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Old 10-02-2017, 07:31 AM   #1
Terry, NJ
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Default Hey, I was wondering (Carburetors)

I was wondering, Has anyone ever bored out a venturi ? I was thinking that maybe the stock venturi might be more of an obstruction than a help. In the original configuration, it was designed to handle the air mass for a for an engine that was 200.5 inches, that really didn't breathe too well. Most of today's engines are larger due to boring during rebuilding, have larger valves, better cams, and many are ported and polished. But i've never heard of anybody boring the venturi say, .020 or .030. I know how venturi's work (Bernoulli's princible) It's the same with an aircraft upper wing shape. But what works in one application, IS NOT correct in all applications. The purpose of a venturi in a carburetor is to speed up the air at that point. However, with the ability to breathe better (Cams, valves, etc) has the shape and size of the venturi become more of a choke point? Maybe a small opening up would help it.
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Old 10-02-2017, 08:09 AM   #2
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Default Re: Hey, I was wondering (Carburetors)

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Originally Posted by Terry, NJ View Post
I was wondering, Has anyone ever bored out a venturi ? I was thinking that maybe the stock venturi might be more of an obstruction than a help. In the original configuration, it was designed to handle the air mass for a for an engine that was 200.5 inches, that really didn't breathe too well. Most of today's engines are larger due to boring during rebuilding, have larger valves, better cams, and many are ported and polished. But i've never heard of anybody boring the venturi say, .020 or .030. I know how venturi's work (Bernoulli's princible) It's the same with an aircraft upper wing shape. But what works in one application, IS NOT correct in all applications. The purpose of a venturi in a carburetor is to speed up the air at that point. However, with the ability to breathe better (Cams, valves, etc) has the shape and size of the venturi become more of a choke point? Maybe a small opening up would help it.
Terry
I've done with holly's on my sbc and it has helped, i would think that it is worth a try, if it don't work out you can always just buy a new one, might need to change the jetting a little to match up,
if you don't try you will not really know!

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Old 10-02-2017, 09:36 AM   #3
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Default Re: Hey, I was wondering (Carburetors)

The purpose of the venturi is to provide suction that will pull out gasoline from the main and cap jets at such a rate that the proper fuel / air ratio is maintained. A small increase in the size of the venturi throat will increase the air flow. The proper fuel / air ration can them be obtained by opening the GAV. Small improvements like this are difficult to measure quantitatively.
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Old 10-02-2017, 12:17 PM   #4
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Default Re: Hey, I was wondering (Carburetors)

Just playing Bernoullis advocate here, but increasing the venturi diameter would lower the speed of the air passing through it. It would increase volume, though.

So, how do you want to experiment with sizes? A 1:1 percentage increase?

Say, you have an A block with stock stroke but bored 0.125" over. This increases engine displacement from 200 in^3 to 213 in^3, or a 6.5% volumetric increase. (This is also an area increase of 6.5%)

The stock venture diameter is 0.843". So, do you want to increase that by 6.5% to 0.897" (+0.054") or increase it 6.5% as an area from 0.558 in^2 to 0.594 in^2 (making the new diameter 0.870") or volumetrically 6.5%... somehow... It's actually going to be tough for me to calculate that without a cross-section of the venture in front of me. If anyone can post one, I can get real scientific for ya.
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Old 10-02-2017, 01:26 PM   #5
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Default Re: Hey, I was wondering (Carburetors)

WELL, there goes the makers' fine ENGINEERING!!!
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Old 10-02-2017, 01:36 PM   #6
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Default Re: Hey, I was wondering (Carburetors)

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This AM, I was thinking about the old N.A.C.A wing profiles and shapes and how different they were and I started to think about the other common use of Bernoulli's principal, carburetors! Specifically, mod. A Zenith carbs and it occurred to me with all the improvement made to the valves, cam, ports, heads etc, nothing much was done to the carburetion. And might not the relatively small bore be an obstruction. Plus, the "Bullnosed"shape of the entrance might be something of a obstruction . I think that if a wing were shaped like that the plane would be said to have built in headwinds.
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Old 10-02-2017, 02:12 PM   #7
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Default Re: Hey, I was wondering (Carburetors)

