340 gas mileage

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kevindahle said:
Wow that's pretty impressive!
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I said that to say I think your car should exceed it easily. Why it's not could be a lot of different reasons, but right now you have something(s) really wrong. We could make a list of things to look at .... maybe some are already listed. I didn't read what others were say'n
 
kevindahle said:
Runs good. 727 8 3/4 with 2:76(?)
Click to expand...

kevindahle said:
Rear tires are 235/60/15
Click to expand...
These are ~26.1 diameter, so 82" roll-out;
Therefore with 2.76s,............................ 65=2310
That 276/288/114 Summit K6901 is your biggest enemy; go do a compression test.
While the engine is warmed up, measure the lowest rpm at which the vacuum first peaks.
And check your cruise timing at 2310rpm, with the Vcan hooked up.

For an eye-opener;
install a timing tape. Inject #1 cylinder with enough air pressure to push the piston down to the bottom. Then reduce the pressure to about 20psi and bring the piston up on the compression stroke. Reduce the pressure when the piston starts to bounce. Keep doing that until you find the crank-angle, as close as is possible to determine, when the intake valve finally closes.

IMO
2310 is just barely enough rpm to maybe clean up the airflow in the intake.
The biggest issue I see from that cam is the very meager 98* of power extraction, (in at 110, and off the advertised mind you), which leaves plenty of energy still in the exhaust; energy that could have been used to propel the vehicle. You can't do much of anything about that except retard the cam, which brings on another set of problems.so don't do that.
The bigger issue is how to give the engine the ignition timing it craves.
Do this; with the engine warmed up; rev it to 2300 and give it more timing, forget the light, just crank some in. Reduce the speed back to 2300, and more advance. Continue this way until the engine no longer increases in rpm with additional timing. Now read the timing,still at 2300.
I'll bet the number comes in very close to 56 degrees.
Push the D back some and idle her down. Then restore to normal timing.
Now; about that number; whatever you got; cruising at 2300, that is close to what your engine is wanting. What have you been giving it? Rev it up and see. I'll bet less than 45 with a working Vcan. I'll bet less than 28 if not running a Vcan.
This is probably your second biggest problem, the first one being cylinder pressure.
I'm just guessing. Go ahead and prove me wrong.
I mean I assumed you know how to calculate mpgs, and that you have proved yourself right,lol.
Timing and pressure walk hand in hand with rpm to make the big numbers.
When looking for fuel-economy,the 340 does not have enough stroke to waste on long slow acceleration ramps. That 6901 is a small 218/228/114 cam,with ~58* ramps and could be had, from another manufacture, with perhaps as little as 44 degree ramps. The ramp difference, and tightening up the LSA to 110 from 114, could increase your cylinder pressure over 18psi!

Lemmee compare;
your current cam in at 110, should have the following numbers;
276/288/114+4/Ica of 68/112 comp/98 power/o'lap of 54@52effective
This is a 218/228 cam, it wants headers.
Lets shorten the ramps to 44*
and tighten the split to 6*
and tighten the the LSA to 110* in at 107*
but keep the 218*@.050
so; we will end up with a 218/224/110, here we go;
262/268/110+3/Ica of 58/122 comp/113power, o'lap of 45@42 Effective
This is is still a [email protected]
But now ;
the effective overlap is reduced to 41 from 52, and
power is extended to 113 from 98, and
compression is up to 122 from 112.
You lost only the overlap, which with log manifolds, you were not using anyway!, so I set 'em on fire and trashed 11 of em.

I'm not saying you should run out and get this cam; for all I know the new Ica of 58* could put your engine straight into detonation. That is why I asked for the cylinder pressure readings.
I just have lots of free time to play with.
And I just wanted you to see how the cam lobes can be manipulated to your advantage.
One thing I can guarantee you; if it is possible to increase your power extraction by 15 degrees, while simultaneously increasing your compression by 10 degrees and reducing your overlap 11 degrees, while yet maintaining your [email protected], phewff there's a mouthful;
as to power, there is no downside to this.

as to low-rpm torque, you will have a good amount of increase.
as to fuel economy,say goodbye to 10 mpgs.
With the new cam, and the higher cylinder pressure, and the fixed timing, your carb will need a new tune , and after you get all that done, you should see mid to hi teens as others have stated.

