Big carb (Quick Fuel 850) on a small engine?

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Up on the RPM's with a down leg booster is a good thing :)
 
I'd say right size is where there's no more appreciable gain in power by going larger minus where you can tune it with acceptable idle driveability etc.. For the application eg.. race only, street strip, street etc.. Probably only one way to find out experiment.
 
Oldmanmopar said:
I ran a 1050 on my small block with down legs. Went mid 10's @130 through the exhaust 3650 lb. street car. A straight leg carb didn't work for me at all.
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1050, I like it. build the engine to work with the carb. :D
I have a sweet spot for the 1050.
I don't think I'd mess with an 850 on a 318, I'd lean more towards a 650.
 
Johnny Dart said:
Ede 600 for the Win !!!!!!!!
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I ran a 600 on my 4 cyl Omni when I found the 500 holley was to small. Worked great on a 4 cyl.

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this myth that dp are harder tune on small engine its just bs its not that hard
set up right they make more power and drive like and other carb small primaries on the themobog or quadabog are nice for crusing
harder to use a dp on an auto tranny is complete bs
what it does do is roast the tires instantly better get the suspension set up right
you are in control with a dp not a vacuum signal even with the weakest spring

1 tunnel ram 660 center squirters
2 dp of your choice
3 thermobog setup right
 
a-bodyguy said:
this myth that dp are harder tune on small engine its just bs its not that hard
set up right they make more power and drive like and other carb small primaries on the themobog or quadabog are nice for crusing
harder to use a dp on an auto tranny is complete bs
what it does do is roast the tires instantly better get the suspension set up right
you are in control with a dp not a vacuum signal even with the weakest spring

1 tunnel ram 660 center squirters
2 dp of your choice
3 thermobog setup right
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I bet a lot of this carb nonsense has come from bolting on like a 750 or 750 dp with zero or near zero tune carb and ignition and wonder why it don't work.
 
273 said:
I bet a lot of this carb nonsense has come from bolting on like a 750 or 750 dp with zero or near zero tune carb and ignition and wonder why it don't work.
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And then, THEN to make it all the worse they read books and magazines and go on the web and YouTube and get taught the exact wrong things to do.

And then they claim ”holleyitis” because they will not learn.

Its sad really.
 
I have not had any good luck with the annular boosters, but i did ran a 850 quick fuel on a 360 in my Duster drag car, and could get the jetting right, i was into the 60s on the primary jets, and still not enough, put a 750 on and no problem with jetting, and pick up a bunch of ET.
 
ValiantRacing said:
I have not had any good luck with the annular boosters, but i did ran a 850 quick fuel on a 360 in my Duster drag car, and could get the jetting right, i was into the 60s on the primary jets, and still not enough, put a 750 on and no problem with jetting, and pick up a bunch of ET.
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Did you buy a carb with annular boosters or did you change from a down leg to them?
 
Here is what I have learned through the years. It is better to have a slightly smaller carb than one that is too big. A smaller carb maintains higher velocity through the venturis at low and medium speeds, and thus the boosters get a better signal. The better the signal at the booster the better the carb works. As always everything is a compromise, the smaller venturis will create a flow restriction at higher engine speeds and thus they will limit how much power you can make. Everything else being equal, a square bore carb compared to a square bore carb or a spread bore compared to a spread bore, the bigger the carb the worse your fuel economy usually is, and the more drivability issues you will have. Although if you are running 4.30 gears, I doubt you care about fuel economy. When I built my last 340 duster in the 1990's I ran a 750 DP modified by Barry Grant to Flow 910 CFM, it ran great. That car had the 509 purple shaft, M1 single plane, ported and polished T/A heads with the T/A adjustable rockers, 10.5:1 compression, 4.30 gears, reverse manual valve body and a 3500 RPM converter. If I was running a similar setup except for 318 cubes, I would still run the same carb. Now, those old 750 CFM Barry Grant carbs may flow 910 CFM, but they would work a lot better than an 850 CFM Holley. They still used the smaller throttle blades and venturis of the 750, and not the bigger 850 throttle blades and venturis, they had much higher velocity through the venturis than and 850 CFM Holley. Those old BG modified Holley 750's ran on the street like any other 750 Holley, but flowed better than an 850 Holley, they were great carburetors, too bad they do not make them anymore. On a mild 318 I would run a 650, and on a street and strip one with 4.30 gears, 4 speed or high stall converter I would run a 750. Unless you are making a ton of power I doubt that you will see much if any difference in power on a 318 cube engine by switching from a 750 to an 850 carb, and the bigger you go the more difficult it is to get it to run right, especially if it will see any type of street duty. As for the boosters, annular booster can mask a lot of the issues of running a bigger carb on the street as they need a lot less booster signal to work, they will not cure a too big carb for the engine syndrome, but they will definitely make it more drivable and much easier to tune. Again, we come to the issue that everything is a compromise, annular booster are much bigger than a dog leg booster and thus restrict airflow, although they have the side benefit of atomizing fuel better than a downleg booster. If I was building your engine and I did not want to spend a fortune on a custom carb, I would just use the old Holley 0-4779. If I wanted to spend a lot of money, for honestly very little gain, I would would contact one of the custom carb companies that are out there and have them build me a carb specifically for what I had. My BG carb was built specifically by BG for my engine and it ran great, now having said that, I am sure that if I took a stock 0-4779 and tuned it right, it would probably run withing one tenth and a tenth and a half, and one mile an hour to a mile an hour and a half from what that BG did, and I spent easily 3 times what a Holley would cost on that BG carb 30 years ago...
 