Terry, look at the dyno results from Piriano's, and Bill Stipe. I think you'll find venturi size increase didn't benefit that much until the rpm was increased. ie, a Weber or Model B carb. Now I have a B carb with a B manifold on my A engine and I can tell a slight difference, also have a Weber that I tried for a month and didn't like it's characteristics. I think if you are looking for more HP, a camshaft and exhaust manifold along with bigger intake valves will be your most likely answer.
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Old 10-02-2017, 04:16 PM   #8
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Default Re: Hey, I was wondering (Carburetors)

I might be on the wrong track here but if the venturi were enlarged, surely the air speed would be reduced and the size of the jets would have to be increased.
Anybody able to confirm or contradict that?
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Old 10-02-2017, 04:46 PM   #9
Terry, NJ
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Default Re: Hey, I was wondering (Carburetors)

I wonder! With the increased ability to breathe better, would the airspeed be reduced? Again, We're not talking a stock engine here, but one that's been ported, polished, larger valves, cam, and a bored cylinders. Cleaning up the ports is fine
better cam and valves is great, but what if the volumetric efficiency is being held up by a venturi that is too small? And what happens if we put a 2 x 1 manifold on and we double the area of the venturi? I was looking at a SBC in a rod recently with 8 (Eight) carbs on it and wondering how the damn thing started. Multi Carb setup have never been my forte! Ah well! I'll just have to try it I guess! I was hoping that someone else had all the specs and dimensions. Ma always said I was a lazy, good for nothing!
Terry



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I might be on the wrong track here but if the venturi were enlarged, surely the air speed would be reduced and the size of the jets would have to be increased.
Anybody able to confirm or contradict that?
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Old 10-02-2017, 05:54 PM   #10
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Default Re: Hey, I was wondering (Carburetors)

I am all for someone just getting out there and gathering empirical evidence.

Take care to only change one thing at a time and to use some sort of repeatable measurement/conditions.

Buy a few aftermarket venturis and cut a few different IDs. Swap them out on a dyno. Then, report back with your findings!
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Old 10-02-2017, 06:09 PM   #11
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Default Re: Hey, I was wondering (Carburetors)

A Club in Gilroy, Ca, along with the Weber Dealer, tries to see who??? can get the MOST carbs on their engines!!--LOL
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Old 10-02-2017, 06:23 PM   #12
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Default Re: Hey, I was wondering (Carburetors)

I have done this. Did it several years ago trying to mimic a 'B' venturi & B throttle blade on an A carb. I was using a Superflo bench set-up for motorcycle & kart cart carb(s). I would probably try to do it again now that I have CAD and access to CNC machining, but if you just try to do it freehand, you have a much greater chance of making flow worse than better.
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Old 10-02-2017, 07:28 PM   #13
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Default Re: Hey, I was wondering (Carburetors)

On Jalopy "cheater" carbs, we bored out the manifold passages, used a 94 & bored the venturis WAY oversize (which bored through the wall in places) press in 2 lengths of BRASS Plumbers tubing & ran #91 main jets. The carb would get COLD as ICE! We also removed the choke butterfly & shaft & filed the throttle shaft REAL THIN! AND, of course, plugged the manifold heat riser.
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Old 10-03-2017, 12:31 AM   #14
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Default Re: Hey, I was wondering (Carburetors)

Some of the best youtube videos started out with the words, "hey, I was wondering."
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Old 10-03-2017, 05:42 AM   #15
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Default Re: Hey, I was wondering (Carburetors)

I am sure there is a better way to do this, but I copied this from a post that H. L. Chauvin wrote on a recent question about a high compression head. This sounds like an interesting experiment to me:

FWIW: One humble opinion heard on one (1) vintage, very inexpensive method to increase an original Model A Engine's performance.

Model A racing engine re-builder, Mr. Ron Kelley, explained to me about many of his past experiences with testing horsepower on Model A engines on his "calibrated" dynamometer, most of which he recorded on printed charts in the past indicating horsepower and overall engine performance.

He said most never realize that one of the most effective horsepower gaining methods at the "least" cost to a Model A owner that he observed and tested is simply trying to increase the velocity and volume of the fuel mixture.

This method is related to what racing engine re-builders refer to as porting; i. e., smoothing engine interior surfaces adjacent to the where the fuel mixture is flowing.

