[BigBlue 94]

I'll have ti look to see what wires I have and coil. I don't really need top end as I almost never Rev over 2500 unless I really have to merge quickly. I'll definitely check out that forum, I've read a ton of posts there, but never actually posted. So maybe I'll give that a shot, thanks!

You are seriously missing out on a lot of fun and power by shifting under 3000 rpms. When i got my 300 it was set up with a 390 cfm holley, Clifford open plenum intake, and headers, with a points distributor and pertronix ignitor. It was a screamer up to 5500 rpms. You arent even opening the secondaries at 2500 rpms.

If you do any towing, rpms are your friend.

 

[Klenahan]

You are seriously missing out on a lot of fun and power by shifting under 3000 rpms. When i got my 300 it was set up with a 390 cfm holley, Clifford open plenum intake, and headers, with a points distributor and pertronix ignitor. It was a screamer up to 5500 rpms. You arent even opening the secondaries at 2500 rpms.

If you do any towing, rpms are your friend.

Yea I open it up when I need to, but with the gearing I have its not too thrilling. I tow a small boat and plan on towing a small camper some day, but for now I would just rather better fuel economy while Im doing longer drives.

 

[BigBlue 94]

Yea I open it up when I need to, but with the gearing I have its not too thrilling. I tow a small boat and plan on towing a small camper some day, but for now I would just rather better fuel economy while Im doing longer drives.

Yeah those 2.47 gears are strictly for mpgs... I have 4.56s turning 37s. But i also have a pickup for towing things. Broncos are not great for towing

 

[Klenahan]

Yeah those 2.47 gears are strictly for mpgs... I have 4.56s turning 37s. But i also have a pickup for towing things. Broncos are not great for towing

Yea, Ideally Id get a bronco and put these gears in it, then build this truck for towing

 

[White Dragon]

So @AbandonedBronco ,

You get that vacuum issue sorted?

 

[robbz28]

@Klenahan im late to the party on this one, but from what i've read i believe you just have a bad combo going on. I don't know much about your intake, but as mentioned its a single plane and they do make less vacuum (typically) and depending on the runner length they can be better for low end or top end. In drag racing a low profile single plane were more torquey and the high rise single planes were for more upper RPM range. Sounds to me like you don't have a good cam / intake / final gear ratio combo to make the most of what you have.

We all have to learn somewhere, i can tell you the exact moment i learned this lesson, it was about 23 years ago and i had a 69 camaro with a 350. I wanted to go faster so i got myself an Edelbrock Performer RPM cam / intake combo kit and a brand new holley 600 double pumper. I didn't know crap about camshafts, and using a vehicles weight, heads, transmission ratio, rear end ratio etc to utilize said camshaft. I installed it all one weekend, did the cam break in, the car sat there and just shook the cam was so aggressive. Then when i took it out on the road, the lift was so high that it pulled half of my rocker arm studs out of the head. I spent the next weekend putting screw in studs in, finally got the car running, but it ran like crap. I had a peppy zippy happy little 350 that ran great and stuck a big stupid cam and intake and carburetor on it that it didn't need. In order to make the new combo work i needed to re-gear up to a 4.11 and get my heads ported. Instead i got some advice on what cam i should get, and got my heads reworked in the process. I finally ended up with a little bit faster car after a lot of unnecessary work and expense that a 19 year college student couldnt really afford.

You may be best putting your old stuff back on, or making the necessary changes to utilize your new components properly. Camshaft, head work, gears is my best guess.

 

[Klenahan]

@Klenahan im late to the party on this one, but from what i've read i believe you just have a bad combo going on. I don't know much about your intake, but as mentioned its a single plane and they do make less vacuum (typically) and depending on the runner length they can be better for low end or top end. In drag racing a low profile single plane were more torquey and the high rise single planes were for more upper RPM range. Sounds to me like you don't have a good cam / intake / final gear ratio combo to make the most of what you have.

