Bottom of booster vs venturi with straight throat section

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BradH
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Bottom of booster vs venturi with straight throat section

Post by BradH »

Sorry, no pics to go with this.

If a carburetor venturi has a short straight section between the end of the entry radius and the beginning of the expansion taper, is there a preferred position with respect to the venturi as described where the bottom of the booster should align?
- Even with the end of the entry radius?
- Even with the end of the straight section of the throat?
- Anywhere between the two is all that matters?

The only thing I can add is an image & associated pressure drop graph that I came across of a venturi that had a short throat section similar to what I described looked like the peak pressure drop occurred at the end of the throat, but it was a pretty subtle difference. Thx
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Re: Bottom of booster vs venturi with straight throat section

Post by Tuner »

Look at all the OE carbs you can. They will all have the booster venturi exit located below the minor diameter of the main venturi. This is because the smallest area of fluid flow, the "vena contracta", is always below the smallest area of the main venturi. Also, with increasing overall flow velocity the vena contracta moves further downstream below the minor diameter.

To obtain the strongest metering signal the location of the exit of the booster should be in the highest velocity air flow, which is lowest pressure, so it should be located below or downstream of the venturi minor diameter.

Good video with animation but the gentleman speaks German and the CC translation is crazy.

https://www.youtube.com/watch?v=EMT6kNNkHzc

A venturi with a straight minor diameter section is less efficient. Why is it like that in your carb?
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Re: Bottom of booster vs venturi with straight throat section

Post by ClassAct »

Looks to me to get the booster in the correct position you’d have to use a down leg booster as a straight leg won’t get you there.

My question is what about applications that need an annular booster. As far as I can tell the bottom of that style booster won’t be below the vena contracta.

I need to install some annular boosters so this interests me.
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Re: Bottom of booster vs venturi with straight throat section

Post by panic »

The discharge end of the booster is frequently 1/32" below (downstream) the smallest diameter of the main venturi.
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Re: Bottom of booster vs venturi with straight throat section

Post by Tuner »

ClassAct wrote: Mon Jan 18, 2021 10:39 pm Looks to me to get the booster in the correct position you’d have to use a down leg booster as a straight leg won’t get you there.

My question is what about applications that need an annular booster. As far as I can tell the bottom of that style booster won’t be below the vena contracta.

I need to install some annular boosters so this interests me.
Original Holley boosters, straight and down leg, the location of the booster exit is the same, 5/16" down from the lower edge of the flange that seats in the notch in the main body. Holley castings (at least old school) are all the same in that dimension so the boosters are interchangeable.

You must be dealing with some kind of billet main body ?? some of them have the venturi minor diameter too low, 1/16" or 3/32" below the bottom of the booster.
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Re: Bottom of booster vs venturi with straight throat section

Post by BradH »

I'm looking over a Braswell Holley 4150 HP main body I just picked up. It's a 1.59" venturi that, unless my eyes & fingers are deceiving me, does have a narrow straight throat section as described. It must have been machined from a 1.56" main body cuz the casting parting lines in the venturi are still visible.

I'm going to put annulars in it to replace the odd straight legs it had. This appears to be an older Robert Yates Racing piece circa 2007, so who knows what they were trying out at the time.

I refer to this as Project $crap Mettle; it's being built around a collection of leftover new or nice used parts I've had on hand for a while. The booster inserts I have available are a set of Quick Fuel .600" ID 12-hole billet and an older set of Mark Whitener's .640" ID 20-hole billet. The skirt length is different between the two, with the shorter MW boosters dropping to right about the end of the radius entry, whereas the longer QFT pieces drop to the end of the short straight section.

Here's a pic of it in its current mocked-up config.
20210118_082459.jpg
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BradH
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Re: Bottom of booster vs venturi with straight throat section

Post by BradH »

FWIW, the two different booster inserts I mentioned above.
20210119_074938.jpg
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Re: Bottom of booster vs venturi with straight throat section

Post by Mike Laws »

Tuner wrote: Mon Jan 18, 2021 10:05 pm Look at all the OE carbs you can. They will all have the booster venturi exit located below the minor diameter of the main venturi. This is because the smallest area of fluid flow, the "vena contracta", is always below the smallest area of the main venturi. Also, with increasing overall flow velocity the vena contracta moves further downstream below the minor diameter.

To obtain the strongest metering signal the location of the exit of the booster should be in the highest velocity air flow, which is lowest pressure, so it should be located below or downstream of the venturi minor diameter.

Good video with animation but the gentleman speaks German and the CC translation is crazy.

https://www.youtube.com/watch?v=EMT6kNNkHzc

A venturi with a straight minor diameter section is less efficient. Why is it like that in your carb?
This.

Having tested Brad's exact scenario and many variations multiple times; the above information is correct. And fwiw: Other than a very slight edge break at the pinch-point, a flat in this area is not advantageous. The main body shown was required to pass many size & shape gauges by rule. Be sure to align the cross channel exit with the booster feed hole or else it will definitely be scrap mettle.
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BradH
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Re: Bottom of booster vs venturi with straight throat section

Post by BradH »

Going w/ the longer boosters after seeing confirmation of what I suspected. Now the trick is to just stuff in what I have and resist the urge to order up some new 16-hole BLPs. :lol: I think the way Mark W described the QFT inserts was: "They're better than stock, but..."

It's an interesting combination of parts coming together here. Those are RapidJet metering blocks I bought from a carb builder who decided to not bother trying them out and cut 'em loose. The main well exit holes and booster entry locations align well based on test-fitting them w/ gaskets to see if any trimming was required.

There's probably a story behind every component being used, but only amusing to me.
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