Improving Air Flow
Firstly, let's explain why the Twin Cam engine is so inefficient. Unfortunately, the stock cams in all of these engines are designed around emission control, not performance. The cam specifications combined with overly restrictive port and valve seat profiles, is what is responsible for only 69 ft/lbs of torque out of a stock 88 inch motor, 75 ft/lbs from the 96 incher, and 80 ft/lbs out of the newer 103's.
The maximum valve lift of these cams is a modest 0.473 inch, and after flow testing many Twin Cam heads I have seen the flow rate consistently stop increasing at about 0.4 inch of lift. The older Evolution engine actually flows more air throughout the range than it's successor. The stock TC castings simply do not flow enough air to support even the stock cam flow requirements, so you can forget about using a higher lift cam with the stock cylinder head! It is near impossible in a mass production situation to produce a cylinder head capable of optimizing any chosen cam profile, so it needs to be ported, if it is to develop the air flow required to maximize its performance.
Shaping and Testing
The heads are the governing factor in how much torque can be produced from the Twin Cam engine. Any cam profile can be fitted, but if the heads are not capable of providing the air flow then you are wasting your time and money - the performance package has to be designed as a whole. Exhaust choice, ignition timing and tuning are also very important, but the flow of air is the key to any internal combustion engines performance.
Long stroke engines will always have inherent limitations on power output, as the maximum RPM is limited by piston speed. What we can do with specialized porting techniques is to exploit the torque output of these long stroke V twins.
The high velocity port designs that I use will feed the motor as much air as it needs, based on bore, stroke and cam specifications to produce additional torque that you can actually use. Where we have the most fun on our Harleys is generally between 2000 to 4500 rpm. In reality we are not drag racers taking these engines up to 6000 rpm. Improved acceleration due to high intake charge velocity is what we need here for street power.
Torque is what is measured at the rear wheel on a dyno, and horse power is calculated from the torque and RPM values. Remember that torque is for riding, HP is for racing.
This is achieved by the combination of careful reshaping of the mouth, throat and bowl to the optimum dimensions within each port and a precision cut multi angle valve job. The final stage before returning the heads to the customer is flow testing on our Superflow flow bench. This bench is equipped with Port Analyzer software and Flowcom and allows me to port heads more efficiently, providing printable results. The Flowcom is a microprocessor based computer which measures flow rate and temperature from sensors.
This also connects to the motor controller to ensure a constant test pressure throughout the range of valve lifts being measured. The Port Flow Analyzer software is the interface between the flow bench and a laptop. With this program I can use pitot tubes for velocity measurement, to identify areas within the port that require more work. It also allows me to produce graphs and detailed performance charts.