Need more MPG's

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robertrpeak

Need more MPG's

Post by robertrpeak »

Looking to try and get a few more MPG's out of my 390, this is what I'm running, 64 390 with Edelbrock intake and 750 carb, Comp Cams 280H with 530"lift and 230degrees@.050, Ford dual points ign, long tube headers, with a C-6 and 3.50 rear gears. Right now it gets in the 4-6 miles per gallon range, I need to get a min. of 12mpg. Possible? If so what do I need to change? I do know that the rear gear is one of the problems, at 55mph its taching in the 2600rpm range. So what do ya'll think? Oh and just for good measure the tune is really good, got clean burning plugs, not to rich or lean!! Thanks
Robert
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Re: Need more MPG's

Post by Walking W »

Wow that doesn't add up! Are you sure about your gear ratio n the rear end? You should not be getting that high of rpm at 55 mph with 3.50s. I would think you should get at least 14 mpg with that set up cruising at 75 mph with that set up. I know I have been able to get 16 mpg with close to the same set up and I ran 75+ in Arizona mountains in a 4x4. Which Edelbrock intake are you running? Good luck.
jamesdfo
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Re: Need more MPG's

Post by jamesdfo »

Robert: If you tell me what size rear tires you're running, I will do the math and tell you what RPM you SHOULD be turning @ 55 MPH (or whatever speed you ask for), but 2600 does sound a bit high??
I would also look at finding a 70's truck with FE & robbing the electronic ignition distributor & CDI box (or converting your dizzy to pertronix).
You didn't mention if the 750 carb you're running is a Vac seconday, double pumper, etc??
You also didn't mention what you are using for a torque converter, and loose converters will not help fuel economy:)
Axle ratio, carb size & ignition (but not necessarily in that order) are where you will get the most gains, but everything you alter is a compromise, the key is finding a combo that gives you better economy without neutering your truck :wink:
robertrpeak

Re: Need more MPG's

Post by robertrpeak »

The Edelbrock carb is mech. secondary, the converter is matched to the cam, the tire dia is 27" as best I can tell, its a 255/70-15. I ran the numbers on several online calculators and they all came up with 2531rpm @ 55mph plus or minus a few rpms. So I guess I need to think about where to go with this, I really don't want to loose the "hot rod" aspect of the engine, but at the sametime I would like to go on trips longer than a few miles....and not go broke.

Robert
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Re: Need more MPG's

Post by jamesdfo »

Robert: I would check to verify that your rear gear is in fact 3.5, and if so, if the speedo isn't reading correctly, then you know it's the gear on the end of the speedo cable, not that you have a different axle ratio (than you thought)

255/70/15 is ~29.06" Tall

So, Running the numbers for your tire height & 3,50 gears, they come out like this:

@50 MPH: ~2026 RPM

@ 55 MPH: ~2226 RPM

@ 60 MPH: ~2430 RPM

@ 75 MPH: ~3035 RPM

and just for the record, when I run it using your RPM # (2531), I get 62.5 MPH
jamesdfo
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Re: Need more MPG's

Post by jamesdfo »

Calculating tire height:

P255/70/15

255 = section width (in milimeters)

70 = Aspect Ratio (ie: the % of height vs width

255mm divided by 25.4 (mm per inch) = ~10.039" Section Width

~10.039" * 2 (sides to the tire) = ~20.078"

20.078" * 70 % Aspect Ratio = ~14.05"

~14.05" + 15" (rim size) = 29.05"

**There is a small difference from my number above due to rounding differently here than in my spreadsheet, but it's close enough for 'gubmint work :):)
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bobenhotep
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Re: Need more MPG's

Post by bobenhotep »

If his stall rating on the torque converter is higher than his cruise rpm, won't it slip like crazy? been a long time since I messed with a non-locking tc type auto transmission.

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LM14
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Re: Need more MPG's

Post by LM14 »

My '66 had a 3.25 and turned 2300 at 55MPH. His RPM is right.

The carb is hurting you. So is the cam.

Gear is OK, I would actually like a deeper gear but you have the wrong tranny.

For mileage, small cam helps a lot. Keep the overlap down.

The carb is awful for mileage. I would suggest a 600 vac secondary.

Pertronix would be a big help. Make sure it's a vacuum unit and the line is hooked in the correct location. Most directions tell you where to hook the line but tell you the smog answer, not the best answer.

Most people lie to themselves on the actual use of the vehicle worry about sounds, looks or bragging rights. For mileage most of that has to leave, they don't go hand in hand. How often do you use full throttle? How often do you turn 6000 RPM or above? That's what that cam and carb are for. How often do you need a mechanical secondary carb (race use only)? Same for the rumble of the cam. We want the sounds and looks but hate the actual results. We've all done it, some of us will admit it.

