Hi All,


Okay, after this weekend, I've been left scratching my head.

As many know, one of the biggest frustrations I've had with my 300 4bbl setup in my '81 Bronco is the gas mileage.

When I had the stock 1bbl, I could easily net 20mpg at 65mph on the interstate. 75mph would bring me down to around 17mpg.

After I installed my Holley 390, my gas mileage plummeted and I've never been able to get it back up. I get around 11 - 12 around town, and my record was 15 on the interstate (at 55mph with a tailwind) and I average around 13. My lowest was 9mpg. I also get worse gas mileage in overdrive than I do 3rd (since it dogs so bad, it stays in the power valve), so I don't use overdrive anymore.

I've spent countless hours tuning it with different jets, powervalves, accelerator squirters, etc, etc. and have recurved the distributor and done my best to fine tune the timing to see if I can ever get it any better, but nothing seems to make any difference.

I've never been able to understand why, and have finally just given up and come to terms with it.

The specs on my '81.

3.00 rear
4spd OD
31" BFG A/T
Holley 390 4bbl
Offenhauser DP
2150 RPMs @ 65mph (1550 in OD)

Pulls around 18hg vacuum at idle, 10hg at cruise in the flats, and will drop into the powervalve with the slightest press of the gas pedal.





However, this weekend I finally took my '84 Bronco for it's first long distance trip (250 miles).

Specs on the '84

3.55 rear
4spd/granny (no overdrive)
31" BFG A/T
Holley 600 4bbl
Offenhauser C
2600 RPMs @ 65mph

I immediately could tell the difference. It was pulling 15 - 16hg at cruise and it felt completely effortless down the interstate. The lowest it would drop going up the steeper grades was 10hg vacuum. Out of 250 miles, it went into the powervalve once. It seemed so much more effortless than my '81.
When I got to my destination, I topped the tank back off and estimated my mileage: 16.8mpg! This is with 3.55s, no overdrive, and a big 600cfm Holley that I've done NO tuning to except for adjusting the idle.
I imagine with some leaner jets, a K&N filter, and some distributor recurve work, I could easily get 17 or more.


As my title says, WHAT GIVES?
What is wrong with my '81? After this test drive, I know something just isn't right with it. I should easily be matching and/or beating the mileage of the '84.
What could possibly be wrong? What am I missing?
The '84 pulls the same vacuum going up steep hills that my '81 does when cruising the flats!







PS: For what it's worth, they both have identical exhaust setups from the engine to the tailpipe. EFI manifolds. Same part numbers, same everything.
They also have identical tires. Both are bare bones models. No A/C, cruise, carpet, etc. etc. so they should weigh roughly the same.

 

sorry man no help here but if you dont mind i am subscribing because i want to know too lol..maybe not as much as you though...good luck man

 

subscribed...

(i'll have the same 390 holley soon to go on)

AB,

I just had a thought or two... what cam do you have in the 81?

I was thinking with the 3.00 rear and small jets you might be running too lean with too low of a rear gear making the engine over work to keep the broc up to speed... Just my 2 cents though...

 

Except for the month that I had my Comp 260 cam (before the lifters chewed it up), this has, and still is, with the stock cam.

Stock jets are 51s and I've tried from 47 to 55.

 

I never did hear what/how the lifters ate your 260h...?

81 vs 84 3.00 vs 3.55 gears... I can't imagine it's the carb that is killing you...

What trannys are you running?
Are they both 4x4?
I'm guessing both are running stock cams then?
Can you swap carbs? Just to rule out the carb being the problem...??? I would think it would only take 1 hr to swap them...

 

Not sure on the lifter. Did everything on the install and break in check list. From what I hear, you have to visually inspect each lifter to be sure the end is convex. If it's flat from the factory, it'll tear your cam up. I'm guessing I got a flat one.


I agree, I plan on swapping them to see if the issue goes with the carb.

The 81 has an SROD 4spd with overdrive and the '84 has a NP-435 4spd with a granny.
Both are 4x4.

 

i'm thinking out loud more than anything else...

the difference in vacuum at cruise should be directly related to the vacuum advance, right? is your vac advance working, or maybe it needs some adjusting?

to figure out how many inches my canister was pulling, i (temporarily) switched from ported to full manifold vacuum, and measured the timing difference at idle with my light.

 

Hey Rusty,
Yes, I've done a lot of work with the vacuum advance setting up my initial, mechanical, and vacuum timing such as internal spring tensions, how quickly the vacuum advance comes in, etc. It's a new distributor and I've tested to be sure everything works.
I also have timing marks on my harmonic balancer up to 60° advance to I can test things like that.

