TT5.9mag
Two atmospheres are better than one
Power valve has nothing to do with economy, in cruise you aren’t anywhere near getting in to the power valve circuit. You need more cruise timing if you want MPGs.
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My 340 getting 6 mpg
- Thread starter 64whiteghost
- Start date
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10 initial seems a little low, but I know it somehow has to do with your camshaft… I think…
On your carb you have 4 adjustment screws correct? I think they should be close to around 1/2-3/4 turns out.
harrisonm
Well-Known Member
Modified 340, 3.73 gears, 750 double pumper; what were you expecting?
I get about 10 MPG in town with 3.55s, a 340 that is at about 375HP, headers and vac secondary carb. I have gotten 15 MPG or so on the highway at 60 MPH. Any faster and the gas milage drops. I am fine with my MPG. I did not build my car for gas mileage. Anybody who says they are getting great gas mileage with a 340 has not built it for any level of performance. In my opinion, you can have performance or gas mileage.
I get about 10 MPG in town with 3.55s, a 340 that is at about 375HP, headers and vac secondary carb. I have gotten 15 MPG or so on the highway at 60 MPH. Any faster and the gas milage drops. I am fine with my MPG. I did not build my car for gas mileage. Anybody who says they are getting great gas mileage with a 340 has not built it for any level of performance. In my opinion, you can have performance or gas mileage.
TT5.9mag
Two atmospheres are better than one
10 initial and 34 total doesn’t really tell us anything about your curve. You need rpm with it to understand what your timing is and when. If it’s all in at 3500 and you’re cruising at 2800 then you really don’t have enough cruise timing.
Plot your timing against rpm and like I said before, if you want MPGs you need ported vacuum advance to bring in another 10-15 degrees at your cruising vacuum.
Definitely have some tuning to do, you should be getting closer to double that for mileage.
For reference, the 340 in my Duster is a '68 340, .060" over, 9.8:1 compression with 308 heads, 2.02/1.60 valves, 65 cc's chambers, stage II ported and flowing 264 cfm at .500", it has a Lunati 60404 Hydraulic cam, .513/.533", 276/284 advertised duration. I run it with an Edelbrock Air-Gap RPM intake, Holley Ultra 750 double pumper, headers, 2.5" dual exhaust. Timing is ~18° initial, 34° all in mechanical.
Even before when I was running it with no vacuum advance, 3.55 gears, 26" tall rear tires and a 4 speed on the highway in with mixed driving I was getting about 12- 13 mpg.
Now that I'm running it with the vacuum advance hooked up with a T-56 magnum, 4.30's in the back and 27.3" tall tires I'm still doing 12-13 around town and I can do closer to 14-16 out on the freeway if I'm in the overdrive gears.
For reference, the 340 in my Duster is a '68 340, .060" over, 9.8:1 compression with 308 heads, 2.02/1.60 valves, 65 cc's chambers, stage II ported and flowing 264 cfm at .500", it has a Lunati 60404 Hydraulic cam, .513/.533", 276/284 advertised duration. I run it with an Edelbrock Air-Gap RPM intake, Holley Ultra 750 double pumper, headers, 2.5" dual exhaust. Timing is ~18° initial, 34° all in mechanical.
Even before when I was running it with no vacuum advance, 3.55 gears, 26" tall rear tires and a 4 speed on the highway in with mixed driving I was getting about 12- 13 mpg.
Now that I'm running it with the vacuum advance hooked up with a T-56 magnum, 4.30's in the back and 27.3" tall tires I'm still doing 12-13 around town and I can do closer to 14-16 out on the freeway if I'm in the overdrive gears.
My guess is it’s probably almost 80% due to the 3.73 rear. I just did a round trip to LA from San Jose in my warmed over 273 Commando with a 3.23. Took it easy on the way down on 101, kept up with traffic on the 5 (meaning 80) on the way back, averaged 12.5mpg. Give me a 3.73 and 6mpg seems plausible… I would probably get a little better because my primaries are way smaller. But I don’t see your setup getting better than 8, realistically. You make your choices and live with the consequences.
64whiteghost
Well-Known Member
I didnt build my car for MPG either. But im just seeing what is possible. I love its power and if it is what it is then I will live with it. Money is not a problem. But this group is to seek help from the community. So thats the nature of this thread. Just openning up a convo
72smallairparticler
Well-Known Member
Sheesh.. my 440 charger with 3:55's and 750DP holley 4 speed got 10 average. Definitely something wrong there that can be tweaked. Honestly, a bit of work but you could weld in an O2 sensor to leave in for readings while you're driving and see if the ratio changes during different behavior.
Outside of that, check for some gas leaks lol
Outside of that, check for some gas leaks lol
64whiteghost
Well-Known Member
I have an 02 sensor unit to read it. That is what i will be doing this weekend.
Good point. I will do that
I definitely didn't build my car for mpg's!
