Performance mixed with fuel economy!?

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Steve Cotter

Steve Cotter

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Green Bay, WI
Last December I picked up a 1969 Barracuda fastback, original 340 car; fender tag, build sheet gone, so no documentation that is a formula S as the seller claimed. The 340 was gone, has a 360 that doesn't run. I've been trying to get it to run but no luck. I had dreamed of building a 340 someday. BUT. With the way the world is going, especially gas prices, I'm considering picking up a 318 instead. The rear end has 3.55 gears and is a posi. (Both wheels spin the same direction off the floor). What do you guys think? Anybody in the same boat?
 
I don’t drive my Dart because it’s good on gas. Even with a 318 and efi I would think breaking 20 mpg would be a struggle.
 
you can get both with a Gen 3 hemi. Sorry to burst the LA bubbles but the Gen 3 is so far advanced over the LA/Magnum it is a no brainer if you want both. If your set on using the 360, getting it to E58 specs would give you a reliable mill. ie. 340 cam, 340 spec intake, dual exhaust. Plenty of 360 power on this board.
 
Your title made me chuckle. Nothing wrong with a 318 but if you build it for performance (depending how far you go) you won't get optimum mileage. A Teen with a 4 barrel, proper distributor advance curve, and dual exhaust can be pretty snappy and still get you upper teens for mileage on the highway. 3.55 is a push for mileage unless you have a overdrive transmission. That's a whole other can of worms as far as modifications unless you are running a 4 speed. I have a 273-4 with a 3.23 gear and a 4 speed in my 66 and a 318-2 barrel automatic with a 3.23 gear in my 70 Swinger. Both get upper teens for mileage and run well. No tire burners as far as performance. It is nearly impossible to have it both ways.
 
Thanks for the input.
A gen 3 hemi would be very cool, but no power steering, (call me a sissy).
Trying to stay with the LA engine to keep the 340 hope alive.
Forgot to mention that I was planning on swapping to a 4 speed also; then I could use the much maligned OD manual. Upper teens for mileage is tolerable, I want to drive the wheels off it in retirement (12 months left of 12 hour rotating shifts!).
 
A 318 gets way better milage than a 360, but the key is the transmission. An OD 42RH or a 46RH will help get you the highway mpg..2wd mid 80s dodge vans.....a 3.9 would have the 42rh a 360 the 46RH 318 either have to check. A gear vendors od might be a less expensive solution ultimately.
 
yeah, there aint a lo compression high performance motor on the planet unless its 2x the displacement of the stock motor its replacing. Most our LA stuff is low (by todays standards) compression. whats up with the 360? we can help....
 
@12many , good book.


Steve Cotter said:
Last December I picked up a 1969 Barracuda fastback, original 340 car; fender tag, build sheet gone, so no documentation that is a formula S as the seller claimed. The 340 was gone, has a 360 that doesn't run. I've been trying to get it to run but no luck. I had dreamed of building a 340 someday. BUT. With the way the world is going, especially gas prices, I'm considering picking up a 318 instead. The rear end has 3.55 gears and is a posi. (Both wheels spin the same direction off the floor). What do you guys think? Anybody in the same boat?
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In my ‘79 Dodge Magnum, thanks was able to get 20 MPG’s. 2.76 rear gear though. If I was in your socks, going with a manual, and had the money, I’d look for a 5spd manual trans and work the math on the final gear ratio once your in overdrive. A very mild cam and best but free breathing intake and exhaust to a long way even on a stock engine.
 
i have 355s and 360, for better mileage go 323s, tight converter and tall tires. of course a manual would help. forget the 318, great motor, but they have to work a little harder to get that beast rolling. make that LA as efficient as possible. most could care less about mileage, rough and rowdy is where it's at
 
Power and mpg are not mutually exclusive. The more efficient your motor, the more you get of both. First, what is up with the 360? Did you take compression readings? Not much difference between a 318 and 340 built the same. The manual 833 O/D is a good move and should work good with your gears, but can be expensive by the time you get everything. A Thermo Quad is the best carb, maybe a Holley Street Demon, TQ square bore copy, but I have not been able to play with one. Get your compression up to work with the gas you want to use. Get a cam close to a stock 340 cam. 20's mpg is within reach even with 3.23's - 2.76 without an O/D. Of course I'm talking highway cruising, not around town. Been there and done that, and loved it.
 
