When is a single 2 barrel carb the best choice?

[EVfun]

I've been pouring through the threads here as best I could search, looking for information about the different choices in carburetion.

As I look at threads here I am trying to figure out when a 2 barrel center mount carb, either a single progressive 2 barrel or strait 2 barrel system (like an IFD or HDMX) is the best choice. So... when is a 2 barrel center mount system a better choice than a single 1 barrel or dual 1 barrel system? I guess the question has to start with the assumption that the builder can set up proper the manifold heat needed for a center mount car system.
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[Coyotemutt]

This is just, like, my opinion maaaan.... But...

I think a centermount true 2 barrel (NOT a progressive) is inherently better than a centermount 1 barrel because it reduces the degree to which the cylinders have to fight each other for food.

Even better is 1 barrel per cylinder (dual 2 barrel). However, I think that if you do alot of really hard off-road driving (hard left/right tilting) the centermount is less likely to spill fuel due to being at the center of the car rather than at the end of a lever.

But then, the better solution would still be dual 2 barrel carbs with some kind of anti-slosh modification.

[vwracerdave]

Progressive carbs DO NOT belong on a VW engine and I will never recommend using them. The best use of a single 2bbl IDF carb would be extreme off road use or racing.
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[modok]

In the beginning....there was one carburetor, and apparently, people liked it.
The manifold it sits on... is overly long, which causes some odd things to happen. At low RPM it's basically ok, tho you have to keep it warm as it won't keep itself warm.
Somewhere between 3300-4000k rpm WOT, there is a period where due to the "phase" of how they bounce around.... both sides are trying to flow forward mid stroke, and one normally sized carb can't feed both sides, so you get a dip in the torque curve there compared to..... what "could be", more pronounced on the rear cyls.
Beyond 4500 increasingly the length becomes just too long, choking off flow to the rear cylinders although the fronts do better.
Motivation to run a single is sometimes for simplicity of air filtration, reduce flooding on bumpy tracks, space constraints, or the desire to keep the engine quiet.

Enlarging the ID of the manifold will improve top end at the expense of -everything- else, so, probably don't do that. Feeding the manifold with a much larger carburetor would improve torque by resolving the dip mentioned above, however, there does not seem to be space to fit a progressive carburetor on there nicely, so although we have many different types they all seem to suck, wet flow problems and bad drivability.
tho a type-3 or 4 flat fan it would be possible, tho I've never seen it done right. One big carb on a plenum sometimes is good choice for a large CC offroader, and a zenith on a deano plenum manifold is ok, or a dcnf. a single IDF isn't necessarily better if on a plenum, in fact might be worse.



Dividing the manifold into right and left halves, and feeding each half with it's own barrel, does eliminate the midrange dip, but makes each side EVEN longer, so it still has the high RPM drawbacks, and a lot more reversion evident too which needs to be handled with an airbox or large air filter housing, open 6-8" above venturi for proper cloud formation.
Works well if you don't need much over 5k rpm
When throttled you get uneven VE again, front cylinders throttled more and get more EGR contamination, but.....so do kadrons, and you get some of the same behavior. Often good for fuel economy but can cause the two harder working cylinders to overheat sooner. Similar to kadrons expect to idle better without a balance tube with cams wilder than engle 100, but will idle better on two than coughing and loping on four, which is what you'd have running the same cam on a single barrel or plenum setup. Possible candidate for split compression ratio.

That's the short and simple. and we all lived happily ever after, the end.

[oprn]

The need for a center mounted carb in my view is very real on open wheel cars to keep the carbs and air cleaners out of the line of fire from water, rocks and mud. Also twin carbs on a Manx style body are a serious pain in the butt to service. I love how they run but dread trying to work on them!

Not quite the end Modoc, please address the center carb manifolds that split front to rear rather than side to side.


Thanks Merv.

[busman78]

Either the single progressive or the IDF type are good choices as long as you can heat the intake, the real "Weber" intakes for both those carbs has that capability, I would also adapt a warm air supply to the charge air. Both are easily tuned and will provide a good degree of performance over a stock single barrel.

[Dale M.]

If you really read the fine print and tuning confusion aside (Weber IDF or HPMX), there are more real issues with progressive carbs tuning than any other carb style.... My impression is progressives are to be avoided....

Dale
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[busman78]

Lack of experience, understanding and general good carburetor tuning ability create the issues, plus a lot of parroting of comments and beliefs from those that have failed.

If a particular carburetor was really that bad, single, progressive, twin barrel, two barrel, three barrel (Weber), four barrel, no barrel, there will always be negatives, there would not be millions of then in use today, minus the three barrel, running, tuned and performing.

Another good choice would be the Weber 38DGS, nice easily tuned synchronized two barrel, drops right on a progressive intake. Not recommended for a stock 1600cc or smaller.