Now someone might say that the 6901 is a great cam. I'm not saying that it isn't. I'm just suggesting that for your needs,it is not the best fit.
Every time someone speaks of the 6901 the fuel-economy issue seems to come up.
 
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kevindahle said:
I have a 71 dart that I over hauled a 68 340, built it as close to stock as possible, electronic hei distributor, edelbrock carb. The cam was a summit, other than that put new rings and bearings had the heads done. Runs good. 727 8 3/4 with 2:76(?). I'm not a super tuner but the car runs well. When I drive it to work about 70 miles, I get about 10 mpg. I had a Duster with a built 360 4 speed & 3:23 and would get 15 ish. My next step with the dart I think is a air fuel gauge. Any other suggestions?

View attachment 1715494735
Click to expand...
Got a picture of how your plugs look?
 
jimjimjimmy said:
thats not very good mileage i get 14 to 14.5 with my 530 hp 418 with a 950 hp and no vacuume advance .

thats an imperial gal .
Click to expand...
Got 15-16 mpg with a 340 .528 cam, headers, 750 holley , 3.73 gears & a 4 spd.
33 degrees, could've gotten better with some vac advance hooked up but that was the average mpg.

My 410 did 15mpg hwy , IF I stayed out of the secondary aka didnt pass people ..

Same demon 750cfm carburetor.
340 like 74's in the primary and the 410 75's.
Ootb was 76/83
 
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You small-block guys have it made for mileage...I sure wish I could get 10 mpg!

Try my 451, huge [email protected] cam, tweaked 800 double pumper, 3.91 gears. I'm still dialing it in (with an AFR meter, so I know it's not running 10:1 pig rich!) and there's a lot of idling and throttle winging in the driveway, and 0-40 mph runs from 1500 to 4000 rpm on the back roads. Think I'm getting mid single digit mpg!

It does better at a steady 60 mph and 15:1 AFR, but not all that much better. The gas gauge literally falls when I stomp it... Oh well, I didn't build it for economy.
:steering:
 
Something is wrong. I have a moderately modified 340 (375HP +/-), 4 speed, headers, 3.55 gears and 205/70/14 tires, and I get about 15 mpg on the highway. With gas mileage like that, something is wrong.
 
Sure there is... lots of overlap, running in an inefficient RPM range for the duration, and constantly stomping the throttle on a double pumper! Plenty of ignition advance, reasonable AFR, and no leaks. What would you say is wrong? :realcrazy:

I used to have an A-body with a basically stock '69 383, .484 purple shaft cam, 750 vac sec on a DP-4B dual plane, 3.23 gears... and it got about 15 mpg on the highway and averaged 12.5. My worst tank ever was 8.3 mpg, all short blasts 1/4 mile at a time around town ;)
 
AJ/FormS said:
These are ~26.1 diameter, so 82" roll-out;
Therefore with 2.76s,............................ 65=2310
That 276/288/114 Summit K6901 is your biggest enemy; go do a compression test.
While the engine is warmed up, measure the lowest rpm at which the vacuum first peaks.
And check your cruise timing at 2310rpm, with the Vcan hooked up.

For an eye-opener;
install a timing tape. Inject #1 cylinder with enough air pressure to push the piston down to the bottom. Then reduce the pressure to about 20psi and bring the piston up on the compression stroke. Reduce the pressure when the piston starts to bounce. Keep doing that until you find the crank-angle, as close as is possible to determine, when the intake valve finally closes.