Best answer: go to a torque/RV style cam that starts pulling at 1000 RPM and runs out of steam by 4500-5000 RPM. Electronic ignition with the proper vacuum advance. Free flowing dual exhaust but keep it under 2.5" diameter (2 or 2.25 would probably really be better for torque). Carb? None. Fuel injection, and that comes from a died in the wool Holley carb guy. Sometimes honesty hurts. You would also do better with an AOD type tranny and a lockup converter or an overdrive and deeper rear gears.

Cheapest fix? Smaller vacuum secondary carb (450-600 for high fuel velocity), electronic ignition with vacuum advance and a tighter converter. Cam will still be hurting you.

Good luck,
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Re: Need more MPG's

Post by ICEMAN6166 »

i got 6 mpg in my school bus with the 330ft engine
11-12 on my 66 y block and 4.56 rear 3000 rpm@ 70
16 in the rambler with the holley 4v and 3.14 rear

all factory ,no mods of any type

build something for real high performance take away from other things like life and mpgs
if you are not a racer and have no place to use all that hp then its really not worth it
you can have both reasonable power and better mpg from a fe engine without spending a lot from stock

Steve has some good suggestions
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bruceandersson
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Re: Need more MPG's

Post by bruceandersson »

In every 77 and older ford truck I've owned, the speed and airflow have been the most critical aspects of MPG. I've not run engine mods beyond headers, and carb but have been able to consistently get at least 16 MPG as long as I kept my speed in check and was running a clean air filter. A dirty air filter could drop the mileage to 6 and running over 70 would do the same. Towing another vehicle did not have that much of an impact. I'd move to an OD trans if possible with a lockup torque converter. I've probably put in a couple 100,000 miles in older Ford trucks, often towing another car for long distances and have got consistent results. Trucks were '74 F250 highboy 360 4spd 4x4, '77 F250 Crew cab 351M 4 spd, '65 F100 300 4spd w/ O/D, and '66 F100 352 4 spd. I always kept them in good tune and checked mileage on each fill-up as I was often ending a trip both low on gas and low on funds. I did have a 2000 F350 5.4L auto o/d. This truck would do better on the mileage at higher speeds, but had some technical advantages over the older ones. It got 16-19 MPG up to 70 MPH. Tires will also have an impact. My trucks have always been run with std tires at max inflation. A real truck tire running at 80 PSI has a lot less rolling resistance than a low profile wide gripper.
robertrpeak

Re: Need more MPG's

Post by robertrpeak »

Thanks for all the great comments, very helpful. I built this engine when I was 40 yrs old, still had a heavy foot like a 19 yo. Always drove "hotrods" and guess its just in the genes.... Anyhow I did a test this weekend, using the GPS to check the trucks speedometer, it checks out within a mph or two, double checked the rear gears just to make sure, tire height etc. Then I layed out a route to town and back, a total of 13.5 miles round trip, [That is GPS miles] topped of the gas tank to a level I could see inside the filler neck, went for the ride {3 times} and came back and refilled to said level, each time it took 1gal + 2-3 oz of gas to return to marked level.. So in my mind and if I'm doing the math correct thats roughly 13mpg.....
And by the way I'm 59 now, but the lead foot has gone to feather foot!!! Most of the time :P

Thanks
Robert
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Re: Need more MPG's

Post by 6166 Junkyard Dog »

Want good mileage out of a 390, trash the carb/intake set up. trash the rear end ratio, get a 3.00 or 2.75, trash the cam,, cams sound good but the harder the hit the more gas it uses, trash the distb., dual points love gas as well.. only reason I say that is we get 17 to 19 out of our 390 automatic with stock Ford 4 barrel carb, stock points rear end 3.08 and out of other trucks I have gotten up to 22 mpg.. Heck last time we run our 427 stroker it got 8 and the harder we run it the better the mileage up to 10,, mostly it's all about the rear end gear
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Re: Need more MPG's

Post by BarnieTrk »

Robert,
Regarding your unknown rear end ratio:
Without needing to know anything else... do what they say here: http://www.how-to-build-hotrods.com/gear-ratio.html

BarnieTrk
robertrpeak

Re: Need more MPG's

Post by robertrpeak »

Thanks BarnieTrk. The gear ratio is 3.50 ...... To be honest, I.m pleased that its in the 13mpg range, after thinking it was 6-8 for so long.

Robert
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Re: Need more MPG's

Post by chris401 »

If it has not been done a timing recurve will help. Not having a vacuum advance will hurt mpg as well. I am still toying with my 352, SP2P-390, 4.10, 4 speed F-250. With a stock 68 Quadra Jet it got 12.99 running 75-80 with about 1,500 lbs in the bed. 15 empty with the tailgate down running 65 with an Autolie 4100. The stock 66 4v cam is +4°. With my vacuum advance pulling in 15° my total is 52°. I would not recommend setting it that high with your cam. I did it accidentally but it works so I left it.
Chris
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