Following the encouragement of a couple of forum members, the following data representing the truck I drive will be presented with the hope that this information will assist others. In no way do I make the claim that this represents the best combination for high gas mileage, but works for me.

1986 F150 SWB with 4.9L engine [Duraspark conversion]
4 speed manual OD transmission
3.08 LS differential
215/75 X 15 Michelin LTX M/S tires mounted on 7" wide factory spoke steel wheels [35 psi front; 32 psi rear]
Rotella T6 [5W40] synthetic oil
Motorcraft FL1 oil filter
Purolator air filter

No EGR or air pump
Factory log style exhaust manifold including original (2) catalytic convertors
195F thermostat
Stock air cleaner [including exhaust preheat & cold air induction]
Original intake manifold
Carter YF 4901 S carburetor [from 1970 F350 with 300 engine]; choke converted to manual
Duraspark distributor recurved to 24 degrees advance @ 3000 RPMs
Motorcraft SP452 spark plugs gapped @ .050"
MSD 6 AL ignition module
Base timing via vacuum gauge = 18.9" Hg @ ~650 RPMs

The truck was purchased from the original buyer in March of 2008. The engine performed terribly in all respects due to missing vacuum hoses and assorted sensors. The "feedback" computer controlled ignition was locked in the "limp home" mode. The decision was made to retrofit the truck with Duraspark components. Presently, the engine has 176,000 miles with no internal work.

3/12/08 11.42 mpg
3/29/08 9.40 mpg

Presently 18.56 mpg avg. [17.49 - 21.16] 17 tanks [3/11 - 7/14/14]

My driving style is very conservative. The goal is to get into the next higher gear as soon as possible without lugging the engine. "Drive like there is an egg between your foot and the gas pedal". Normally, the engine speed is kept between 1200 and 1800 RPMs. [With 3.08 rear gears, 2000 RPMs = 70 mph].

My daily drive is very repeatable and records have been kept since 2008. Last Fall, the speedometer was checked against Interstate mileage markers. The truck was driven for 20 miles while staying in the same lane. After 20 miles, the odometer indicated 19.65 miles [1.75% slow].

The mileage is indicative of what I call urban driving... small towns with stop signs, lights and 25-45 mph. Probably 50-60% is @ 45 mph.

The average mileage drops 1.5 - 2 mpg in the Winter.

In September, a trip is planned which is ~575 miles, one way. This will present a good opportunity to check the highway mileage. At the destination with assistance from other forum members, plans have been made to "map" the initial + centrifugal + vacuum timing as well as use an AFR meter [wide band] to record the fuel mixture. This data will be added to this thread as an update.

Knowing actual timing will enable ones who use timing lights to duplicate the settings, if desired. The present set-up as listed above, was tuned using a vacuum gauge. The engine is capable of pulling 21.5" Hg, but due to pinging while under load, the vacuum was retarded to 18.9".

I do hope this information will be beneficial. I am always trying to figure "other" ways to improve, such as perhaps a free flowing exhaust and lighter wheels and ?????

 

Good job, David. The info we get in Sept should really help nail down the setup.

As for improving, I think you can get a bit better by adding more vacuum advance. That way you can have as much advance as you did at the highest you got, but when you give it more throttle the vacuum will drop and so will the timing so you won't have pinging.

 

You're doing good! I have the same setup but an '83. Two things strike me. Why are you using a spark plug gap other than stock? The other is....you would get a jump in mileage with higher tire pressure, especially in the rear. Will those Michelins take 40 psi? Ford recommended 41 in our version. The tires were Extra Load and rated at max 41.

 

That's impressive mileage, especially for town driving

Is the 24 degrees just your centrifugal advance?

 

oneowner83, the wider spark plug gap is possible due to the higher voltage output when using the MSD ignition module. A wider gap facilitates a more complete burn, but at the same time tends to advance the "timing".

With 35 psi in the front and 32 psi in the rear [normally unloaded], the tread lies very flat and wear is very even.