But with a 340 and your basic engine information, even with 3.73's you should be doing better than 10 mpg, forget 6 mpg. My 340 has a bigger cam, higher compression and the same sized carb, and even with a heaver car I can get 12 mpg. And I drive it to have fun, not to get mpg's, so it's not because I putt around.
mopar head
Well-Known Member
.30 over 10-1, similar cam, Ede. air gap, IMM built Indy X heads, Holley 770 street avenger, Vac sec. headers, 323 gears , and my vac advance isn`t hooked up either.
18* btdc
I think double pumper and gears are killing economy.
junkyardhero
Well-Known Member
you sure your speedo is correct?
(ie. right drive pinion for the gear ratio and tire size)
(ie. right drive pinion for the gear ratio and tire size)
340/6= 56.6666667 smiles per gallon on my calaculator!
Jeff Seighman
aka jeffnmo
I get around 10 MPG if I'm lucky, but I didn't build it with gas milage in mind!
MOPAROFFICIAL
Oogliboogli
I thought it was 12 and 34.
The carb isn't right yet, may be the wrong carb 'not size' for the aspirations of decent mpg, but it still needs to be jetted down and driven hwy speeds..reducing the primary jet size until it surges, then go back up 2 jet sizes. Next dial in the secondary jets within 9 sizes of the primary, check vacuum and install pv at 3 to 4" below idle 'close enough for now' then the one of the biggest influencers... the distributor. It's a pro billet msd non vac advance 'which would help if it had one if mpg was a concern'.. it needs a sdvance bushing made to eyelid a mere 8 to 10 degrees so that he can have a higher initial timing.. like 24 degrees 'wherever the starter kicks it back warmed up, then back 2 degress'... and now the carb throttle position can be lowered.. may or may not be tickling the mains atm... and voila... it could get 12 or 15mpg.
If ya follow those instructions... as I said...you'll fix your mpg issue mostly.
Car rips tires well. It's just the final touch left.
As for 340 mileage...
Best I've gotten with a ld340/750 .528 solid 284 112 in at 105.5 was 15 mpg with no vac advance. 3.73 gear at 60 mph
The carb isn't right yet, may be the wrong carb 'not size' for the aspirations of decent mpg, but it still needs to be jetted down and driven hwy speeds..reducing the primary jet size until it surges, then go back up 2 jet sizes. Next dial in the secondary jets within 9 sizes of the primary, check vacuum and install pv at 3 to 4" below idle 'close enough for now' then the one of the biggest influencers... the distributor. It's a pro billet msd non vac advance 'which would help if it had one if mpg was a concern'.. it needs a sdvance bushing made to eyelid a mere 8 to 10 degrees so that he can have a higher initial timing.. like 24 degrees 'wherever the starter kicks it back warmed up, then back 2 degress'... and now the carb throttle position can be lowered.. may or may not be tickling the mains atm... and voila... it could get 12 or 15mpg.
If ya follow those instructions... as I said...you'll fix your mpg issue mostly.
Car rips tires well. It's just the final touch left.
As for 340 mileage...
Best I've gotten with a ld340/750 .528 solid 284 112 in at 105.5 was 15 mpg with no vac advance. 3.73 gear at 60 mph
64whiteghost
Well-Known Member
The Maestro has spoken
Mopar official is my friend that has helped me through out this whole build. He is a testament to how bad *** the 64whiteghost is and a major reason as to the power I have gotten.
Thanks brotha ! Look forward to learning more and getting her dialed in more
Mopar official is my friend that has helped me through out this whole build. He is a testament to how bad *** the 64whiteghost is and a major reason as to the power I have gotten.
Thanks brotha ! Look forward to learning more and getting her dialed in more
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6 mpg entitles you to Holley Gold membership. I remember a mate walking into the workshop one day with a big smile; he changed one POS Holley for another & doubled his mileage! Yep, went from 2 to 4 miles per gallon.
10* initial timing is nowhere near enough with that combo. And if you are not using vac adv, that is only making it worse. Worse, because the engine is inefficient with it's current ign timing; inefficient, gobbles more fuel.
Your engine needs at least 30* of timing at idle [ not 10 ], & maybe as much as 40*.
Read these links & learn.
10* initial timing is nowhere near enough with that combo. And if you are not using vac adv, that is only making it worse. Worse, because the engine is inefficient with it's current ign timing; inefficient, gobbles more fuel.
Your engine needs at least 30* of timing at idle [ not 10 ], & maybe as much as 40*.
Read these links & learn.
64whiteghost
Well-Known Member
Thank you for your advice. I will review thank you !
Slantsix64
Well-Known Member
I’m thinking its two things it’s your timing you need more initial 15 initial and 14-16 mechanical
And carb double pumper equals 2 accelerator pumps which drinks gas. Also 67 jets that’s way too small I would think your engine would want 70s
I have a 68 340 10.5 compression also similar cam to yours it takes 16 initial and 14 mechanical I get 12-14 mpg 4 speed 3.55
And carb double pumper equals 2 accelerator pumps which drinks gas. Also 67 jets that’s way too small I would think your engine would want 70s
I have a 68 340 10.5 compression also similar cam to yours it takes 16 initial and 14 mechanical I get 12-14 mpg 4 speed 3.55
they effect idle,jetting is where the consumption would be
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