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1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

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Retoropak
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1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #1 by Retoropak » Mon Dec 18, 2017 9:22 pm

Hello,

Couple of months ago i acquired 1986 f-150 regular cab with long bed.
It is a 2wd with c6 and 3.55 gears.

When i got the truck it had a feedback carb and eec ignition. It had a massive whistling vacuum leak from the intake gasket and was getting constant 9mpg no matter what.

I replaced the intake and exhaust gaskets and removed the catalytic converter. After that I swapped efi manifolds, removed egr and run 2.5" into single thrush muffler. I adjusted the fuel mixture and idle and i got 12mpg mixed driving which was the best that the truck got so far.

Later on, I bolted the offy C manifold and holley (classic) 600cfm carb. I added duraspark 2 ignition and set to 14-15 initial timing with 18L plate (36 degrees mechanical).

With this setup I was getting a constant 10 mpg no matter what.


So i started tunning it a bit.

I set the secondary throttles to get me a nice square transition slot on the primaries.
I jetted it down from 66 to 62. Added 8.5" power valve and I left the stock secondary springs and pump nozzle.
Guess what? No change still 10 mpg.
I am getting around 17" of vacuum at idle with the truck in gear. I set the float levels multiple times with no change in mpg.
All ignition components are new.
Also i have the heat plumbed to the manifold.

The truck will burn rubber and its fairly quick.
Except slight bog from the start it runs nice and smooth.

I hoped for at least 14mpg highway with the 3.55 and c6.



Do you think its the gearing/c6 problem or something is off with the carburetor?

Thank you

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Haywood
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #2 by Haywood » Tue Dec 19, 2017 6:44 am

What is generally thought of the C6 is; it is one of the least efficient transmissions out there, but it is also the one of the easiest to cure of that. I decent shift improvement kit will do wonders with it. On my 79 F-350 I went whole hog and drilled the hole out, per instructions included in the B&M kit, and it would bark the tires on the 1-2 shift, occasionally on the 2-3 shift. I wasn't chasing MPG so I didn't keep records, but it had to help. What did you base your power valve selection on? Generally it is best to drive it around with a Vacuum gauge where you can see it, to get an idea of how much vacuum is present at cruising speeds. Basing the PV selection on idle vacuum is BAD advice that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close. You want the PV to be closed while running a steady cruise, and open when the load is enough to require more fuel. Idle has nothing to do with it

Retoropak
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #3 by Retoropak » Tue Dec 19, 2017 11:30 am

Haywood wrote:What is generally thought of the C6 is; it is one of the least efficient transmissions out there, but it is also the one of the easiest to cure of that. I decent shift improvement kit will do wonders with it. On my 79 F-350 I went whole hog and drilled the hole out, per instructions included in the B&M kit, and it would bark the tires on the 1-2 shift, occasionally on the 2-3 shift. I wasn't chasing MPG so I didn't keep records, but it had to help. What did you base your power valve selection on? Generally it is best to drive it around with a Vacuum gauge where you can see it, to get an idea of how much vacuum is present at cruising speeds. Basing the PV selection on idle vacuum is BAD advice that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close. You want the PV to be closed while running a steady cruise, and open when the load is enough to require more fuel. Idle has nothing to do with it


I chose the pv based on cruise vacuum. I am getting nice 10-12" cruising at 55-65.

So, do you think that the shift kit might improve the milage?

pmuller9
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #4 by pmuller9 » Tue Dec 19, 2017 12:05 pm

Would you consider installing an air fuel ratio gauge so you can finish fine tuning the carb?
Have you checked the vacuum advance?

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Haywood
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #5 by Haywood » Tue Dec 19, 2017 5:24 pm

Retoropak wrote:I chose the pv based on cruise vacuum. I am getting nice 10-12" cruising at 55-65.

So, do you think that the shift kit might improve the milage?

