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S14 Cylinder Head Flow Bench Results |
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vs. the Honda S2000 cyl. head |
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A flowbench is a tool.
Throughout the remainder of this article try to keep this in mind. A flow bench is a tool.
This point is emphasized so that the results that follow are not misconstrued. A good operator will be able to use a flow bench as a tool with which to optimize cylinder head ports in order to increase engine power. But a flow bench is not a dyno, and the dyno is the only true test as to whether one cylinder head works better than another. That being said, a flow bench is still a very useful tool to use in extracting more power from an engine. The key is that the user must be experienced and knowledgeable. One can often make a port flow better on a flow bench by simply making it bigger, yet this does not necessarily yield more power on the dyno. That is mainly because a flow bench does not give very beneficial information about flow velocity. And flow velocity is critical to inertia supercharging, which along with intake and exhaust pulse tuning are the main reasons why it is possible to achieve greater than 100% volumetric efficiency in a naturally aspirated internal combustion engine. The way that air is caused to enter the combustion chamber can also have a strong impact on power production. For instance, it may be desirable to cause the incoming air to "swirl" into the chamber. This swirl is initially induced within the ports, but cannot be measured on a flow bench. As a general rule of thumb one would like the smallest port cross section that will yield good flow. Yet there is much more to it than that. Factors such as port profile and valve seat angles must also be addressed. Through careful attention to these parameters a good cylinder head technician might well be able to increase the flow of a given port without substantially modifying its cross-sectional area, thereby increasing power over a broad range of the useful rpm band. Another important fact to keep in mind when examining flow bench results is that a flow bench operates with a constant pressure drop across the port(s). In reality however, the pressure drop across the ports is constantly changing (along with valve lift) as the piston moves down the bore on the intake stroke (and beyond), up the bore on the exhaust stroke, and as any pulse tuning effects play their part. And the manner in which the pressure delta across the ports changes with piston position in the bore is also dependent on RPM as well as rod ratio (an often overlooked contribution). So in theory a flow bench test session should be run with the test pressure varying according to valve lift, with the pressure vs. lift data having been determined from measurements on an actual running engine. And even that data would only be applicable to one throttle setting, presumably full throttle. This could present quite a challenge. However, flow bench experts have found over the years that quite good results can be obtained by running the flow bench somewhere close to the "average" pressure drop that the port is likely to see in actual application. After all this, it must be decided at which particular pressure to run the flow bench. Test pressure is usually indicate in inches of water (in. H2O). The actual pressure drop across an intake port might vary from 0 in.H2O to 250 in.H2O during an engine cycle, but flow bench operators have found the the "shape" of the flow vs. lift curve is relatively unchanged when varying the test pressure. Test pressures that have been used by successful flow bench operators appear to have varied from as low as 3 in. H2O to as much as 80 in. H2O. The most important factor is that the same pressure be used for all comparison testing. Certainly running the bench at 200 in. H2O would produce very high absolute flow numbers, and this might highlight certain aspects of the port flow such as separation, which would not occur at lower pressure differentials. But in practice this appears useful only in extracting the very last 1-3% from an engine. And it makes for a very expensive flow bench. All of the charts in this article were obtained at 10 in. H2O. | |||
Note: If someone says "You should see the flow chart for the XYZ head!" be cautious. Sure that head may flow real well, but a flow chart on it's own is not very meaningful. Flow data cannot generally be compared from one flow bench to another, except perhaps by someone with a lot of flow bench experience. Flow data is most useful when used to compare cylinder heads (or cyl. head modifications) back to back on the same flow bench (at the same test pressure of course). You can't look at a flow chart like you look at a dyno chart, and pick off the peak power and torque. It just does not work that way. This is an important distinction to keep in mind. | |||
Good technical information regarding the theory of flow benches can be found on the Superflow website. | |||
Test Description: This article compares flow bench test results for 3 different cylinder heads. The first is a European 215 HP S14 cylinder head. This head uses 37.0 mm intake valves (just like the standard head), but it is factory ported on the intake side to mate up with the larger 48 mm throttles and intake trumpets. The second head is a genuine, brand new Evolution 3 cylinder head. This head is factory machined to accept bigger 38.5 mm intake valves (the exhaust valves are unchanged at 32.0 mm). For this test the Evo 3 head is fitted with the unique valves from the European 3.8L E34 M5 which neck down to a 6 mm stem inside the port. The last cylinder head featured in this test is what many Japanese car tuners refer to as the "King of Heads", the one from the Honda S2000. | |||
OK then... on to the results » | |||
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