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Variable Venturi Sidedraft Carburetors

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peeeot
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Variable Venturi Sidedraft Carburetors

Post #1 by peeeot » Wed Jan 15, 2020 10:19 am

I have been reading up on posts and the old Classic Inlines tech pages about carburetor conversions to improve upon the single-point, single-barrel stock carburetor. The consensus seems to be that a modified log head to accept a 2-barrel is the way to go, if the aluminum head isn’t an option.

I am wondering why I rarely if ever see anyone following Ak Miller’s footsteps and modify the log head to accept multiple variable Venturi sidedraft carburetors. I haven’t even seen a sidedraft intake on the aluminum head besides the ad banner for VI on this site. I am talking about CV carbs, not the race-oriented Weber sidedrafts.

As I see it, multiple sidedrafts have all the advantages:
-multiple points of discharge closer to cylinders
-constant velocity for proper atomization regardless of engine speed or load
-friendly to low hoodlines
-linear throttle actuation rather than staged (in the case of the Weber or any 4-barrel)
-easy to tune with needle adjustments

Is there a design advantage to using downdrafts? If not, why are they the near-universal choice?

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chad
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Variable Venturi Sidedraft Carburetors on falcon motor

Post #2 by chad » Wed Jan 15, 2020 11:02 am

not sure abt all this but the 'falcon' motor is a passenger side intake/exhaust.
The shock tower is tight for side drafts.
I lub me some carter YH (the 'vette) and even more - the 6 cycle or bike carbs on there
but it's tight & throttle comes frm entire other side...

then we got some wet blankets too aaahahahaaaa
8^0
Last edited by chad on Wed Jan 15, 2020 2:03 pm, edited 1 time in total.
"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11), 250, NV 3550 & DSII to B transplanted, "T" D20/PTO, 2" SL, 1" BL, 4 discs, 33"X15", tool boxes, etc. Seeking: Hydraulic gear motor for Koenig pto. chrlsful@aol.com (413) 259-1749

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Invectivus
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Re: Variable Venturi Sidedraft Carburetors

Post #3 by Invectivus » Wed Jan 15, 2020 12:38 pm

I think there are a few issues with fitting side draft carbs:

Multiple carbs are seen as a pain to synchronize, so added time and/or money.
Multiple carbs increase costs. I'm looking at Mikuni HSR carbs, and unless I buy 80 dollar Chinese rip off units, I'm looking at $300/per for three to six units. dropping $900-$1800 is a big pill to swallow.
Head work can be extensive. Whether you're machining down the head and using custom runners, or welding mounts onto the existing log, both are more time and money than a single 2V conversion.
viewtopic.php?f=1&t=80532

I'm personally super leery of a carb leaning out and messing up one or two cylinders. It might be my ignorance, but I'd want a wideband on an exhaust pipe for each carb, so that's 3-6 sensors. AFAIK, that would be a chunk of change and a pain to set up in the car. AEM’s 4-Channel Wideband UEGO Controller could work, but that's about $600 dollars, plus the 3 bungs welded in, time and money.

I'm there with you, multiple side drafts are great, sexy as all get out, and I really want to do them before I die. But it's money and skill set, and most people don't have either, let alone both.
1967 mustang coupe - 200ci, 69 dizzy, adjustable rockers, headers and dual exhaust, T5.

1964 convertible falcon - Gutted.

1973 EB Bronco - 302ci (sorry!) DUI ignition, C4/D20, Dana 44, 33's

peeeot
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Re: Variable Venturi Sidedraft Carburetors

Post #4 by peeeot » Wed Jan 15, 2020 2:01 pm

I can appreciate the extra cost of multiple carbs, and wariness about synchronization, which applies to downdraft multi-carb configurations as well.

Ak just made it look so easy! Bore a couple holes, braze on a couple of mounting plates, put together a linkage, voila. As I think through downdraft carburetor selection, I keep hitting against the necessary compromises: too big, you lose velocity (and economy and responsiveness) at the most common real-world engine speeds to make the most of the top end. Too small, it’s gutless when you need full power on that highway on-ramp. With a staged carburetor you cover both ends but have to nearly bury the pedal to access the second stage. A CV carb is always the right size for what the engine needs, and the throttle actuation is synchronous.

Maybe in practice for a mild street car the difference between a downdraft 2 barrel and a pair of CVs just isn’t that noticeable. In that case, I can see an edge in ease of installation for the 2-bbl.