IMO
2310 is just barely enough rpm to maybe clean up the airflow in the intake.
The biggest issue I see from that cam is the very meager 98* of power extraction, (in at 110, and off the advertised mind you), which leaves plenty of energy still in the exhaust; energy that could have been used to propel the vehicle. You can't do much of anything about that except retard the cam, which brings on another set of problems.so don't do that.
The bigger issue is how to give the engine the ignition timing it craves.
Do this; with the engine warmed up; rev it to 2300 and give it more timing, forget the light, just crank some in. Reduce the speed back to 2300, and more advance. Continue this way until the engine no longer increases in rpm with additional timing. Now read the timing,still at 2300.
I'll bet the number comes in very close to 56 degrees.
Push the D back some and idle her down. Then restore to normal timing.
Now; about that number; whatever you got; cruising at 2300, that is close to what your engine is wanting. What have you been giving it? Rev it up and see. I'll bet less than 45 with a working Vcan. I'll bet less than 28 if not running a Vcan.
This is probably your second biggest problem, the first one being cylinder pressure.
I'm just guessing. Go ahead and prove me wrong.
I mean I assumed you know how to calculate mpgs, and that you have proved yourself right,lol.
Timing and pressure walk hand in hand with rpm to make the big numbers.
When looking for fuel-economy,the 340 does not have enough stroke to waste on long slow acceleration ramps. That 6901 is a small 218/228/114 cam,with ~58* ramps and could be had, from another manufacture, with perhaps as little as 44 degree ramps. The ramp difference, and tightening up the LSA to 110 from 114, could increase your cylinder pressure over 18psi!

Lemmee compare;
your current cam in at 110, should have the following numbers;
276/288/114+4/Ica of 68/112 comp/98 power/o'lap of 54@52effective
This is a 218/228 cam, it wants headers.
Lets shorten the ramps to 44*
and tighten the split to 6*
and tighten the the LSA to 110* in at 107*
but keep the 218*@.050
so; we will end up with a 218/224/110, here we go;
262/268/110+3/Ica of 58/122 comp/113power, o'lap of 45@42 Effective
This is is still a [email protected]
But now ;
the effective overlap is reduced to 41 from 52, and
power is extended to 113 from 98, and
compression is up to 122 from 112.
You lost only the overlap, which with log manifolds, you were not using anyway!, so I set 'em on fire and trashed 11 of em.

I'm not saying you should run out and get this cam; for all I know the new Ica of 58* could put your engine straight into detonation. That is why I asked for the cylinder pressure readings.
I just have lots of free time to play with.
And I just wanted you to see how the cam lobes can be manipulated to your advantage.
One thing I can guarantee you; if it is possible to increase your power extraction by 15 degrees, while simultaneously increasing your compression by 10 degrees and reducing your overlap 11 degrees, while yet maintaining your [email protected], phewff there's a mouthful;
as to power, there is no downside to this.

as to low-rpm torque, you will have a good amount of increase.
as to fuel economy,say goodbye to 10 mpgs.
With the new cam, and the higher cylinder pressure, and the fixed timing, your carb will need a new tune , and after you get all that done, you should see mid to hi teens as others have stated.

Now someone might say that the 6901 is a great cam. I'm not saying that it isn't. I'm just suggesting that for your needs,it is not the best fit.
Every time someone speaks of the 6901 the fuel-economy issue seems to come up.
Click to expand...
If you read all that you could get separate award..:realcrazy:..:poke:..
 
