One carb vs duals, theoretically speaking

I ran a DCOE40 for many years on my Commando.



Drivability, starting and idle were excellent. Weighed about a half ton.

But this carb setup made considerably more power although it did suffer from uneven mixture distribution. Left side was always richer. Fuel milage sucked.



Why- because with the single you have one 45 mm carb feeding one cylinder at a time. With the dcoe you have one 36mm choke feeding one cylinder at a time. The 45 offers less restriction.
The port tuning effect with the single carb was done in the long manifolding and head ports. Just think of a big carb as a plenum. It offers little restriction to the entry of the manifold. Jim

comnoz said:

I ran a DCOE40 for many years on my Commando.
Drivability, starting and idle were excellent. Weighed about a half ton.
But this carb setup made considerably more power although it did suffer from uneven mixture distribution. Left side was always richer. Fuel milage sucked.
Why- because with the single you have one 45 mm carb feeding one cylinder at a time. With the dcoe you have one 36mm choke feeding one cylinder at a time. The 45 offers less restriction.
The port tuning effect with the single carb was done in the long manifolding and head ports. Just think of a big carb as a plenum. It offers little restriction to the entry of the manifold. Jim

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Not surprised as you have 2 different size carb port sizes.

Not surprised as you have 2 different size carb port sizes.[/quote]

Not sure what you mean.

When I ran the dcoe I had 34mm ports at the cylinder head flange tapering to the 40mm bore of the carb.

When I ran the single carb I had 32mm at the cylinder head flange and the manifold stayed 32mm until it opened up into the plenum that the carb is attached to.

Carbonefiber, please send us the link to these OKO 32mm carbs. Please include the jetting requirements, needle dimensions, cable lenth and any particular needed to make these work.

By your firm opinion it is safe to assume that you yourself have done this and that the information is readily available.

What sort of mileage do you get?[/quote]

No real point in once more posting the link to the $25 OKO carbs, as jetting a carb to work properly seems to be pretty much a black art nowadays, even though it really isnt that difficult for anyone with a basic understanding of whats involved.

Carbonfibre said:

pvisseriii wrote,
Carbone-fubar, please send us the link to these OKO 32mm carbs. Please include the jetting requirements, needle dimensions, cable length and any particular needed to make these work.

By your firm opinion it is safe to assume that you yourself have done this and that the information is readily available.

What sort of mileage do you get?
Carbone-fubar wrote,
No real point in once more posting the link to the $25 OKO carbs, as jetting a carb to work properly seems to be pretty much a black art nowadays, even though it really isnt that difficult for anyone with a basic understanding of whats involved.

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How utterly predictable, you are!

Rohan said:

SteveMinning said:

The comments about twin carbs accelerating better sound a bit like oversimplification.

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You are assuming.
The BSA Star Twin was one of the first bikes to come out with twin carbs, as a production bike. You could buy the same bike as a single carb version.

All other things being equal in its manufacture, its spec sheet quoted better 1/4 mile times and better acceleration times 20-60 40-60 etc etc than its single carb brother. Very little difference in top speed, however.

This had long been known - the Vincent twin came out with twin carbs too, for precisely this reason if you read Phil Irving (and also that the layout didn't allow a single carb, but that was a deliberate choice.)

Cheers.

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So, you're saying that any dual carb setup will out-accelerate a single carb setup? So you could replace a single 34mm carb with two 10mm carbs and get better acceleration? Or perhaps replace one 34mm carb with two 100mm carbs and get better acceleration? I think not. I think it's just a little more complex than you imply with anecdotes.

I imagine that Rohan is right about twin carbs. I think a completely optomised twin carb setup will outrun a completly optomised single carb setup. The problem is neither setup when competely optomised would fit on a Commando.


Just like the old exhaust system builders always said. "you know when it is designed for the best power when it will no longer fit on the bike."

Aha, we are seeing a consensus here ?

P.S. Didn't I see that a Weber DCOE 45 won't even fit between the frame rails on a Commando. ?

SteveMinning said:

So, you're saying that any dual carb setup will out-accelerate a single carb setup? .

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Where did I say ANY twin carb setup. ?
Jim summed it.

There seems very widespread agreement that a properly sized twin carb set up with flow more air and provide more acceleration from mid range right through to redline.
I sure think so, after having single Mikuni for 15 years and over a year ago switched to Jim Schmidt's 32mm flatslide the difference is astounding from 4500rpm on up.

BUT, to my question, I personally feel that the twin carb set also feels more grunty, powerful, even right off idle and ALL the way through the rev range than my single Mikuni did.

Just my own seat of the pants feel.