I feel like you though invectivus—I can’t get the thought of those sidedrafts off my mind and want to see it through someday.

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cr_bobcat
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Re: Variable Venturi Sidedraft Carburetors

Post #5 by cr_bobcat » Fri Jan 17, 2020 2:49 pm

Not cheap, but these guys advertise like they have it down with the Mikunis. Obviously this isn't for the average cat, but if you're someone willing to spend the time to synch, you'd end up with quite the setup. I would think that having a cross-channel between carbs would help reduce the risk of running too lean if one were to go awry.

http://www.v-performance.com/products/air_fuel.html
Block: Stock C8 Block/pistons, C9-M head, 1.75/1.5 valves, dual spring, 1.65 RAU Rockers, port divider, direct mount Holley 4412-500, HEI w/20* advance, manifold vac, dual-out Arvinode exhaust, Clay Smith 264/274 110* installed w/ 4* advance, adjustable dual chain timing, C4 w/ shift kit, 3.20 (TBC) rear ratio, 9.44 SCR / 7.97 DCR

Build that Six with parts from http://www.vintageinlines.com

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chad
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Re: Variable Venturi Sidedraft Carburetors

Post #6 by chad » Fri Jan 17, 2020 7:22 pm

that one's run on here B4 (link), lots more if interested
"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11), 250, NV 3550 & DSII to B transplanted, "T" D20/PTO, 2" SL, 1" BL, 4 discs, 33"X15", tool boxes, etc. Seeking: Hydraulic gear motor for Koenig pto. chrlsful@aol.com (413) 259-1749

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cr_bobcat
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Re: Variable Venturi Sidedraft Carburetors

Post #7 by cr_bobcat » Fri Jan 17, 2020 8:07 pm

Yeah. Was probably me that did it. I've been kicking around ideas for a custom intake on a sawed off head for awhile
Block: Stock C8 Block/pistons, C9-M head, 1.75/1.5 valves, dual spring, 1.65 RAU Rockers, port divider, direct mount Holley 4412-500, HEI w/20* advance, manifold vac, dual-out Arvinode exhaust, Clay Smith 264/274 110* installed w/ 4* advance, adjustable dual chain timing, C4 w/ shift kit, 3.20 (TBC) rear ratio, 9.44 SCR / 7.97 DCR

Build that Six with parts from http://www.vintageinlines.com

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chad
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Re: Variable Venturi Sidedraft Carburetors

Post #8 by chad » Fri Jan 17, 2020 11:13 pm

"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11), 250, NV 3550 & DSII to B transplanted, "T" D20/PTO, 2" SL, 1" BL, 4 discs, 33"X15", tool boxes, etc. Seeking: Hydraulic gear motor for Koenig pto. chrlsful@aol.com (413) 259-1749

peeeot
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Re: Variable Venturi Sidedraft Carburetors

Post #9 by peeeot » Tue Jan 21, 2020 8:47 pm

Thanks for sharing the links with the Mikunis! I’m curious to know how well they would work on a six in an American car or truck. They are flat slides, not CVs, so it seems like if they were installed on an engine mated to a heavier vehicle with taller gearing than a sports car that there would be a major loss of air velocity/vacuum signal to meter fuel when opening the throttle suddenly at low speeds—same as with an oversized downdraft carb. But I’m just guessing!

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chad
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6 Keihins

Post #10 by chad » Tue Jan 21, 2020 9:17 pm

this may B a thread

viewtopic.php?f=1&t=12675&hilit=6+motorcycle+carbs

just skimmed it, don't know. But there's guys on here (a la AK Miller) done it. Check the Index page under the ' hot rod magazines' scanned in here, yeah post #6:

viewtopic.php?f=1&t=12675&hilit=6+motorcycle+carbs

We have one inc a uTube of a fella runnin his Cali stang down the road (hood off near final test). Pic of his adaptor (I think it was 2x3 Keihins offa two 3 cyl bikes) justa DIY plate, the rubber boots still used to go on the intakes (chopped off log). That's the most extensive thread we have...
"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11), 250, NV 3550 & DSII to B transplanted, "T" D20/PTO, 2" SL, 1" BL, 4 discs, 33"X15", tool boxes, etc. Seeking: Hydraulic gear motor for Koenig pto. chrlsful@aol.com (413) 259-1749

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