Thank you, Ken. Yes, the 24 degrees is strictly the centrifugal advance.

 

I run high air pressure. I never see a tire worn down. They dry rot first. You're giving up some serious gas mileage. Why not at least put a pencil and paper to it eh?

 

We played with David's truck today to determine what his AFR is and what his timing is. First, we installed my AEM wideband meter and looked at the AFR:

• Idle: Once warmed up it was about 12.5:1
• Cruise: At 62 where he ususally runs it was in the mid-15's on level ground, but climbing hills it got as high as 17.0 at one point.
• Power: Once the throttle was open it went as low as 11.5 but normally at about 12.0 - 12.5.

That said the carb, a 1970 YF carb off of an F350, is jetted just right. It really shouldn't be any leaner, but the truck runs well so it isn't too lean. And it certainly isn't rich.

Then we checked the timing. The initial timing was set at 18 degrees, which was getting good MPG but made the engine slightly hard to start sometimes. And, it pinged at full throttle, so the overall timing was too much.

As for the mechanical timing, at 1700 RPM where David cruises it was giving 10 more degrees advance. Then we checked the vacuum advance and found that it was starting to advance at 10" and was giving a total of 18 degrees 16" of vacuum, and at the 14" of vacuum the truck had at 62 MPH the advance was 12 degrees. So, total advance at 1700 was 18+10+12=40.

Then we turned the vacuum advance two turns clockwise, which made the vacuum start coming in at 8", gave the full advance of 18 degees advance at 14". Then we set the initial timing to 17 degrees, and the drive determined that it pinged both at part throttle as well as full throttle. So we put the initial timing at 14+, giving a total of 14+10+18 = 42 degrees at 1700 RPM. And the truck felt happier with that setting than it has.

 

Would it benefit from a power valve that opened slightly earlier under load......but keep the current jetting for cruise economy?

Also, does the centrifugal advance still need 3000 rpm to be in fully (as in Post #1)?

Sounds like a good day!

 

We didn't test the centrifugal beyond 1700, but we are sure it does max at 3000. As for having the metering rod come up earlier than it is, which looks to be at about 3", I don't think it is needed. The truck doesn't act like it is lean at all when tipping in. At one point David eased into it until he reached WOT and at no time during that did it sag or buck. In fact, it wasn't obvious when the rod popped up.

But, it was a very good day. We got essentially everything accomplished we wanted to accomplish. We were able to dial 2 more degrees of timing in at cruise while backing off the initial by 4 degrees, making it start easier. Plus, with the vacuum advance coming in with less vacuum he's going to hold the advance under more driving conditions - w/o pinging. It really looks like a win/win/win.

 

Good to see that level of efficiency from a 44 yo carb

 

I expected the thing to be jetted rich, but nothing of the sort. It was set leaner than I would have specified, but it works and works well.

 

I'm sure he's driving home with a big

 

Subscribing just for Davids report on driveability and return trip mileage.

I really should do something with my Holley but I don't have a place to deal with it.
It certainly doesn't have the 'economy' the 4180 did.

 

David is on his way home as of about 6:00 this morning. And, he was wearing a smile. His only concern was whether to get gas here, making this tank lower MPG due to the ~40 miles of testing we did, or getting gas at the end of the range to pull this tank's average up. Obviously there are swings and roundabouts in that, but the fact that he's thinking about those things shows his attention to detail.

And, he is very thorough. He pulled receipts from his wallet for the last six or so tanks of fuel and each had notes on it describing the conditions under which the tank was burned. Like "hot" or "75 miles of city" or "all highway". And one notes when he re-installed the chin spoiler, which has made a noticeable difference. So, I'm confident we will get a good and accurate report on his MPG.

