We have an 85 Big Bronco with a 300 I6 4spd manual. everything is stock and this truck only has 45k original miles. when we got it it had a blown head gasket but was still running. after doing a new thermostat, head gasket and re seating the valves it was back on the road. there were(maybe still are) some wiring quirks but its being driven everyday and doin ok. my questions are 1) im only getting about 10 mpg and i drive 55. no hotrodding. was hoping id be closer to the 15mpg i see a lot of other guys say they get. 2) a six inch lift and 35x12.5 tires are already on the way to get some clearance and traction. will the 300 push these tires comfortably? This is our first Bronco and it seems very low geared. 3) how involved is doing the offenhauser manifold and holley 4bbl swap? any help will be appreciated. I already know these motors are usually bulletproof and ours runs pretty well but doesnt really seem to have any pep.

 

I'd be looking at carburetor tuning to be certain you aren't dumping too much fuel to begin with. If you get a lot of detonation, especially while climbing or passing, check your base timing and come up a fuel grade for a few tanks.

If your fuel economy is that bad now, those tires will drop you into the realm of "Gallons per mile" unless you plan on re-gearing the axles. The 300 will pull those tires around just fine but with the OEM axle gearing (typically 3.08:1), you are going to drop the RPM's well below optimal torque range even for a 300. (which develops its peak torque about 600-800 rpm LOWER than the average V8).

At 60 MPH you are turning about 2142 RPMs with stock tires and 3.08:1 axle gears.

At 60 MPH you are turning about 1870 RPMs with 35" (nominal) tires and 3.08:1 axle gears.

That represents a difference of just a hair under 272 RPMs. Considering that the 300 reaches peak torque at about 2300 RPMs, that puts you more than 400 RPMs BELOW optimal output range.

Best advice I can think of... rectify the fuel economy problems that exist already, then seriously consider what adding 35's without re-gearing will do to your performance.

 

Your lack of "pep" and poor mileage is from the stock feedback carb and TFI distributor. When not working correctly you aren't getting any distributor advance. That makes the engine sluggish and gives it crappy mileage. The easiest fix is to dump it and do the mods below.

If you want some more power look into the following mods.

DS II distributor & ignition module swap.

Offenhauser C-series 4V intake.

For the carb you don't need a 4V unless you really want one. A 2V to 4V adapter with an Autolite/Motorcraft 2100/2150 2V carb ( 1.21 or 1.33 size ) or a Holley 500 cfm 2V work great. If you want a 4V an Edelbrock 500 or 600 cfm are the simplest. If you really want a Holley 4V the 1850 600 cfm are the cheapest and will work. The 390 cfm Holley is more trouble then it's worth.

A pair of 87-96 EFI 300 ( 4.9L ) exhaust manifolds work great and flow much better then the stock carb exhaust manifold. These are basically a pair of cast iron shorty headers that came stock on all EFI 300's. Then add a freer flowing exhaust system.


These mods will give the best bang for the buck without tearing into the engine internals ( cam swap, head work, etc. ) .

 

forgot to mention the truck is currently on 31x10.5's. Regearing is certainly an option. I dont mind spending a little time and money to have a reliable wheeler. any sugestions on what gears to go with? this will be primarily a wheeler but will also serve as a backup vehicle in case something goes wrong with one of our daily drivers. we both work within 20 miles of home so the mpg doesnt have to be great. if it stayed where it is i could deal with it. just wondered why everyone else got so much better than i was. Timing i can do. Carberator tuning... never done it... first time for everything tho. can u steer me in the right direction to get the info to tune the carb. should i buy a rebuild kit? Im mechanically inclined and willing to attempt anything once or twice before i pay to get something done. a locker was in the future plans so if i regear i could probably put that in my budget. thanks for your advice greystreak!! look forward to more of it!!

 

fordman75 is there a writeup somewhere that details the ignition conversion? ive been looking for a offy manifold and i dont need the 4bbl just didnt think about adapting the 2bbl to the manifold. THANKS!! the exhaust manifolds are at the local wrecking yard by the dozens i already called and checked on them. no smog here so i was just going to cut out the cat and put on a flow thru muffler. would that flow well enough or should i increase pipe diameter as well?

 

ok just found the duraspark thread. sounds easy. until i get the manifold can i use the carb off the older trucks to get me going? the non feedback carb off say an 80?

 

If you are going 35's you should really go with at least 4.10/4.11 gears. If it's mostly for wheeling then even 4.56 gears would be a good choice. But if you want to keep it a back up vehicle for street use I wouldn't go steeper then the 4.10/4.11's.


The problem is your 85 would have came stock with a computer controlled feed back carb and TFI distributor. If there is a bad sensor or other issue it will cause you problems ( lack of power and crappy mileage ) . It's not really worth the time and money to try and fix that set up.





Check out the inline 6 engine forum here or even the 240/300 forum at Fordsix.com and you'll find all the info you need for the Duraspark II distributor swap. It's pretty simple if you get your hands on a complete system with the harness.

The EFI exhaust manifolds are a great upgrade. They are easy to find and pretty cheap. But you will need to either get a Y-pipe for them or run dual exhaust. If you are going new on the exhaust a single 2 1/4" or 2 1/2" system would be the way to go with a fairly mild engine. Or if you go dual exhaust there is no need to go bigger then dual 2". A slightly larger then stock exhaust system will perform better. But if you want to work with your stock exhaust that's fine too. But like I mentioned you'll need to get a Y-pipe for the dual outlet EFI manifolds. Dumping the cat and adding a freer flowing muffler will help a lot.

 

Yea just get a carter YF off a 82 or older 300. But you'll want to do the DSII swap too. otherwise you won't see a whole lot of improvement. Going to the YF won't help your power a whole lot but the YF swap and DSII should improve you mileage issues.

 

it seems to have low end power as it is now just no giddy-up. ill be shopping for a yf carb when i pick up the rest of it. thanks for all the help guys. really steered me in the right direction!! now to start looking for some 4.11's