Good thinking on the fuel ratio - That would be a very good indicator if the carb was too small, . . . I do have an AEM 02 sensor. I try to keep an eye on it. . Its hard to look at that while racing but the fuel ratio is normally between 12 and 13.
The plugs look good after a few races - grayish black ...Not super white or dry . I don’t think its running lean. (also -the car does stay cool - almost never runs hotter than 190 degrees)
The way I am thinking is - a 4bbl probably won’t help much ...maybe a little, but I could also be opening up pandoras box for over fueling or stumbling or some other gremlin. And I guess at partial throttle the 2bbls might actually have more cfm avail before the secondaries open on a comparative 4bbl.
AFR is important to know, but it won't tell you if the carb is too small. All it lets you know is if you have the correct amount of fuel for the air flowing through the carb to achieve your desired AFR.
Sadly, your AEM O2S is a narrow band sensor, and provides about the same level of information as looking up into the sky to place the Sun for determining time of day. You get a general idea of AFR, but the narrow band sensors bounce around a lot.
Looking at your plugs is a good way to get closer to proper AFR for the situation, but you have to be careful about how loaded the engine was before you shut it down. If it idles for a while and your idle AFR is lean the plugs will be cleaner than when it's loaded, and vice versa.
Now on to the meat...If you ran a 500 CFM 2BBL and a 500 CFM 4BBL at WOT they should come close to making the same level of power. However, manifold accommodations made to fit each one can change the power level somewhat. Where the 4BBL is superior, all other things being equal, is how fast the air flows through the primaries to feed the engine. Smaller diameter primaries of the 4BBL mean that the air has to move faster to get the same volume through. Higher air velocity means more inertia, and better air/fuel mixing. This means that the engine is more efficient, with a bit more cylinder filling at part-throttle. That means more torque at lower engine speeds, and better acceleration out of the corners. Opening the secondaries would then provide the max power available with 500 CFM.
You will notice a more responsive throttle with a 600 CFM 4BBL over a 500 CFM 2BBL, AND you can control when and how fast the secondaries open.
Another benefit is somewhat better fuel economy, due to the greater efficiency of the smaller primaries.
Quite literally, YMMV.
(If I have made any errors in the above reasoning, would someone with greater knowledge please correct me. I don't want to propagate "bad" information.)
Capt. Delinquent Racing
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