mid 80's 318 mpg economy numbers...

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BigBlockMopar

BigBlockMopar

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Anyone know first hand (or 2nd hand from literature) what the average mpg was of mid '80s (pre-roller) 318 engines?

I think the compression ratio was still down the drain in that era.
Want to compare the lousy mpg of the '80s 318 I have in my '73 Dart.
 
We could supertune a 75-76 Cordoba 318 and get low to mid 20's highway. Left Chrysler about then and never had any 318's from the 80's
 
My 80 power wagon was getting 11 mpg,35x12.50-15 inch tires. Was getting better until ethanol showed up. Dont drive it now,needs a sheet metal overhaul.
 
I wouldn't take too much stock in fuel mileage of these engines. They were low compression, small valve, weak cammed pigs. I'll bet most had a less than 3:23 rear gear. If they got mid 20's you will have to duplicate those parameters to get the same. My 70 318 Swinger with a 3:23 gear and 27" tires gets right at 20 on the highway if I drive 55. I have no idea what it's taching. I don't have one. It seems if you can keep the rpm below 2000 at highway speeds you will get pretty good mileage. That of course makes a difference how mild the engine is built. You can add a lock up converter or a o/d tranny to help get bigger numbers.
 
Let's say my engine is getting the same mileage as the Vans where getting during the '80s.
About 6-8mpg too low, although I'm running on propane fuel.

Engine setup;
318, no overbore that I know off. Block casting dated 1984 or 5. Heads are from '86 I recall.
Stock cam and rockers.
Replaced timing chainset 2 years ago.
Changed to 4-bbl Edelbrock intake and '4-bbl' propane carb.
Stock thermoclutch fan (clutch is worn and freewheeling at all times, no drag. Doesn't overheat at all).

Ignition is fully digital driven from a LeanBurn distributor with MegaSquirt ECU, GM HEI module and MSD E-core coil.
Timing map and vacuum advance curves have been optimised for propane.
Full length Doug's headers, dual 2.5" X-pipe exhaust with Borla ProXS mufflers.
Cold air intake into closed airfilter housing from a late '70s 400 engine.

Transmission:
A518 overdrive with a large '60s stock stall convertor.

Rear Axle:
Narrowed (to stock A-body) 8-3/4" with 3.55:1 gears

Wheels:
No drag on the brakes.
17" lightweight wheels and tires, properly aligned.

Engine Vacuum:
Engine has a rock steady and high vacuum at idle and during driving.
It can idle @ 500rpm all day with transmission in Drive, and hold the vacuumneedle steady at close to 21".
On flat stretches of highway it pulls 18-19" vacuum, running 60-70mph (2200-2400rpm) in Overdrive.

Driving style:
Certainly not "Granny", and I drive usually a little faster than other traffic, but not flogging it often.
Daily driving distance consists of 90% highway, keeping it below 80mph usually.

Car weight:
1440kg according to the title, but I've added a 2nd propane fuel tank in the trunk (but also removed the stock gasoline fuel tank).
 
Wait a second, how can you have a O/D trans with a 60's stock stall convertor. You need a lock up for it to work.
 
Not all 518-OD transmissions have Lockup.
My current one only has OD, so 'any' older 727-stall convertor will work.
This '60s stall convertor was the only one I had around that suited the combo.
Well, I did have a slightly smaller TCI 'Saturday Night Special' convertor. Tried it once, and the convertor-slip made the car feel like it had jelly inside the transmission.
Took it out again the very next weekend.

Mileage 'card';

DartMileage.jpg
 
I like to believe a more modern, smaller, but still tight stall convertor might be helpful.

I've ran an A518 with OD+Lockup shortly, and it's smaller convertor was noticable even when revving the engine in neutral. The less weight made the engine accelerate just a tad quicker.
Unfortuntatly that transmission ate its OD-unit and I had to replace it with a non-lockup version A518.
 
