Hi all,

Just got back from a 1200 mile round trip in my '84 Bronco. Drove to northern California and back for Thanksgiving.
Overall, the Bronco drove like a champ. Not only were there clear skies, clean roads, and no wind the whole way, but I didn't have a single problem with the Bronco.
Plus that bench is so incredibly comfortable! Usually I'm dancing in my seat to turn and find a comfortable position, but after 11 hours in the vehicle (one way), I really just wanted to get up and move around for the sheer sake of having sat too long. So compfy.

ANYWAY.

I calculated the gas mileage for every leg of the trip, and it was all over the place.

Most all of the terrain of western Idaho, southern Oregon, and northern California is nearly identical. Rolling hills and mountains. For the most part, it was very easy going.

First leg:

Boise, ID to Lakeview, OR (OR/CA border).
335.2mi 24.17ga. = 13.9mpg (freaking out at this point)

Second leg:

Lakeview, OR to Red Bluff, CA
225 mi

While we were there, we put on 100 or so miles driving back and forth to town, going grocery shopping, etc.
Before the trip back, I rejetted the carb by dropping it 2 sizes, and then putting in a smaller power valve.

Then, since I didn't want to fill up on CA gas, I put in 12 gallons to get back to Lakeview.

Sunday:
225 miles back to Lakeview. (The last stretch of this trip had me a nervous wreck. When you're way out in the middle of nowhere, going 30 miles with the gauge below E isn't fun).

But total round trip Lakeview to Lakeview:
600.7mi. 39.23ga. = 15.3mpg. Much better! Even with the 100 miles of grocery getting and city driving.

Lastly, Lakeview to Ontario, OR.
272.7mi. 15.75ga. = 17.3mpg!!

After filling up in Ontario, I drove back to Boise, but didn't top off, so I don't know what it did on the last 60 miles of the trip home. I was just glad to be back in Boise and didn't bother. However, I could tell that it was just sucking gas down again between Ontario and Boise. After having put that many miles on it and watching the gauge, I knew where the needle should have been if I was getting decent mileage. I'm guessing it was getting 12 - 13 again on the way back.



The odd thing is, the trip between Boise and Ontario is pure, straight interstate driving. It's the only interstate stretch on the whole trip, so I can fly with ease on this stretch. There are no hills or mountain passes like the rest of the trip had. It should have, theoretically, been the best gas mileage, but it was getting its worst. I'm guessing that the first leg of the trip (the 13.9mpg) was due to terrible gas mileage between Boise and Ontario (like 12 or so) that was brought back up by decent mileage through Oregon.



The only difference between the driving I did with the good mileage and the bad was speed, and it wasn't much of a difference.
Boise to Ontario is 75mph, and I usually do 65. I think I averaged around 65 to 68. (2550 - 2650RPMs)
The rest of the trip was 55, and I usually averaged around 58 - 60. (2250 - 2350RPMs)

That increase in speed shouldn't earn a drop from 17 to 12mpg, but it did. I really don't think it's aerodynamics since my other Bronco gets decent mileage at that speed.

Could the secondaries be opening?
Mathematically, they shouldn't be with a 600cfm carb, (that should be more around 3000 - 3500) but something's happening right around 2400 - 2500 RPMs to shift the carb into a gas guzzler mode.
The power valve is a 6.5, and even though I didn't have my vacuum gauge with me, this Bronco has a history if averaging around 13 - 15hg on the highway. I've done 75mph for short stints and it's usually around 10 - 12.

Any ideas?

 

AbandonedBronco,

I always enjoy reading your postings!

Let me share a couple of ideas...

First, FTE has posted in the past, that there is an appreciable drop off regarding gas mileage when our beloved "bricks" exceed 62 mph. It takes much more power to go faster. The easy way to look at this is that @ 60 mph, double the speed [60 X 60] and you get a 3600. At 75 mph [75 X 75 = 5625]. At 75 mph, the power required to overcome the wind drag is 56.25% more!!! Imagine what that does regarding gas mileage. Shocking, isn't it?

Second, I know your carburetor is not like mine [Carter YF], but follow this comparison. On the highway @ 60 mph, my engine returns ~22 mpg @ ~1650 RPMs [3.08 gears and 215/75 X 15 tires]. My son's 1984 F150 [feedback carburetor] used to have 2.47 rear gears and stock sized tires. At 60 mph, it would return 26 mpg [pre 10% ethanol] and was turning ~1300 RPMs.

