Where is my dang MPG?

[Secret Chimp]

I'm still searching for decent MPG on my 318. My most recent change was going to a clutch fan to a fixed fan - some improvement, but not the 19+ mpg people are getting with less mild setups than what I have.

Here's my setup:

1967 Coronet station wagon

318/727
Split dual exhausts with Flowmasters of some kind ahead of the rear axle on the stock manifolds
2.93 rear
Summit 6900 cam
Edelbrock Performer intake
MSD 6AL ignition on older Mopar Performance distributor
625 cfm Carter AVS 4966S leaned down to a rod change away from a light lean miss at cruise on a 4 hole spacer

Here's my distributor curve/setup:
15 initial
15 degrees vacuum at 18inHg
19 degrees mechanical all in by 3100
(this was set up by FBO)

The absolute best I've gotten out of this car is around 14.5 mpg all highway at 60-65 mph and around 2100 rpm.

What else can I try? I'm out of ideas short of just trying a different carburetor ($$$ with unknown benefits). I fiddled with a strip kit but I can't lean the carb any more than it is already. Choke comes off all the way.

 

[Cranky]

15 initial with 15 more from the advance or just 19 total period? If it's 19 total, then that may be one of the problems and 3100 is too high. I like mine all in by around 24-2500 and around 33 degrees depending on the combination.

 

[Secret Chimp]

15 initial and 15 additional from vacuum. I've monkeyed around with the vacuum pot some and tried having it come in at a lower vacuum level but that just made the engine upset at midtown speeds at a certain point.

 

[Dako]

1st off: With 2.93s, you are right...you should be getting a little better on the mileage. Not necessarily in town, but definitely on the highway.

I know you don't want to go slower, but 55-60 is the optimum speed range for fuel conservation on the older cars. That is why speed limits were dropped to 55mph in the 70s. Cars of today?...not so much, as they are much more aerodynamic. I'm guessing this would drop your rpms under 2,000. Most v-8s with over-drive run under 2,000 when in the 'hi gear'.

Three other things to check/ try. Only one may cost you and it shouldn't be over $70. they are: tire pressure, alignment, and ride height.

Tire pressure and alignment have bigger effects on mileage than most folks realize. Make sure they are both good. A front end alignment may cost you $60-$70, but if it hasn't been done in a while, it's a good investment. At a minimum I get one every time I replace tires. It also maximizes tire tread life, helping ensure even wear.

You can try playing with ride height too. This you can do yourself. Lower the front end by adjusting the torsion bars. This will reduce the amount of air that gets under the car, creating turbulence and drag. There is a balance between function and appearance...What looks good asthetically, yet doesn't go so low you start bottoming out the front end going through low spots, driveway entrances, etc. That, you will have to determine.

Tire compound can make a difference (the harder the better for fuel conservation), but I wouldn't switch out good tires just to increase mileage. That's why I didn't mention it earlier.

I'm assuming the plugs, wires, points, rotor cap, and all are fairly new? While it may run 'okay', if worn too much these will also rob fuel economy.

How wore out is the 318? a compression test will tell you a lot. A tester can be 'borrowed' from most chain parts stores...again something free. Good luck, hopefully some of this will help.

 

[Secret Chimp]

I've had the car aligned at its current ride height - it tracks straight and I'm not seeing any funny tire wear. I've gotten it about as low as I'm comfortable with (i.e. not bottoming out on big bumps)
The tires that came on it are some kind of cheapo Cooper that seem to have the squishiest sidewalls on the planet. I'm running 38 pounds at all four corners and the tires look like they're underinflated. They keep a lot of tread on the ground.

Plugs and wires are new, Blaster 2 coil isn't but it works, rotor's new. I haven't done a compression test on the engine yet but it doesn't go through oil and pulls pretty strongly.

Even if my compression's down a little, 67s have closed chamber heads so I'd think I'm still in the neighborhood of 8:1, and people can get high teens on late 70s 318s with party balloon levels of compression.

Could I have another bum carb even though I can't get it any leaner? I've had it open and cleaned it out twice but I don't really know what could be goofing up inside of it.

 

[PettyBlue72]

I know I am going to take some flack for this but here it goes.

You have a very heavy car with a low hp motor and I am assuming an automatic transmission. Your power to weight ratio is way unbalanced not to mention the power you are losing through the automatic. This might sound nuts but you might be better off with a big block and a 4 speed because the motor will have more power and will not have to work as hard to move the weight and you will lose less power through the 4 speed. The car should be better balanced as well.

Just my 2cents

Joe

 

[Secret Chimp]

I know it's a big beefer of a car and I'll never expect good mileage out of city driving with it with the weight it has to haul up

However on the highway I'd expect a lot better - around here there aren't any strong grades or anything else that should put more demand on the car vs the slight aerodynamic hit I have for having a wagon rear end instead of a sloping coupe or hardtop rear and trunk.

I've driven it on the highway with my shop vacuum gauge poked out from the hood and I can keep it inside of 16 degrees of vacuum on the slight hills we have and well over that on the flats, so I know I'm not asking too much of the engine to keep it going. I don't coast down too quickly in neutral either.

I'd dig a 383 or 440 with a 4 speed, but I don't have that kind of money. That's a good few years of gas with my crappy mpg right now! I'm trying to improve what I have to what others are reporting (i.e. a guy on Mofarts with a 77 Coronet and an 8:1 motor that will get 19-20) so I can take my family on trips without a big gas bill.

