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Old 02/12/07, 06:02 AM   #1
broncobillyusmm
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Broncos will inherit the Earth.

Mountain Home
Joined: Feb 07
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Thumbs down 351W Looking for better fuel economy

I have full restored 69' Bronco that I use as a daily driver. It has a 351W and I'm only getting 7 mpg maybe 8 on a good day. Below is a list of all the things that will give you a feel for what I'm running.

351W w/GT40 cast heads bored .040" over
RV cam with 302 timing
Motorcraft Aluminum Intake
Holley Truck Avenger Carb
2" Low Profile K&N Air Filter (14" dia)
Long headers from Wild Horses
2 1/4" dual exhaust with high flow mufflers with turn down right after muffler
4 core radiator w/mechanical puller fan and electric pusher fan
NV4500 and dana 20
4.11 gear ratio with detroit locker in rear and detroit true trac in front
33" tires
Net vehicle weight w/me 4900 lbs

With all these it seems everything is against me. Any help would be appreciated. Thanks.
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Old 02/12/07, 06:58 AM   #2
Apogee
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Tangent, OR
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Buy a manifold vacuum gauge and install it in or under your dash. Take your EB to a shop with an exhaust gas analyzer and tune the carb for maximum economy, not performance. You may have a vacuum leak or things other than just the configuration of your engine contributing to the not so impressive gas mileage. How aggressively do you drive?

I have a 429 in my F150 PU that gets better mileage than your 351...pretty much always and it has a 750 cfm Edelbrock, cam on the lumpy side, dual 2.5" exhaust, NV4500, 35" tires, etc...not to mention weighs in several hundred pounds heavier.

Did you know that 95% of all statistics are made up on the spot?
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Old 02/12/07, 07:02 AM   #3
bmc69
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IF you can borrow a diagnostic O2 sensor and drive around with it in the tailpipe, that would give you a lot of good info on what yr engine is contributing to yr lousy economy. FWIW, my 408 (built..and EFI) rig with 39.5 IROKS and 4.88 gears...T-18 tranny (so no OD) gets 11 on highway. So you are correct in thinking yrs should be doing better than what it is.

'69 w/ Oz-408, T-18/NP-205, D44/ARB, D60 Detroit
39.5 IROK Radials/Champion Beadlocks
Hydroboost- F-250 dual-piston frt disk/ F-250 drum
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Old 02/12/07, 07:10 AM   #4
Scrapper_MV
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I just bought a 73' with a 351W. I drove it home to So Cal from Tucson AZ and got 15 MPG! Thats 6 MPG better than my 02' Superduty! The Bronco is now my economy car! So I think yours could do a lot better.
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Old 02/12/07, 10:04 AM   #5
broncnaz
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Proper jetting and timing will increase your mileage. Get the distributor recureved to get all the advance in by 3000 rpm and then make sure the jetting is on a O2 sensor helps but you can just pull the spark plugs to check the jetting you want them to be light brownish to grey. Not black or white.

73 Ranger 2 1/2 lift 302 TFS heads NP435,PS,PDB, tierod over, 33x10.50x15 BFG M/T
71 Sport 63,000 miles all orginal except for cut fenders 302 3sp 4.11's D44/BBTrac lok rear
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Old 02/12/07, 11:31 AM   #6
70_Steve
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Quote:
Originally Posted by broncnaz View Post
Proper jetting and timing will increase your mileage.
Speaking of timing.... You are running a distributor with vacuum advance, right??

'70 Sport, 351W EFI, NV4500, D20 w/Teralows, 4.56s Dana 44 w/Ox and 9" 31 spline Detroit, 35" KM2's, 3 1/2" lift, 2" body lift, 4x4x2 PS, some other stuff I forgot about...

"If it's got tits or tires it's gonna be a problem"
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Old 02/12/07, 01:39 PM   #7
broncobillyusmm
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I lowered the jets by 5 sizes and it began running rough so i went 3 sizes smaller than orginal. It smoothed it up a bit but i didn't see any change in mpg.

My distributor is made by motocraft with vacuum advance. Another thing that might be a concern is how the fuel gets from the tank to the carb. First it goes through an in-filter and then through the holley red pump but after two failures (deep mud was probably the cause) I switched to a pump that delivers a much lower output. Before the carb I have a return valve from holley and a fuel pressure gauge. The recommendation from holley I believe was between 3-4 psi.

Another strange thing is after a 30 min or so run on the highway and then park it, the fuel gauge is always lower then when I left it. Its usally a significant amount on the guage. I'm thinking that this is just do to heat making the fuel expand and when it cools it gives me a better reading. Its very frustrating to leave a Bronco with a 3/4 tank of gas and come back to 5/8 or less.

Would adding a crossover pipe in the exhaust help at all? Because of the headers a fairly large bend was made to go over the crossmember.
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Old 02/12/07, 02:27 PM   #8
76Broncofromhell
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Campbell, CA
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Search "fuel economy" and "best mileage" it's probably been covered 1000 times on here.

You need to correctly curve your distributor to your engine and cam combination, but even then don't expect much more than 10 mpg.

'70 Bronco - 408W
'45 Ford GPW - 351W
'84 Toyota - 5.0L HO
'13 Shelby GT500 - 5.8L
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Old 02/12/07, 02:45 PM   #9
csangster
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Golden, CO
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Poor mileage

Quote:
Originally Posted by broncobillyusmm View Post
I lowered the jets by 5 sizes and it began running rough so i went 3 sizes smaller than orginal. It smoothed it up a bit but i didn't see any change in mpg.

My distributor is made by motocraft with vacuum advance. Another thing that might be a concern is how the fuel gets from the tank to the carb. First it goes through an in-filter and then through the holley red pump but after two failures (deep mud was probably the cause) I switched to a pump that delivers a much lower output. Before the carb I have a return valve from holley and a fuel pressure gauge. The recommendation from holley I believe was between 3-4 psi.

Another strange thing is after a 30 min or so run on the highway and then park it, the fuel gauge is always lower then when I left it. Its usally a significant amount on the guage. I'm thinking that this is just do to heat making the fuel expand and when it cools it gives me a better reading. Its very frustrating to leave a Bronco with a 3/4 tank of gas and come back to 5/8 or less.

Would adding a crossover pipe in the exhaust help at all? Because of the headers a fairly large bend was made to go over the crossmember.
I hate to point out the obvious, but have you ever considered that you have a leak somewhere?

I would recommend looking over your rig with a fine tooth comb for any leaks.

I just found a fuel leak on my rig today......so it never hurts to inspect your rig from time to time.
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