I believe these engines return the best mileage when operated at or below the peak torque speed. On stock carburated engines, this is ~1400-1600 RPMs.

Both of our trucks have the 4 speed manual O.D. transmissions. Also, I believe out trucks weigh a bit less than the Bronco. Mine weighs ~3750# when empty.

 

Thanks for the ideas!

I know the faster you go, the more power it takes. Plus, these things aren't all that aerodynamic. But, if truck A and truck B are for all intensive purposes equal, and driving side by side, and truck A is getting 20mpg, then aerodynamics and speed aren't the reason truck B is getting 12mpg.
My '81 used to be able to get 16 - 17mpg going 75 - 80mph when it had the OD, smaller tires, and 1bbl. That's pretty fast for these, and reasonable mileage!


It's an argument I've presented quite a few times on the Bronco forums when people say that speed/aerodynamics are the reasons ALL Broncos get terrible gas mileage (usually 10 - 12). If that were true, then X amount of energy would be required to go that speed, Y amount of fuel would be required to supply that energy, and my 6 cylinder wouldn't get any better than V8. .

My '81 can get 17 - 19 going 65 - 70, so the '84 should, theoretically be able to too.

That's why I imagine it's gotta be something do with RPMs, secondaries, or some power mode is switching on, etc. The '84 is at higher RPMs than the '81. (3.55 vs. 3.00 rear end) so it'll never really get as good as mpg (not a fan of its 2500 RPMs at 65), but going from 60 to 68 mph and it drops from 17 to 13mpg? That's fishy to me!

 

Heres just a random idea..elevation. I know it may not matter..or if elevations changed very much..but I wonder if the elevation and pressure could change the fluid density of the fuel..thus changing the way it is burned and how efficient?

 

That's why I imagine it's gotta be something do with RPMs, secondaries, or some power mode is switching on, etc. The '84 is at higher RPMs than the '81. (3.55 vs. 3.00 rear end) so it'll never really get as good as mpg (not a fan of its 2500 RPMs at 65), but going from 60 to 68 mph and it drops from 17 to 13mpg? That's fishy to me! [quoted from above]

Okay, if my poorly worded mathematical explanation is anywhere near being correct [I believe it is], then if you take 60 mph as your reference point [60 X 60 = 3600] and compare to 68 mph [68 X 68 = 4624], there is a 28.44% increase in power required to overcome the wind resistance. If you multiply 17 mpg by 28.44% [4.83] and subtract, the projected number is 12.17 mpg [17 - 4.83]. Of course, this theory is not absolute, but does strongly show a trend.

Again, thanks for sharing as this causes all of us to think and hopefully learn. <!-- / message -->

 

You changed the PV to a smaller one...6.5? What were you running prior to the change?

 

As far as elevation goes, most of the drive was around 4500 - 5000 feet with mountains.

The bad gas mileage stretch is around 2000 - 2500 feet and is basically steady and flat between the two.


I thought about that math, and it works in small doses, but not in a bigger picture.
My original setup got 20mpg @ 65 and 17mpg @ 75.

If using the math above and, just for example, saying I got 20mpg at 60mph:

60: 20
62: 18.7
64: 17.6
66: 16.5
68: 15.6
70: 14.7
72: 13.9
74: 13.1
76: 12.5


I have a really hard time believing that a well tuned and nicely running truck is going to drop from 20mpg at 60 to 12.5mpg at 75. That's just over the top. That'd mean that one of those gas saver cars would go from like 45mpg to 29mpg. No one would ever go 75 if that were the case.

I mean, I know it'll drop, that's just the nature of the beast.

At least, that's what I'm thinking.





I was running an 8.5pv prior. Reason being that the '84 operates really high on the vacuum gauge. It takes some serious hills to get it down to 8 to 9. I've done my fair share of steep grades at 65 - 70 where it sits around 11 - 12 vacuum.

I dropped it in size "just in case".

 

Hmm..I fins this to be a mystery and I'm thinking of all sorts or crazy ideas..what about temperature changes? Could affect your tire inflation..thus less mpgs? Stupid..but I thought is toss it out

 

I'm inclined to think that most of it is in the speed. 60-62 mph is about tops for mpg in these bricks.

 

I feel compelled to throw in my two cents: My stroll down the yellow brick road of carbs has led me to a rebuilder of Autolite 4100's who has 40 years experience with them. When I asked him about building one for my 300, the first thing he asked was how much manifold vacuum it had. It seems he always builds them with Holley pv's, at 1/3 the manifold vacuum at idle.