For the records, here are our notes:

• Overall Timing
  • On arrival:
    • Initial: 18 degrees
    • Centrifugal: 10 @ 1700 RPM/62 MPH
    • Vacuum:12 @ 14"
    • Total: 40 degrees @ 62 MPH on level ground
  • On departure:
    • Initial: 14+ degrees
    • Centrifugal: 10 @ 1700 RPM/62 MPH
    • Vacuum: 18 degrees @ 14"
    • Total: 42 degrees @ 62 MPH
• Vacuum Advance:
  • On arrival:
    • 10" = 8 degrees (advance starts here)
    • 12" = 10
    • 14" = 12
    • 15" = 16
    • 16" = 18 (max)
  • On departure (2 turns)
    • 8" = 7 degrees (advance starts here)
    • 10" = 10
    • 12" = 14
    • 14" = 18 degrees (max)

 

Okay, where to start? Gary and I had a discussion centered around the fact that when setting up and conducting experimentations, one is supposed to enter it with no preconceived ideas. However, that is hard to do. It is possible and I am trained to do it, but I admit, I was shooting for increased gas mileage and even shared with the OK GTG attendees what my hoped for figures were. Sometimes things turn out differently and one has to accept the facts and then think about what really happened...


In other threads, some of my truck's gas mileage records have been shared and I have gone on record as saying that with my very conservative driving style and the routine route driven daily, the truck averages 17-22 MPG. Gary mentioned the gasoline receipts in my wallet. That is where I keep them until my wallet gets uncomfortable to sit on [notice, receipts, NOT dollar bills! ]. The data is then transferred to a more permanent file at my house. I will now list what Gary looked at just to provide recent mileage figures and then discuss the ones for the drive to and from Skiatook, OK. All the local ones are from the same pump, which has a very slow delivery rate.


7/23/14 203.8 miles 11.172 gal. 18.24 mpg town/warm advanced timing
7/30/14 197.3 miles 11.079 gal. 17.81 mpg extra town/mild some ping
8/7/14 212.8 miles 11.939 gal. 17.82 mpg town/very warm 20.0" vac. @ 95 miles.
8/15/14 203.9 miles 10.207 gal. 19.98 mpg town/warm 130 miles highway @ 60-65 mph
8/21/14 205.3 miles 11.294 gal. 18.18 mpg town/hot 20 miles highway
8/30/14 240.9 miles 13.020 gal. 18.50 mpg town/hot * chin spoiler @ 50 miles
9/10/14 206.9 miles 10.857 gal. 19.06 mpg town/warm-hot * air line to choke housing


Then, with perhaps 250+# of parts and luggage, I departed for OK after having driven 58 miles in town.


9/12/14 251.0 miles 11.620 gal. 21.60 mpg 58 town/193 hwy 55-62 mph; rain/hilly
9/12/14 233.6 miles 10.043 gal. 23.26 mpg hwy @ 58-62 mph cool/lt. rain


See Gary's comments regarding adjustments. [above]


9/14/14 227.9 miles 11.634 gal. 19.59 mpg ~40 miles test./62 mph
9/14/14 305.2 miles 13.984 gal. 21.82 mpg hwy @ 62-65 mph*/cool
9/14/14 251.0 miles 10.736 gal. 23.37 mpg hwy @ 60-62 mph.


I believe the 21.82 mpg achieved on the next to last tank was due to the engine running more freely after the adjustments. I had mentioned this while in OK. The engine seems to want to rev more easily. Perhaps there is a little more power in my crusing range [~1650 RPMs]? Anyway, if I did not pay close attention, instead of maintaining what previously had been ~59-62 MPH [no cruise control], I would look down to discover that I was going 62-65 MPH, sometimes while going uphill [gasp! ]. One time I even slipped to 70 MPH.


I paid closer attention on the final tank.


Well, I was wanting to increase the gas mileage, but what was accomplished was to maintain the mileage and stoping the pinging while under load and making the engine start and run better.


I have noticed that it does not ping at all, even when under a light load. Would the next step be to again increase the sensitivity of the vacuum canister so the timing advance will come in sooner?


Hope this is not to long!