Max MPG- slant vs 318/340

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replicaracer43

replicaracer43

Grumpy Old Man
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Lets discuss MPG... I see guys with slants talking about getting somewhere around 20-22 or so with good tuning, and I wonder what peeps are getting with a mild, sharply tuned 318 LA or even magnum. Sorta thinking a good 318 magnum, low gear A999 lockup trans and a 2.94 gear in a A body could possibly get better milage than the slant..... Thoughts?
 
guy on .org gets repeated 24-25. also really depends on area/temp/time of year
 
I got 20-21 with my 3.32 geared 71 Dart with a 360 and a TQ. That's if I drove with my big toe, but it got it.
 
The conditions of the day effect mileage some no doubt.

I'm surprised some of the slant 6 guys didn't blow more smoke up your *** with some of the ridiculous reports the have me of north of 35mpg's. A few Bafoons report and swear they got 40.

Here is a case of real life & no bull **** with the parts used.

A 3700+lbs.. 1979 Dodge Magnum. 360 cid. Best of 20mpg Hwy.
Parts used on this good running condition mill were;

An OE trap door air cleaner '72 or '73 model year IIRC???
K&N air filter
Carter 625 AFB
OE iron 4bbl. Intake from '79
MSD ignition coupled to a OE vacuum advance distributor
(Not a stock distributor on that year.)
Dual exhaust off the manifolds @ 2-1/4
(Due to the law....)
An H pipe was used with Twin high flow cats into cheap Thrush turbo mufflers
(Quite and no droan, loud enough to raise and eyebrow when you mash the go pedal)
904 trans - stock, deep pan added.
8-1/4 rear with 2.76 gear and 245/60/15's

Things I wished to get but never did get a chance to get and experiment with we're;

Headers
Split duration cam (in a useable power band fitted for the cars set up)
An LD-340 intake manifold
A TQ carb with it's tiny primary side
 
Here's my combo on my daily driver which gets around 20city/25hwy mpg:

Body: 72 dart 4 door w/ 74 duster front clip, no frills, power nothing just a place to sit and steer, no carpet, no radio, extra steel reinforcement behind the front bumper removed, weight unknown but I would guess just under 3000 lbs.

Engine: Stock 72 225 with 170000 miles, offenhauser 4bbl intake w/2bb carter bbd, stock exhaust manifold, 2 1/2" single exhaust w/flowmaster 40 and a thrush glasspack at the end (was too loud and annoying with just the flowmaster) Burns about a quart of oil every 1000 miles.

Transmission: OD 833 from an 82 Dodge pickup

Rear: 8 1/4" w/3.23 suregrip

Performance: best 1/8 mile time I've got so far is a 12.55 @58mph, I would imagine 1/4 mile time would be measured with a sundial.

There's no reason why a good running 318 couldn't get 20's mpg in a light car with conservative driving, with a overdrive transmission mileage should be in the mid 20's.
 
see, that my thinking, a 318 COULD get better mileage than a 225.... the 318 wouldn't have to work as hard....I would rather have a 25 MPG 318, than a 25 MPG slant anyday...
 
You probably could not do this today with our drunk gas, but ............

in the early 70's I swapped a 71 hi compression low miles 340, with headers, into my 70 RR 440-6 body, so this is a heavy car..............3.54 Dana, A933 18 spline box, "trak pak" suspension, and hang on aftermarket A/C. All that crap adds up..................

I had various carbs on it, everything from the original TQ which had problems, to a QJ, and Holley SB. That thing would get 17.5, and I was never all that careful a driver. More careful driving could have wrung more out of it.

My present Dart, when I had the AFB on the 360, "touched at" 17, but I don't drive it on long trips. I also don't drive it very carefully. At the time it had (Ford) 3.89 and 28+ " rear tires, about 3200 RPM at 70
 
67Dart273 said:
You probably could not do this today with our drunk gas, but ............

in the early 70's I swapped a 71 hi compression low miles 340, with headers, into my 70 RR 440-6 body, so this is a heavy car..............3.54 Dana, A933 18 spline box, "trak pak" suspension, and hang on aftermarket A/C. All that crap adds up..................

I had various carbs on it, everything from the original TQ which had problems, to a QJ, and Holley SB. That thing would get 17.5, and I was never all that careful a driver. More careful driving could have wrung more out of it.