I do, it tightens it up considerably, all that slipping is $ out the tailpipe.

wallen7
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #6 by wallen7 » Tue Dec 19, 2017 6:34 pm

Now I know opinions vary widely, but did you degree the cam. I know you said the distributor was on the 18l slot and you were running 12 - 14 degrees of initial timing. 36 + 14 is 50 degrees. What about vacuum advance? I was taught that you should never have more than 42 degrees total advance. Have you checked the color and condition of the spark plugs. Does the engine overheat and are you drowning it with too much fuel or carb?

Retoropak
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #7 by Retoropak » Tue Dec 19, 2017 6:46 pm

pmuller9 wrote:Would you consider installing an air fuel ratio gauge so you can finish fine tuning the carb?
Have you checked the vacuum advance?



Sure i considered it. I guess i should have megasquirted the truck instead of going carb. I just wante to make it a bit more simmple wiringwise.

No i have not checke the vacuum advance. Ill do it next.


wallen7 wrote:Now I know opinions vary widely, but did you degree the cam. I know you said the distributor was on the 18l slot and you were running 12 - 14 degrees of initial timing. 36 + 14 is 50 degrees. What about vacuum advance? I was taught that you should never have more than 42 degrees total advance. Have you checked the color and condition of the spark plugs. Does the engine overheat and are you drowning it with too much fuel or carb?


Yep that 36 mechanical advance is way too much. It pings a bit at cruise around 65-70 on the slightest hills.

It does not overheat and i have not degreed the cam.


Color of the plugs? Hmm Ithink Ill get the wideband before i pull the plugs.

I dont think that i am flooding it with fuel.
Someone on the bronco forums had the same setup with 4spd OD and 3.05 gears and was pulling 18-20mph out of it using 64 main jets. I am using 62s

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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #8 by wallen7 » Tue Dec 19, 2017 6:53 pm

As far as a C6 goes a shift kit will help but I doubt that you will gain mileage from that. The things that really kill on gas mileage on a pickup truck are bug shields ( about 1-2 mpg ), Running boards ( about 2 mpg ), tool boxes that are loaded to the hilt, big heavy bumpers & winches and the number 1 culprit low tire pressure. And of course poor driving habits. When I was working ( Retired now ) we would have people come in and complain of poor fuel mileage, We had a calibrated fuel mileage tester that we used to verify gas mileage with customers. It held exactly 1 quart of gas and when you hooked it up you would record odometer reading and drive it til the engine ran out of fuel and check odometer then multiply by 4 to get true mileage. Most of the time it would show that the vehicle did exactly what the factory said it would do. Now that being said I still believe that with a little more tuning that you can get your mileage up to where you want it.

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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #9 by wallen7 » Tue Dec 19, 2017 6:57 pm

The pinging is what I kind of figured that's the reason I asked about the plugs. I like a smaller carb with stock cam and head but a 600 should work. You will be suprised what a 4 degree advance on a stock cam will do and consider a rocker arm change also.

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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #10 by Retoropak » Sun Oct 07, 2018 4:03 pm

Damn... a year later i finally installed a wideband.
I put back 66 main jets and 31 pump nozzle. I use orange cam on position 1 and the brown secondary spring.

After couple of tweaks it does the following.
Idle : 14.2-14.85 AFR
Light Cruise 5-10% throttle: 16.0-17.5 AFR
Cruise 10-15% throttle: 15.5-16.2 AFR
Cruise 20-25% throttle : 14.8-15.5
Light acceleration 50% throttle- 12.5-13.5
WOT: 11.7-12.2 AFR

I am happy with how it drives, there is barely any stumble when i mash the pedal. I might want to go with a bit more aggressive accel pump cam.

Do you think that exposing a bit more of a transition slot will cure the lean light cruise?
I might also try 68 main jets to see if it will get that light cruise to 15-15.5 AFRs
Last edited by Retoropak on Sun Oct 07, 2018 4:04 pm, edited 1 time in total.

Retoropak
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #11 by Retoropak » Sun Oct 07, 2018 4:03 pm

Damn... a year later i finally installed a wideband.
I put back 66 main jets and 31 pump nozzle. I use orange cam on position 1 and the brown secondary spring.