Why then, do (some) people maintain that a single carb will "outperform" a twin set up in the lower rpm range?

If a well set up twin carb is definitely better flowing at higher rpm, then is it not also better flowing at lower?

Rohan said:

Aha, we are seeing a consensus here ?

Maybe -but I have yet to build a twin setup that will outperform the single setup and still fit on a Commando. But I haven't given up.

P.S. Didn't I see that a Weber DCOE 45 won't even fit between the frame rails on a Commando. ?

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Mine is a dcoe 40 and I had to do some machining on the carb, along with removing most of the gusset at the top of the rear downtubes to get it in there. The carb weighed about as much as the head and required a mount from the rear of the carb to the engine cradle to support it. Won"t ever do that again. I probably have some pictures of it on the bike if I go back to some of my old pre-digital albums. Jim

1up3down said:

BUT, to my question, I personally feel that the twin carb set also feels more grunty, powerful, even right off idle and ALL the way through the rev range than my single Mikuni did.

Just my own seat of the pants feel.

Why then, do (some) people maintain that a single carb will "outperform" a twin set up in the lower rpm range?

If a well set up twin carb is definitely better flowing at higher rpm, then is it not also better flowing at lower?

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A single carb setup will provide a better controlled mixture at low engine speed because the carb sees twice as many pulses at the same engine speed as a twin carb setup. That is why you can use a larger carb on a single carb setup and still make it work at low speed.

I haven't found a single carb setup will produce more low speed torque by itself but the mildly tuned engines they are usually fitted to may make more torque at low speed because of the tuning. Jim

Rohan said:

A single carb and manifold feeding 2 cylinders in theory can NEVER be as efficient as a pair of individual carbs - the gas flow having to constantly change directions to feed different cylinders ensures this, this is an overhead to the flow that always detracts from performance ?

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I am rooting for the single carb setup. Flow is not the only dynamic to an intake system.

It is well known that exhaust systems that use a collector to join two or more pipes use pressure waves and sound waves to attain efficiency much higher than separate exhaust pipes can.

Keeping the inertia up in the intake system instead of starting and stopping it is going to be worth something, and of course so is tuning the runners and plenum and overall length to take advantage of wave energy and signals that do not exist in individual intake tracts.

American v8 racing engines make great power with single carburtors or throttle bodies located over a plenum feeding runners of dimensions tuned to rpm bands.

Chassis design on the Commando would not be as friendly to experiments with exotic single carb intakes as other chassis that are more wide open behind the engine like the featherbed.

Writing off the possible advantages of sharing the dynamics between intake tracts that could compliment each other in the manner that exhaust collector systems do before giving it a real scientific investigation just might not be the smartest way to go.
Great tuners like Helmut Fath etc. are those with open minds, free minds, and are willing to try fantastic ideas that others might laugh at initially, until they are left in the dust.....

Also, don't forget Ron Gardner as a source, who has been making and selling flat=slide carbs for over sixty years now....

Let's see where we are at:

Jim Comstock says he personally is NOT aware of a single properly sized carb putting out MORE low speed torque in the same engine that if replaced with a properly sized twin carb set up (34 Mikuni versus 2 32mm carbs).

My own personal experience is that my twin flat 32mm flat slides DO put out more grunt at lower rpms say under 4500rpm than the single 34 Mikuni they were preceded by.

FACT: There seems overall consensus that a properly set up twin carb WILL flow more and produce more acceleration than our properly sized single carb AT higher rpms, presumably this means from about 4500-redline.

Conclusion: There is no dyne tested proof that a single carb puts out more "torque" at ANY rpm than our twin set up
There is dyne tested proof that our twin set up DOES produce more "higher rpm" horsepower and consequently faster acceleration.


Did I get this right?

More than one person here has a dyno, maybe you should ask them. Oh, wait, they've already told you. :roll:

Sorry about having no pictures to accompany my post Dave!

1up3down said:

Let's see where we are at:

Did I get this right?

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I think you got it right if you are talking about what is commonly available.

However I still believe that a single carb on a correctly designed manifold can produce results every bit as good as twin carbs. Maybe even better. Especially if powerband width is taken into account. Jim

comnoz said:

However I still believe that a single carb on a correctly designed manifold can produce results every bit as good as twin carbs. Maybe even better. Especially if powerband width is taken into account. Jim

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So are we talking theoretical pie-in-the-sky here, or where can we get this perfect manifold and single carb setup ready to go ?

No question- it is pie in the sky, backed up by little bits of experiance over many years.

If you have the time you are welcome to come on over to my shop and use all of this great fabrication equipment and the dyno and see what you can come up with.

for ' esspecially ' read ' particularly ' .