Even the posted mileage was probably 3-4 higher than anything you could eek out outside a roller dyno with a 'program' to simulate driving. What a crock. Check Consumer Reports on real world mileage data. I recall that ugly Honda van lookin thing got about 14 mpg against its quoted 19. Thay sucks. The magnum 360 in the 4x4 trucks got like 8?;
 
LP doesn't have as many BTU's per gallon than gasoline. There's your problem I'll bet. Switch it over to gas and you will probably get 20 (miles per gallon)
 
I agree LPG/propane has less energy than gasoline, but I've gone and done quite a bit to optimize timing and OD to just be able to at least reach the same mpg-level again I would think.

Few things that could still imo improve mileage; are the engine's compression ratio, the 3.55 rear gears and perhaps the stall convertor.

I will most likely be changing the rear axle to 3.23 gears fairly soon, and I've also slowly started the overhaul of the OD/LU A518 (with its smaller stall convertor).
I will also be 'fixing' the engine's compression 'issue' kinda soon hopefully, but not by changing pistons, but changing the entire engine with a high CR 360.
But in the mean time I would still try to get some more 'normal' mpgs out of the 318.
 
A bit off topic, but I could use some anecdotes, or actual data for that matter. Just acquired and drove 2000 miles in two day a 65 Barracuda, 273 2 bbl. It has the automatic transmission (nifty console shift). The trip was problem-free, but the car only managed 11miles/gallon. The car is a keeper, runs strongly, but I am not sure what to do about its gas guzzling ways (which I think must be abnormal--yes?). Thanks for nay thoughts you might have. jjc
 
I would not put ANY credence to mileage numbers. "The 80's" had so darn much smog crap, lean burn, EGR, retarded timing, all the rest of it, what you would have to do is set one up "back then" in a time warp just as you'd set it up NOW...........bypass all the junk, leave the engine "as is" with exhaust, heads, cam, internal parts, Get a decent carb and timing curve into it and THEN ask it for some mileage
 
JACOBI said:
A bit off topic, but I could use some anecdotes, or actual data for that matter. Just acquired and drove 2000 miles in two day a 65 Barracuda, 273 2 bbl. It has the automatic transmission (nifty console shift). The trip was problem-free, but the car only managed 11miles/gallon. The car is a keeper, runs strongly, but I am not sure what to do about its gas guzzling ways (which I think must be abnormal--yes?). Thanks for nay thoughts you might have. jjc
Click to expand...
2000 miles in two days? how fast were you driving? that may have something to do with the poor mileage.
Anecdotally, my wife and I were traveling on I-5 thru the mountains of So Oregon/No California and she got 24+mpg, in our 76 dart 318 ex-cop car.
 
rustycowll69 said:
my wife and I were traveling on I-5 thru the mountains of So Oregon/No California and she got 24+mpg, in our 76 dart 318 ex-cop car.
Click to expand...
That is pretty damn good for a 318!

To the OP: your mileage seems decent considering: your 3.55 gear set, your driving style (80mph is almost 130km/h after all), and you using LPG.

Anecdotally I never got my 70 Challenger with its anemic 318 and no tune whatsoever better than maybe 13mpg, and the best with a 68 El Camino SS396 was 16mpg highway.
Both long gone....
 
I'm not hitting 80mph on a daily basis, just very occassional.
Average highway speed is around 70.

I went to the Mopar Nats in Germany 2 weeks ago which was some 350 pure 'highway'-miles roundtrip and still only netted 13.3mpg.
I did drive a bit faster then usual evenso but expected/hoped for better.

Fun fact is that when I had just bought the car, I made a 300mile roundtrip and got 17mpg.
Few things were at hand though; first the radiator was clogged inside and I could really only drive 60mph. At 65mph the engine started overheating.
Also, the engine had a lousy tune, running very lean on the highway, and it still had its 904 transmission and tiny 7-1/4" rear axle. All mostlikely having less friction losses than the current A518 and larger 8-3/4" axle.
 
I have a 1988 Diplomat with 318, 122,000 miles since bought new. My average is about 19-20 mpg, all around. Made a trip, last spring, with a combination of 70-75 mph interstate, "normal" 2 lane 55-60 mph, and mountain roads with 30-35 mph. My average for the trip (approx 900 miles) was 23.7 mpg
I did remove the lean burn system, and used a earlier mechanical/vacuum distributer with a GM HEI system, and earlier 2bbl carb. 904 lockup trans, and 2.7x rear.
 
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