One carb vs duals, theoretically speaking

Classic Norton Commando Motorcycles.

One carb vs duals, theoretically speaking

Postby powerdoc » Mon Oct 10, 2011 4:56 am

OK, before anyone calls me daft for asking a potentially stupid question, think about this. If, in a 4 cycle engine, you're only pulling air through the carb half the time, or less, with each cylinder's cycle, why do dual carbs give more power? With one carb, you'd still get the same flow to each cylinder, and potentially more because of the inertia of the air constantly flowing through the carb rather than pulsing. Is it the manifold that interferes with flow thereby lowering the potential flow and power?. Do we just need a better single manifold or do we all just like to suffer a bit with having duals?
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Re: One carb vs duals, theoretically speaking

Postby GRM 450 » Mon Oct 10, 2011 5:25 am

Single carb manifold is not ideal for flow. There is no straight path to the valve. I have 1 mikuni which is ok till higher revs, but separate manifolds should flow more. Graeme

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Re: One carb vs duals, theoretically speaking

Postby highdesert » Mon Oct 10, 2011 5:33 am

+1 on what Graeme said about manifold flow.

Personally, I don't "suffer" one tiny bit with my twin carbs. Lots of people on this website are very, very happy with their twin carb set up and would never consider a single carb.

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Re: One carb vs duals, theoretically speaking

Postby comnoz » Mon Oct 10, 2011 7:44 am

You are right, the carb only feeds one cylinder at a time and it is the manifold that kills the power. Whoever designed the popular single carb manifold obviously did not have a flowbench or was not concerned with making power.

A single carb manifold can be designed that will flow as well and produce as much power as twin carbs but unfortunately it will not fit a Commando without modifying the frame. If you hang a single carb on my fuel injection manifold it will run with twin carbs but a slide type carb will not clear the backbone tube. Jim
"If the facts don't fit the theory, change the facts." Einstein
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Re: One carb vs duals, theoretically speaking

Postby hobot » Mon Oct 10, 2011 8:09 am

One factor most fail to mention/consider on this, is if the single and dual carbs are the same size or not. In general a bigger single can flow more than a pair of smaller carbs at hi rpm WOT. My surprise on past Ms Peel was going from single 34 mm Miki to dual 32 Amals was the loss of her top speed power. But the response was slightly duller at slow speeds, below 60 mph with the bigger single than the 2 smaller Amals. Most detectable on THE Gravel as I had to twist further-faster-throttle to break loose with the Miki than the 2 Amals.

Of course the split manifold is an issue, just ask the the Big Vtwin crowd. I must of lucked out on Peel because I got like 15 mph higher speed on the 34 vs 2-32's. If not trying to spank new sports bikes I prefer the smaller dual carbs for the better low end up to legal+ speeds on my factory Combat but I liked/loved the bigger single for over the ton thrills on a special built to strain things.

There is another aspect to consider in dual vs single, the size of the air plenum, if a carb only feds one jug it takes a rather bigger carb to supply it at top end than if a smaller carb is used with a bigger manifold volume to feed two or more jugs. This is why fuel injected stacks were so big per jug in past racers and why smaller throat fuel injectors with big air boxes are used in the inline 4 sports bikes nowadays. Length of manifold can make up some speed-momentum of intake charge at lower rpms on a bigger carb that can fully feed at top end.

And another thing, if the head ports are 'too' big, like the factory Combat, the intake velocity is sluggish to fill jug with turbulence so smaller carbs can help response d/t a bit faster mixture speed compared to a bigger carb on same head/cam.
Comstockt-Fullauto head use smaller better shaped ports to get its advantage over the factory heads so could get away with bigger carb and still kick ass down low and keep on increasing as rpms rise.

I my case I'm sticking with smaller dual carbs for the response on a stock Combat but on my big block Peel, with big ports and big cam I'm going with a bigger 40mm single. There are many reports on other lists of the joys of putting on a 32 single Amal over the duals for normal everyday ease and response short of land speeder use. Its a better more efficient mixture feeder when getting the double sucks on each turn, just as the initial poster described.

What would be the cats meow, Quadrajet like single carb with two barrels with progressive linkage, small barrel down low spunk and no end to the top end flow as the big throat opens up as wind spreads your grin.
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Re: One carb vs duals, theoretically speaking

Postby frankdamp » Mon Oct 10, 2011 8:53 am

When I was at N-V, Plumstead took my ride-to-work 650SS back for a couple of weeks to test a single carb configuration. As far as I know, the manifold went from the single carb throat and split into a Y shape to feed the two cylinders. I never ros it in the single carb configuration, and I don't know what size the single carb was.

The oral report I got from the guy who ran the tests was that low-end torque increased about 8 percent, but top-end power was down. He thought maybe it was air-flow restricted at high rpm.

The reason for the tests was to see if a single carb would make the 650SS more suitable for sidecar use, as the Commando couldn't be used for sidecars. The sidecar fraternity were a bit ticked off about Norton "abandoning" them with the Commando.
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Re: One carb vs duals, theoretically speaking

Postby Rohan » Mon Oct 10, 2011 9:25 am

comnoz wrote: A single carb manifold can be designed that will flow as well and produce as much power as twin carbs


On the face of it, this is a pretty wild statement ?
And while there is an element of truth in it as to maximum flow (and fuel injection ?), twin carbs main benefits are always touted as improved acceleration over a single carb setup.

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 ?

