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Miles Per Gallon and other questions

Posted By: Anonymous

Miles Per Gallon and other questions - 12/10/08 04:38 PM

How many miles per gallon does a 1971 340 plymouth duster get? Here are the specs.

650 cfm carb
1971 340 block
.30 over 10 1/2 to 1 compression pistons
stock steelcrank
edelbrock airgap intake
mopar purple cam shaft 484 lift
360 j heads (small valve)
727 trans with shift kit
7 1/4 non posi rear

Would i see a big performance increase if i but 340 x heads on this car with a 8 3/4 posi rear?
Posted By: DJVCuda

Re: Miles Per Gallon and other questions - 12/10/08 04:48 PM

what gear?
Posted By: Anonymous

Re: Miles Per Gallon and other questions - 12/10/08 04:52 PM

I honestly dont know. The trans is automatic. When im doin 60-70 mph the tach says 2700-3000 rpms.
Posted By: Jim_Lusk

Re: Miles Per Gallon and other questions - 12/10/08 04:54 PM

Some 340s do real well while others not so good. I've heard of over 20mpg and under 12mpg on the highway. State of tune, cam, and gears have a lot to do with it.
Posted By: Mr.Yuck

Re: Miles Per Gallon and other questions - 12/10/08 05:16 PM

wow 4 1/4 rear be careful. my 72 swinger had a mild 340, 10:1 stock 340 cam w/ 1.6 rollers, headers rpm air gap and a 750 holley dp, 727 and 3.91's. I could pull 17mpg highway at 65-70.
Posted By: patrick

Re: Miles Per Gallon and other questions - 12/10/08 05:53 PM

you won't see a noticable power difference between the J and X heads.

you might see an appreciable performance improvement by going to a different gear ratio (say from a 3.23 to a 3.91), but you might sacrafice fuel economy and comfort (buzzing along at 3300-3500 at freeway speeds vs. 2700-2900)

that cam, you'll be lucky to get much better than ~13mpg....
Posted By: MoparforLife

Re: Miles Per Gallon and other questions - 12/10/08 06:09 PM

With the RPM speed and the MPH given I would say that you have a 3.23 gear given the low and high RPM and low and high MPH.
I had a pure stock 71 that I bought new with a 3.91 factory installed and at 65/70 it would pull an honest 15-16 on the highway on a trip.
Posted By: Anonymous

Re: Miles Per Gallon and other questions - 12/10/08 06:12 PM

youve got the 284/484 cam in there. I ran that with 3.91's and a 3000 stall converter and got between 7 and 9 mpg. looks like you have 3.23's so you'll do a bit better. I'd say 8 to 10 mpg
Posted By: okie

Re: Miles Per Gallon and other questions - 12/10/08 09:51 PM

I've got about the same motor with 323 gears but I'm running a 750 Edlebrock carb and it's not too good and I get around 10 MPG's out of it. The carb isn't good and it hasn't been since it was new and I've got a new Carter to put on it and I think it will do a lot better then. Good Luck! Ronnie in Okla.
Posted By: 4speeds4me

Re: Miles Per Gallon and other questions - 12/10/08 10:24 PM

with a 7¼ rear you don't have likely more than a 3.23...probably less. IIRC, the LSA on MP's hydraulic cams really sucks for mileage, but it's been awhile.

hotroddave40's mileage thread in the Projects section has some really good info for choosing parts with mileage in mind.

And changing your heads would be a colossal waste of money. Except in extreme (7K+ rpm/race) scenarios, you will not see enough improvement going to a 2.02 head from the 1.88's...likely none at all.
Posted By: DaytonaTurbo

Re: Miles Per Gallon and other questions - 12/10/08 11:29 PM

That cam is killing you. If there's one thing the 284/484 was not made for, it's fuel mileage. That's a big cam for a small block and it's got a narrow LSA. You'll never get good mileage with it.
Posted By: MoparJ

Re: Miles Per Gallon and other questions - 12/11/08 06:59 PM

I am pulling 16-18 mpg out of mine at 65 mph on the hwy, with 26.5" tall tires, 3.21 gearing, and a stock converter. It turns about 2700 rpm at that speed. The motor is a 73 318, with stock compression replacement pistons, stock heads, Comp XE268 cam, headers, 2.5" exhaust, and a holley 600 carb on a performer intake.

The trans is a 904. I am hoping the milage might make an uptick when I move up to a looser 2-2,200 stall to complement the performance gain.

