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Why so many carbs?

  • Thread starter Thread starter t3rmin
  • Start date Start date
T

t3rmin

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This isn't a question with any practical application (so if I'm posting in the wrong forum, I apologize!), but as a car guy making a first foray into the cycle world, I've been wondering...

Why a little carb for each cylinder? Why not one big carb and a manifold with runners? Seems like it'd be much simpler to maintain/tune, cheaper to manufacture, and less parts to break. Is there a real "scientific" reason for this or is it for the coolness factor or perhaps tradition or superstition? Just curious...
 
This question has been asked before, so if you search you'll find many opinions......I have my own opinion on the engineering of so many carbs, and there are also some very definitive answers from people that have owned their bikes longer than I have. I'd point you to the thread, but it's a lot more fun looking it up (and I don't have time to do it right now anyway).......:-D
 
Well I did find one thread on this topic but it was kind of shallow and I was a bit more curious than that. ;-) If there's another one I'm missing, please do tell!
 
I'm not sure if that was the one, but anyway, bikes really have very little room for a well tuned intake manifold design. 1 carb per cylinder with a boot attachment to each cylinder seems to do much more for HP increase and overall performance than attempting say, a 4bbbl of some sort on a manifold in the space allotted between the intake and the battery box.

Some automobiles were also fitted with multiple carbs due to space considerations, notably the Jaguar "E" types (V-12, six downdraft SU's) Volvos (twin SU's), MG's (twin SU's), '65-67 Corvair Corsa 140 hp (4 single barrel Carters), all in the interest of max performance and HP while keeping it all under the hood of the vehicle.
Hope this helps, I wasn't being facetious, I just knew I'd seen this question before.
Dan
 
Yep, that's what someone told me when I asked that question once. I still think somehow having one carb would make more sense and no more carb synching!
 
Without carb synching, what would we do with our time?
adult032.gif
Could see a Holley 750 or Rochester Quad being much more practical. But then again, there were plenty of Big Block 60's cars with dual quads or six packs, guess to squeeze out the horsepower without taking up too much room under the hood.
 
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Desolation Angel said:
Yep, that's what someone told me when I asked that question once. I still think somehow having one carb would make more sense and no more carb synching!
Click to expand...

this was actually practiced on Goldwings for a number of years in an after-market environment... I can't recall the name of the guy who did it, but he took a single car bar (a small one if I remember the tale), made a manifold, and sold many of them as kits for Goldwings... they offered what seemed to be just as good of performance, without all the work.

But, as the design/layouts of the bikes changed with model years, his design didn't evolve and eventualy he went out of business... But, while it lasted (like Vetter), it went like hotcakes.

Not sure why they can't do it now... sure would make sense to me, except that eventually on new stuff carbs are going to go away anyway...
 
The way iunderstood it was 1 carb per cylinder more closely approximated the performance of fuel injection, which was to cost-prohibitive at the time to use on bikes. Nowadays it is hard to find bikes without injection, and the ones that don't are usually lower-end or dirt bikes.
 
I've always thought the multi-carb thing (on musclecars and such) was mostly for bragging rights -- i.e. "I've got more carbs than you!". I don't see why dumping the same amount of air/fuel from either one carb or multiple carbs would make a difference. Of course that's just my impression without any hard facts to back it up.

The only "legitimate" reasons I can think of for multiple carbs are:
* Your engine is built so much and its breathing requirements are so high that you have trouble finding a big enough single carb.

* Your cylinders do not all have the same breathing requirements (either due to old age and wear or just natural machining discrepancies or something) and dumping the same amount of air/fuel into a manifold and into all the cylinders equally does not account for slight variations in the cylinders. Running more carbs would give you the ability to fine tune each cylinder according to its particular requirements. This seems kind of unlikely, since if you've got much variation between cylinders there's probably a problem, and if not, different cylinders putting out different amounts of power seems like it would eventually cause problems.

* As mentioned before, lack of space for a unified manifold. I don't really buy this, since if you had a single carb model in mind from the getgo, you'd design the frame and such with enough clearance. Besides, I don't really see how a manifold with runners plus one carb would have to be any bulkier than FOUR separate carbs.

...Or maybe they made 'em this way to make more money on parts and service for all the people who don't want to/can't work on the four carbs themselves.
 
