Dual Carbs with a 1949 Dynaflow - Bad Idea?

[Dan O]

A mechanic told me the other day that it would be a bad idea to put a dual carb setup on a 1949 320 engine with a Dynaflow as it would just "bog down" and slow down the takeoff speed.  I realized that the dual carbs were used on the earlier manual Buicks but never the Dynaflows.  He said it's because it just does not work.  I'm no mechanic so I am asking here for your experience, knowledge and opinions.  I have an old Edmunds dual carb intake fyi.

[Beemon]

I put dual carbs on my 56 with Dynaflow and I felt the car performed better than before. It didn't go any faster by any means with the Dynaflow, but it didn't bog down either. Accelerating from Low was leagues quicker but in normal Drive there was no noticeable difference other than my gas gauge accelerating faster than the car.

[Guest]

35 minutes ago, Dan O said:

A mechanic told me the other day that it would be a bad idea to put a dual carb setup on a 1949 320 engine with a Dynaflow as it would just "bog down" and slow down the takeoff speed.  I realized that the dual carbs were used on the earlier manual Buicks but never the Dynaflows.  He said it's because it just does not work.  I'm no mechanic so I am asking here for your experience, knowledge and opinions.  I have an old Edmunds dual carb intake fyi.

Engine vacuum decides when more fuel is needed. That is why Buicks original setup was controlled by vacuum. When new and operating correctly if the rpms were not sufficient to generate enuff vacuum, the rear carb wouldn't open, no matter how hard you stomped on the pedal! Once it got moving and revved up, the vacuum increased and overcame a mechanical counterweight under the rear carb, vacuum pulled fuel through the rear carb. Takes a little patience to explain the way it operates to non-mechanics, maybe he didn't have that. On a Dynaflow car with the slippage(stall speed) in the convertor, it might kick in a little faster? If you have both carbs open at the same time with no vacuum control, you could over fuel the engine and create a bog and maybe even kill the engine over fueling it, but if your aware of it you learn to feather the foot feed while the engine gains rpms. What fun though! Yeah!

[Ben Bruce aka First Born]

Dan O, mechanics can be wrong. I was told that  TBI would not work properly on mine. Come to Burnett and see.

 

  Just try it and fiddle with it until it works.

 

  Ben

[smithbrother]

If you use progressive throttle linkage, then the engine is on just one carb until you floor it so to speak, so I don't see why it would bog down your car, IMO

[Beemon]

If doing dual carbs, unless tri-power, I was always told to run straight linkage because you'll over richen the cylinders closest to the master carb and lean out the cylinders closest to the slave carb when running progressive. It works on the track to run progressive because there isn't a lot of low RPM driving.

[smithbrother]

Don't TELL all the 41's running progressive linkage for the past 75+ years as, THEY DON'T KNOW THAT.

 

Dale in Indy

[Guest]

The Edmonds custom is a "single plane" intake. Pretty much open inside, unlike the "dual plane" Buick original dual carb intakes. That makes them kinda apples vs oranges with regards how they respond to dual carb control. When you look at the number of burnt-up exhaust on the same ends of those dual carb engines, it leads me to think, maybe when the rear damper started sticking, the rear leaned out so badly it overheated the rear exhaust manifold causing warping and cracking. I have found those systems with tin under the rear carb and folks from that era told me when gas was rationed, that was a common trick to obtain better fuel economy. No studies have been done to prove or disprove my wild imaginings, so your left to your own math, but anecdotally I'm just saying 2+2 usually equals 5 in my world! All seriousness aside! They make a little gauge that sits on top of the carb with the air cleaner off, I believe it reads vacuum and with it you can match the carb flow front to back. Called a synchronizer(Sp?). People smarter than me have made these dual carbs work, so it's got to be doable? Caveat! The author is not responsible for any inaccuracies due to probable, at least partial, ignorance on the subject!

Edited September 2, 2016 by Guest
Dim bulb! (see edit history)

[Guest LONG]

I have a factory dual manifolds and plan to use 2 strombergs this winter.  they will be linked together.  going on a 263 dflow.  currently running an Edelbrock 1406 and it actually works pretty well.  most of the time i'm not in the 4bbl.  but at highway speed and go to pass, it works well.

[NTX5467]

IF the dual carb set-up was a real issue, per se, it'd show up MOST on the manual trans cars.  Whereas the DynaFlow torque converter would let the engine rev more on take-off than the manual trans would allow, without seriously slipping the clutch on initial take-off, or "launch" as they now term it.  I remember how those older DynaFlows sounded when moving from a stop, and it sounded like a good bit of slippage to me (which could also be a result of starting in "high" gear rather than a geared "low" gear).

 

What IS the issue is, as mentioned, the correct functioning of the "secondary" carburetor.  If it opens with the primary carburetor, especially if you floor it at lower rpm, there will be too much air flow for the motor and it will bog, just like a QuadraJet spreadbore with the secondary air valve opening way too early.  Same thing, a big bog until the rpm can rise to where the additional air flow is really needed, whether on take-off or at lower "in town" vehicle speeds.  That's the purpose of the weighted counterweight "air valve", just as the earlier AFBs had counterweighted air valves over the manual-actuation secondary throttle plates.  Air flow through the open secondary throttle valves was what the counterweights modulated, such that smooooth transitioning between the lower air flow demands of lower speeds and to attain higher engine/vehicle speed when the engine

could use it.

 

One thing I've noticed is that with an "under-geared" or "lower torque output" engines, with carburetors which are now deemed "big enough" for the engine size (rather than the smaller carbs of prior times), you DON'T floor it from a standing stop.  You punch the throttle and THEN apply full throttle after the car is moving, when the additional potential air flow can be used.  Otherwise, the "gutless" feeling ensues the driver.  In the earlier times, with smaller carburetor air flow capacities, plus the milder cams of those times, the general air flow needs for a 300cid/5.0L engine were well-served by a carb that might not flow 500cfm (on a good day).  I suspect that some of the earlier 4bbls might not

 

flow 400cfrm, but if the engine only needs that much, all's well.  I also suspect that the TBI unit on First Born's Straight 8 is probably about 400cfrm (with the larger 1.69 throttle bore, 454-equipment being similar to the Holley 500cfm 2bbl carb), which would be close to what the Straight 8 4bbl was, or a little more.

 

The "hot rod" intakes for the inline 8 cylinder engines were designed to have "more generous" internal cross-sections and smoooother transitions from the main "log" into the ports for the cylinder intake ports.  All for more air flow and higher horsepower, which usually happened in the 4000+ rpm range.  These "single plane" intakes would also have weaker throttle response at sub-2000rpm levels, I suspect, due to the lower air/fuel mixture's velocity in the manifold itself, due to the "larger size of the pipe".  Once the air flow velocity increases, then throttle response would be much "tighter", I suspect.  Also, those intake manifolds would also work better with a hotter aftermarket camshaft, so the engine would need the extra air flow to produce more power at higher rpms.  Remember that cam card posted in these forums a year or so ago, with the stated rpm in the 8000rpm range?  Everything just needs to work together!  Few things, especially when "modified" are "plug and play", but need some tweaking to attain their full potential.  Whether carburetor jet changes, accelerator pump settings, distributor advance rates/base ignition settings, OR combinations thereof.  Wasn't that the FUN of a hot rod?  Getting it tuned "just right" and then it working as good as it could?  Then maybe fudging a little as you might not tell everybody ALL of what you did?

 

NTX5467