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To mill Choke Air Horn from Holley carbs or not

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To mill Choke Air Horn from Holley carbs or not

Postby Dodge Freak » Tue Jan 22, 2008 8:28 pm

Question for you carb pro's, is milling off the choke towers off Holley DP carbs-and the others too-a good idea or not?

I know the carb looks better-IMO-having it milled off but for some reason I believe some think the carb flows better with the choke tower and just removing the choke plate. Could somebody explain why? Not running a mechanical fan, just electric in front of rad, so theres no "wind" from the fan like a stock motor would have.

Getting my main body hot coated a pretty red--for show not racing--and would like to know if I should leave the tower on or have it milled off. Motor starts OK with no choke-just need to pump the gas a few times.
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Postby David_D. » Tue Jan 22, 2008 9:38 pm

I am in no way a carb guru, but I believe there will be some tuning involved when milling the choke tower. I don't believe it is as easy as changing the jets, but rather tuning the metering circuits, etc. If it's not an all out performance car, I would leave it alone.

Hopefully a guru will chime in.
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Postby fishman » Tue Jan 22, 2008 9:39 pm

Not sure what size of carb you have but replacing the body with a proform body is cheap and will do the trick for you........unless you are milling the choke horn down yourself......carb will flow better with out the choke horn as the carb works better with axial flow and the choke horn restricts the flow
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Holley carbs

Postby bigjoe1 » Tue Jan 22, 2008 9:49 pm

I have done testing of milled and nonmilled carbs, and unless there is lots of radius work and smoothing of the milled surface, Nothing will happen. I have done extensive rework on Holley carbs,and a properly rework carb can be worth 10 to 20 HP in certain conditions, but just cutting the air horn off does nothing.

JOE SHERMAN RACING ENGINES
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Postby Nitro » Wed Jan 23, 2008 10:00 am

Is that to say it doesn't hurt, either? I may end up with a hood/air cleaner clearance problem, and since I don't need a choke...
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Postby #84Dave » Wed Jan 23, 2008 10:30 am

Many moons ago, we played with milled choke horns on a 350 and 500 cfm Holley 2-bbl. Removing the choke horn did increase the flow through the carbs. However, there was an adverse side affect. Booster 'buffet'. I built a 'poor boy' manometer using clear plastic tubing on a 6' piece of board. Used rubbing alcohol as a fluid. Removed the metering block and shoved/wedged small tubing into the booster channels on the main body. Then we flowed the carb, while monitoring the boosters, milling .100" at a time off the choke horn. When we got the horn milled down to about 1.400" in height, bad things began to happen through the boosters. The mano began to bounce around erratically. By the time we had all of the choke horn milled off, the booster signal was all over the place. Our simple experiment told us it wasn't a good idea to mill the horn off a Holley 2-bbl. Dave
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Postby PackardV8 » Wed Jan 23, 2008 12:09 pm

1. this is a show, not go engine. properly radiused, the carb "looks" better, more symmetrical, without the choke apparatus. however, no pro would ever allow strangers to wander around an engine without it having a carb cover, so who's gonna see the bling? (my first pit crew job was to put a cover over the Enderle injector stacks as soon as the modified pulled in and shut off. never forgot that.)
2. as far as tight hood clearance, if the choke-less carb is professionally shaped and dialed in and the air cleaner lid is shaped correctly, air flow at full throttle will be improved over having the lid sitting flush on the choke tower.
3. as bigjoe1, david_d and others correctly said, takes a lot more work inside to make a performance difference. #84 Dave has seen removing the choke apparently worsen airflow. maybe troy can share general directions to avoid that problem.
4. as far as performance goes, look at any NASCAR Holley. never, ever a choke tower, but $2k of work in, out and around to make it fly. that is why guys who really want to go fast go to troy, braswell and other pros. easy to make it worse. difficult to make it faster.

thnx, jack vines
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Postby #84Dave » Wed Jan 23, 2008 10:36 pm

Jack.... I may not have made myself totally clear. Removing the choke horns on the [2] Holley 2-bbls did not decrease the total airflow. It did, however, cause variations in the booster signal. The more the horn was milled, the more dramatic the booster variation. When I formed a mass of clay around the flat-milled area and shaped it similar to a velocity stack entry on a Weber, the booster immediately 'quieted down'. Therefore, I assumed, booster buffet with a milled choke horn. At least on the 2-bbl's. Dave
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Postby Dodge Freak » Wed Jan 23, 2008 11:54 pm

But all the HP and racing Demon carbs have no choke horn and those boosters look the same :?:
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Postby #84Dave » Thu Jan 24, 2008 1:00 am

But the smoothed, expanded entry area around the tops of the venturi's and whether the boosters are straight or down-leg, may make a considerable difference for the better? Maybe? I've never run the booster manometer test on a HP Holley or Demon. And does steady-state flow bench booster signal variations translate to degraded performance in a running engine with a pulsed induction? Have no idea! Dave
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no change

Postby raynorshine » Thu Jan 24, 2008 1:14 pm

i'm with BigJoe on this one.

i removed the chokehorn on a 750 gas carb, was on a 427 BBC, thought it would be good for some hp, et.... the car ran to the hundredth afterwards...no change at all! probably to the thousandth..
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To mill Choke Air Horn from Holley carbs or not

Postby Troy Patterson » Thu Jan 24, 2008 5:47 pm

Generally speaking, removing the choke air horn on a four barrel is an excellent thing to do. It's provides two benefit's: 1) it increases clearance between the bottom of the air cleaner / air box assembly and the entry surface of the carb, and 2) makes air flow into the primary more uniform to air flow into the unobstructed secondary barrels.