I hope that adds something useful to this thread.

P.S. Was that you I saw with the bull horns wired to your hood? Come on, AB, tell the truth.

 

I know they certainly aren't the most aerodynamic things around, but I've done much better mpg at much higher speeds than 60-62. When mine was stock, I used to get 17mpg at 75mph. Tires are a little bit bigger now, but not a whole lot, and I'm not doing 75 either. It shouldn't be getting 12 - 13 at 65 - 70, when 60ish is 17mpg. That's a FAST drop.
I'm inclined more to think that something kicking in fuel guzzle mode, like too high RPMs, or the secondaries coming on.
It MAY be speed alone, but I'm having a hard time swallowing that.

I've often thought that. Recommendations from Holley and tuners are either 1/2 the hg of idle, or idle hg - 2. For a good, healthy idle of 19 - 21, that'd be a 9.5 to 10.5 power valve. That's really, really big! Especially since, on the last motor in my '81, I'd have a idle vac of 19, and a cruising vac of 8 - 9. Yeah right I want an 9.5.
I think the hg - 2 is more for drag motors that are pulling only 8 - 10 vacuum at idle.
I'd agree that for a good, strong stock engine, 1/3 is much more like it. Which is usually around a 6.5.

LOL, I'm trying to visualize a bronco with bull horns on the hood. Someone loves their attention.
On that note, if I was ever down that way, I'd letcha know. I was way up north, though. Around Redding.

 

AB--

I've been collecting 4100 p.valves, and now have, IIRC, 4. The highest is 7. The others are all close to 5.

As with all things automotive, the I6 crowd finds that the world revolves around the v8! Every tuner we speak to, every kit we buy, 99% of the time they are speaking v8. Something always gets lost in the translation.

 

Hi, just found this thread. I do like my inline engines.

Reading about your trip, it's hard to truly figure having the mods done at the halfway point. But a couple of things do occur to me. One is the gasoline itself. Varying amounts of alcohol might account for part of it. And something you can check, see if the carb is loose. These things are famous for that. Both the base and the top and bottom sections of the carb will loosen up over time. I thought of this only because you said the mileage dropped off at the lower altitude, which is exactly backward from what you would expect.

 

It'a a shame you didn't run the whole trip with a vac gauge installed. I run one all the time and it tells you a lot. My vac is completely different than yours. We have no mountains or big hills here, but it's easy to jump down to 3,4, or 5" vac just going over an overpass. Idle is 20, putting in drive drops it to 16.
Cruising at 70mph level stays around 14".
I've run 3"4" and 5" power valves, even 2 stage power valves. They may work for you. I would never want one to open unless carb was floored or under very heavy load, no matter what anyone recommends.

About the only way to really find out is to install a wide band to monitor it.

 

Reminds me of how sometimes when I'm in a parts store and they ask what motor is in it, they try to stab a guess, and 99% of the time they guess 302 or 351. I've never had a parts store guy guess, "300?" I'll be shocked if I do.


Yeah, changing the jetting half way didn't let me see what it did at the 'high speed' portion of the trip back, but the rest of it was definitely better mileage. Maybe a 1 - 1.5mpg increase.

I doubt the carb is loose since I just had it off a week or so before the trip. Torqued it down properly, checked for leaks, etc. Even had the manifolds resurfaced and a fresh gasket at the same time.

The mileage dropped at a lower altitude because with lower altitude came higher speed limits.

I had thought about gas, which is why I filled up at the same gas station there and back. I thought I might have had some bad Idaho gas, so after getting good mileage from the Oregon gas in Lakeview, I filled up there on the way back (and I know it was ALL from there because I was below the E when I filled it). Got 17 some on it on the way back. Then, topped off in Oregon before coming back into Idaho. It was a lot less, so there would've been plenty of "good" gas still in there.

I wish I had had my vac gauge with me. It was in my '81, which is usually where I have it. I didn't want to mess with it since I was traveling with my wife and didn't want a vacuum line running across the cab and the gauge sitting on the bench. But, it would have been good to have. It's always such a strong runner with high vacuum that I didn't think it was too important. But, in hindsight, I'd have been really curious to see what it read.

I really, really wanted a wide-band before I left for the trip, and had one scoped out. Just didn't fall into the budget in time, unfortunately. THAT would have been great to have on the trip. No guess work anymore!