This is why I'm so frustrated. I've spent a lot of time on this.

Technically, this setup should be ideal for gas mileage (it was with the 1bbl) and yet it's abysmal. Then, along comes a setup that is notorious for not getting the best gas mileage (aggressive gears, big carb, no overdrive) and it does great out of the box.

 

it would at least eliminate one variable to swap carbs; or find the problem...

Sorry I'm not familar with SROD and NP tranny's... Should have seen them in you sig and figured it out... Maybe you need a 5spd OD ZF for your 81!!!! LOL... They are both pretty identically set up except for the offey dp vs offey C and the rear gear... the 3.00's I would think should get better mpgs but the gears could be low enough to be making the engine work harder...

How many hg's are you pulling on the 84 at idle?

What is the final drive ratio on the SROD vs the NP????

 

Granted, I'd LOVE a ZF but the transmission shouldn't be a problem.
The SROD is a 1st, 2nd, 3rd, OD
The NP is a Low, 1st, 2nd, 3rd
The ZF is a Low, 1st, 2nd, 3rd, OD.

So, if I never used Low, the ZF and SROD are fairly identical (aside from the ZFs extra duty strength).


I've thought about the lower gears making it work harder, but it shouldn't be that much. It's still at 2100 RPMs which is plenty of strength. Besides, I believe Harte3 has 2.73s in his truck and gets 18ish and everyone's always talking about "getting your RPMs down" for better gas mileage.

The '84 is pretty similar to the '81 at idle. Around 18 - 19hg.

 

ok not to totally sound like an **** but are you going by your trucks Odometer??? Did you change tire size? Have you made sure that your speedo cable has the right driven gear on the end with the right number of teeth? you probably have checked all that but just wanted to check the basics to make sure all the inputs to the mpg calculation are correct...

I've got a ZF waiting for you here in Iowa if you want to come help me tune my 4.9L in LOL! I know I keep joking about this... so i'll stop...

 

The fact that the power valve keeps kicking in has to be a symptom. I would start there. Why is it needing to kick in to supply fuel? I don't know a tic about Holleys so I can't help you, but it seems like a logical place to start.

I got a great Qjet for ya, lol. I think you need 750 cfm!!!!

Good luck.

 

hmm.
one last spark-related question, then i'm switching my vote over to the carb:
what kind of ignitions are you running?

can you put an o2 sensor in somewhere? the Walker y-pipes and (some of) the EFI manifolds have bungs for them.

 

Swap carbs, and I'll bet the MPG readings will swap as well.
My theory, the 390 is just too small for the 300 and 3.00 gearing combo. You're running close enough to the max volume of air that can flow thru the primaries, before the PV comes into play, just to maintain hyw speed. Putting it on the 300 and 3.55 gearing combo will have the RPM higher, where less throttle input is needed to maintain speed, and the higher velocity thru the primaries will give even better fuel atomization. However, it won't take much to get into the PV again.

Of course, 31" BFG's and 3.00 gears is a rather tall setup for the engine to deal with, especially in OD. I know, as I have 2.75 gears and 31" BFG's on my truck. Gonna be dropping the gears to either 3.25 or 3.50 soon......
I've suggested it before, but I'd bet both trucks would get better MPG if you were to swap the trans between them. Pair the OD trans with the 3.55 gears, and the non-OD trans with the 3.00 gears. Besides, might as put the bulletproof trans in front of the bulletproof axle, and put the weaker trans in front of the weaker axle......
The 3 + OD trans really should not have been used with gears higher than 3.25 or 3.50, in my opinion. I know it was used with 3.00, 2.75, and even 2.47, but that's just getting a bit ridiculous to me. Works ok on the flats, but in the hills, it really sucks the gas......
Prolly why I chose the 3.55 option when I bought the 94. I took a test drive in a 300/5 speed truck that had 2.73 gears, and said no way. Found one with the same engine/trans and 3.55 gears, and was very satisfied. Enough so, I took it home with me.

FWIW, my 95 B2300 can get well over 30 MPG, but thanks to a 2.3L/5 speed and 3.45 gears, it won't pass anything BUT a gas station...... Gonna swap 3.73 or 4.10 gears in it. Someday.......

 

My personal feeling is to put the original carb configuration back into place - IOW undo the changes - and see if the mileage & performance also return to how they were.