Dang skippy.
What MPG are you 340 guys getting?
- Thread starter 340Dust-her
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Dang skippy.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Some,
to a lot,
of fuel mileage gains
can be found in the cruise timing. Sometimes (depending on the cam) it is possible to run the engine at too low an rpm, and to not be able to give her the ignition timing she desperately wants.
For instance, if you gear your car for 65=2000, and the engine wants 52* of timing...... how are you gonna meet that need? With factory parts, it is nearly impossible. What if your cam is big enough to still be in reversion at 2000? What if, because of that reversion, your Effective Dynamic Compression ratio is just 4 or 5/1?
It's all in the combo, and especially in the ignition timing.
A manual trans, traditionally, has been cited to make about 10% better fuel economy, cruising at the same rpm. Part of that is in the TC slip; and that is why the factories have gone to loc-up convertors. And another part is inside the autos themselves. And a third part is in the transmission ratios.
Another major factor in fuel economy is your running cylinder pressure, which is governed by your Dcr (Dynamic Compression Ratio), the Ica (intake closing angle), and the throttle opening. This runs in conjunction with your carb, and carb size. If you gear your cruise rpm too low, and your throttle opening becomes too large, then she will get up on the mainjets. But now, the airflow thru the venturies will be quite low, because the power-requirement is so low. And it won't take much error in MJ selection to totally negate, or worse, all the work you put into the low-rpm running.
I used to run hi-compression 340s for years, beginning in 1970, in hi-school. 340s were never known for fuel economy except to say it was lousy compared to any other Mopar offering. And part of the problem, perhaps most of it, was the factory 268/276/114 cam.
That's part of the reason, as a streeter, I switched to 360s years ago. And the biggest reason I installed alloy heads on mine, was to get the super-high cylinder pressure..
to a lot,
of fuel mileage gains
can be found in the cruise timing. Sometimes (depending on the cam) it is possible to run the engine at too low an rpm, and to not be able to give her the ignition timing she desperately wants.
For instance, if you gear your car for 65=2000, and the engine wants 52* of timing...... how are you gonna meet that need? With factory parts, it is nearly impossible. What if your cam is big enough to still be in reversion at 2000? What if, because of that reversion, your Effective Dynamic Compression ratio is just 4 or 5/1?
It's all in the combo, and especially in the ignition timing.
A manual trans, traditionally, has been cited to make about 10% better fuel economy, cruising at the same rpm. Part of that is in the TC slip; and that is why the factories have gone to loc-up convertors. And another part is inside the autos themselves. And a third part is in the transmission ratios.
Another major factor in fuel economy is your running cylinder pressure, which is governed by your Dcr (Dynamic Compression Ratio), the Ica (intake closing angle), and the throttle opening. This runs in conjunction with your carb, and carb size. If you gear your cruise rpm too low, and your throttle opening becomes too large, then she will get up on the mainjets. But now, the airflow thru the venturies will be quite low, because the power-requirement is so low. And it won't take much error in MJ selection to totally negate, or worse, all the work you put into the low-rpm running.
I used to run hi-compression 340s for years, beginning in 1970, in hi-school. 340s were never known for fuel economy except to say it was lousy compared to any other Mopar offering. And part of the problem, perhaps most of it, was the factory 268/276/114 cam.
That's part of the reason, as a streeter, I switched to 360s years ago. And the biggest reason I installed alloy heads on mine, was to get the super-high cylinder pressure..
Last edited:
Gator
Mopar or No Car
10 if I dont hammer it and if I do I get about 8 in my Hi Po 340...
Back in the day, 71 340 Duster, 4 speed, 2.76 gears, 20-21 mpg highway. I did have other gears that did not do as well.
Dartsun
Mopar Dude
My 340 in my 67 Belvedere 4 spd 3:23. Got around 10mpg combined. Mild driving edelbrock 600cfm.
Ahhhhhh, there’s that GV/OD unit again!
If these things are in your budget, there really cool.
Expensive stuff.
If these things are in your budget, there really cool.
Expensive stuff.
Yote
Well-Known Member
340, 4 speed, 3:55 rear - my last 90 mile steady drive got 17 mpg
512Stroker
We are all here because we are not all there.
AHHH! My 67 LA 340 Barracuda gets double digit fuel mileage.
Like 9.2-9.8 mpg on the highway going down hill with a strong tail wind.
72Dart6pack
Harder Better Faster Stronger.
I'm in the 7 mpg range. And happy as hell.
Last Dart cruise I put in 7.5 gallons, drove 70 miles and came back with the gauge back in the same place. 9.3MPG and I was being pretty gentle with it.. lol.
K.O. SWINGER
Meeting in the alley since 1976
Anywhere from 4 to 16 mpg. 391 rear end and 240 at 50 camshaft oh yeah I run a over jetted six pack 
AJ, kinda surprised about the 340 fuel mileage comment. My experience has been the low compression 360's were the gas suckers. I guess it is the overall combo. Do you think that a 360 scienced out combo would make significantly better mpgs over a 340 optimized combo?
GAS MILEAGE???
Oh that's right, just one 600 - maybe you'll get twice the mileage!
