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.
Why Carburetor Bowl Vents Require Attention
Think these are innocuous little vents that don't affect overall fuel enrichment? Will exposing them to non-atmospheric pressure conditions affect fuel delivery? Maybe some of the following will help point out their importance.
Simplistically described, a carburetor provides fuel as a function of its delivery system (passages and jets) that relies heavily on the difference in pressure between fuel bowls and throats. If for some reason bowl pressure is caused to deviate very far from atmospheric pressure, especially if this deviation varies over a range of engine or vehicle speed, net fuel delivery can become influenced by a condition other than delivery passage sizes. You'd actually like for bowl pressure to be constant throughout all conditions of engine rpm and vehicle movement, as near to atmospheric pressure as possible. That's in a perfect world. In reality, making an effort to not create pressure fluctuations at the point of bowl vent entry is a fundamental requirement to allowing stable fuel delivery. Exposing vents to irregular pressure conditions or pulses should be avoided wherever possible, even if you only use the tried-and-true method of joining primary and secondary bowl vents with a section of hose and providing a small vent-hole around the mid-point of the hose.
Is Valve "Float" Really Float?
Maybe this is an issue of semantics. Do valves really experience "float," or does the condition amount to a lack of valve control? Regardless of how the issue may be defined, you can boil it down to two primary periods during valve motion when it becomes problematic. One has been termed valve "pitch," when camshaft followers are briefly separated from the nose of a camshaft lobe, particularly at higher rpm. The other occurs at the instant of valve seating when a lack of motion control is lost to what we'll call "bounce" that can immediately follow the failed initial seating. In either case, barring mechanical contact between a valve head and piston crown, valve-springs face immediate damage.
Years ago, well in advance of when commercially-available valvetrain "spin" machines became available, we built our own at Edelbrock. It was during a rocker arm design and development project and we were doing our best to examine as many dynamics variables as possible. What we quickly learned was that an unstable valvetrain (for whatever reason) could cause spring damage prior to any audible evidence of the problem. And, in fact, when the problem became heard, chances were good that damage had already occurred. We also discovered that the subsequent loss of valve-spring pressure did not transpire in a linear fashion. In other words, when we created and repeated the conditions of valve motion instability (on our spin device), spring pressure would decrease by multiple times what had been lost from the previous such test.
As a result, each period of instability caused the same condition to occur at a lower rpm, during subsequent run-ups of the machine as a further sign of rapid spring pressure decay. Of course, today valvetrain component design and the tools by which they are configured are far more sophisticated. But the problem can still be created by racers and engine builders, virtually independent of how well the camshaft and valvetrain component manufacturers do their jobs.
Small Holes in Header Pipes Near Their Cylinder Head Flange
Years ago, Smokey was doing this little trick, causing broad speculation about his reason for doing so. All sorts of theories abounded, some so complex it was difficult to separate fact from fiction from speculation. One frequently-heard reason was he'd discovered some innovative way to control reversion, if you can believe that . . . and on and on. During one particular late-night engine dyno session in his shop, I asked him point-blank if it was a quick way of seeing if any cylinders were misfiring. His answer? "Yep. But I like being told all the theories because it ain't often I get to hear from geniuses."