After couple of tweaks it does the following.
Idle : 14.2-14.85 AFR
Light Cruise 5-10% throttle: 16.0-17.5 AFR
Cruise 10-15% throttle: 15.5-16.2 AFR
Cruise 20-25% throttle : 14.8-15.5
Light acceleration 50% throttle- 12.5-13.5
WOT: 11.7-12.2 AFR

I am happy with how it drives there is barely any stumble when i mash the pedal. I might want to go with a bit more aggressive accel pump cam.

Do you thing that exposing a bit more of a transition slot will cure the lean light cruise?
I might also try 68 main jets to see if it will get that light cruise to 15-15.5 AFRs

Retoropak
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #12 by Retoropak » Mon Oct 15, 2018 7:58 am

Little update.

I did 150 miles over the weekend and the fuel consuptionncame ar whooping 15mpg. Oh and 90% of those miles were cruisng at 75-80mph.



I also changed the mains to 68 and put a #35 pump shooter.

It cured the lean cruise. It is now cruising at 15.5-16AFR. However it also richened up the WOT. It is runnibg 11-11.5 at full tilt.
I might need to drop the size of the secondary metering plate.

Also the bogger pump nozzle cured some of the lean bog.

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THE FRENCHTOWN FLYER
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #13 by THE FRENCHTOWN FLYER » Mon Oct 15, 2018 8:07 am

Retoropak wrote:... 90% of those miles were cruisng at 75-80mph.




This may sound crazy, but try a tank driving at 65 mph instead. At least then you will know what that extra ten mph is costing you.
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THE FRENCHTOWN FLYER
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #14 by THE FRENCHTOWN FLYER » Mon Oct 15, 2018 8:09 am

Retoropak wrote:... 90% of those miles were cruisng at 75-80mph.




This may sound crazy, but try a tank driving at 65 mph instead. At least then you will know what that extra ten mph is costing you.

And modify that Holley for changeable air bleeds. Requires 10-32 tap.
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1986F150six
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #15 by 1986F150six » Mon Oct 15, 2018 11:33 am

Multiple post... sorry.
Last edited by 1986F150six on Mon Oct 15, 2018 12:23 pm, edited 1 time in total.

1986F150six
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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #16 by 1986F150six » Mon Oct 15, 2018 11:33 am

Multiple post... sorry!
Last edited by 1986F150six on Mon Oct 15, 2018 12:23 pm, edited 1 time in total.

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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #17 by 1986F150six » Mon Oct 15, 2018 11:35 am

THE FRENCHTOWN FLYER wrote:
Retoropak wrote:... This may sound crazy, but try a tank driving at 65 mph instead. At least then you will know what that extra ten mph is costing you.


I agree... here is some mileage data from a 1200 mile round trip [several times]:

2014 55-62 mph with no aerodynamic modifications = 21.93 mpg [highest tank = 23.26 mpg]

2015 55-62 mph with aerodynamic modifications = 24.13 mpg [highest tank = 25.72 mpg]

2016 55-70 mph [weighted towards 65-70] with aerodynamic modifications = 22.05 mpg [highest tank = 23.46 mpg]

2017 Missing data due to having driven another vehicle.

2018 65-75 mph [closer to 75] with no aerodynamic modifications] = 19.87 mpg [highest tank = 21.3 mpg]

If you look at the data [above], and compare 2014 against 2018, you will see what a little speed does with regard to gas mileage. This is my truck being driven to and from Skiatook, OK.

I am sorry for the multiple posts... I was attempting to highlight some areas and had bumbling fingers...

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Re: 1986 F-150, c6, 2wd, offy, holley 600 - bad mpg

Post #18 by F-250 Restorer » Tue Oct 16, 2018 9:18 pm

Surely it would be wise to classify mileage claims into rear end ratios. At 60 mph, the guy with the 3.00 rear is going to be spinning far fewer rpm than the guy with the 3.74 rear.

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