Stock Vehicle Analysis:
The image below shows a side view of the Suzuki GS550. Relevant parts of the induction system are numbered. Intake air enters each cylinder through a single port (1) in the cylinder head, controlled by an overhead intake cam. The intake system faces the rear of the motorcycle so that the exhaust side may be exposed to the cooling effects of frontal airflow. Air enters the induction tract through a sheet-metal airbox (2) located under the seat, which contains a 180°, reusable foam-type air filter. From there it flows into an adjoining plenum of injection-molded plastic (3). The plenum divides the flow into four intake pipes (4), each leading into a side-draft carburetor (5). Another short section of pipe then connects the carburetor to the engine flange
Suzuki GS550 side view.
near the induction port. The stock intake pipes are formed of flexible rubber tubing to facilitate easy assembly/disassembly and also to isolate the carburetor somewhat from the vibration of the engine, which may otherwise interfere with the fuel mixing process. An obvious question would be whether the intake system was not already properly tuned. Using the analysis method presented later, however, it was found that the dimensions of the stock system would correspond to tuning for a speed range far beyond the redline of the engine. Therefore, the stock system is not optimized for this particular engine. In addition, the airbox and plenum contain many plain orifices and square corners which suggest that the system may exhibit a high level of flow resistance, thereby limiting the engine’s volumetric efficiency. Below is a 2-D representation of the stock plenum. The stock air filter is also highly restrictive, and its location beneath the cycle seat prevents any helpful utilization of onrushing wind to aid flow.
Stock plenum
The stock system was modeled using ALGOR finite element analysis (FEA) software to provide a qualitative model of the internal flow. Similar simulations were performed on the final design selection. Results of both cases are featured in a further section of this report.
Prior to alteration, the motorcycle was tested on a chassis dynamometer to determine its output characteristics. This test was performed under the team’s supervision at a local motorcycle dealer. The motorcycle’s rear wheel was placed against a revolving drum which applied a constant load, and the throttle opened slowly to increase the engine speed. Strain-gauge-based measurements of torque and horsepower were then output to a computer acquisition system. Measurements were taken from just above idle (about 2000 RPM) to near-redline (about 8000 RPM). The resulting output curves are shown below. In general, the engine produced a steadily increasing torque output peaking out at about 25 ft-lbs (7000 RPM). Power output followed a similar trend, as power is defined as the product of torque and engine speed.
Stock torque curve
Stock power curve
In observing the driving habits of the operator under a variety of conditions, it was determined that an increase in output across a speed range from 4000 to 7000 RPM would prove most useful. Further design was targeted at this speed range for improvement.
Of particular importance in any possible design configuration is the space on the motorcycle chassis allotable to the intake system. The available internal frame space was found to be slightly less than a cubic foot, located in the cavity which contained the original airbox. Other spatial constraints include avoidance of the battery tray and electronic components. In addition, any protruding parts should be routed to the rear so as to avoid interference with the rider’s seating. Although the specific details will be omitted from this report, any design concepts generated would necessarily adhere to these constraints.