You also must keep in mind that any manifold is a compromise. fuel distribution will not be equal across all cylinders with a one-carbed/manifold setup.
 
The REAL reason for the multiple carb setup:

My local motorcycle repair shop charges $65 per hour for carb balancing. It's all about job security...
 
Arkaloid said:
You also must keep in mind that any manifold is a compromise. fuel distribution will not be equal across all cylinders with a one-carbed/manifold setup.
Click to expand...

I believe that is a big part of it...that's why car fuel injection is now using individual injectors instead of the older throttle body injection.

Also less distance for the fuel mixture to travel.

Bob T.
 
Arkaloid said:
The way iunderstood it was 1 carb per cylinder more closely approximated the performance of fuel injection, which was to cost-prohibitive at the time to use on bikes. Nowadays it is hard to find bikes without injection, and the ones that don't are usually lower-end or dirt bikes.
Click to expand...

You might be on to something there.

I always thought fuel injection was more efficient because of the more precise, computer-controlled metering through the RPMs, rather than the cumbersome multi-circuit richening system in carbs. Seems like you're still bound by the limitations of carburetion with mutliple carbs. There must be more to it, though, because multi-point injection is better than throttle-body injection (or so I assume, since that's the modern FI incarnation).

Is there something inherently better about multi-point aspiration? Why??? Inquiring minds want to know! ;-)
 
I think it must have been strictly marketing hype at the time. I can't see that you'd get any appreciable performance increase with multiple carbs on bikes with less than 1000cc engines.
 
Arkaloid said:
You also must keep in mind that any manifold is a compromise. fuel distribution will not be equal across all cylinders with a one-carbed/manifold setup.
Click to expand...

Hmm. Didn't think of that. If multiple carbs or multi-point FI is a way to guarantee each cylinder gets its fair share of air/fuel, I can see that being an advantage.

Baatfam said:
Also less distance for the fuel mixture to travel.
Click to expand...

What about tunnel rams and high-rise intakes on hotrods? I always thought more distance/more intake volume made more power. Maybe this comes back to the even distribution thing. More distance and more intake volume means more time/space for the mixture to... well... mix?
 
I think it has to do with engine design. An inline 4-cylinder motorcycle spins fast and makes its power at high rpms. A single carb with an intake manifold would not allow the engine to breath as fast at higher RPM. Not only that, but I think the long carb to cylinder distance would have an effect on throttle response, especially for the two outside cylinders. In cars, this is countered with big flywheels - something M/Cs don't want.

I'm no engineer, so this is all semi-educated guesses.
 
flyingace said:
I think it has to do with engine design. An inline 4-cylinder motorcycle spins fast and makes its power at high rpms. A single carb with an intake manifold would not allow the engine to breath as fast at higher RPM.
Click to expand...

Well I suppose it all depends on size. A single carb with the same CFM rating as the combined capacity of multiple carbs should breathe just as much. But there may be a point at which it's not practical or possible to make a carb big enough to meet the requirements of a motor. Although I can't see that happening in the context of a small motorcycle engine. A single Holley double pumper can easily meet the requirements of a big block V8...

flyingace said:
Not only that, but I think the long carb to cylinder distance would have an effect on throttle response, especially for the two outside cylinders. In cars, this is countered with big flywheels - something M/Cs don't want.
Click to expand...

Yeah, I think carb to cylinder distance would have some effect on throttle response. Though you probably have quite a bit of leeway before it becomes a problem (again, consider high-rise air-gap intakes and tunnel rams on muscle cars). AFAIK the varying distance between the outside and inside cylinders is usually accounted for by way of putting extra twisties in the inside intake runners, so they all end up being the same length, which would be just as possible on a motorcycle.
 
I guarantee you if they could have figured out a way to put a carb on each cylinder of a muscle car, they would have. It would have kicked the living daylights out of anything with a manifold. Breathing isn't just about CFM, it's about efficiency as well. A direct path into the cylinder will always mean better breathing than going around corners.
 
They did/do put multi-carbs on cars (like this), and I think it is more efficient for the same reason we've figured out in this thread: more even mixture to all cylinders. All I'm saying is I don't think it's for capacity or carb-cylinder distance reasons.
 
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