We all have to learn somewhere, i can tell you the exact moment i learned this lesson, it was about 23 years ago and i had a 69 camaro with a 350. I wanted to go faster so i got myself an Edelbrock Performer RPM cam / intake combo kit and a brand new holley 600 double pumper. I didn't know crap about camshafts, and using a vehicles weight, heads, transmission ratio, rear end ratio etc to utilize said camshaft. I installed it all one weekend, did the cam break in, the car sat there and just shook the cam was so aggressive. Then when i took it out on the road, the lift was so high that it pulled half of my rocker arm studs out of the head. I spent the next weekend putting screw in studs in, finally got the car running, but it ran like crap. I had a peppy zippy happy little 350 that ran great and stuck a big stupid cam and intake and carburetor on it that it didn't need. In order to make the new combo work i needed to re-gear up to a 4.11 and get my heads ported. Instead i got some advice on what cam i should get, and got my heads reworked in the process. I finally ended up with a little bit faster car after a lot of unnecessary work and expense that a 19 year college student couldnt really afford.

You may be best putting your old stuff back on, or making the necessary changes to utilize your new components properly. Camshaft, head work, gears is my best guess.

I was thinking about that, I have the stock cam and planned on putting a zf5 in, but am having trouble finding one. I don't think I would go back to the old setup though because the truck runs great, pulls hard, and is fun to drive. It just gets much worse fuel economy now. With this being my first road use carburetor, I'm trying to keep my motivation up and just claim it to bad tuning on my part. I'm just a bit surprised I would need more work done to the cam or head, I feel like most of the applications I hear people using similar setups on are pretty much stock

 

[robbz28]

I was thinking about that, I have the stock cam and planned on putting a zf5 in, but am having trouble finding one. I don't think I would go back to the old setup though because the truck runs great, pulls hard, and is fun to drive. It just gets much worse fuel economy now. With this being my first road use carburetor, I'm trying to keep my motivation up and just claim it to bad tuning on my part. I'm just a bit surprised I would need more work done to the cam or head, I feel like most of the applications I hear people using similar setups on are pretty much stock

Well if it runs good, then thats fine. If its already been bored out it may have had some kind of cam installed?? The camshaft completely changes an engine, where it makes power and how much power it makes. If you are having brake issues due to the low vacuum maybe you can put a vacuum reservoir in the brake booster line, they definitely help. Those vacuum secondary carbs are great in the fact that you are only running on two barrels most of the time and that also means youre only running on about half the CFM of the carb (until it needs it). Im not an edelbrock carb expert, but i believe you can adjust when those open with a screw. IM not a 300 expert either, just most engines work the same but each has their quirks, @BigBlue 94 and @AbandonedBronco or go try out fordsix.com forum to get the most out of your 300.

Im jealous, i'd like a 300, mine came with the EFI 5.0...and it runs good, it definitely suits my needs, but oh the possibilities and the room to do so with 6 in a row.

 

[Klenahan]

Well if it runs good, then thats fine. If its already been bored out it may have had some kind of cam installed?? The camshaft completely changes an engine, where it makes power and how much power it makes. If you are having brake issues due to the low vacuum maybe you can put a vacuum reservoir in the brake booster line, they definitely help. Those vacuum secondary carbs are great in the fact that you are only running on two barrels most of the time and that also means youre only running on about half the CFM of the carb (until it needs it). Im not an edelbrock carb expert, but i believe you can adjust when those open with a screw. IM not a 300 expert either, just most engines work the same but each has their quirks, @BigBlue 94 and @AbandonedBronco or go try out fordsix.com forum to get the most out of your 300.

Im jealous, i'd like a 300, mine came with the EFI 5.0...and it runs good, it definitely suits my needs, but oh the possibilities and the room to do so with 6 in a row.

Yea, I was thinking about that last night but im not sure if there's a way I can check for an aftermarket cam without pulling it all apart. And yea I've played with the vacuum secondaries and adjusted the flap to stay closed longer because I thought maybe they were opening a little during cruising, but I dont think they are.

 

[BigBlue 94]

im not sure if there's a way I can check for an aftermarket cam without pulling it all apart.

Not really. You could pull the side cover and measure lift but thats only part of it. The factory 300 cam is actually pretty good. Comp Cams's offerings suck imho, catering to the always shift below 3000 crowd. Schneider or Erson are the good 300 cam manufacturers.

 

[Klenahan]

Not really. You could pull the side cover and measure lift but thats only part of it. The factory 300 cam is actually pretty good. Comp Cams's offerings suck imho, catering to the always shift below 3000 crowd. Schneider or Erson are the good 300 cam manufacturers.

I've heard talk of an RV cam for lower rpm drivers, are they still offered?

 

[BigBlue 94]

I've heard talk of an RV cam for lower rpm drivers, are they still offered?

