The Flow Test
Before starting the procedure a leak test needs to be performed. This is done with both valves closed. There should be no leakage. If leakage is present in any area it should be corrected or the amount of leakage needs to be subtracted from the test data.
During the flow test, the cylinder head is mounted on the flow bench and secured to the test fixture through an adapter. This maintains the relationship of bore shrouding that the cylinder head will experience on the engine. An opening fixture is used to actuate the valve to specific lift points. Lightweight test springs are installed to keep tension on the valves. The radiused inlet along with the exhaust extension allows for a smoother transition. Without these fixtures, the air will shear and give readings that are lower than the actual potential of the port.
Working with a cylinder head, a large variation in flow numbers can be achieved by using different-sized bore adapters. This is a common mistake in many shops.
The test pressure is read on a manometer attached to the bench while a rotary valve is controlled by the operator to arrive at this value. When discussing flow numbers for a potential porting job or the purchase of a set of heads, always identify the bore size used along with the test pressure, or you may not get what you paid for.
To get the most benefit from flow testing it needs to be realized that either valve spends more time traversing the lift range than dwelling at maximum lift. Herein lies the fallacy of most published flow numbers, i.e., that they are at maximum lift and may also be at lift values higher than your cam will generate. A port is considered stalled when increased valve lift yields little or no increase in air volume, or an actual decrease. Low valve lift flow is critical for power production and a head that has better low lift numbers in most cases will outperform one with superior high-lift results. The recommended procedure is to calculate the test heights with an equation that takes the valve diameter and then multiplies it by a factor to determine seven test points.
The equation is: Net Valve Area =0.785 (valve diameter2-stem diameter2).
Once this is derived, it's multiplied by the Lift/Diameter (L/D) equation of .05, .10, .15, .20, .25, .30, .35, to determine the specified lift points for testing. This is done to make an accurate comparison of flow for different valve sizes in the same head. It is an acceptable practice for an experienced operator to flow at .050-inch lift graduations and not do the (L/D) calculations. L/D calculations are necessary when comparing the flow results of different size valves since they can be further broken down into valve efficiency, which represents cfm/square inch of valve area.
When we work with a flow bench to quantify changes to a port as it is being modified, it is essential to make our procedure as repeatable as possible. Inaccurate flow data is worse than no data at all.
A velocity probe is used to measure the speed of the intake or exhaust port flow while the head is being tested. Using an additional manometer that's attached to the bench, velocity in feet/second can be derived. If the air is traveling too fast around the short-turn radius, it will shear or skid, reducing the effective valve area and choking flow. In most intake port designs, a threshold of 350 feet/second is the maximum speed for the air to follow the contour of the port. It must be remembered that when checking velocity on intake ports, the dynamics of inertia supercharging cannot be duplicated. Subsequently, exhaust port velocities do not calculate the thermal expansion and contraction from heating and cooling that occurs during blow-down and the pumping loop.
Charge motion is evaluated when developing a port since it has an effect on combustion.
We record the test results on our computer system for easy referencing, because of the high volume of customers and tests we do, it is just as important as performing an accurate test.
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