budnmic
Member
How to adjust for fuel economy? '70 AAR 6 barrel motor in a 1969 fastback with overdrive 4speed. any thoughts???
six barrel
- Thread starter budnmic
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First, 340's aren't economy engines. Second 340 TA engines damn sure aren't economy engines. They have high compression, a large factory cam,and huge cfm's for a fuel system. Disconnect the front and back carbs and see where your mileage is.
Front and rear carbs are vacuum controlled...you can adjust there opening by replacing the spring in the vacuum canister.....and your right foot is the best control of the secondaries...
actually high compression engine get better gas mileage then a low compression engine...
actually high compression engine get better gas mileage then a low compression engine...
budnmic
Member
thanks, did that already. played with timing and I'm at 18 highway.
budnmic
Member
wired front and back shut, played with timing. thinking about jet size next
Probably not going to get better than that.
Lean it out too far and it will detonate and burn pistons.
Connect a hose to manifold vacuum and run the guage inside the car. Drive at cruising speed in high gear, flat to moderate grade, take a reading of the guage and report back.
Yunick13
2 Time FABO NASCAR champ 2023 FABO NHRA champ
Check your air/fuel ratio. Make sure all your plugs read good.
Is the rest of the car set up for fuel mileage? Narrow tires set to right pressure? Correct gear ratio?
Is the rest of the car set up for fuel mileage? Narrow tires set to right pressure? Correct gear ratio?
Getting your air idle adj right can be interesting. Do you have the ProMax throttle plate for the rear carb? If not, how are you going to adj it? Getting the proper power valve for the center carb/jet size can be interesting. What color springs did you go with? What thermostat for engine temp? Ign timing? Air filter type? Fuel bowl levels. AFR for both sides of the engine, not just one. End carbs, start with 1/8 turn out to start, make sure that your needles are good, have not been overtightened and needles bent. All three carbs need to be in sync, they all idle together, not just the center carb. Every engine is different. I also run ProMax jettable plates instead of the metering plates on the end carbs. I was getting in the ball park last fall before winter came, will pick up where I left off in about a month or so. Never checked for fuel mileage, but 18 would be stellar in my book. My engine is a 418, will be slightly different, but not that much, they just draw a little harder.
I’d leave it. The new challengers with the 5.7s barely get 20 and they’re EFI
K.O. SWINGER
Meeting in the alley since 1976
Factory settings on a six pack/six shooter are pretty lean. I could squeeze about 17 out of mind(4speed) before I rejected it. Once outboard carbs are engaged all bets are off in my opinion jetting down on the primary carb is just looking for trouble in a lean condition as mentioned before.
JDMopar
Well-Known Member
If I were getting 18 mpg out of a 340 Six Pak motor, I'd be doing back flips! My 70 Challenger 340 used to get 20 mpg on the interstate. That was with the 2.76 gear that I swapped into it because I was doing a year of road tripping between Candler, NC and Clarksville, TN.
Right?! I’m hoping for 12-15 out of my 440 with a 6 speed. I’d be doing back flips as well if I hit 18. I mean it’s not like this is a Toyota
What’s the fuel economy goal your trying to achieve? You must have a number considering you’ve already measured an actual 18mpg. You looking for 19 or 35mpg?
Keep in mind a lightweight Feather Duster/Dart Lite got over 30mpg but that was with a heavily tuned for fuel economy slant 6.
I wonder if salesmen at Ferrari/Bugatti/Lamborghi/Land Rover and other fuel thirsty vehicle brands hear clients complaining about how their vehicle has hefty fuel bills.
Keep in mind a lightweight Feather Duster/Dart Lite got over 30mpg but that was with a heavily tuned for fuel economy slant 6.
I wonder if salesmen at Ferrari/Bugatti/Lamborghi/Land Rover and other fuel thirsty vehicle brands hear clients complaining about how their vehicle has hefty fuel bills.
I remember talking with someone when the 400hp outboard boat engines came out. He wanted to know economy if he upgraded from 4 - 350hp engines to 4 - 400hp engines. Explained old rule of thumb was 10% of hp at wide open. He had a meltdown. 160 gallons per hour...his boat only has 400 gallons. Had to put it in perspective for him. That is 1600hp at WIDE OPEN for hours at a time. Who does that? What country does he wish to be in?
K.O. SWINGER
Meeting in the alley since 1976
I had to reject my center carburetor because I run a 242 at 50 camshaft, it really smoothed out the motors operation but shot my MPG right in the butt. So that 17 miles per gallon I spoke of earlier was before my rejetting now I think I'd be lucky to get 13 or so.
Are you running a MSD or similar like box? That will get you a gain in MPG’s.
moper
Well-Known Member
Do the usual stuff: verify tire pressure is a few under the max rating; make sure the alignment and brakes are good; make sure your vacuum advance is connected and operational; rest of ignition curve is right and working... I would expect 20+ with what you have, depending on the camshaft choice.
