Even prior to emergence of the Internet and its "instant information" channels and high-speed means of communication, racers and engine builders were exchanging ideas and opinions about a range of topics. Among them were claims and explanations that sometimes led those seeking correct answers down less than accurate paths of information, not entirely unlike today. In fact, perhaps more so in current times because of how information is occasionally derived and shared. As some of the subjects have been and continue to be focused on racing engines, we thought a brief discussion about some of them might be helpful. So in no particular order, consider these subjects.
You've likely read or heard about an assortment of consequences when a carburetor is raised (or lowered) relative to an intake manifold's plenum floor. More plenum volume helps high rpm, for example. Or maybe, smaller carburetors like more spacer. This isn't to say these analogies are incorrect, but maybe the reasons are a bit obscure.
For example, let's look at it this way. A carburetor is a pressure differential device. It delivers fuel into a region that's designed to be at less than atmospheric pressure. It's a fuel metering "signal" that allows atmospheric pressure to force fuel into the incoming airstream, acting through the carburetor's bowl vents. By elevating a carburetor, all else being equal, it becomes more remote to runner entries and thus can experience a weaker metering signal. The carburetor tends to act "leaner" in the presence of a decreased signal. Aggregate airflow tends to be the same but there's less fuel flow, so fuel enrichment is decreased.
However, as Smokey often said, "there's one more little item." Once discharged from the carburetor, air/fuel charges are required to make a relatively abrupt turn into the manifold's runners. Air, being compressible can navigate this sudden change more easily than fuel. Increasing carburetor height allows air/fuel charges more time to slow down and make the turn more effectively, often reducing the possibility of air and fuel separation. As carburetor size is decreased and no spacer is used, the problem becomes more critical. In fact, carburetors placed too near a plenum floor can be akin to sticking a hose in a bucket with fuel impingement materially upsetting proper air/fuel mixtures.
One key here is to map brake specific fuel consumption (BSFC) data as carburetor height is changed, assuming carburetor sizing remains the same. If a disproportionate amount of fuel is required for best power and BSFC data are trending higher and higher, chances are good mixture quality is being upset and raising the carburetor is a potential solution.
On the other hand, if you discover that a four-hole spacer is beneficial, possible reasons include the fact the fuel metering signals were insufficient (for the calibration or jetting being used) and stronger signals provided by the spacer helped deliver additional fuel. But in virtually any case, power changes from raising or lowering a carburetor affect more areas than plenum volume.
Why Exhaust Crossover Pipes Are Beneficial
It's not necessarily about "smoothing out exhaust pulses." This Enginology column has previously contained discussions about how and where in an engine's rpm range exhaust collectors tend to work. You'll likely recall we suggested they are most effective below and up to peak torque rpm. In fact, if an engine operates primarily above its torque peak rpm, collectors are essentially out of play. You've likely seen some engine applications where low-rpm torque is unnecessary or unwanted, so employing little or no collector volume is an effective tool. Further, as collector volume is increased (to a point), additional torque is produced up to this engine speed. Consequently, joining two sets of collected primary pipes with a connecting cross-pipe effectively adds functional volume to both sets of collectors. It's also always wise to revisit air/fuel calibrations after adding a cross-pipe.