I have a fairly large cam, but that 284 is even bigger.

The relatively stock 70 340 with J heads in my 340 Formula S 69 Cuda pulls about 15-16 with 3.23s and a 2000 stalled 727. The manifold is stock and the carb is a worn out Edelbrock performer 600 cfm.
Posted By: dave571

Re: Miles Per Gallon and other questions - 12/12/08 02:54 AM

Quote:

That cam is killing you. If there's one thing the 284/484 was not made for, it's fuel mileage. That's a big cam for a small block and it's got a narrow LSA. You'll never get good mileage with it.






I think the 8 to 10 mpg estimates are pretty close. Still better than the 6 to 7 I get
Quote:


Would i see a big performance increase if i but 340 x heads on this car with a 8 3/4 posi rear?




I doubt you'de see a perf gain from the heads, but it's hard to say. Has either set been ported>

The ring gear size won't gain you anything. It's the ratio. Stick with a sure grip instead of GM posi unit though.

An upgrade to an 8 3/4 is probably a necesity, simply from a strength standpoint. I had a friend with an aspen with a 340 in it. It broke a couple of those 7 1/4's in really short order.
Posted By: Mark340A66

Re: Miles Per Gallon and other questions - 12/12/08 09:14 PM

My old stock 70 340 4spd A66 Challenger with 3.23's, a good working vac. advance system and a real sweet 800 TQ was getting 18 US MPG/22 Imp at 65-70. Before the TQ, I had an AFB 625 that got 2-3 MPG less. Bring on the Thermobog bashers! I sold the car and kept the TQ (#1966 rods)

Posted By: MoparJ

Re: Miles Per Gallon and other questions - 12/12/08 11:27 PM

Quote:

My old stock 70 340 4spd A66 Challenger with 3.23's, a good working vac. advance system and a real sweet 800 TQ was getting 18 US MPG/22 Imp at 65-70. Before the TQ, I had an AFB 625 that got 2-3 MPG less. Bring on the Thermobog bashers! I sold the car and kept the TQ (#1966 rods)






Thermoquads= small primaries for fuel savings, huge secondaries for power. Great carbs when set up right.
Posted By: MoparforLife

Re: Miles Per Gallon and other questions - 12/12/08 11:38 PM

Unless the combustion chambers on the X heads are smaller or the heads are ported to flow better than the 1.88 heads you will gain nothing with the switch. In fact you could go the other way and loose slight low end performance unless you are going to run in the higher RPM range. (like 6000+)
Posted By: 68Bullit

Re: Miles Per Gallon and other questions - 12/12/08 11:48 PM

With your current setup, try out a TQ I've got a new rebuilt one that is on the way now, and I've run one before on a mild 360. The fuel mileage versus other carbs is quite noticeable, especially when compared to any Holley that I've run
Posted By: thedriver

Re: Miles Per Gallon and other questions - 12/13/08 12:41 AM

i used to daily drive a BONE stock '70 340 duster. the only change it had was a thermoquad over the factory afb. it had 3.23 gears and 205/70/14's that car would get me 18mpg on the hiway at 65mph. very respectable for what it was really.
Posted By: Rapid340

Re: Miles Per Gallon and other questions - 12/13/08 02:13 AM

A stock 71 340 with a correctly setup Thermoquad carb(800 or 850) and distributor can achieve at 20 mpg highway with 3:23's. To be honest, mid twenties wouldn't surprise me. The 284 MP cam will knock that down quite a bit.
Posted By: dd340

Re: Miles Per Gallon and other questions - 12/13/08 02:21 AM

Under ideal conditions, warm day, flat road, continous driving at 60mph i have gotten an honest 20US mpg, closer to 26 imp mpg. I have the 600 eddy carb and it is tuned lean at cruise, cam is a fairly small 268* and 3.23 gears with an auto and stock convertor with stock exhaust setup. It's not a dog either cause it wil run a 13.7 in the 1/4. I think the secret to good mpg is a fairly small overlap cam and lots of compression. That and a properly tuned carb.
Posted By: Anonymous

Re: Miles Per Gallon and other questions - 12/17/08 05:37 AM

I still run my original ThermoQuad on my 360.
(yea...I re-built it a few times too).
But in my 30 some years of running my Chally, I've tried a variety of new carbs...nothing beat that TQuad for milege, and stump pulling power when you need it. BooWaaaaaeeeeeeh
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