Why Multiple Throttle Bodies?

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QUESTION: This is a question I've pondered many times. With many cars and motorcycles on equal footing with respect to specific output (i.e., horsepower per unit of displacement), why is it that motorcycles persist with employing an individual throttle body for each cylinder whereas in automobiles a single throttle body suffices? Granted some bikes do produce higher specific output than many cars, so I assume there's a performance advantage. Still, many bikes overlap cars with respect to horsepower per liter yet still come equipped with individual throttle bodies. Seems to me that motorcycle manufacturers could take some cost out both for themselves and, later, for customers by adopting a single throttle body, since the maintenance to synchronize them would be eliminated.

Jeff Bertrand

ANSWER: The first problem is to avoid fuel or air maldistribution, either of which can cause one or more cylinders to receive an incorrect mixture. Incorrect mixture produces higher emissions and lower power than having correct mix on each cylinder.

At the races one can often see, on four-cylinder bikes, that although only one oxygen sensor is fitted in the collector pipe, there are positions (all plugged) for oxy sensors in each of the four head pipes. What this indicates is that the engine’s fuel map was “trimmed” on the dyno to establish the small corrections required to bring all cylinders to the desired mixture strength. Once that is done, all four fuel maps (one per cylinder) can be raised or lowered together to compensate for changes in atmospheric density or humidity. Once all cylinders are trimmed, a single oxy sensor is all that’s needed for trackside tuning.

Something on the order of a 10 percent torque boost can be had over a modest range of rpm by choosing the correct intake length. While pipes of this correct length could all be fed from a single “log manifold,” it has always been quite difficult to get fuel added at a single point to proportion itself equally among the four cylinders at all rpm and throttle angles.

Therefore it has proven best to treat each cylinder as essentially a separate engine, with its own separate intake system and its own fuel and ignition maps, optimizing not for an average but for its specific optimum.

Another point is that motorcycles, having so little rubber on the road as compared with cars, need utmost accuracy in torque control to apply power in corners. Four throttle butterflies close to the cylinders do a good job in this respect.

As an example of worst-case, early development examples of the Wright 18-cylinder R-3350 engine (later powering the B-29 bomber) added fuel at a single point upstream of the supercharger, resulting in a nearly two-to-one variation in mixture strength, from the leanest to the richest cylinder. Achieving even workable improvement in this proved to be an enduring nightmare for both development engineers and flight crew (lean backfiring and induction fire on takeoff caused many aircraft losses).

Honda some time ago revealed that its fuel-injection control models all three forms in which fuel reaches engine cylinders: (1) as evaporated fuel vapor, (2) as entrained droplets moving with airflow, and (3) as wet fuel, migrating along the walls of the intake pipe. Steady-state cruise, as in the case of a touring bike on an interstate highway, is an easy problem, but when the throttle is constantly moving, the differing time constants of the three delivery modes begin to affect mixture strength.

Bonneville is another venue in which fuel or air maldistribution in convoluted manifolds all too frequently leads to the loss of a piston. When people in the work area hear a run by a strong V-8 beginning, they stop what they are doing and turn toward the “music,” willing the engine to make it through all its upshifts. One of the occasions when such a car “popped,” I heard an older man standing next to the line of waiting cars say, “There’s a waste of good ice.” He was referring to the use of ice-and-water mixtures in the charge air coolers of supercharged cars. When you run out of ice, you have to drive into Wendover to buy more.

If you remove the four-barrel carb from an old-time V-8 engine and look straight down into the manifold, on its floor you will often see little guide channels. These were intended to guide wet fuel to weaker cylinders. In the case of “log” manifolds, points of junction were often sharp rather than smoothly radiused, the aim being to allow wall flow fuel to strip off at the sharp edges, rejoining the airflow.

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