Small Block Chevy - What is a D-Port Cylinder Head
It is important to understand the exhaust port shape on small-block Chevy cylinder heads, especially when selecting exhaust headers. With so many OEM and aftermarket cylinder heads available for these engines, the exhaust port shape can vary dramatically. In most cases, this is not an issue. Round port, square port, something in-between, it really does not matter as long as the header flange seals properly to the cylinder head. Of course, race-only applications may want an exact match between the port and header. However, most of the time, this is a non-issue and provides little to no benefit in street applications, hot rods, cruisers, or even many street/strip engines.
One of the most confusing definitions we often hear relates to D-port cylinder heads. A customer opens a conversation by stating that they need a header that matches their D-port cylinder heads. Some time later, after receiving and attempting to install their headers, they contact us and state that the headers will not seal against the cylinder heads and are leaking at every port. Why?
The problem here is that the customer does not have what we identify as a 'true' D-port cylinder head. However, they do have cylinder heads with D-shaped exhaust ports. With some header flange designs, it is important to understand the difference. Please review the slightly exaggerated image below; it should be clear how this might be confusing. The D-Port roof extends well above the center line of the exhaust port, whereas the D-shaped port fits nicely inside what is deemed a standard port dimension.
Added confusion arises with some cylinder heads that have exceptionally large exhaust ports and a very pronounced D-shape, but do not quite fit the definition above. One example is the Pro-Filer 176 head shown below. Is this a D-Port or D-Shape? In this example, either a large port flange or a D-Port flange could work — as long as the header flange seals properly. The concern would be whether or not enough machined surface area exists above the port. The top of this port does not extend far above what would be described as a large square port. Additionally, Pro-Filer does not state anywhere in their documentation that this is a D-Port exhaust, what the port dimensions are, or what specific gasket to use with them.
In contrast, the GM part #12556463 ZZ4 cylinder head is a true D-Port (shown below), as is the GM part #19300955 Fast Burn cylinder head (at bottom). Notice how high the port extends above the mounting bolt centerline.
In Closing:
Knowing the exhaust port style or shape is important when selecting exhaust headers. The manufacturer's notes often provide information on header compatibility with various port styles. For most street-use applications, the priority should be proper sealing. For racing applications that spend the majority of their time above 5,000 RPM, a header flange opening that matches the exhaust ports on the cylinder head may offer a few more horsepower.
Examples:
- The majority of street or street/strip headers will cover the upper portion of the port opening on D-port heads. This should not be a concern, because the typical operating RPM is between idle and <4,000 RPM.
- Sanderson Headers offer a patented raised flange design that does not use a gasket. To ensure a proper seal, it is important to choose the correct header flange on small-block Chevy applications. It is not about matching the port opening as much as ensuring there is adequate sealing material in the correct locations. A common standard-port cylinder head may not provide sufficient sealing area above the exhaust port when using the D-port flange header.
Have Questions? Please contact us.
Related Articles
Sanderson Limefire - Header Tech
The Sanderson Limefire header offers a lakes-style nostalgic look for outside chassis header needs, but is it the best header choice? Lakes-style headers replicate the header style used on '50s and '60s salt-flat racers, and the design is ...Sanderson Header Warranty
Limited 5 Year General Warranty Sanderson Street Rod Headers strives to supply our customers with the highest quality parts and service available in the street rod industry. In keeping with this, we warrant and will repair or replace, at our option ...Basic Header Installation Procedure
Besides a few common tools, patience is the most important tool for installing a set of headers. Sanderson Headers are built by hand in the USA with the finest materials. When properly installed, Sanderson Headers will provide many years of ...Exhaust Header Cleaning and Upkeep
Maintaining the Header Finish and Ensuring Longevity Exhaust headers are an investment in our vehicles. The installation of headers offers performance and efficiency gains, for a cost that is often substantial. In addition to making sure we select ...Headers and Starter Clearance
Starter clearance can pose a unique challenge for many header installations, especially when installing headers on a classic vehicle application. Just imagine how many times the starter may have been replaced on a 30+-year-old vehicle. Was the ...