HSDemon said:
Here is what I have learned through the years. It is better to have a slightly smaller carb than one that is too big. A smaller carb maintains higher velocity through the venturis at low and medium speeds, and thus the boosters get a better signal. The better the signal at the booster the better the carb works. As always everything is a compromise, the smaller venturis will create a flow restriction at higher engine speeds and thus they will limit how much power you can make. Everything else being equal, a square bore carb compared to a square bore carb or a spread bore compared to a spread bore, the bigger the carb the worse your fuel economy usually is, and the more drivability issues you will have. Although if you are running 4.30 gears, I doubt you care about fuel economy. When I built my last 340 duster in the 1990's I ran a 750 DP modified by Barry Grant to Flow 910 CFM, it ran great. That car had the 509 purple shaft, M1 single plane, ported and polished T/A heads with the T/A adjustable rockers, 10.5:1 compression, 4.30 gears, reverse manual valve body and a 3500 RPM converter. If I was running a similar setup except for 318 cubes, I would still run the same carb. Now, those old 750 CFM Barry Grant carbs may flow 910 CFM, but they would work a lot better than an 850 CFM Holley. They still used the smaller throttle blades and venturis of the 750, and not the bigger 850 throttle blades and venturis, they had much higher velocity through the venturis than and 850 CFM Holley. Those old BG modified Holley 750's ran on the street like any other 750 Holley, but flowed better than an 850 Holley, they were great carburetors, too bad they do not make them anymore. On a mild 318 I would run a 650, and on a street and strip one with 4.30 gears, 4 speed or high stall converter I would run a 750. Unless you are making a ton of power I doubt that you will see much if any difference in power on a 318 cube engine by switching from a 750 to an 850 carb, and the bigger you go the more difficult it is to get it to run right, especially if it will see any type of street duty. As for the boosters, annular booster can mask a lot of the issues of running a bigger carb on the street as they need a lot less booster signal to work, they will not cure a too big carb for the engine syndrome, but they will definitely make it more drivable and much easier to tune. Again, we come to the issue that everything is a compromise, annular booster are much bigger than a dog leg booster and thus restrict airflow, although they have the side benefit of atomizing fuel better than a downleg booster. If I was building your engine and I did not want to spend a fortune on a custom carb, I would just use the old Holley 0-4779. If I wanted to spend a lot of money, for honestly very little gain, I would would contact one of the custom carb companies that are out there and have them build me a carb specifically for what I had. My BG carb was built specifically by BG for my engine and it ran great, now having said that, I am sure that if I took a stock 0-4779 and tuned it right, it would probably run withing one tenth and a tenth and a half, and one mile an hour to a mile an hour and a half from what that BG did, and I spent easily 3 times what a Holley would cost on that BG carb 30 years ago...
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That’s the benefit of annular boosters. You can run a carb with bigger venturii and have the same (or better) bottom end and make more power up top.

The AB increases signal strength so what small amount you give up in airflow is negated by the increase in venturi diameter.

That being the case, the AB even on the bigger venturi will have a far greater signal than a down leg booster on a smaller venturi carb.

Plus you get better atomization with them.

Of course, they require a different tune up than a down leg.

And, since pump gas is now (and has been for long while) formulated for EFI the AB has a far greater affect as far as power output than it did before that change.
 
My 360 has a Quickfuel 850 on it same carb I ran on my 416 excellent carb the best I ever bought :thumbsup: :thumbsup: ..
 
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