When is a single 2 barrel carb the best choice?

[busman78]

Yep, it is all about making the mix warm & fuzzy.

[ccowx]

I am working on an engine that will be used in the Yukon, where it will quite possibly be expected to run well at -40 (basically the same in F or C).

The carburetion is going to be a PICT 34-3 with a 28mm venturi. I have chosen that for the following reasons:

-It is going into a 70 Westy, and duals mean you can not have either the BN4 heater or the oil bath air cleaner. I see a lot of dust here and also, did I mention the temperature?! Btw, I am married, so the heater is not going anywhere!
-Duals are more of an issue for cold starting.

My solutions are as follows:

-My headers are going to be modified for proper cross flow intake heat.
-The thermostat for the heads will be functional.
-Functional choke of course.
-I am going to make a shround around the exhaust pipes for intake air. I will run it to the stock pre-heat location on the air cleaner. I have also figured out how to use the coiled spring from a Holley choke mechanism to thermostatically control the pre-heat inlet by having the coil mounting inside the air cleaner and an acuator rod move the stock flapper. This means the incoming air will be heated even when the heads are at full operating temp, unlike the stock system. It should provide hot air quite quickly upon start up and once it stabilizes it should maintain around 10* C/45* F for running.

My hope is that effectively my engine will run as if it is a cool spring day, even at extreme low temps. It may also answer some of the reasons why someone would choose a single carb over duals, despite the acknowledged superiority of duals in nearly every way.

Chris

PS: I am aware that a PICT 34 is not a progressive carb, I simply jumped in because the issues of why and also heating are relevant.

[busman78]

Yes Chris but not exactly, several models of dual carbs use short manifolds which the heads heat up quite effectively, pin some case too much, VW used a phenolic spacer on the dual's (T3 & T4 style) to keep the the transfer of heat down or fuel would boil out of the bowl. Also the distance the atomized fuel had to travel was shorter, less drop out. So dual carbs on short manifold with the proper insulator can be run in rather cold weather.

With a single manifold there is a lot of runner distance, atomized fuel needs the warmth to keep the marriage together, good you are planning ahead for that.

[ccowx]

When it comes to dual carbs I am more worried about startup than I am about heating of the carbs, though both matter. Duals are harder to get running and choke options are more limited. Doing the toe dance on the gas and trying to get it started at those temps is more of an issue than you think. Add in that my margin for battery power is less and you can see the issue. Things start to get weird below about -20 C!

None of that solves the issues of filtration and heater installation. I am not scrapping my heat at -40!!!

I did not want to hi-jack the thread, but the points of body structure and start up were relevant to the original question.

Thanks!

Chris

[EVfun]

[busman78]

Starting up in cold is the same, the engine is cold, the air is cold, the oil is cold, the fuel is cold, dancing the foot with or without a choke a single barrel, two single barrels or host of barrels does not change the cold factor. Until the engine warms up and either transfers heat to the manifold or the warm air tube life in the frigid carburetor world is a bit doggy.

Now torching off that BN4 before starting up and dumping a bunch of hot air in the engine compartment wold make it easy all the way around.

In decades past they use to make an adapter plate that bolted to the oil strainer cover, could plug your VW in, have a warm engine when you decided to drive.

Not done the -20 or -40 but have driven in 10F to 20F, the dual preformed better, no icing or stalling.

As for ease of running nothing like a single.

[[email protected]]

SVDAs also give increased idle timing when the choke is in operation, that helps a LOT over the uh-oh-nines.
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[Alstrup]

[W1K1]

[mikedjames]

Take stock air system, slot in a progressive and tune with a PLX AFR meter.

Not a cheap option - I have $100+ of jets alone for this carburettor, $200 for the AFR setup .. And since that picture a custom linkage giving a 1:1.2 step up in movement to get to WOT without wires stretching up at stupid angles.

It does not stumble.

The exhaust rattling between #2 and #4 just behind the block still gets hot enough to burn you if you touch the riser pipes under that less than excellent EMPI manifold.
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[busman78]

I am a seat of the pants HP tester, if it feels good, sounds good and puts a grin on my face then obviously it worked. I would prefer 3" but VW failed to make the metal tin piece in that size, piss poor engineering in my opinion.

