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  #1  
Old 01-28-2011, 03:43 PM
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Carb Size vs. Gas Mileage

Hi everyone,
Was posing a question over on a carb tuning forum as I was thinking of shopping around for a replacement to the big 600 cfm Holley that came on my 84's 300. Was thinking about either Holley's 650cfm spreadbore (smaller primaries), or the smaller 465cfm Holley.

I was thinking that a smaller carb would net better gas mileage for long distance trips and such, but was told otherwise.


This is what was said:

Quote:
The main reason for cruising type fuel economy is the tuning of the carby not the size of it. High load driving like towing etc is a little different, the sizing of the carby makes a difference then. High load fuel economy is best if the carby is on the small side. But that doesnt mean the maximum power needs to suffer. The right size carby has to match the power output expected from the engine, too big and the benefits of manifold vacuum wont be received on some design engines. Most stock cam and compression motors need manifold vacuum to achieve efficient running and fuel economy at all load conditions. Thats why the engines have the OEM carbys on them. Tuning them for fuel economy would be possibly the best option. If the vehicle is older then the original carby setting wont be suitable for the fuel today and a re-calibration will net more fuel economy than even when it was new.
To which I replied:

Quote:
The stock carburetor was a (roughly) 200cfm 1bbl which was simply way too small for the motor and from my history lessons of the engine, was simply to keep costs down during production, not to make a great running engine.

The max RPM of the engine is roughly 4000 - 4500 RPMs, so standard CFM calculations (CFM * RPM)/3456 would state around 350 - 400 CFM is about all I'll ever need, even at WOT. I'd have to do some serious work to probably even get the secondaries on my current 600cfm to even open.


So, are you saying under normal driving conditions (ie. going down the highway, no load, no trailer), a 600cfm 4bbl will get the same gas mileage on a 465cfm 4bbl if the engine vacuum signal is the same?
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Quote:
Yes because cruising down the highway requires very little hp and at that power the mains will probably not even be functioning so the running is done primarily on the T-slot and idle circuitry. And that has to be tuned to the engine.
What're the thoughts on this? Does a bigger carb use more gas just cruising down the highway than a smaller one?
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-28-2011, 07:20 PM
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Depending on how much bigger it might actually draw less gas. Big carbs run sloppy down low because there's not a big enough vacuum signal to pull enough fuel through the boosters - i.e. an 800cfm carb might need bigger jets to get the AF ratio in range for low speed driving...and would be too rich at rpm. Bigger carbs use a bigger pump shot too.

The calculator I used said a stockish 300 (~75% VE) would need 350cfm @5,000 rpm. I had a 500cfm Edelbrock already so that's what I'm using. It runs fine with just a smaller accelerator pump nozzle.

I don't think a 600cfm vacuum secondary carb is "too big" for a 300. The carb will only let through as much air through the secondaries as the engine requires, assuming everything is dialed in correctly. It's more carb than the engine needs, but no worries considering how the carb actually works. On the highway it's all primaries, so tune those as lean and with as much timing as you can without pinging.

Since it's a C, I'll be able to give you a scouting report on a 351cfm (1.21" venturi) 2150 2bbl in a week or two. I've rebuilt it, now just need to modify the throttle cable and find an air cleaner before I can slap it on. It's the carb calculator "ideal" size below 5,000 rpm, I'm curious to see how it works in practice.

Being an Edlebrock guy, if you do go with a smaller 4-bbl get a 500cfm Performer carb. So easy to tune...
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Old 01-28-2011, 08:18 PM
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I got given a 600 CFM Holley carb by my F.I.L. to go on my Cliffy "C" intake. I've been wondering if this is way too much carb, or just a PITA to tune for the big sixes need of barely 350-400 cfm bone stock.

I may port the head, I might throw on the chevy rockers, but otherwise my engine is going to remain stock. It just makes me wonder how much of a headache its going to be to tune it right.
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Old 01-28-2011, 09:57 PM
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I ran a 2150 1.08 carb, and while I loved it, I think it may have been slightly small for running higher RPMs. Fortunately for me, I stop at about 3K (I'm just not a revver) but my observations were this:

The 390 was larger than the 2150 (~287 CFM) however, neither got better or worse mileage. What was noticeable was that the 2bbl was more driveable (less stumble)

My personal assumptions is that a 400ish CFM tuned lean would be the best overall compromise.