RV cam is a generalized term for a cam that makes a bit more torque than normal, at lower rpms. Unless you wanna make more power at higher RPMs, the factory cam is more than adequate. The Comp 268 cam would be a good cam for you too.

 

[AbandonedBronco]

Hey, sorry for the late reply.
It's so hard to remember if I got that sorted out. My Bronco has changed SO much since then, and I don't remember which steps of the process changed what.

There were a few things I learned though. Now, I don't know if this is the same with an Edelbrock, but it is with a Holley. Holleys can struggle a bit to give good mpg when you lug them too low. Like, low vacuum at cruise. The reason for this is because the engine, even though it's spinning really slowly, has to push really hard to turn the gears over. I had a 3.00 rear end, the 2.47 is going to be even worse. When it has to push that hard to spin the engine, you're in the power circuit the entire time. With a Holley, this has the power valve constantly open, even at cruise. This'll be dumping fuel. I imagine the Edelbrock has something similar. I've never tuned an Edelbrock so can't say for sure.

Back when I made my original post 11 years ago, I had 31" tires and the SROD transmission. I found out that I would get better mpg in 3rd gear than I would in overdrive. Even though the engine was spinning faster in 3rd, it was working so much less.

When you originally had the 1bbl on there, it doesn't have that kind of enrichment circuit like a power valve, so even though it's tossing in some more fuel, it doesn't toss in that much. So when your engine can't keep the power up, you just lose speed. The 4bbl now can introduce the power, so you maintain speed but use more fuel.


As for the coil, if you're using the stock system, there's a resistance wire that lowers the voltage to the coil to 8 volts. If you have a 12v coil, it's not getting its full voltage. Either remove the resistance wire, or get an 8v coil. That might help a little with your spark.


Lastly, as far as the C vs the DP Offenhauser, it doesn't necessarily increase the vacuum. The DP design increases air speed at low RPMs, which helps with atomization and your torque. The atomization can also pick up a little gas mileage.

That said, I'm not a fan of the DP and in all my experiences, the open plenum C is just a better design for almost all applications.

 

[AbandonedBronco]

Also, I remember being very frustrated when I originally put a Holley 390 on mine. I went from 21mpg with my 1bbl to 12mpg with the 390 (on a good day). It was awful. I tried so many different things and finally what resolved it was going to a Holley 600. I got 15 in town and 17 - 19 on the interstate. Sometimes, it's just the carburetor itself.

 

[Klenahan]

Also, I remember being very frustrated when I originally put a Holley 390 on mine. I went from 21mpg with my 1bbl to 12mpg with the 390 (on a good day). It was awful. I tried so many different things and finally what resolved it was going to a Holley 600. I got 15 in town and 17 - 19 on the interstate. Sometimes, it's just the carburetor itself.

Okay wow that makes so much sense. I didnt know power circuits were a thing and Ill have to do some research on them. So for you, going to a bigger carb helped? I checked and I have a MSD Blaster 2 coil and it came with a resistor that I wired in. I guess something I can try is staying in 3rd and seeing how I do on fuel. If it seems to do a lot better then I might just have to look into a t19 or t18 instead of a zf5 since the OD will be useless and Ill still be able to tow just fine with those.

 

[AbandonedBronco]

I wouldn't necessarily say it was going to a bigger carb. Just that sometimes you just get a carburetor that has some weird quirks to it and you don't know why. I know plenty of people who've gotten good mpg out of a 390cfm Holley. I sure didn't though! Something must've just been up with that particular carburetor. Dunno.

With your coil, you wired in a resistance wire? If so, you might have two resistance wires. The original one in the stock harness, and the new one you wired in. That'd really lower your voltage at the coil. Maybe take a volt meter and check what voltage it's getting at the power wire at the coil and make sure it's correct. The coil itself should say what voltage it needs on it somewhere.

The only way I'd recommend a ZF5 for you is if you were planning to regear your differential(s). 2.47 is already very long gears, and is essentially the "overdrive" of the times. The ZF5 is designed for more aggressive gearing and its gear ratios reflect that. For example, a 3.00 rear end with a T18 is very similar to the gearing of a 3.50 rear end with a ZF5 (minus the overdrive).
If you combine a ZF5 with 2.47s it's going to be a total dog all around. I don't think you'd like it.

