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DCOE double vs. single choke theory

Nader

Senior Member
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For a tuned Spridget road car, convention dictates that a single DCOE be 45 with chokes of 34 or 35. Going to double DCOE dictates 40s with chokes of 28 or 30. There must be a good reason for it, but in my simplistic view, I don't see why there should be a difference in size. The volume of the intake stroke is constant, so each throat of the double DCOE setup will be supplying the same volume of an intake charge but half as often as the two throats of the single carb setup. So why should choke sizes diminish with more barrels that will flow less frequently? Why even bother with extra carb barrels?
 
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Because of charge velocity and time b/w strokes. With dual carbs you just doubled the time b/w stokes and the amount of time the charge "lies dormant". When the valve shuts, fuel is still suspended in the air, velocity drops off and fuel falls out. Smaller choke means higher velocity which results in longer fuel suspension times. Think in terms of long much distance and/or energy is required to stop a car going 60 mph vs 30mph and that should give you a better idea. Granted that example is apples and oranges but that's the best I can put it at the moment.

While overly simplistic, one barrel per cylinder is to allow the motor to flow as much as it will take or need. It is also to create a ramming effect at high velocities and RPMs. Single runner lenth can be tuned for a desired rpm. IMO 45 is too big for even a healthy 1500. I'd rather a better signal response from a single 40 with the same size chokes as a 45. That's probably why I have one on my car..
 
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The volume of the intake stroke is constant, so each throat of the double DCOE setup will be supplying the same volume of an intake charge but half as often as the two throats of the single carb setup. So why should choke sizes diminish with more barrels that will flow less frequently? Why even bother with extra carb barrels?


yes and no, you still need to factor in changing VE. Once again to over simplify (and completely change directions). Think of each as one big hole, a single being a "70" (35+35) and duals being a "112" (28X4). At any given RPM the motor will require the same amount of air through each set up, however; one hole is 1/3 larger than the other, therefore; will have a velocity 1/3 less (not exact but oversimplifying here). Now consider that the duals DID NOT have the original single choke size of 35 (which would be 140 if they did). If that were the case, then the duals would have a hole twice the size with half of the velocity AND would only need to move air half the amount of time as before. In effect (but not really) you decreased velocity by a factor of "4" using the same size chokes in duals as a single.

I hope this makes sense. In a street car I'd start w/ 26s or 27s ( in duals). IIRC I have 28s or 30s in my single 40 and that's still a bit too big for my stock 1500 7.5-1 motor. You have to remember, you'll be spending WAY more time below 4K than above and when you are above 4K, and above that is mostly will be part throttle highway speeds.
 

aeronca65t

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For a tuned Spridget road car........

First:

What engine are you referring to?

The A Series (1275, 1098, 948, etc) only has two intake ports, so it's kind of hard to see two twin-choke Webers going into two siamesed intake ports.

(I have actually seen those unusual "two half-carb" manifolds, but I do not think they make a lot of sense)

Unless you're talking about the custom A Series heads with 4 ports?

Or a 1500?
 

nomad

Yoda
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I have not done that math.....radius squared X pi..... but I'll bet the difference between the single DCOE throat's and the double setup would be about the same per cylinder. Personally I prefer a single SU! Soooo much easier and cheaper to tune though not quite as powerful. But very close!


Kurt.
 

Roger

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(I have actually seen those unusual "two half-carb" manifolds, but I do not think they make a lot of sense)

Nial, just for historical interest, you might want to know why those manifolds came about .Some say it was to give a straighter inlet tract, but given the contortions of an A-series head that doesn't really ring true.
Back when BMC ran a Rally team. the Appendix J regulations for modified production cars Group whatever stipulated that cars in competition must have the same number of carburetters as standard production cars. The Mini-Cooper "S" had 2 carbs, so the team cars had to have 2. Of course, the rules were drafted to stop people fitting more carbs, not less as would have been the case with a single DCOE. SO there you had it - half a DCOE on each inlet port.
 

Pythias

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I run a single Weber DGV. I couldn't find a 28/36 as is recommended but have a 32/36 jetted down. All the tuning ease of a single with all the power of a Weber. No off idle stumble, no power loss spots. But then I had it dyno tuned rather than trying to find all the right jets for myself.
 

Roger

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I run a single Weber DGV. I couldn't find a 28/36 as is recommended but have a 32/36 jetted down. All the tuning ease of a single with all the power of a Weber. No off idle stumble, no power loss spots. But then I had it dyno tuned rather than trying to find all the right jets for myself.

Ah, but the DGV, or DCD like we used way back, are totally different from the DCOE.

Nial, I don't suppose the dual half-carbs were any better than a single DCOE, but they complied with the rules where the single wouldn't. I don't suppose they wanted the extra complication, not the modifications to the firewall to fit them in.
 

DanLewis

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I hope this makes sense. In a street car I'd start w/ 26s or 27s ( in duals). IIRC I have 28s or 30s in my single 40 and that's still a bit too big for my stock 1500 7.5-1 motor. You have to remember, you'll be spending WAY more time below 4K than above and when you are above 4K, and above that is mostly will be part throttle highway speeds.

'Just thought I'd add my two cents to this. I have a single 40DCOE on my 1275. When I first installed the Weber, I spent a long time fussing with jets trying to eliminate the infamous off-idle stumble. After many weeks of not being able to solve the problem, I decided to play with the choke size. It came with 30 mm chokes, so I ordered a set of four - two 28's and two 26's. I began by installing the 28's and although there was a significant reduction of off-idle stumble, I still found myself having to baby it everytime I started from a dead stop. I had some free time tonight with nothing to do, so I replaced the 28's with the 26's. Now the progression from idle on up is very smooth - no hint of a stumble at all. Although I may have lost some top end performance, the loss really isn't enough that I even notice it and is certainly worth the improvement in drivability.

Dan
 
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Nader

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Thanks for your input, Dan. What you and Kellsguy are saying is exactly what I suspect about these single Weber DCOE carburetors being prescribed in too large of a size for these little engines in street tune, at street RPMs. I have a Weber DCOE 45 on the car with a choke size of 34. It has that stumble as well, which on my air fuel meter shows to be a very lean condition. I just got additional chokes in size 32 and 30. I may just go straight to the size 30 based on what you guys are saying.
 
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