Some Model A owners never understand and consider that smoothing the interior fuel passages to increase fuel flow is similar to one barely depresses a Model A accelerator where the carburetor butterfly throttle opens ever so slightly; hence, more of the fuel mixture is entering the combustion chamber at a more rapid rate which means the Model A engine's rpm's are increased.

This fuel/liquid/ air faster flow hydraulic engineering theory is not any different than that of using "Manning Charts" to determine the flow of fluids for designing engineered storm drainage.

For example, for fluids, because of interior surface friction, one can observe that:

A. Water flows faster in a new, clean concrete culvert than in an exterior drainage ditch full of thick grass.

B. Water flows faster in a new, ultra smooth PVC pipe than in an old very rough interior surfaced cast iron storm drain pipe.

Mr. Kelley further explained that while re-building stock Model A engines that he sees today, he observed several minor things can be done to increase engine performance only by slightly increasing fuel flow.

He recommends trying to provide smooth uninterrupted surfaces, (which causes turbulence and reduces fuel mixture velocity), where the fuel mixture flows such as:

1. Start by verifying that the interior of the carburetor venturi is smooth as opposed to having some of the earlier 1950's reproduction venturi's which had an internal lip which should be removed.

2. Verify that the carburetor to intake manifold gasket interior hole is enlarged where it does not protrude into the intake manifold thus causing turbulence. (Also, an old sagging Model A manifold with non-aligned openings at the engine block does not help to increase engine fuel flow.)

3. With a Dremel tool, smooth out the cast iron interior surfaces of the intake manifold and the engine's cast iron intake and exhaust ports. Smooth out very rough surfaces, protruding cast iron fins at corners and sharp corners; however, never cut deeply into the thin cast iron inside the engine ports such that one cuts into the engine's water jackets.

4. Remove and round off the sharp bottom lip on all valves and slightly round off the sharp top lip found on valves.

5. Insure there is no carbon build-up on the top surface of the engine block which can reduce valve opening size and fuel flow.

Just hope this can help someone some day.
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Old 10-03-2017, 06:22 AM   #16
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Default Re: Hey, I was wondering (Carburetors)

While I appreciate Henry's thoughts as portrayed by Mr. Kelley, some of what is being recommended is actually false based on R&D. First, while I agree regarding a smooth pipe flowing a solid liquid, the problem with that analogy is we are not flowing a solid liquid, but we are flowing a gaseous liquid while trying to keep other heavier gas particles suspended. A smooth surface transition does help keep airflow moving but much of this is dependent on the speed of the flow. One also needs to factor in abrupt changes in direction which occur too frequently to only consider the flow efficiency. These directional changes are caused by a intake valve closing and an adjacent port providing a higher vacuum signal with the opening of that valve. So the air must keep the fuel suspended while moving but what happens when the valve closes and the moving air must change directions? A smooth surface will allow the atomized fuel to cling to the surface and fall out of suspension. My son's early racing career involved little flathead engines, and the amount of money that has been spent on R&D in kart racing and jr. drag racing is phenomenal. If you want to learn how to build a powerful Model-A/B engine, go study what the Briggs & Stratton builders do.
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Old 10-03-2017, 08:17 AM   #17
Terry, NJ
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Default Re: Hey, I was wondering (Carburetors)

Bill! I wonder if you ever considered "Carb Heat" as is on all airplanes. You bet it gets cold! It can freeze and clog in an instant. That's why you use "Carb Heat" while landing. It's in the category of water injectors, something that's needed on airplane engines (sometimes) but almost never used on automobiles.
Terry


Quote:
Originally Posted by BILL WILLIAMSON View Post
On Jalopy "cheater" carbs, we bored out the manifold passages, used a 94 & bored the venturis WAY oversize (which bored through the wall in places) press in 2 lengths of BRASS Plumbers tubing & ran #91 main jets. The carb would get COLD as ICE! We also removed the choke butterfly & shaft & filed the throttle shaft REAL THIN! AND, of course, plugged the manifold heat riser.
Bill Genius???
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Old 10-03-2017, 10:38 AM   #18
BILL WILLIAMSON
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Default Re: Hey, I was wondering (Carburetors)

I ran that "cheater" carb & manifold on a stock '48 Flathead, in my '32 Coupe & made many trips to the Coast & it did well & got "fairly" decent mileage.
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