[EVfun]

A couple of thoughts are running through my head about 2 ICTs or Kadrons vs. 1 centermount IDF or HPMX...

The duals have quite the proven history of use on stock engines. They don't need exhaust heating. On the downside, throttle shaft wear could be a problem as there are twice as many throttle shaft holes. Depending on the linkage, synchronisation of the carbs could be troublesome too. I know from experience that the steel rod connecting 2 Kadrons meant they where only working together at one engine temperature. (It did prop one side open for a cold start idle boost.) Oh, I'm surprised more carbs are not made with one throttle shaft hole a blind hole to cut wear leakage issues in half.

The centermount options have the advantages of not needing to be balanced, both at idle and in the linkage, to insure they agree across the entire range. They also have the upside of using sealed ball bearing throttle shafts to stop the wear leakage issues. Lastly, if I understand those carbs it looks like jetting changes are easily handled by just removing the air filter. One likely downside for use on a stock 1600 is that the carbs and runner volume looks a bit large for good lower end (and that is where a stock cam is happy.) Also, most of these systems are a bit tall and may not clear engine covers in some applications. Lastly, any of the centermount choices require a good exhaust heat riser setup.

O.K. what have I missed or gotten wrong?
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[oprn]

[67rustavenger]

[modok]

We have mastered running in the cold with DRLAs/alfa IDFs, I can't see much to improve.
It was easy because Italy did the work for us, and having a intake manifold only 6" tall, not a lot of ways to screw that up.

Now I look at the OE vw setup and I see there details that can be improved.
Steel riser and aluminum end castings......oops, that's backwards.
Put the heat right under the throttle, because that's where it gets cold.

[modok]

[EVfun]

[modok]

I don't consider manifold heat to be highly important. one thing that helps is making the key parts of the manifold out of aluminum. Actually the plenum area eing aluminum and then steel runners is GOOD idea, but yet again only one detail of many.
Yes you need to heat some areas of the manifold that get cold, and you need good wet flow and velocity, and a good carburetor, and intake heat so the carburetor itself does not freeze over.
How the fuel comes out of the carburetor and hits the manifold is very important, both for fuel distribution and drivability. A lot of people only imagine what happens at WOT, but MOST of your driving the throttle is not full open, so what comes out of the carburetor is all about the throttle plate.


THINK about the four tube progressive manifold and how that's going to work at part throttle. The fuel stream coming out of the primary, which is on one side, will hit the floor of that tiny plenum, which has a dividing ramp, and get swept into mostly TWO of the runners, and once in those runners there isn't any heat there nor any kind of turns or anything to throw the fuel back into the airstream at all, until it goes through the turn, and that turn is not going to help either.. So you'll have two cylinders drowning in liquid fuel and two going lean. Not only does that run terrible but it also might kill the rings. Liquid fuel going into the engine is NOT good, and it should be avoided. A lot of good designs have grooves or ribs some little feature to keep the liquid fuel IN the heated area of the manifold until it has evaporated or there is enough airspeed to pick it up.

Turns are also tricky. A gentle turn ends up with all the fuel on the outside and most of the airspeed on the inside, which means a gentle turn wrecks your atomization. One would think the AIR would get thrown to the outside by centrifugal force, but actually....... if it does, or does not, depends on the raduis to size ratio, and if it is converging or diverging diameter through it, and other factors that aren't even constant, and in fact it's to a degree unavoidable, but even so you can minimize the areas of the manifold that are doing that. You look at the subaru intake the plenum floor is certainly a tight enough turn after the throttle that the air will take the outside, so have airspeed on the floor, and the floor is WIDE and will spread out what fuel collects there, AND that area is heated by water and an EGR port. You don't need to build a fire under the whole thing, just where it needs it.

[ach60]

When was a single 2 barrel carb the best choice?
When I was 24, just moved to Ca. and didn't know any better.
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Good Luck
Al

[modok]