean the mixture a bit. This should not be necessary if the fuel injection has an oxygen sensor. The oxygen sensor will report the air fuel ratio to the computer. The computer should lean the fuel mixture to what ever setting is programed into the the computer's memory. Of course, this also assumes that the oxygen sensor is capable of reading real lean mixtures. Some sensors are incapable of reading extreme mixture settings.

Getting the distributor tuned could help the mileage and power. If you have a dial back timing light, it can be done at home. The cost for parts is usually minimal, but it will take some time. Unfortunately, I don't know of any good online sources which tell how to do it. Trying to explain how to do it without pictures would be a bit difficult or impossible. Getting it to the optimal setting could easily be worth several MPG's though. (depending on how far off it is)

If you keep the 727 transmission, 14-15 mpg is probably a reasonable expectation for mixed driving. It should do a bit better with 100% highway driving. If you want to go beyond this changing the transmission and/or rear axle will be required.

If you want to achieve some real substantial fuel economy, a lower speed, high torque cam will probably be required.

A 2.2 or 2.45 rear axle ratio would help economy a bit more. The 1980's M bodies (such as the diplomats) used these ratio axles. A second and better option is to install an overdrive transmission. The overdrive will effectively allow a 3.55 rear axle to turn into a 2.45 axle when the overdrive engages. The 3.55 ratio maintains performance, while the overdrive allows for fuel economy and more comfortable highway travel.

A Lock-up torque converter will be required to slow the engine down enough for best economy. This is particularly true when running a 2.2 or 2.45 axle. (or an overdrive with that has an overall ratio in this range) Gear ratios in this range are much harder for the transmission to pull, so the torque converter slips far more than it would with 3.23 or 3.55 gears. The extra slippage isn't good for the transmission or fuel economy.

If swapping in an overdrive transmission you could try running the 2.76 axle. It may work. This would give an overall ratio of around 1.90, assuming a .69 overdrive ratio. Most of the guys on this forum would consider this idea crazy. But lets look at a newer, large car which get decent fuel economy.

Some of the Buick Roadmasters from the 1990's used 2.56 axles with a similar overdrive ratio. These cars weigh around 4200 pounds,which is probably more than a 1956 Dodge. These cars were more aerodynamic, and had a bit smaller engine.(350ci) But these cars would often get 25 mpg on the highway. With a 56 Dodge the aerodynamic drag will be higher, but 18-20 MPG on a 100% highway trip seems reasonable. It will take some tuning, parts swapping, and messing with it to perfect the performance and economy.

My thoughts:

If you're wanting MPG, make sure you have a heat riser passge thru your intake, especially with TBI systems like the P-J. Heat helps atomization, and atomization = better MPG.

For a cam, get a Lunati Voodo 60302 or something similar. Err on the small side.

If the P-J has an O2 sensor setup (and hopefully it's a wideband), turn the FP UP, not down, IMHO. Increasing pressure increases the atomization, which helps MPG. The ECU should watch via the O2 sensor and then shorten the injector pulsewidths to reduce the delivered fuel back to target. Depending on the P-J's learning capacity, this may take a few days of running to adjust .

Depending on how much you can program the air-fuel settings, leaning the AF to 15.5-16 on the highway will help. You will need more ign advance at those spots to help the burn. If you get a misfire the AF is too lean (keep in mind a lean misfire will make the O2 sensor show rich, so don't get confused when it happens).

Don't get terribly concerned if the engine wants to be a little rich at idle. That's somewhat normal, some engines aren't happy idling at 14.7.

-Quickdodge--I looked at doing the overdrive thing at the beginning of building the car, but a local custom shop here told me I should be able to get 18 to 20 hwy miles with a fuel injection and 3.23 rears (driving 65 of course)

Has anyone done this swap? Overdrive 518 on a 727

I will work on the distributor first, then a cam swap, and the overdrive as a last resort. I am afraid if I put an overdrive in, I will have to cut the driveshaft tunnel again.

My P-J system is the older kind without the oxygen sensor. I can upgrade it to the new system with the oxygen sensors, but that is a computer change and an adapter for the wiring harness. I may have to do it later.

thanks for all the replies.

You might get 18-20 mpg without the overdrive. Limiting cruising speed would likely help with 3.23 gears. Personally, driving 65 is difficult because it just seems so slow!

