sb difference fuel consumption

-
madmax2

madmax2

Active Member
Joined
Aug 28, 2004
Messages
37
Reaction score
3
Location
Buenos Aires, Argentina
hello, I have a 318 and 340 block, and like build the engine for daily use and long trip in highway (holidays). I think that 318 are more economic, but if little difference I want do a step away (340, 349 stroker or 390 stroker).
what is the difference MPG between a 318, 340 or some stroker with the same stuff... my car it's a early a-body with manual trans.

thanks...
 
Generally speaking less cubes = more MPGs.

But other things play into this......

Often peoples driving stile changes as power goes down....

For example you may have a tendency to push a 318 harder than a 340..... Resulting in the 318 getting worse MPGs simply because driving style.

So part of this is mindset..... Do you have the mindset that will allow good MPGs?

You really need to decide what you will be happy with... Pick a 1/4 ET for example...


If you build a 318 that gets 25 MPGs but only runs 16.5 1/4 mile times would you be happy?

If not you will have your foot in it all the time trying to make up for the missing power and bleeding off its MPG ability....

Really.... It's impossible to answer your question without more info.

Give us some numbers.... Yet another example...........

Guys.... I'm doing a 65 dart 4 speed build (rear gears are... ). What would be my best choice/build of small block if I'm looking for at least mid 14s in the 1/4 while returning at least low 20 MPGs on the highway........
 
My 72 Dart gets 20MPG without OD. I have a mildly built 340. With OD, it could get 23-25, depending on driving style.
 
As listed by the manufactures when new in the same car, only 2 -3 mpg's difference.
 
1968FormulaS340 said:
Generally speaking less cubes = more MPGs.

But other things play into this......

Often peoples driving stile changes as power goes down....

For example you may have a tendency to push a 318 harder than a 340..... Resulting in the 318 getting worse MPGs simply because driving style.

So part of this is mindset..... Do you have the mindset that will allow good MPGs?

You really need to decide what you will be happy with... Pick a 1/4 ET for example...


If you build a 318 that gets 25 MPGs but only runs 16.5 1/4 mile times would you be happy?

If not you will have your foot in it all the time trying to make up for the missing power and bleeding off its MPG ability....

Really.... It's impossible to answer your question without more info.

Give us some numbers.... Yet another example...........

Guys.... I'm doing a 65 dart 4 speed build (rear gears are... ). What would be my best choice/build of small block if I'm looking for at least mid 14s in the 1/4 while returning at least low 20 MPGs on the highway........
Click to expand...

I agree. I only average around 15 MPG in my '70 Duster (318, A-904 auto) because with the single exhaust, only primaries working on my ThermoQuad, 2.76:1 rear gears and the thin air from the high altitude that I live at combined with low compression (~8.5:1) I'm constantly pushing the pedal to the floor just so I can get up to cruising speed in a decent amount of time. I'm sure if I had a good dual exhaust system, a properly-working 4-bbl. carb, and maybe some shorter gears I wouldn't be flooring it all the time because I'd have enough torque at lower throttle openings to get me moving.

With that said there isn't much of a size difference between a 318 and 340. The real difference comes in the port/valve sizes for 340 heads versus 318 heads as well as the cam/intake/carb setup. Build a 340 with stock-valve J heads, decent compression (9.5-10:1), a fast-ramp cam on the mild side, a good dual-plane intake, headers, dual exhaust, and a 650 cfm. square-bore (or ThermoQuad) and you'd probably get mileage in the low 20's on the highway, depending on your gearing of course.
 
A big factor in the fuel mileage will be how much disipline you have in controlling the extra HP/fun-factor that a bigger engine would provide! :)

But a longer stroke will provide more torque and require less gear and converter for less rpm's, especially on the highway.
 
excuse me my English...

my stuff are: Edelbroxk RPM heads, Air-gap intake, other xcelerator intake, holley 600 dp, holley 750 vac sec, cartr AFB 750. MP cam .528" 284º and I like swap to Comp cams xs268s (adv 268/274, 230/236 @050, .488/.501, 110 ICA) solid, 318 and 340 blocks, 4 speed manual transmission, 3:45 and 2:87 gears.

Before i was thinking to assemble a performance engine.... but now i like switch to a daily driver engine.... and can go to fishing with my father or holidays trips.

¿what would you build... (a 318... a 340... a Stroker)?
 
Mad Max2

I would use your Edelbrock heads and RPM intake and the 600 DP Holley with the 2.87 gears.

Those cams I would not use. I would seriously look at a smaller cam for daily driving. I would seriously consider the Comp Cam XE262H with dual exhaust at a min. and headers a serious plus in power/mileage abilty's.

IF I had my choice in engines, it would be a 360.

Are YOUR 2 choices a 318 and a 340? I would do the 340 then.
 
My 318 gets 12 mpg at best :-D 3.91 gears and 26 inch tires means the rpms stay up.

I found the more power you have the more you will use it...not all of it but enough to eat up your mpg.

I dive my old mans new Buick car and it passes cars on the freeway so slow...even when floored. My ride I can zip pass cars at 1/4 throttle and do

Be interesting what the mileage would be at a steady 55 mph but I just can't do it long enough to check and see.
 
You can grind the crank mains, no problem. (and it's inexpensive). It's the custom pistons that you need for a .030 over 340 block, that will cost a lot of coin. If the 340 block can be cleaned up with only .020" removed then standard off the shelf .060" over 360 pistons will work and not break the bank.

also,

I would go with Rumblefish's sugestion of the XE262H cam. Anything larger and the mileage will suffer.
 
You must log in or register to reply here.
-

Back
Top