My present Dart, when I had the AFB on the 360, "touched at" 17, but I don't drive it on long trips. I also don't drive it very carefully. At the time it had (Ford) 3.89 and 28+ " rear tires, about 3200 RPM at 70
Click to expand...

my current slant, 10:1 with a 246 @ .050 solid flat cam, 600 DP mech secondaries, 3500 stall with 3.91 gears and 27" tall tires. going 65mph @ 3 grand i got 20mpg from Phx to Vegas, then again from Vegas to Phx.

it really confirms my theory that a race motor prepper and tweeked is cam really be an MPG motor... someday i'll really test it.
 
This is the whole reason most people a small block instead of a slant. A warmed up 318 can get the same mileage as a slant but have twice the power.
 
I agree, the 318 can get 23 mpg, the same as my slant six and it is in a much lighter car. But I do have a slant 6 car and will keep it that way, I like the slant 6 engine and the Teen. :thumblef:
 
Improving the efficiency of an engine yield both more power and more MPG.

Keeping that efficiency in the desired operating range of RPM is the key.
 
I would even go so far as to say, having a fuel injected 318 (stock truck motor) would be even cheaper than trying to fuel inject a slant... I guess what I am saying is the slant seems like a lose/lose situation, as far as milage, unless you just want to keep your slant for the sake of keeping it. I wonder in my hypothetical car... 318 magnum, headers, good exhaust, low gear 904 (A999) and a 2.94 or so gear in a 3100 lb A body, just how high you get get the milage....25-28 possible, or perhaps more????
 
I'd say no. 25 would be my guess for the max.
 
You might could bump 25 with some kinda good sequential EFI on top of a mild build with an OD transmission of some sort. It would be interesting to find out.
 
The Feather Dusters had, what- 2.4x rear and .73 o/d?

I guess the theory we're working with is that an efficient 318 wouldn't have to work as hard to get into the sweet spot.

I'd like to see what that combo would do.

I've often wanted a 3.73 rear with that .73 o/d and a moderate build motor.
 
well, my thinking is that the mopar overdrives eat a ton of power, and MPG as a result... so, using a 904 with the low first gear, will let it take off pretty decent with the 2.94 rear gear, but still be mellow on cruise rpm...
 
The low cr 318 in my dart managed to get almost 19mpg on a 200mile highway trip just last week. Running on propane, tuned very lean.
The car has an A518 OD but 3.91 gears in the 8-3/4".

I'm fairly certain if the lame compression ratio in the 318 had 2 points more it could very well mpg into the 20's somewhere. On propane.
Gasoline should be able to get 1 or 2 points more perhaps.
 
Got 20 mpg average coming back from Minnesota in july, and that was fully loaded with parts and other crap that I brought along!. My setup is a 5speed with 3.25 gears, late 80s roller cam block, cam is stock, roller rockers, autolite carb, which made a big difference in performance and fuel economy, stock manifolds, HEI ignition and eddy intake. Did get on it a lot and cruised it at 75 mph most of the way but if I was driving 65, and not loaded up and wasn't getting on it a lot I can see getting 25 mpg.
 
How does an overdrive "eat" power?
 
The engine struggles a little bit against the steep ratio. The engine rpm needs to be in the sweet spot where the most torque is made for best mileage. This is where the engine makes the most Efficent use of the fuel. How much fuel it needs vs the amount of power it needs to produce to turn the wheels is show in resistance and the gear ratio is the resistance. Along with the rest of the drive train and wind resistance.
The faster you go, the more air/wind you must overcome.
Same idea with the gears. This is how I explain it.

Now how much more fuel is required to propel the car @ speed in drive (A 1:1 ratio) vs. overdrive (A.69 in the auto trans) is a good question!
 
Well, I am surprised to a degree since it was not a dead stock engine enhanced with parts but rebuilt to do the task. Still, bettering 25 as my guess for best possible mileage by a good margin is showi g excellent results from actually thinking about what it is your doing.

Hats off to Hot Rod Dave.
 
I would argue that by the time you are in o/d, that max power output is not the most pressing issue.

That's why there's downshifting and kickdown.
 
rumblefish360 said:
Well, I am surprised to a degree since it was not a dead stock engine enhanced with parts but rebuilt to do the task. Still, bettering 25 as my guess for best possible mileage by a good margin is showi g excellent results from actually thinking about what it is your doing.

Hats off to Hot Rod Dave.
Click to expand...

My thoughts as well. I need to go back and reread it again soon!
 
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