I drove a mid to later 70's Chrysler Newport years back. It had a 400 with a 2 barrel carb. I'm uncertain on the axle ratio now, but that old car would cruise comfortably on the highway, so I'd guess it may have been a 2.76. That big old car managed to get a fairly consistent 17 mpg on the highway. I was pleasantly surprised.

The 400 is slightly bigger than your 383. The Newport may have been a bit heavier. Your 56 would likely have more aerodynamic drag. Your engine may have higher compression which would help fuel economy. Comparisons are difficult because there are many factors which affect fuel economy!

Looking at the distributor first is a good plan. Maximize your current combination and see what it will do before making major changes. The only down side to this is that changing the cam will mean redoing the distributor to obtain maximum performance and economy with the new cam.

An overdrive conversion is nice, but it's somewhat expensive on a big block. It would take quite a few miles to save enough gas to pay for the swap. Swapping in a 2.45 or maybe a 2.76 axle would be easier and cheaper. If the M-body 2.45 axles are to narrow or you can't find an 8 1/4 axle with this gear, some of the late B bodies had 2.45 which were wider. A LOT of the M-bodies with 2.45 gears had 7 1/4" axles. I believe the 8 1/4 was the smallest axle offered in the late b-bodies.

Suggest increasing intial timing until the rich smell at idle goes away, if possible. Then, shut the engine off and see if it will start. If so, let it sit a while to heat soak, then see if it still starts. If that's all good, take it for a spin and see if you have any pinging. If so, adjust the total advance of the distributor. Lighter springs will bring the advance in earlier, I suggest all in by 2500 or so. Probably want to change that camshaft to a current-technology profile if this doesn't get you closer to where you want to be. I'd recommend the Comp Cams tech line for a recommendation.
I got 13.3 mpg with my 446 in my Charger at a careful 70mph on the highway, and it ran mid-12s with 3.23s in the same configuration. Keep us posted!

Thanks for the replies guys. I changed my plug wires out (was using an old set from when the motor was in the ramcharger, possibly had 10,000 dirty miles on them). I also advanced the distributor until it backfired through the carb (supposed to be at 38 degrees, but my father in law didn't let me rev the engine up above 2500, so I think it was a little more). I then backed it off a little bit and it runs a lot better.

I noticed when I was changing the wires that probably 5 of the 8 cylinders were pretty wet with fuel (checking the plugs at the same time). Only 3 cylinders had a nice clean burn to the spark plug. I haven't checked them again yet, but I do know that the occasional miss is not occuring.

Also, I forgot that I have a 2,000 rpm stall converter in the transmission. It is a B&M unit. The only time it seems to affect the driveability of the car is after cold start up to get the car to move, you have to rev the motor a little. After that, all driving seems normal. Will this hurt fuel economy?

One problem I have on fuel economy is when I am checking out the running of the car, I have a hard time keeping my foot out of it . It runs great from 3,000 RPMs up. I will see what this cam runs on fuel economy, but I have a feeling I will need to swap them out. Surely this can be done in a weekend right? Also, can you get the timing chain cover off with the motor still in the car?

I guess my question should be: "Do you have to remove the oil pan to take the timing chain cover off?"

I have plenty of room in the 56 to get to the front of the motor, however, there is only about 3/4" between the oil pan and the cross member (fat man fab front end kit), so I have to remove the motor to get the oil pan off, or at least unbolt it and lift it a little.

There are 2 bolts that screw through the pan into the timing chain cover. Those 2 bolts are in the front of the oil pan rail. If you can access those bolts to remove and reinstall them, (they are about 3/4" long) I see no reason you can't change the timing chain while the motor is still in the car. The timing chain gasket set comes with a front pan gasket piece. Gene

Finally bit the bullet and put on an MSD box on my 74 truck.
Want to burn every last hydrocarbon going the chamber.
To recent to tell mileage but it runs like a top and I should have done it long ago.
Stock ecu just to outdated anymore.

I'd agree with a number of these suggestions, but not all of them.

IMO, to get the most efficiency out of the engine you need to maximize the use of each power cycle. So make sure the charge is as lean as practical, ensure it is squeezed as tightly as octane permits, then burn it as completly as possible.

So, based on what I've read so far, I think the best bang for the buck will be a small duration cam. Getting the the cylinder pressure up to more completly burn the charge can be accomplished by altering the cam timing events.

Tuning the carb with a O2 sensor will give you a good picture on where in the range of use your mixture can be dialed back some.

Others have already brought up the distributor and ignition issues. Find The Dr's Guide to Improving Your Ignition for more info on how to step these system up.