The best thing you can do for mileage that won’t hurt performance is in the transmission department. Get an overdrive of some kind. Then, get your tuning skills up and dial the engine in, and then……..wait for it………TURBOS and fuel injection.
Been sayin it for years.
 
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if you're going for performance, the 318 is not the engine for you
(and neither is the 340)
 
66fs said:
That is very subjective. He actually wants to drive it, a lot. What is low performance about a 318 or 340? What is your criteria?
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doesnt matter much what the criteria is, if he wants 300 or 500 hp
a 360 making a certain amount of hp will always be more streetable then a 318, or even a 340 making the same hp

especially since he has the 360 allready
 
You cannot be worried about fuel mileage if you have a V8 of any size or year. Just use it for cruze nights. Get a Tesla for your daily.
 
Our family's then new '69 4 door 318 Belvidere got 21 mpg North Jersey to Fla panhandle. Factory w/ A/C, using American 'Premium' with the timing slightly advanced (like 2*). Automatic transmission. Plain Jane family car. Reasonably snappy thottle response.

It's 65 mph up full throttle acceleration where a 4 bbl and more hp will really shine.
Off the line, its the gearing, traction and torque that will launch it.
Sitting at stop signs, traffic lights, and put putting in parades (or traffic) the bigger engines eat more fuel per minute.
Its all trade offs.

The current 340 in my '67 Barracuda gets anywhere from 6.5 to 13.5 mpg depending on what I'm doing. Longer interstate drives its averaged 15.3 mpg. That's with a 727, 3.23 gears and 215/70r14 tires. Quarter mile here

Tuning process burns a fair amount of fuel too!
 
The current 360 was installed and worked on by some unknown amateur 10 years ago and the car has not run since. I think I have the timing set where it should run but it just won't light up. As weather and time allows, I have been working on new plug wires and rebuilding the edelbrock carb. (No heated garage) I don't have the correct battery so it might not be getting full voltage, plus there was a starter in the trunk which way mean the one in the car is not the best. These days don't we go with the Dakota starter? around 1994?
 
i don't know...build a 340 to Factory specs including wheels and tires with a 3.23 gear. Stay away from a big cam, gears and tall tires, air gap, converter and all that stuff.
The factory got it right and a good one would run lo 14's and get hi teens for mileage!
 
66Valiant528 said:
A 318 gets way better milage than a 360, but the key is the transmission. An OD 42RH or a 46RH will help get you the highway mpg..2wd mid 80s dodge vans.....a 3.9 would have the 42rh a 360 the 46RH 318 either have to check. A gear vendors od might be a less expensive solution ultimately.
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I disagree about the gear vendor OD.
But an OD trans is the way to go.

@Steve Cotter
No matter the displacement, a dual plane with a mild cam will be very good. A free flowing exhaust and a good intake track so it all breathe easy.

I found that a good working multi spark ignition helps very with mileage as well.
 
If it was a 340 car, it would have a "P" as the 5th VIN digit
 
ir3333 said:
i don't know...build a 340 to Factory specs including wheels and tires with a 3.23 gear. Stay away from a big cam, gears and tall tires, air gap, converter and all that stuff.
Click to expand...
Agreed except the air gap.
Should an OD trans come into play, then it’s multiplying the OD ratio by the gear ratio for the final drive ratio which would allow a change to a deeper gear (numerically lower) while still getting a good final drive ratio. 3.55 X .69 = 2.45, final drive ratio.
3.91 X .69 = 2.69
4.10 X .69 = 2.829 (2.83)
 
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