Just bought an 81 f250, light duty, 4wd, t18, 300 six, flatbed. Use it on the farm, general transportation. It already has dual exhaust (factory split cast manifolds, 2.25 pipes, single dual inlet/outlet magnaflow(?) muffler).
It gets 12.5 to 14.0 mpg with my light foot. This is 15% more than what I get with my '77 F250, but I'd like to do better. I haven't checked the distributor yet, but I'm planning on 12 initial, 38 total by @3000rpm, plus 10-12 vacume, just for starters.
I'm open to suggestions for the intake. Ideally I'd have a couple of SUs on a log, but that's not realistic at this time. Are Clifford and Offenhaeuser (still in business?) manifolds just for speed, or would they work for my application?
Can anyone tell me what my stock cam specs are?
Any comments welcome.

 

Clifford and Offy are still both in business. Check E-bay for used ones at a good price. Also you could go with a Holley 390 cfm 4 bbl. You get the advantages of 2 bbl primary with a vacuum secondarys for times that you need power.

 

I had about the same truck you mentioned and that is what I got doing about the same type of work. I had an all stock engine excpet it had been rebuilt before I got it and the pistons were stamped ".040" when I replaced a head gasket. With that engine I wouldn't expect much more mpg, as a daily driver with three different 300-6's in three different trucks and I never got any better with any of them.

 

14mpg is great for an '81 4X4. Leave it alone! The initial out of pocket costs far outweigh the fuel savings benefit. If you want better mileage you have only one choice...EFI.

 

Yea, a bigger carb isn't going to help mileage. If your set up is running right, the only thing you are missing is overdrive. And about the only way to correct that is a ZF tranny. Check around for one, then figure out how many miles you can drive on the money it takes to swap. If your tranny is tired or croaks, then it will make more sense.

Check down in the tranny threads.....there's a guy who wants to put a Spicer auxilary tranny in his....and it looks like it's been done before. You need to start drinking beer if you do that, as you will need to steer with your gut as you handle two shifters at once.....

 

Come on a ZF will run, on a very good day, $1500 and a big job to install

 

Don't forget the easy stuff. This stuff costs a little more than stock, but it is all maintenance stuff, so it might save you some $$$ on repairs later as well as giving you a little better mpg.

Plugs, check for vaccum leaks, timing, tire pressure/size
Change the differntial fluid and trans fluid to Synthetic
Change the air filter (even to a high flow)
if you don't leak change to synthetic oil. If you do, use the proper oil for your average temps. (thicker is not always better )
Check the settings on the carb/rebuild if necessary. Make sure the choke is opening fully.
Make sure you have the right thermostat (more important in FI vehicles)

Not a maintenance but...
If you don't use the bed every day - I put a tonneau cover on mine and it made a 2mpg difference on the highway.

Jim

 

Well, it's a flatbed, and I doubt I'll find a tonneau for it (have thought about one for the wife's Nissan). For the moment I have three potentially rewarding ideas:
- Different wheels/tires. It now has cast 16.5x8's with 31x10.50's (worn to 29.5). They look neat, but the tires are well worn and I believe every thing from milage to mud and snow would go better with 7x16's with, say, 235/85x16 tires. In my experience, the wide tires just won't "dig in", or maybe they just resent menial farm life. Wide tires also have greater rolling resistance. 235/85s generally have an OD of 31", which is what it want.
- Recurve the distributor. My 400m (77 F250) runs and pulls well with 16 initial, 38 total by 3000-3200, and another 10-12 vacume. I suspect a similar curve would work on the 300. (If my milage seems good, it may be that the distributor is already set up - there are painted timing marks to 50 degrees on the crank pulley)
- A thermostatic fan clutch. The fan is currently direct-drive. I'm told the fan on the M11 Cummins in my big truck can draw 40hp at 1800rpm (engine) when it cuts in. The 300's fan is much smaller, but I'm sure it unnecessarily draws power. The engine also runs cool, even with a 195 thermostat. The local Autozone has a thermo clutch with a lifetime warrenty (good til it dies???), I just have to check for parts compatibility before I buy it. $28 plus tax, and I think it'll pay for itself.
As for carbs - bigger can certainly be thirstier, especially if you can't keep yer foot out of them, but I'm an old f*rt with patience and discipline even if the old lady doesnt' see it that way ("Another truck????!!!!"). Smaller primaries can give better economy, and I've seen just that on V-8s. I suspect the real problem here is even distribution, which is why I like the idea of a log manifold with dual carbs, especially if they are SUs. At any rate, that's the last thing I'll do.
As for the ZF - trashed f250s with efi300s and ZFs aren't the hottest wrecking yard commodities. I'll bet I could score one on the cheap.
Thanx for the input!