This has been interesting reading. I won the Small Block portion of the first Carb Shootout (there have only been two years) at BLP, the second year ended up third. Considering the company I was in and no test time the second year because I had open heart surgery earlier that year I don't feel too bad. The top 8 or 10 in both the big block and small block portions had less than 10 HP between them, between my third place and Quick Fuels first place finish the second year on the small block there was about 2 HP. And it was a privilege to be around all the competitors involved, there were quite a few interesting stories told...
Finding the ideal carb setup for any engine is a balance a lot of factors, and every engine will want things tailored for that engine. If you build the carb to deliver fuel correctly and with boosters that provide a well atomized mix of air and fuel the difference from one engine to the next will be minimized. There are no secrets, carburetor engineering has had over a hundred years of development and while there have been small refinements there has been nothing new for quite some time.
As far as how many circuits... Here are my thoughts. On a racing engine what a carb needs to accomplish is the following:
1. Deliver the correct amount of fuel in the entire operating range it runs. This means primarily not being so lean it misfires, or so rich it causes the plugs to foul. With methanol you can tolerate being on the rich side, because it is such a simple hydrocarbon with only one carbon atom it will have very little free to leave deposits. Gasoline on the other hand is very sensitive to being overly rich, so making sure the idle, transition, and if equipped the intermediate are not so rich that it fouls plugs driving thru the pits or part throttle in the burnout. This is what get a 3 circuit carb into trouble, because of the location of the intermediate tube it delivers the most fuel at part throttle. Now the original 3 circuit Dominators were designed to do just that, however the reason was to be able to deliver enough fuel at that point on an engine with an individual runner manifold. Cylinder pulsing with individual runner intakes do not have a plenum to dampen the pulses and at part throttle will disrupt the fuel flow from the booster. 4 and sometimes 6 cylinder engines even with a plenum may have strong enough pulsing to gain from an intermediate circuit. Since the intermediate will still flow at WOT some will use it to even out distribution, but I believe there are better ways to help that area. And if you have to add too much to even distribution at WOT you end up with way too much at part throttle. Black carbon fouled plugs dissipate heat faster making the plug act colder, and having carbon on the porcelain is also a conductor and will give the spark an alternative path it can take.
2. Provide the least amount of restriction possible. Bigger is better, however only up to a point. That point comes when the equality of distribution to the cylinders falls too far, that comes because the fuel does not stay suspended in a relatively equal and homogenous mix with the incoming air. Higher vacuum from a smaller carb improves that, but at the expense of the power needed to create that higher vacuum.
3. Provide a well atomized mix of air and fuel. This is a big one, when you can provide a well atomized mix of air and fuel distribution improves, carb size can go up, and power improves because you don’t lose as much pumping the air in. There are other things that help here as well, using a fuel that vaporizes well for the intended use and engine heat range, this also improves distribution. A quality intake that distributes fuel as equally as possible, and prepping the surface of the plenum and runners. When fuel properties are poor or suspension and distribution qualities of the induction are lacking this is where a carb with those qualities will shine the most. Booster selection is key, and as long as the carb size is not restrictive using an annular booster is my preference. Using a 12 hole like Thumper is using can help over a stock booster, I use inserts with even more holes and smaller than some. Combined with a design that provides a stronger signal it can in most cases eliminate the need to use the intermediate circuit to equalize distribution.
As far as the 4 and 5 circuit stuff Braswell is doing, the extra circuits are only extra bleeds that feed different sections of the emulsion channels. It still has the idle/transition circuit, intermediate circuit, and multi emulsion main circuit.
Which is best? Within a limited RPM range you can make most anything work, within a wider range is where it gets tougher. Tuned correctly for the engine they will all fall within a few HP. For me the simplest that provides the correct fuel is best. There is an old saying, the more complex the plumbing the easier it is to stop up the drain…
Now for those that want to learn to do things on your own, follow the link below, lots of free info there and lots of help. I’ll be happy to answer any questions here or there and with no strings. Any carb work I do is only if someone does not want to attempt it, and since I have a regular job outside of carb work I try to limit what I do anyway. It’s a free forum, I make nothing from it either, it’s there only to help. And for everyone to learn.
http://racingfuelsystems.myfunforum.org