The only two barrels I have played with and had problems getting to perform on a air cooled engine is the Holley 2209, a Holley 2300 series 350cfm and a Rochester 2GC, just too much volume and not enough jetting to choke them down.

[[email protected]]

Progressives are fine.

The issue that makes them difficult is the 2 bbls are tuned totally differently, and this confuses a lot of people. By this I mean the primary and secondary bbls are jetted differently. That and out of the box their jetting is horrible, totally wrong. At least on a syncronous 2bbl the 2 bbls are jetted the same (square jetting), what you do to one you can do to the other, so it's much easier for the novice tuner.

Preheat (lack of) is the other huge mistake people make. The need for this is also a problem, since most VW exhausts don't have a good preheat SUPPLY, so that attempt is already doomed.

Dual carbs don't need the preheat, so that is a battle already won.
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[Krochus]

I really would like to take a stab at a single point 2 barrel based fuel injection setup on my 2165 turbo. But I really want to try it on one of those mythical “cheater” style intakes to get away from the poorer idle qualities of a shared bank plenum tend to have.

In my experience preheat is a huge help but with the exception of absolute carb icing a digitally controlled ignition can alleviate many of the off idle drivability problems associated with such an unheated setup via carefully tailored ignition mapping.....within reason.

I would say that with any aftermarket exhaust you need to look at oil or some other means of heating the manifold. Even with preheat tubes hooked up on aftermarket exhaust you just trade pulses back and fourth between exhaust tube 2 and 4 with no real flow taking place.

[Bugsy61]

I've always thought the progressive would be a great carb if the secondary were opened gradually by a diaphragm, like a vacuum secondary Holley. I've never owned one, so can't attest to it. Other than that, it theoretically should be a decent carb. I know, theory verses reality, reality wins every time.

[busman78]

The extreme of using oil is a bit much, warm/hot air can be obtained either off the original preheat VW used, even better would be to run a hot air hose off the heat exchanger which would make for a forced hot air feed.

Here is the current set up I use to warm the little carb, has both original exhaust and preheat off the left cylinder, runs smooth, so far only dropped into the 40's, may add another 2" (50mm) to feed the left side for unleaded fuel likes being warm. Oh by the way the only hesitation I get is at first start up till the engine warms, there is no choke on that progressive.

[nextgen]

Back in the 60s's Empi was it for carbs and they were serious. If you bought an extractor exhaust it came with a larger main jet for your stock Bug carb. Twin Solex and Zeinths were the max. at that point guys like me were taking single webers from Fiats, went back to stock with bigger main.

But then came the BUG SPRAY, bad on gas horrible. Next the progressive Holly/Weber on a Raja Plenum manifold. worked on my bugs and when I got rid of the F.I on Type 3's. For some reason guys said that one pipe for each intake was better then the Plenum which was a chamber below the carb to the single intakes. I did not see the difference and like the single Holley Weber. I think it was that I could not fix the F I if it went bad. I needed those cars running as commuter vehicles and if there was a problem with the FI, I was down and had to spend big bucks at VW to get it fixed. Verses the carb, the may run like crap but get me to work.
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[Alstrup]

[jeffrey8164]

I think it boils down to personal preference. Dual carbs are the bomb. Essentially each dual carb acts as a single carb per cylinder with a common bowl. Single dual throat carbs require bigger jets all around(venturis too) since they are feeding two cylinders per throat.It is best to have an isolated manifold for tuning.
I ran a Weber Progressive 32/36 for years across a few different engines from 1641 to 1914. I never thought is was difficult to tune and seemed to run strong. When I switched to dual DRLAs was when I found out what I was missing.
On another note, I set a guy up with dual 36 DRLAs on an 1776 and he didn't want to or wasn't competent in tuning the duals so I switched him to a singe 40 DRLA. No synchronization rigormoral and on his motor the difference was miniscule.
Back in the day companies that sold the dual throat carbs changed the jetting etc. to be close out of the box. Not sure if that is still done.
I can't speak to the different angles a carb will experience during off road use but the angle is the same regardless if it is center mounted or duals on a traditional manifold. The only difference is that a single mounted horizontally will be different than duals mounted perpendicular. As long is the bowl is full enough there really shouldn't be a starvation issue.
I could just be talkin yang.
I do agree that the preheater tubes are necessary to help keep the fuel in suspension and not puddle up.
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[busman78]

Alstrup, that 50mm hose alone would easily feed a 2.0L engine, but the entire left side of the air cleaner is not blocked off, getting plenty of air.

[Coyotemutt]

[Krochus]

[modok]

[nextgen]

Busman78 this is the same Plenum manifold I used iwith my Holley Webers.

Worked fine.
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