Question: Do three circuit carbs really pose a fuel flow restriction and go lean up high?
An internet carb guru was proclaiming this to be fact and asked for even a single data log displaying a 3 circuit carb that did not lean out as it traveled down track.
Answer: Actually I have years of “real world” data showing the exact opposite to be true. It was first discovered in NHRA Pro Stock (the first cars to have wide band data acquisition in drag racing). Now that data acquisition systems are more attainable and many racers have them, folks see that these old wives tales are false. They are simply sales tools to get folks to send a carb to them to get converted to their program. We were amazed at how different the ratios were from the dyno to the vehicle and were doubting what we were seeing back then.
However, we wish two circuits were better than three circuit carbs. Two circuits are cheaper to build and easier to build as well.
On a three circuit design, you have to spend the time to lay back the intermediate circuit to get them clean at part throttle, but only lay it back as much as you need. As it is a valuable circuit that fills in the gaps in shift recovery. It makes the ratio stay flatter and the vehicle more consistent and accelerate faster.
Question: Can you make a 2 circuit flll in these gaps?
Answer: Yes, however, if you can make a 2 circuit fill in these gaps, then you could simply add a third circuit which would fill them in EVEN MORE. This allows you to make the carb EVEN bigger to take the pumping restriction off the engine and make even more power. Remember carbs and throttle bodies are restrictor plates of sorts so they need to be as big as possible but still maintain velocity and signal. So the 3 circuit will always be king in a properly setup combination. Bigger carbs that can still supply the fuel at the proper air to fuel ratio and in a burnable froth, will allow for easier breathing, faster acceleration and more power.
There are alot of internet gurus that will mislead folks with old technology or old advice. But watch what the big boys are running and know they’ve tried and continue to test it all.
Who knows maybe someday a One circuit carb will be the deal or even a Fiver.
But until then, as requested here is a shot of an unfiltered download of a 3 circuit carb (from a test session a few months ago). Notice the carb is not going lean up high. Watch the green line transition from a clean crisp idle to a loading air to fuel ratio. You will see the idle rpm raise up to stage as I rest my foot on the throttle and the system will go a little rich to maximize the intermediate circuit. Then you’ll see it spike up when its decked (moments before release of the transbrake button). This spike denotes the fact it needs a little more squirter fuel on the stab of the throttle. Then watch as magically this three circuit carb defies all statements by internet gurus and does not go lean up high. You will see it recover from the shift and then flatten right back out. This is on a 1100 cfm carb with 88 main jets and 31 hi speed air bleeds in a mild 800 h.p 540 c.i. program.
I have years of this data available to debunk the guru’s statement and anyone is welcome to stop by our race trailer after a run and look at these years of downloads.
Question: Does the well tube in a 3 circuit metering block pose a fuel flow restriction for high powered engines?
Answer: That is yet another myth. There is more than enough material available in 3 circuit Holley CAST metering blocks to support over 1100 h.p. of fuel at a 12.8:1 air to fuel ratio on a single carb with only a 98 holley jet. We use that setup on our 1600 cfm carbs everyday with only a .189 well tube passage diameter and you can take the well out to .213 with room to spare. However, be aware that most of the currently available 3 circuit Billet metering blocks (because of their required passage layout) actually do not have the capability of being enlarged to these dimensions and those versions WILL suffer a flow restriction in the above 900 h.p. engine programs. We do not use those blocks, our Billet blocks for hi flow applications that will support flow rates of even Alcohol and E85 fuels, have more material in the design to support these large passage layouts.
Question: Are three circuit carbs rich at part throttle?
Answer: They can be, but that can be changed, you typically want them to be a little on the rich side at part throttle so the system is as active as it can be as it goes down the race track. If you lay it back too much, you will have slower shift recovery and slower rates of engine acceleration.
So like anything, if you have to remove or drastically restrict the third circuit and lose the advantages it offers there is something that needs to be addressed. If your combo is so unlike the vast majority of the folks in the world that love the 3 circuit and you find you have to remove or crutch this system and lose this good option. It usually means the engine combo is either really so different than what the main stream engine builders are doing that it’s ground breaking. Or something is wrong on the combo, cam, headers, intake or exhaust and one or some of these items is not what the engine wants. Its usually the latter.