Other things to consider is that smaller intake runners increase velocities, and this can help sustain atomization. In that regard, I think my Offy C was too big for the RPMs I used the engine at.
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Old 01-29-2011, 08:07 AM
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I have a 600 cfm holly on my 82, it does better on gas than it did with the 1bbl. you have to keep your foot out of the secondaries [hard too do] for it to show good gas mileage.if I were to change, it would probably be to a edelbrock 500, or a quadrajet. I have found in the past with the small block chevys, they do better with a spread bore, for economy, and I'm sure the 300 is no different.
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Old 01-29-2011, 02:37 PM
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Like Dusty said "...you have to keep your foot out of the secondaries...". And what BVA said "The carb will only let through as much air through the secondaries as the engine requires, assuming everything is dialed in correctly. It's more carb than the engine needs, but no worries considering how the carb actually works. On the highway it's all primaries, so tune those as lean and with as much timing as you can without pinging."

Also, IIRC, Vizard said that in theory, there is no such thing as a carburetor that is too big. BUT, I don't think I would put a Dominator on my engine and expect to get it dialed in.

I have a 465 with vacuum secondaries on my engine...I have had a 450 Economizer on it which comes with mechanical secondaries. They both virtually run the same except for the lack of tuning capability of the Economizer which is pretty much limited to jet changes...it takes Weber jets. With either carb the secondaries aren't effective until the engine speed gets to around 3000 rpm MOL. It can be made to bog with the 450 at lower engine speeds because of the mechanical opening of the secondaries...it's all in the foot and knowing what the engine can handle. Vacuum or flow operated secondaries like on the Holley or Edelbrock or Qjet can be adjusted so that they will only allow what the engine can take. Ya just can't force-feed the engine more then it can handle without problems of driveability.

At some point each driver must take responsibility for their manner of driving and finding the manner that will result in the balance of mpg and performance that they want regardless of which carburetor they use.

Having said all the foregoing tripe, it's JMO that a spread bore carb like a Qjet or Holley 80555C with small primaries for the 98%+ driving one does and large adjustable secondaries for the "Exxon" moments would fit the bill on even an otherwise stock engine.
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Old 01-29-2011, 02:52 PM
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I have considered a Q-jet for this very reason. Also, they are somewhat complex, and designed for good driveability. I would think an old chevy 305 carb would be pretty good for a 300
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Old 01-29-2011, 03:01 PM
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The Qjet is quite common on V-6 carbed boat engines like the 4.3 Mercruiser.
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Old 01-29-2011, 03:34 PM
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many of the astro vans with the 4.3 had a 450 q jet. you just have to keep looking. rebuilding a q jet can be a bit of a chore, but nothing beats the baroooomph sound when the secondarys open, except maybe a 3 duce set up, but that's another story.
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Old 01-29-2011, 10:30 PM
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Quote:
Originally Posted by teds74ford View Post
I have considered a Q-jet for this very reason. Also, they are somewhat complex, and designed for good driveability. I would think an old chevy 305 carb would be pretty good for a 300
I'll bet you could use the biggest Qjet mad and it would run great on a 300. It has tiny primaries for good driveability and vacuum and large vacuum operated secondaries that open as much as the engine needs. It's the best of both worlds; no wonder even Ford used it on a couple cars.
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Old 01-30-2011, 02:03 AM
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some of the caddys and old's had 750's on them, that would be a bit much, I would think that would leave a puddle lying in the intake
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Old 01-30-2011, 11:21 AM
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Quote:
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some of the caddys and old's had 750's on them, that would be a bit much, I would think that would leave a puddle lying in the intake
I'll bet we'd be surprised. Since the secondaries only open as needed, I doubt you'd have much problem with it that the secondary spring tension adjustment couldn't fix.
But a 750 would certainly be a waste on a 300.
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Old 01-30-2011, 12:54 PM
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A spreadbore carb is a better design, in most cases, for a street driven engine. The smaller primaries allow for a greater velocity of air going thru the venturi, which leads to better atomization of the fuel. They also tend to have better throttle response. These 2 factors alone can yield an imporovement in MPG when driven in a reasonable manner.
A few trips into the secondary zone can rapidly negate any fuel savings, though.
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Old 01-30-2011, 03:47 PM
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This is why I liked swapping in my 2150 over the 390 I had- no secondaries ever!
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Old 01-31-2011, 05:41 PM
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Hey everyone, thanks for the great discussion to read! Interesting stuff.

I'd just been used to the idea that bigger carbs just use more gas, even under the same work load, since the primaries are larger.

However, that makes a lot of sense that the same work load would require the same AFR. It's just the tuning, and the off-the-line that can suffer with a larger carb. The 600 CFM that's currently on my '84 300 works really well and has lots of power, but I can tell that the bottom end off the line isn't so hot.

If road cruising gas mileage won't change when properly tuned with the 600 (Offy C), I may not mess with it for the time being. As stated on a few other threads, the vacuum reading on it is amazing. 19hg at idle, 15hg in 3rd gear at 35mph around town. 18hg in 2nd gear @ 35 (barely off idle!) Around 14hg or so on the highway at 65.
Haven't taken it on a trip yet, but I'll be curious how it'll do.
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Old 01-31-2011, 05:41 PM
 
 
 
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