Two barrel carbs are really no different. There is no fundamental difference which would cause buffeting or flow disturbance to a two barrel that a four barrel would not see.

I can't speak of 84Dave's tests, I know nothing about what was run for air hat, or air cleaner / filter for the tests. These may be factors in what he experienced. Additionally, Dave didn't mention the test depression the carbs was flowed at. Keep in mind the carburetor doesn't see a continuous draw such as a flow bench would generate, an engine generates a pulsing, largely pushing into the carb versus drawing into the carb although, I believe it is an alternating sequence, but still primarily pushed in by atmospheric pressure. So, there are lot's of variables to consider. Maybe, Dave could shed more light on his tests.

I'd point out that both Chrysler and Chevrolet used shortened choke towers on the two barrel carbs they used on tri-power systems.

Personally, I'd mill the choke tower in any maximum situation where carbs rules limited the carb to a specific size / number of barrels, but allowed milling. I'd be very careful about the air approach and entry over / through the air cleaner / box to smooth, straighten, stabilize and for the air flow to make as gentle a transition into the carb as possible.

The effects of changes of this nature are like any other, change one parameter and you most certainly affect a number of parameters. For the comparison to be meaningful, all affected parameters must be reset to "zero" so to speak to make a fair comparison.

Generally, milling off air horns would / should net a gain on most applications - as all serious high performance / racing carbs do not have choke towers.

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Postby bill jones » Thu Jan 24, 2008 7:08 pm

-of the 4 barrel holley's I've cut the choke horns off of---I have never been able to prove they make a car significantly quicker or faster---but they never slowed down---and they usually would show some very slight improvement on the track.

-Personally I feel like having a choke horn up inside of inside of a short airfilter like say a 3" x 14" with a dropped base is probably about the worst case.
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-Now with a 500cfm two barrel it's an entirely different deal.

-The two barrels I've worked on all would flow more air and run faster on oval track cars by leaving the choke horn on---which is usually required by the rules-----but the real trick to the 2 barrel came with the various carburetor hats (inside the filter)---and those usually liked at least a 4 x 14 filter with a dropped base.
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-I spent much time redoing old junk carbueretors that nobody wanted to run and making them into better carburetors than these same people ended buying to replace their old junk.

-During this era I also chased the airflow on my SF300 flowbench----and for a while I also checked the main airbleed suck numbers before and after removing the choke horn---always with a 4 x 14 airfilter-----and the numbers dropped as soon as the choke horn was cut off.

-Other racers I was working with and I tried to find a correlation to how this affected the jetting and never found a definitive answer---ended using virtually the same jetting

-The worse case scenario about losing signal to the main air bleed happened when I decided to epoxy the machined top as shown here:

http://www.ryanbrownracing.com/Bill_Jones_Page_4.html

-this total work as described with the photos including the epoxy was worth something like 10 to 12% airflow on a flowbench.

-The main air bleed signal was lower yet with the epoxy contour---yet the engines still ran good and were easily predictable towards jet changes.
---------------------------------------------------------------
-I can say this about the 500 two barrel carburetors----they are very responsive to changes in filter height--filter diameter---and every car we ran that had a filtering lid---this was before the K&N extreme lid----always ran measurably faster and quicker on oval tracks.

-There used to be nearly 1/4 of a second on short tracks just on what could be done Nascar legally inside the air filter---probably still a 1/4 second available to the guys who aren't winning on a regular basis.
Last edited by bill jones on Thu Jan 24, 2008 9:13 pm, edited 1 time in total.
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Postby #84Dave » Thu Jan 24, 2008 7:48 pm

Troy...... on the 500 Holley 2-bbl we tested, we ran our normal race filter setup. A 12" ID pleated filter, 4" in height, sitting on a slightly curved/dropped base with a slight under-side concave lid. We ran the bench @ 10" of H2O, just short of 1" of Hg, because that was about the value of vacuum at the port in the carb baseplate under WOT and 8000+ rpm on a 267HP unlimited 4-cylinder. Not a very scientific test with a steady-state flow bench, in my opinion. But we theorized the boosters themselves liked the relatively straight air path down through the choke horn at full height, versus the milled height and the air coming up/over the filter base then trying to enter the milled choke horn as well. As long as the milled horn height was about even with the top apex of the curved filter base, the boosters didn't bounce around much. But continue to mill the horn on down from there and the boosters didn't like it. The overall carb flow improved slightly, to about 265 cfm @ 10" if memory serves, but the booster depression bounced around more, the more we milled the horn. In terms of moving the heavier fuel into the lighter airstream at the booster, maybe the rapidly-changing depression looks 'average' to the fuel and it just doesn't make any difference? Steady-state or bouncing around somewhat? Dave
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To mill Choke Air Horn from Holley carbs or not

Postby Troy Patterson » Fri Jan 25, 2008 3:40 pm

Interesting how removing the choke tower reduced signal strength at the main jet, but had little or no effect on jet requirement isn't it?

Turbulence past the booster is the answer. Turbulence can artificially boost the "apparent" depression created at / by the booster, or I should say I believe it to be turbulence. This isn't necessarily a good thing however. You've got to consider what effect this has on fuel droplets and flow patterns downstream of the booster as well as overall flow of the carburetor.

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