One of the most common modifications performed during a high-performance engine rebuild is a three-angle valve job. Most hobbyists know that the task consists of cutting multiple angles into the cylinder heads' valve seats and that it's intended to improve performance. But how is the task performed and just how much of an airflow affect does a performance valve job have on a typical cylinder head? Follow along as we explore exactly that!
Valve-Seat Basics
The opposing contact points of a valve and its seat are machined at nearly the same angle. As the valve comes to rest on its seat and the two surfaces contact one another, it results in a positive seal that prevents cylinder pressure from escaping during the compression and power strokes. The general wear that occurs during normal operation can compromise performance over time, and simply remachining the valve seat and/or valve can restore it.
A three-angle valve job has been a popular way to improve performance for many years. It consists of additional cuts above and below the seat angle. The additional angles improve the transition from the intake port into the combustion chamber, and from the combustion chamber into the exhaust port, allowing air to pass over the valve seat with less turbulence. When properly executed, the effort can produce a noticeable airflow improvement at every lift point.
A ’70 455 No. 15 cylinder...
A ’70 455 No. 15 cylinder head we had left over from a previous project proved the perfect subject for a performance valve job. Even though it uses standard-performance 1.96/1.66-inch valves, its existing 45-degree valve seats were completely original and hadn’t ever been modified.
In days past, the task involved using individual grinding stones to cut specific angles into a valve seat. Each cut was performed manually, which made it costly from a labor standpoint, and created a margin for error and the potential for inconsistency. Modern valve seat machining equipment like that from Serdi has greatly improved the productivity and consistency of high-performance valve jobs—its single carbide bit can create several seat angles in a single pass.
While some machine shops still use grinding stones and can produce excellent results, better machine shops tend to use modern equipment. That has made performance valve jobs much more commonplace in typical rebuilds, as well as more affordable for consumers.
Pontiac Valve-Seat Angles
Most regular-production Pontiac engines produced during the '60s utilized 1.92/1.66-inch-diameter intake and exhaust valves. Beginning in '67, engineers rolled the valve angle of its cylinder heads by 6 degrees, and increased the intake- and exhaust-valve diameters in performance applications to 2.11/1.77-inches, respectively. Standard-performance engines retained the 1.66-inch exhaust valve, but the intake valve was enlarged to 1.96 inches in 1968.
Chuck Willard of Willard Auto...
Chuck Willard of Willard Auto Machine (WAM) in Omaha, Nebraska, was enlisted to perform the performance valve job for us. Using his Serdi 3.0 valve seat grinder and a specific carbide-cutting bit, he cut three fresh angles into the valve seat in a single pass. He routinely charges $225 for the service, and it can include minor hand blending into the seat, if required to further improve airflow.
Pontiac's cylinder heads were originally equipped with a single-angle seat, and two distinct valve seat angles were commonly used—30 and 45 degrees. Generally speaking, a 30-degree seat tends to favor low-valve-lift airflow when compared to a 45-degree seat, while a 45-degree seat tends to improve airflow at high valve lift when compared to a 30-degree seat.
Knowing that the large 2.11-inch intake valve would provide relatively good high-valve-lift airflow, Pontiac used a 30-degree valve seat to improve airflow at low lift and maintain excellent port velocity in all of its production engines, except for the Super-Duty 455. By design, smaller-diameter valves tend to promote better off-the-seat flow. Pontiac used a 45-degree seat for its exhaust valves, as well as with its 1.96-inch intake valve to maintain good high-lift flow, except for a short period in the late '60s when it used 30-degree seats with the 1.96-inch intake valve in an attempt to improve emissions.
The Valve Job
Most every aftermarket Pontiac cylinder head and any highly modified cast-iron original prepared by a reputable Pontiac builder will be equipped with multiple-angle valve seats. The exact seat angle that's best suited for a particular cylinder head is dependant upon combustion chamber and port shape, valve diameter, and intended airflow capacity of the casting, and the seat angle can be more or less than what was originally used by Pontiac.
When rebuilding a cast-iron Pontiac cylinder head for a high-performance application where stock-type components will be used, there may not be much to gain by varying from the original seat angle used by Pontiac. There is, however, a distinct airflow advantage with the addition of a performance valve job, and it's one that we set out to quantify.