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#393504 - 08/10/17 07:04 PM 28 special Carby options
RAYCYCLED Offline
Shade Tree Mechanic

Registered: 03/10/11
Posts: 196
Loc: Sydney Australia
I've decided this group needs a bit of activity, I'm putting together my 28 Chev special finally , and first race is Sept 22nd. I'm not going to get all I want done by then, but I do want to try a pair of 1-1/4" SU carbies on her , would I be best with a common inlet manifold or just go one Carby per port? As the 4 cylinders fire 12-43 I wondered if the second cycle would not be pulling enough fuel because of this configuration. Ray

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#393507 - 08/10/17 07:18 PM Re: 28 special Carby options [Re: RAYCYCLED]
RAYCYCLED Offline
Shade Tree Mechanic

Registered: 03/10/11
Posts: 196
Loc: Sydney Australia
Some pics of the progress. Ray Raycycled.com

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#393521 - 08/11/17 07:24 AM Re: 28 special Carby options [Re: RAYCYCLED]
wisebri Offline

1000

Registered: 08/25/12
Posts: 1145
Loc: Long Island, N.Y.
Ever hear of carb venturi sleeves to control the air/gas intake on more than one carb intake?
Frank Whitby makes them (he resides across the pond). I have them on a pair of one bbl Strombergs. He is very knowledgeable.

Venturi sleeves from Frank Whitby ( mango235aayahoocm.au).

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#393558 - 08/12/17 04:44 AM Re: 28 special Carby options [Re: wisebri]
tonyw Offline




Registered: 05/23/02
Posts: 4051
Loc: Goulburn Australia
While I havent read the article on the sleeves what I know of them they are for fixed venturi carburettors but the SU and CD Strombergs are variable venturi controlled by the throttle so I doubt they would work in this case.
Tony
_________________________
1938 1/2 ton Hope to drive it before I retire

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#393641 - 08/13/17 09:45 PM Re: 28 special Carby options [Re: wisebri]
RAYCYCLED Offline
Shade Tree Mechanic

Registered: 03/10/11
Posts: 196
Loc: Sydney Australia
Not heard of them, have U got a link to some technical info on them? I'm more concerned with getting even amount of vapour to the 2 and 3rd cylinders as they are the second draft on each port due to the Chev 4's unique firing order. My thinking, Pros; if I have a dual manifold then there is 2 carbies worth of vapour for every stroke, but the con is that getting the balance on the carbies is much harder so that they draw an even mixture . Also if the carbies are mounted direct to each port they won't protrude past the bonnet, Ray

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#393643 - 08/14/17 02:42 AM Re: 28 special Carby options [Re: RAYCYCLED]
tonyw Offline




Registered: 05/23/02
Posts: 4051
Loc: Goulburn Australia
If you plan on using twin carbs I would use 1 carb for cyl 1 & 4 and the other for 2 & 3 with all tubes the same length or both carbs fully linked for all cylinders.
Tony
_________________________
1938 1/2 ton Hope to drive it before I retire

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#393646 - 08/14/17 04:40 AM Re: 28 special Carby options [Re: tonyw]
RAYCYCLED Offline
Shade Tree Mechanic

Registered: 03/10/11
Posts: 196
Loc: Sydney Australia
Hi Tony, it would have to be for all cylinders then, there are only two ports , one for 1-2 and one for 3-4, the Great Fangio won his first series in A 1928 Chev and had it doing over 100 mph, though I've yet to find details on what was done to the motor. But Carburation is most definitely a big factor. Ray

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#393650 - 08/14/17 05:21 AM Re: 28 special Carby options [Re: RAYCYCLED]
carbking Offline
Shade Tree Mechanic

Registered: 02/03/04
Posts: 59
Loc: Eldon, Missouri
(Opinion only, I have never built a dual manifold for a 1928 4-cylinder).

I believe you need a single manifold with two interconnected plenums. Each carb attached to a plenum, and each plenum attached to a port. The interconnection tube should be roughly half the diameter of the runner diameter. Attaching each carb directly to a port, I think, will require significantly larger carbs. Research the difference in CFM requirements for a plenum intake manifold versus an individual runner manifold.

The old standby equation:

CFM = [CID times RPM] divided by 3456

for CFM is for a four-stroke multi-cylinder engine of at least 4 cylinders. Running the carbs attached directly to the ports would require carburetors capable of twice the CFM of those used in a plenum arrangement.

Jon.
_________________________
Good carburetion is fuelish hot air

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