It is oft quoted that the early Model 7 Dominator twin that Bert Hopwood designed circa 1948-9 initially had a Y-branched manifold (cast into the iron head no less) and this gave terrible performance when actually run on the road. The solution was to cut off the y-branch, and substitute a U-shaped bolt-on alloy manifold. Which gave better all round performance. Be interesting to see dyno figures for both, since no examples of the y-branch version seem to have survived ?? (although pictures of it appear in all the Norton brochures for the new twins for early 1949 ).
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Last edited by Rohan on Mon Oct 10, 2011 9:30 am, edited 1 time in total.

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Re: One carb vs duals, theoretically speaking

Postby hobot » Mon Oct 10, 2011 9:30 am

So same size single carb had more spunk until it run out of flow capacity, cool.
My Ms Peel is being configured to fit a side hack to join the joys of other Cdo side cars, at least 3 examples I've collected so far, mounted on either side too.
sidecar-rigs-commando-hacks-t7007.html

I've had 3 engines that really responded perked up in response and hi rpm power by restricting the port size and putting a flow trip up turbulizer at the head/manifold gasket, by rough edged lips intruding into the air flow. One was 2 stroke out board, one a V8 and the other Ms Peel Combat with standard small port head. Fun experiments by accident and on purpose.
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Re: One carb vs duals, theoretically speaking

Postby hobot » Mon Oct 10, 2011 9:38 am

Rohan, the best place to search up this contest between low end response vs top end power is the fuel injection history. Basically single throats feeding single jugs needs to be rather bigger ID than if a larger manifold and smaller throat[s] feeding multiple jugs. Don't confuse manifold effects on dual vs single carb or injectors. Jim Comstock has switched from two smaller throats to one bigger throat for better results, so ask him if I'm misleading the anyone on this.
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Re: One carb vs duals, theoretically speaking

Postby comnoz » Mon Oct 10, 2011 11:13 am

Rohan,
Most of the testing I have done between single throat and dual throat have been with fuel injection. But I have run a single carb on my engine with the prototype of the fuel injection manifold. I had a single 45mm carb on it and performance was similar to the fuel injection with a 45 mm throttle body. Low speed responce and drivability were good but not as good as injection.

I have also run my fuel injection with dual 34 and dual 40mm throttle bodies mounted on the stock curved manifolds before I went to the single 45mm body on the new manifold. Power was similar to the 40s and drivability was better with the single 45mm body.

Dyno runs have shown the high speed power from the single throat , the big dual throat and a pair of 35mm FCRs to be very close with the midrange advantage going to the single throat with injection.

I think the advantage in power would go to the dual throat if it was not for the curved manifold needed to fit the dual throat on a Commando. That theory is something I have yet to test. I have been working on a design to mount dual flatslide throttle bodies directly to the head to eliminate the curved manifold. Just need more time and money. Jim
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Re: One carb vs duals, theoretically speaking

Postby hobot » Mon Oct 10, 2011 2:43 pm

You just can't rest can ya Jim, very good for us but likely kinda wears on you in intervals. If your injection system can be triggered by another brand of digital ignition I'd sure be restless to acquire your left over 45mm single throat feeder. At some point I want to shoot up Peel for a 160 hp attempt at Big D's dyno. The draw through blowers are famous for helping a big draw though carb flow and to fully atomizing mixture before consumption. Just Once - maybe. I suspect would take like 8000 rpm if tire could even stay hooked.
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Re: One carb vs duals, theoretically speaking

Postby Rohan » Mon Oct 10, 2011 3:07 pm

hobot wrote:Rohan, the best place to search up this contest between low end response vs top end power is the fuel injection history. Basically single throats feeding single jugs needs to be rather bigger ID than if a larger manifold and smaller throat[s] feeding multiple jugs. Don't confuse manifold effects on dual vs single carb or injectors. Jim Comstock has switched from two smaller throats to one bigger throat for better results, so ask him if I'm misleading the anyone on this.


If I'm not mistaken though, Jim is using one injector per cylinder on his testing, so effectively he is running the equivalent of dual carbs for all his testing. ?

You never see race bikes running less than one carb per cylinder - anything less reduces the acceleration. Although may not affect the top speed in the slightest....

If you hunt around, some variants of Commandos (Seeleys ?) can get a straight run for the carbs down into the throats, no curved manifold.
And if you recall, when the Fullauto cylinder heads were announced, all the racers rubbed their hands in glee and asked if the port for the inlets could be had with more downdraft angle and 1/2" more metal above. (no such luck, but anyway...)

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Re: One carb vs duals, theoretically speaking

Postby jseng1 » Mon Oct 10, 2011 3:09 pm

The aftermarket dual carbs I use idle at 800 and don't miss a beat. Feels smooth as an electric motor. The single is definitely easier to adjust and you don't have to worry about balance. But I would never trade off the flood of power when the duals just need a few minutes of tuning, and stay tuned by the way. Same cannot be said for the original carbs.
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Re: One carb vs duals, theoretically speaking

Postby Brooking 850 » Mon Oct 10, 2011 4:19 pm

Hi jseng1 have just checked out your link, how much for a set of carbs to suit a 73 850 sent to Orlando FL so my friend can hand carry them to me in China?
I like the idea of twin carbs for all the above reasons and makes the look of the back end of the head look more balanced. At this stage, just normal road use, but not scared to get after it when required!!!
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Re: One carb vs duals, theoretically speaking

Postby Danno » Mon Oct 10, 2011 4:33 pm

GRM 450 wrote:Single carb manifold is not ideal for flow. There is no straight path to the valve. I have 1 mikuni which is ok till higher revs, but separate manifolds should flow more. Graeme


There is also sonic intake tuning to consider. The pulses of one cylinder might interfere with the pulses of the other, dropping power and efficiency. Two separate systems don't conflict with each other.

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