Cylinder Head Flow and Horsepower
There is a lot of debate over head flow numbers and their relationship to horsepower. Here we will try to clarify this subject and provide a simple way to convert cylinder head flow numbers into horsepower potential.
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Before we start, there are a lot of factors that influence horsepower like engine size, camshaft profile and timing, engine compression, fuel type, RPM capability, exhaust design, intake design, ram air effect and the list goes on. For this article we will consider that all these factors have been addressed, meaning that we have a well balanced engine that has all the right ingredients to produce very good power.
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Of course an engine may produce less power or more power than what we say here and that is fine because each combination is different…and we are not even talking about differences in the dynos. The formulas here will give an estimation of the horsepower potential of a certain cylinder head.
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What is a cylinder head flow test
Cylinder head flow test measures the amount of air that flows through the intake valves and exhaust valves at the different valve lifts. This results in a chart of airflow versus valve lift in which the airflow generally increases with valve lift, with a tendency to taper off as the valve opening (lift) keeps increasing.
These tests are done on a flow bench. This equipment applies constant suction (measured in inches of Water) on the combustion chamber side of the head, while the intake or exhaust valves are opened by a certain amount.
Naturally the air will rush through the port and past the valve – it is this airflow and valve lift that are recorded and represented on the flow chart.
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On the right is a Superflow Flow Bench testing a motorcycle cylinder head. Here the operator even used a velocity stack to better represent true running conditions.
On the right is the result of the air flow test for both intake and exhaust side of a Ford V8 Engine, used as an example.
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In this test they used 28 inches of water of pressure drop / suction and the valve lift is measures in tenths of inches which is common in the USA.
The Flow is measured in CFM - cubic feet per minute. This is the standard and what we will use in the following formulas.
As usual, intake flow is bigger than exhaust flow and both have a stronger increase in the smaller valve lifts and start to taper off in the high lift areas. The intake flow actually decreases past 0,5 inches of lift.
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First Formula – Different Test Pressures
​ The airflow in these tests is greatly dependent on the amount of suction applied to the port. Therefore the test depression (pressure drop) should always be specified and the results from two different tests can only be compared after being converted to the same pressure drop.
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The most common test pressures are 10 and 28 inches of water. The conversion between the two is an approximation and not exact but it works well in most situations.
To convert flow numbers between the two we can use the following formula:
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If you are using other pressures, the general formula is:
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​Second Formula – Turning air into Horsepower
We finally arrived at our destination, but the results must be used as guidance and not as definitive or exact. From our experience to obtain these results you need a highly tuned and efficient engine – basically a full race engine running on good fuel.
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This doesn’t mean that bellow this horsepower number there is no benefit or horsepower increase by improving the air flow with some porting or oversize valves. What it means is that the benefit from porting and bigger valves will not be as great because ultimately the cylinder head air flow is not the limiting factor yet.
The following formula uses the intake air flow in CFM:
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Okay So What?
Now we can transform flow bench numbers into horsepower potential but testing a head is expensive and time consuming and while the results will surely be interesting, sometimes all this work is hard to justify for a simple one off project. This is why most people port the heads and install oversize valves without ever testing them.
Porting the head and installing oversize valves is a worthwile modifications on most medium or heavily modified classic motorcycles. Of course some of them will gain more from these modifications than others, depending on how restrictive the stock cylinder head is but generally speaking, if we do a moderate port, polish and bigger valve head with other supporting modifications the result will be greatly improved engine power and personal hapiness (hopefully).
