Miles Per Gallon and other questions
#165376
12/10/08 12:38 PM
12/10/08 12:38 PM
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Anonymous
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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?
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Re: Miles Per Gallon and other questions
#165377
12/10/08 12:48 PM
12/10/08 12:48 PM
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Joined: Aug 2003
Posts: 11,179 Atco NJ
DJVCuda
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I Live Here
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Re: Miles Per Gallon and other questions
[Re: DJVCuda]
#165378
12/10/08 12:52 PM
12/10/08 12:52 PM
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I honestly dont know. The trans is automatic. When im doin 60-70 mph the tach says 2700-3000 rpms.
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Re: Miles Per Gallon and other questions
[Re: Mr.Yuck]
#165381
12/10/08 01:53 PM
12/10/08 01:53 PM
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Joined: Jan 2003
Posts: 16,123 Grand Haven, MI
patrick
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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....
1976 Spinnaker White Plymouth Duster, /6 A833OD 1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!*** 2011 Toxic Orange Dodge Charger R/T 2017 Grand Cherokee Overland 2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: Miles Per Gallon and other questions
[Re: MoparforLife]
#165383
12/10/08 02:12 PM
12/10/08 02:12 PM
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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
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Re: Miles Per Gallon and other questions
[Re: okie]
#165385
12/10/08 06:24 PM
12/10/08 06:24 PM
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Joined: Jan 2003
Posts: 8,089 Sorrento, BC, Canada
4speeds4me
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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.
2 Demons...no, not my kids!
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Re: Miles Per Gallon and other questions
[Re: DaytonaTurbo]
#165387
12/11/08 02:59 PM
12/11/08 02:59 PM
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Joined: Feb 2008
Posts: 992 Simi Valley, CA
MoparJ
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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.
Last edited by MoparJ; 12/11/08 03:04 PM.
2016 Ram 1500 Crew Cab: 5.7, 65RFE, 4.56 gears with locker, Hemifever tuned, AFE intake, 87mm throttle body, JBA headers, 3" Flowmaster exhaust, split to dual 2.5" exits. 13.57 best ET so far.
Searching for new A or B Body Project!
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Re: Miles Per Gallon and other questions
[Re: DaytonaTurbo]
#165388
12/11/08 10:54 PM
12/11/08 10:54 PM
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Joined: Dec 2003
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dave571
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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.
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Re: Miles Per Gallon and other questions
[Re: Mark340A66]
#165390
12/12/08 07:27 PM
12/12/08 07:27 PM
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Joined: Feb 2008
Posts: 992 Simi Valley, CA
MoparJ
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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.
2016 Ram 1500 Crew Cab: 5.7, 65RFE, 4.56 gears with locker, Hemifever tuned, AFE intake, 87mm throttle body, JBA headers, 3" Flowmaster exhaust, split to dual 2.5" exits. 13.57 best ET so far.
Searching for new A or B Body Project!
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Re: Miles Per Gallon and other questions
[Re: 68Bullit]
#165393
12/12/08 08:41 PM
12/12/08 08:41 PM
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Joined: Jan 2003
Posts: 1,481 Outside
thedriver
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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.
Last edited by thedriver; 12/13/08 03:52 AM.
1973 challenger Dana. 4 speed. Low deck.
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Re: Miles Per Gallon and other questions
#165394
12/12/08 10:13 PM
12/12/08 10:13 PM
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Joined: Sep 2004
Posts: 2,093 oberlin, Ohio
Rapid340
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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.
1971 Factory Appearing Duster 340 11.000 @ 122 mph
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