I took a 48 mile drive last Sunday,, some back roads 45 miles an hour and 55 mile an hour highway for a while and definitely some wide open throttle blast.. 2- 625 Street demons on a dual quad tunnel ram with a 4-speed and 355 gears 238-248 duration cam. I put 4 gallons back in to fill it back up. That was 12 miles to gallon I was a little surprised but a bigger surprise is your car getting only 10..
 
really, you cannot check gas mileage on a 30-50 mile run. In my opinion, you need to go at least 200 miles. That way, if there is an 1/4 of a gallon difference at fill-ups, it won't throw it way out of wack
 
318willrun said:
really, you cannot check gas mileage on a 30-50 mile run. In my opinion, you need to go at least 200 miles. That way, if there is an 1/4 of a gallon difference at fill-ups, it won't throw it way out of wack
Click to expand...
If you fill a 10 gallon fuel cell and refill it with a clear measuring fuel can with four gallons and one gallon left LOL it's pretty accurate...
 
j par said:
If you fill a 10 gallon fuel cell and refill it with a clear measuring fuel can with four gallons sand one gallon left LOL it's pretty accurate...
Click to expand...
ok, I guess I wasn't thinking fuel cells...
 
Let's put it another way
It's generally accepted knowledge that it takes about 40 hp to push one of these classic cars thru the air at 60 mph.
At 1/2 pound of fuel per hp per hour, that would be 20 pounds of fuel per hour,=3.33 gallons per hour.
If you traveled 60 miles during this hour then;
60 miles/3.33=18 mpg.
So; 18 mpg is your target.
whether or not you get it, or more, would be in the chassis or in the tune, or in the driver.
I had a 223* cam in my 367 equipped Barracuda at one time, that did that and more, a lot more mpgs. So I know it is possible.
 
AJ/FormS said:
Let's put it another way
It's generally accepted knowledge that it takes about 40 hp to push one of these classic cars thru the air at 60 mph.
At 1/2 pound of fuel per hp per hour, that would be 20 pounds of fuel per hour,=3.33 gallons per hour.
If you traveled 60 miles during this hour then;
60 miles/3.33=18 mpg.
So; 18 mpg is your target.
whether or not you get it, or more, would be in the chassis or in the tune, or in the driver.
I had a 223* cam in my 367 equipped Barracuda at one time, that did that and more, a lot more mpgs. So I know it is possible.
Click to expand...

Is that 40 flywheel hp or 40 wheel hp? Big difference depending on drivetrain loss maybe?
 
If you can't get 18-20 IMO you're overlooking something. Read books on tuning - and not the ones on tuning for max power or that don't include running vacuum advance on ported vacuum.
 
moper said:
If you can't get 18-20 IMO you're overlooking something. Read books on tuning - and not the ones on tuning for max power or that don't include running vacuum advance on ported vacuum.
Click to expand...
I agree. We got 13 mpg in our '77 full-time 4x4 Ramcharger 360 4bbl, 3.55 gears on an almost 4k mile trip
 
kevindahle said:
I have a 71 dart that I over hauled a 68 340, built it as close to stock as possible, electronic hei distributor, edelbrock carb. The cam was a summit, other than that put new rings and bearings had the heads done. Runs good. 727 8 3/4 with 2:76(?). I'm not a super tuner but the car runs well. When I drive it to work about 70 miles, I get about 10 mpg. I had a Duster with a built 360 4 speed & 3:23 and would get 15 ish. My next step with the dart I think is a air fuel gauge. Any other suggestions?

View attachment 1715494735
Click to expand...
Smaller less fun camshaft for the economy. @318willrun used a crane cams 693901
Camshaft in a duster with 2.45 gears and a crate 360.
Even steeper gears, and even more low down torque

if you want economy instead of horsepower I’d use that camshaft in your 340 or a comp cam extreme energy 250 or 256 camshaft.
 
greymouser7 said:
Smaller less fun camshaft for the economy. @318willrun used a crane cams 693901
Camshaft in a duster with 2.45 gears and a crate 360.
Even steeper gears, and even more low down torque

if you want economy instead of horsepower I’d use that camshaft in your 340 or a comp cam extreme energy 250 or 256 camshaft.
Click to expand...
I haven't checked the gas mileage that Duster yet, but I know I can drive 1 hour to the drag way, run it through about 4 times, and drive it back home off of about 7 gallons.
 
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