Dartswinger70
Well-Known Member
i was going to chime in "what is the MPG on a 6 pack...?" I seem to think a 4 barrel would be some where in the neighborhood of 13 MPG
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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You didn't mention your rear gears.
But if you are running the factory 114 LSA cam, and getting 18mpgUSg out of her, that is phenomenal.
The factory 268/276/114 cam on the advertised numbers and in at 110LSA, has only 104 degrees of exhaust duration, which is really lousy for fuel economy. But the compression degrees while only 116,with a true 10.5 Scr , the Dcr is nearly 8.4@800ft elevation, which is really good, if you manage to keep it out of detonation at WOT.
The thing is; that cam likes to be spinning,
at the very least, 2200rpm at your chosen cruising speed, 2400 would be better.
The slower you spin it, below ~2000 with headers, the worse the economy will be. And the faster than about 70 that you drive, also the worse it will be. Taking all those things into consideration, and with the 3.09 first gear, the optimum rear gear will be about 3.55s; for 65= 2120rpm with 27" tires.
3.73s would get you 2230, 3.91s would be 2330, but now the starter gear, at 11.53 is getting a lil excessive.
At these low rpms, it is doubtful that your MJs will be flowing more than a trickle, but since it's a 6bbl, you can lean them out until the you can't any more,lol.
However, the BIG secret is timing. At 2000 rpm it will be impossible to give the engine optimum Cruise Timing.
Even at 2400 it will be tough.
I estimate your engine will like 54* at 2400 rpm.
If you have set your PowerTiming to 34* at say 3400rpm,
And she idles on 18*
And doesn't start to advance until 1000rpm,
And has a single-stage curve;
THEN
the rate of advance will be;
(34 less 18)/(3400 less 1000) x 1/100 = .6* per 100 rpm, therefore
at 2400, the mechanical in the D is 8.4. and so, to that you add the 18 =26.4. to get to 54, your Vcan needs to bring in ~28 degrees, which, AFAIK is impossible.
You can mod almost any Vacuum canister to get 22 degrees, maybe 24 tops, and so if you did that, you would only be short 4* from my estimated 54*.
How much Cruise-Timing your engine actually wants is easy to figure out.
1) determine your cruise-rpm.
2) rev it up to that, and put it up on the fast idle cam.
3) grab the D and start advancing it. you will notice that the rpm goes up. Just keep advancing it until the rpm no longer rises, then
4) put the rpm back to your cruise rpm with the fast-idle screw.
5) now reset the timing to get the highest rpm.
6) adjust the mixture screws for the highest rpm
7) put the Rs back to Cruise-rpm, and one more time; adjust the timing for highest rpm.
8) finally, set your mixture screws 1/4T rich; and read your timing on the balancer. Subtract 3 degrees, and write it down!
9) return the engine to idle, restore the timing to what it was at the first, and let it cool off.
What you have just done, is determine the ideal cruise timing at cruise-rpm at speed. The 3 degrees I took out is for load compensation at speed.
If the mixture screws are too far from in the center of their adjustment range, or the engine will not idle on the new setting, then you will have to fix something.
If your engine truly is at 10.5Scr and has a factory 340 cam, in at +4*, then the compression readings should be close to 170 psi. And that is a great number to make fuel economy with; BUT; the engine has to be reving high enough to be out of reversion. How can you know when this is? But a vacuum gauge on the intake , then slowly rev the engine up until the vacuum peaks, and no longer rises. The lowest rpm that it peaks at should be considered to be your lowest cruise rpm. Just match that to your chosen cruise roadspeed , and yur done. A lil higher is Ok, but slower, will lose efficiency, until you change the Ica, by either changing the Cam, or the installed Centerline.
If your engine was a 360 with a fast-ramp 268/276/110 cam, I'm pretty sure that it would not yet be on the mains at 65 mph. But your 340 might be dribbling some. Since it hasn't maxed out the PMJ; for cruising on, it don't matter what size MJ is in there...... don't even bother with them, for this mode of operation.
Well maybe I shouldn't be so cocksure of myself; cuz one time I got a tank of something that was not 87E10, and I had to stop on the side of the road TWICE, to upjet the primaries, just so it would cruise. Man that was awful.
But I just gotta say, 18 mpgUSg in a 340 is like Mike said
BTW;
For cruising;
If you get the engine revving fast-enough, and the AFR set right, and the timing set right; then it should not matter what kind of carb you are using, nor how many barrels it has, nor how many total cfm it has...... as long as the primaries are sized decently for cruising. How you get to your cruise speed is an entirely different matter,lol.
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moper
Well-Known Member
i was going to chime in "what is the MPG on a 6 pack...?" I seem to think a 4 barrel would be some where in the neighborhood of 13 MPG
422” 480hp, 17mpg w4bbl and 3.23s, no OD, in a 3600lbs E body.
496” 580hp, 14mpg w4bbl and 3.23s, no OD, in a 3900lbs e body.
450-6bbl, 450hp, 16mpg with 3.91s, no OD, in 3900lbs e body.
a mild 340, in a 3100lbs A body, with OD should clear 20 if you know what you’re doing.
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