FWIW, I have 4.11s with my ZF5 and am around 2600 RPMs at 80mph. I get about 15mpg at that speed.

 

[Klenahan]

Ah gotcha. As for the resistance wire, it was originally a feedback system, so I got a wire harness from somewhere online that said the resistance wire wasnt included. But it came with a ballast resistor that was in the instructions, and the coil came with a resistor too so I only used the one that came with the coil.

The reason I was thinking the zf5 was because it had the granny first, and besides that the gears all seemed a bit better than the SROD I have now. The OD is the same ratio as 4th in my current transmission. So I figured I would basically just gain a gear lower than my 1st and it would be easier to start out.

If I could cruise at 80 Id be thrilled, I cruise at 70 at around 1650 RPM and it will get up to 80, but its not happy about it...

 

[Stevo440]

As @AbandonedBronco said about the power enrichment system in the carbs that's why you need to setup/tune each carb differently depending on its application. On a dual plane manifold each side of the manifold basically sees half the carb so on a 390cfm carb each side sees 195cfm and a 600cfm each side sees 300cfm which helps with more low rpm torque production because of . Now with a single plane manifold the engine sees the whole 390/600cfm and you give up some low rpm torque production in favor of higher rpm HP production. So back to the carb, you have springs/jets/power valves/metering rods/fuel level and whatever else depending on the brand carb you have that need to be tweaked and that 2.47 makes things difficult to get the carb to deal with it because of the load it presents to the engine. But on a side note you said you got 19-22 mpg's on a long trip but then the next day after your mileage has been at 14 or less. It appears something else happened that caused the big drop in mileage, it is hard for "us" to diagnose this over the internet without having eyes and ears on this. There are so many places/systems that could be causing this.

 

[AbandonedBronco]

Ah gotcha. As for the resistance wire, it was originally a feedback system, so I got a wire harness from somewhere online that said the resistance wire wasnt included. But it came with a ballast resistor that was in the instructions, and the coil came with a resistor too so I only used the one that came with the coil.

The reason I was thinking the zf5 was because it had the granny first, and besides that the gears all seemed a bit better than the SROD I have now. The OD is the same ratio as 4th in my current transmission. So I figured I would basically just gain a gear lower than my 1st and it would be easier to start out.

If I could cruise at 80 Id be thrilled, I cruise at 70 at around 1650 RPM and it will get up to 80, but its not happy about it...

Ah, I gotcha. I missed it'd been converted from DuraSpark II. I was thinking with two resistance wires in there, you'd have a weak spark.

That makes sense on the ZF5 if you were looking to use the creeper 1st gear as your regular starting gear. You'd go from the 3.25 of the SROD to the 5.72 of the ZF5. If you were going to be starting in 2nd like most do, you'd be going to a 2.94, which would lack in power.

On a dual plane manifold each side of the manifold basically sees half the carb so on a 390cfm carb each side sees 195cfm and a 600cfm each side sees 300cfm which helps with more low rpm torque production because of .

The Offenhauser DP for the 300 is slightly different. It's a "Dual Port" as opposed to a Dual Plane like the V8s.
All 6 cylinders still see all the CFM from the full carb. What's divided is the primaries from the secondaries. The primaries go through the smaller passageway on the bottom and the secondaries go through the larger passageway on the top. This makes it so that when you're only on the primaries, it's like your intake "shrinks" and you get higher air velocity since it's going through smaller runners. As far as I understand it, a Dual Plane, like on a V8, sends one primary and one secondary to each bank on each side of the engine.

I have seen where some have put a divider plate inside their Offenhauser C (the open plenum one) to act like a Dual Plane. I'd be curious how it ended working out for them.

But on a side note you said you got 19-22 mpg's on a long trip but then the next day after your mileage has been at 14 or less. It appears something else happened that caused the big drop in mileage, it is hard for "us" to diagnose this over the internet without having eyes and ears on this. There are so many places/systems that could be causing this.

I missed that this mpg drop was a sudden change. If that's the case, yeah, that's difficult to diagnose. It could be something as random as your parking brake sticking.

 

[Klenahan]

Yea, I replaced the brakes and emergency brake because I thought they were rubbing just before the trip. The change in MPG came right after changing to an 8" spring in the rods. I wonder if something got into the carb and is blocking a port or clogging something. Wasn't expecting to have to rebuild it so soon