MobileCustoms
You can twist perceptions, reality won't budge
With My 340/416 sroker with a 750 Brawler on a LD340 intake, (engine dynoed at around 530hp), 727 reverse manual VB with 4400 stall and 3.73 gears with 295/55/15 MT drag radials on the back, I got 16mpg during my 101 mile round trip to Mopars In the Park this year after spending some time tuning/jetting. I was really happy with that!
Since then, I've added a GV Overdrive and my last 140 mile cruising trip, it came out to 18mpg. Planning to put 4.10s in the rear soon and will see what that does to it.
Since then, I've added a GV Overdrive and my last 140 mile cruising trip, it came out to 18mpg. Planning to put 4.10s in the rear soon and will see what that does to it.
^^^^ you're doing REALLY well!
I estimate our show pony 71 Demon 340 does 9-10 mpg's when we take it out. .030 over 340, 9.5 CR actual, Thermoquad, etc, etc. As original as we could make it except it has the old "hemi grind" purple cam (.471 lift). 3.55 gears. Frankly, mpg's don't much matter on this car.
I estimate our show pony 71 Demon 340 does 9-10 mpg's when we take it out. .030 over 340, 9.5 CR actual, Thermoquad, etc, etc. As original as we could make it except it has the old "hemi grind" purple cam (.471 lift). 3.55 gears. Frankly, mpg's don't much matter on this car.
Watch your trans coolant temps. That 4400 stall with an overdrive could be a problem if it isn't a lock up converter.
MobileCustoms
You can twist perceptions, reality won't budge
I've got a temp guage and cooler and so far all is good. Didn’t go over 160 on my last 140 mile trip.
Divenut
Well-Known Member
Seems like I sometimes get gallons per mile vs miles per gallon when doing spirited driving lol
Dartswinger70
Well-Known Member
I read somewhere that that's what they got new...I was still in diapers in 1970 so I cant really say
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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How did I miss this?
Long overdo, but
as to fuel economy, on the hiway, the factory low-compression 360 should theoretically annihilate the factory hi-compression 340; never mind after being scienced out.
There are several reasons for this;
Firstly; the Scr of the 340 even tho advertised at 10.5, it never was.
But it doesn't matter to this conversation because to go 65 mpg with a certain car, requires an exact amount of power. which means it should require an exact amount of fuel.
If you ran those two engines in identical cars so that the load would be the same, now you are down to camshafts and timing.
The throttles will control the power output, by the EffectiveCompression ratios while being throttled, so the Static Compression ratios are meaningless, same for Dynamic Compression Ratios ...... because both are calculated at WOT.
That leaves just;
the duration of the PowerStroke and The amount of Overlap, and ignition timing.
To that end,
As to the Power extraction,
installed at 4* advanced, the 340 has just 104* of Power extraction, to the 360 at 116*, which is a huge difference of 11.5%.
As to overlap;
the 340 has 44* to the 360 at 32*, which is just 73%= 27% less for the 360.
With log manifolds most of that is gonna go dead, but deader for the 360 .
As to ignition timing.
I don't have the timing curves but, this I know, NEITHER of them ever had an optimum curve.
Lets say the 340 ran 3.23s as was typical, and the 360 ran 2.76s which was also typical. and lets assume both on same 27" tires. and both with autos and the 340 at 3% convertor slip to the 360 at 2.55, BOTH at 65 mph.
Ok so the 340 is cooking along at 65=2690, and the 360 car at 2180.
At 2690, the 340 car might want cruise timing of 56 degrees, but only gets say 35.
At 2180 the 360 car might want 48, but only gets say 29*
So where is the Fuel economy going?
Well, the 340 is opening the exhaust way too early, dumping pressure, which could have gone to the crank. But to it's credit it is cruising at an rpm at which the reversion should not be an issue, is running at 500rpm higher than the 360, and, it's way short of cruise timing.
Whereas the 360, is pulling more useable pressure out of the expanding gasses, is cruising 500 rpm slower, is also clear of reversion, but is operating closer to ideal ignition timing.
Now, if you build both engines the same, and tune them the same, then about the only difference is the stroke ...... right? Which means the 360 gets more effective stroke, runs a smaller throttle opening, and gets better steady-state fuel economy because of it.
Where most guys fail, is in not giving their engines enough cruise timing. Mine likes over 56* at 2800=63 mph, in direct. In overdrive at 85= 2160, it has liked over 60 degrees, up to 63 actually
How do you get there with a factory-type distributor?
Answer; you cannot. .
If you max your power-timing out at 36*, say at 2760,to run 3.23s, and modify your VA to the max of say 24*, that totals 60* allright .................. but now just try and drive it from idle to 65mph without detonation, with all that PowerTiming, in so early. You'll need best gas, which around here is at least 20% more money. Well then, you'd have to make at least 20% better fuel economy to make it worth while..... right?
How then do I do it?
My secret is a stand-alone, dash-mounted, timing module that has an electronic range of up to 15 degrees.
with a heavy foot you are right
back in 1970 mpg was not something to worry about 30cents a gallon . 15 mpg with a 340 was about average IF the car was in tuneI read somewhere that that's what they got new...I was still in diapers in 1970 so I cant really say
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