Finally, aftermarket heads for the Dmax. Pretty exciting stuff! I can't wait to get my hands on some!
Everyone has been racing dynos for a few years, dynos have pretty much become advertising tools, rather than R&D tools. Most operators have figured out how to make the dyno read anything they want. It has become a race to see who can make the biggest number real or imagined.
I guess now it's time to race flow benches.
The problem you have in both instances is the lack of accurate repeatable testing.
On to cylinder heads…………
Most people don't realize how sensitive the Dmax head is to bore diameter. This is actually true with most cylinder heads but more so with the Dmax than any other head I have seen in my 31 year career working with and flow testing cylinder heads.
The Dmax valves are so close to the cylinder wall, that the effective flow area of the valve is greatly reduced. This is one of the reasons Mr. Wagler chose to relocate the valves more to the center of the cylinder. An excellent idea in my opinion, great job!
As you can see, flow testing a cylinder head on a bigger bore than what will actually be used, will result in inflated numbers and inaccurate data. Worse yet is not having a fixture that accurately locates the head as it will be used on the block.
Most flow benches are purchased with some generic acrylic flow fixture and the user is instructed to “eyeball” the head on the fixture and hold it down with some C clamps. I’ve tested this on a Dmax head and missing the location by .010 can cause differences in flow of 10-15 cfm. Push the head to one side or the other by .060 and you magically pick wondrous amounts of flow. Pretty cool stuff for advertising!
Below are some pictures of our flow fixture and flow bench. Our flow fixture allows us to change bore sizes very quickly with interchangeable cylinders, The top plate is also easily interchangeable for different bore sizes as well as cylinder head configurations. We have manufactured Dmax plates in most of the common Dmax bore sizes 4.055, 4.075, 4.095, 4.115, and 4.200.
The top plate allows us to mount the cylinder head once, and then slide from cylinder to cylinder in order to test all four cylinders without ever unbolting the head. The top plate is located for each bore center location with dowel pins as well.
So let’s all be sure to ask a couple of things when we see flow numbers posted.
1) Was the head flowed on a fixture with the correct location for the block? I.E. dowel pins on the correct bore center and location on the block.
2) What bore size was used?
3) What pressure was the head flowed at? Typically 28” of H2O. Although there is a very good argument for testing at higher pressures, let's just be sure we are comparing apples to apples and not apples to rutabagas.
Below is the acrylic type universal fixture that is shipped with most flow benches.