 

[Moparsmitty]

With the combination you described I wouldn't expect to get much more than 15 mpg on the highway. That's assuming that the engine is in good condition (good compression etc.) and is well tuned. I get a little better mileage using non ethanol gas. In our area some Union 76 and Conoco stations sell it. Also, it's been my experience that some guys may be fibbing a little about their mileage, not that I ever would.:no:

 

[Dako]

Sounds to me like you are looking in all the right places.

You mentioned Dual exhaust. Not that this will get you massive MPG increases, but does it have a cross-over pipe (H-pipe)? This helps balance out the back pressure and increase performance, thus increase MPG. Something a muffler shop can add in relatively cheap...just make sure they put a slip joint in the center in the event work ever needs to be done. Makes it a lot easier.

Cranky mentioned to someone on another thread about validating the odometer...in their case it was because they calculated (don't hit me, I'm just repeating what they stated) 26.5mpg. :eusa_think: Do you know your actual trip length...say by google map? Is it 'on' with your odometer?

Something else:
A larger diameter tire than original will log less miles on the odemeter. My `95 Dakota 4x4 is off 1 mile for every 10(!) because I now have 33"s on it. I doubt you have that much of a tire differential change (in my case it's about 4 1/2"s), but do you have some? Did the car come with a 25" dia. tire and you now have a 27" or 28" dia.? You could you be experiencing a bit of that.



BTW Cooper makes some very good tires.

 

[Secret Chimp]

I'm not running an H-pipe, but I've been mulling over replacing the Flowmasters with Super Turbos and will probably install such a pipe at the same time if I ever get around to it.

I got the car with an incorrect odometer gear (ran too slow, the car came with 14s but the previous owner replaced them with 15s and taller tires) but I've since replaced it with a smaller one from a box I got for free from someone here awhile ago, and it's almost exactly on the money. I've verified it with GPS Google Navigation on my phone both in terms of instantaneous speed and the odometer function. I tracked my miles with a speedometer/odometer app on my phone before I fixed the speedo and usually got around 10-12 on the old BBD.

I forgot to mention this (always forget something): the car was ordered with a towing package and came with a fixed fan and a big 26 inch radiator. I don't have a temp gauge on it yet, but the car never runs down the road at more than the 2/3 mark or so (just before the A in TEMPERATURE).

I read somewhere else that Mopar gauges were set up so the rightmost line of the inner bracket was 195 degrees, which is around where I want to be running. If that's the case for late 60s gauges, could I be losing some MPG from over-cooling? I installed the clutch fan to try and mitigate this, but I think I should have gone for a standard-duty clutch instead of the heavy-duty one.

 

[Dako]

Okay then, I'm about out.

I might question the aftermarket intake manifold (usually more power, but less fuel efficient), but at a constant (approx.) 2,000rpm I don't know that it would make much difference. Same with the carb, since you have leaned it out so much.

Slow your roll to 58-60mph and see what you notice...besides the trip taking just a bit longer. The up-side?...there will be that many more folks seeing your vintage ride and remembering 'when'. :headbang:

 

[Powerflite]

If you are only getting 14.5mpg, something is wrong. I have a '70 Satellite wagon with a 2bbl 318 and 2.76 gears. It got 17 mpg with single exhaust that was VERY restricted - the whole motor passed through a 1.5" pipe at one point. I swapped to 700-R4 overdrive and headers, and it gets noticeably better mileage now, but I haven't had time to figure out what yet.

My advice:
Look for gasoline leaks in your fuel line. You will be surprised how a bad your mileage gets from a slow drip. Lift your tires off the ground and rotate them to make sure your brakes are not sticking. Etc.

Then:
Get a newer 904 trans with a lockup converter and lower 2.75:1 1st gear. Then swap your gears to 2.76. This one change will make a significant difference on the highway. Not much change for surface driving though. Or swap to an overdrive :headbang:

Get rid of the original exhaust manifolds. Get 340 manifolds or newer dakota ones if they will fit.

 

[Secret Chimp]

Might it be worth trying a different carb? I was just reading a thread where a guy had leaned his Thermoquad out as much as possible for cruise, but somehow an AFB he had got 3-4 MPG on the same setup.

 

[Secret Chimp]

I'm also wondering if my timing setup is right at all. If my mechanical isn't coming in until 3100 rpm, I have maybe... 40-ish degrees at cruise (2000rpm) I would think? (15 initial+15 full vacuum = 30 + half of mechanical?) Is that enough for good economy or should I aim higher than that?

 

[Cranky]

Most low compression engines I've worked with like a lot of timing and imo, getting it all in by 3100 is kinda late because you don't run 3000 rpm at highway speeds with the gear ratio you have......and the only way to know how much total you have without a balancer that is marked up to at least 50 is to use a dial back timing light.

 

[Secret Chimp]

It's in at around 3000... maybe I should throw in some Mr. Gasket 925B springs and see what happens? I'm never cruising at more than 2100.

 

[Secret Chimp]

Well let me put it this way, ought I to chase carburetion or ignition first at this point?

 

[Supershafts]

I'd start with the low timing, having duals or 2 single exhaust is not helping performance or mileage, the other issues are these..

tires and weight of tires and wheels.

your rpm, you might think 2000 rpm is the golden rpm, but it may not be, many motors will make best performance or best efficiency at a specific rpm. My jeep is best mpg at 2400, my truck likes 2400, the dak likes 75/80 mph

Now since some want to throw in weight and hp lets try this, i have made performance additions but for now, 3900 lb dakota, 444,000 m's, v-6 between 75/80 mph i do over 19 mpg....

The truck has no mechanical fan, light tires, lightened AL shaft which made the biggest improvement, performance exhaust from the factory logs back. .
All that gets my dak over 19 mpg at 75/80 with a 3.73 final.