Profiles critical to performance
There is a conventional wisdom that has developed around intake valve configuration: cylinder heads equipped with ports inclined at narrow valve angle to the valve stem should have conical, or ‘tuliped’ back faces, and ports that have significant bend at the intake valve should have shallow back angles i.e. the classic ‘nail head’ shape present in so many large pushrod engines. While this is true much of the time, there are exceptions, and frequently the desired point of improvement in the power curve can be as much a factor in the decision as the port configuration.
This became abundantly clear to the team that is performing the ongoing work on the Anhared Design Dual Swirl cylinder head (see RET issue 013 March/April 2006; 015 – June 2006 and; issue 025 – September/October 2007). At first blush, this looks like a classic tulip valve application: the ports are almost perfectly straight, and are angled only 30 degrees from the valve stem centreline. Despite this, flow testing on the cylinder head showed only a slight advantage with tuliped valves at the largest openings and flow rates, while nailhead-type valves significantly outperformed their conical counterpoints at low-to-moderate valve openings and rpm. This was confirmed in dyno tests, where the engine maintained a smooth idle at 800 rpm, as well as being able to accept full load at full throttle from 3000 rpm all the way to over 11,300…at 179 bhp/litre.Even if there is a reasonably close port/valve angle as on the classic Lotus twincam, for example, a heavily tuliped valve may not be the best flow solution. This is because with the Lotus, the intake port intersects the valve stem measurably above the valve face; the designer bent the port just before the valve in order to create a very short port segment normal to the valve stem in an attempt to provide annular flow. The problem is that this segment is far too short to restore the fluid stream normal to the valve stem. The result is that on the Lotus, the ‘near side’ of the port is not as active as it might be at low rpm, and almost all of the flow under these conditions must be directed to the ‘far side’ of the valve. Under these conditions, the nailhead or flat back faced valve is superior; with less obstruction than a tuliped valve, the nailhead performs this flow deflection more efficiently, and produces a higher velocity zone in the portion of the valve annulus that is receiving flow.
With either style valve, the near side of the port is relatively inactive, as far as gas flow is concerned, which makes it an ideal point of entry for residual backflow gases during valve overlap, but at least maximum gas flow energy is being retained for that part of the port that remains active, and as long as the cams are not too aggressive, good torque can be maintained. For ultimate power, the Lotus ‘twink’ will benefit from a tuliped valve at high rpm, when the combination of high gas speed and ramming density eliminates the ‘dead zone’ on the near side…but this comes at the expense of midrange torque. The choice of valve shape is therefore best determined by where the power is needed.Ultimately, the engine builder must first decide whether it is better to redirect airflow, or deflect airflow. Properly applied, the tuliped valve redirects airflow to the inside radius of the port most efficiently, as demonstrated by virtually all modern performance DOHC engines. When used correctly, the flat-backed valve deflects airflow to one side of the valve opening, in order to use that part as effectively as possible; frequently this is the only strategy possible with horizontal or near-horizontal port engines.So back to the Dual Swirl engine: this is a case of a modern, low angle cylinder head…why did the ‘nailhead’ valve perform better here? The answer is found in the unusual configuration of the porting: two ports supply each valve from equal and opposite positions, offset from the valve stem. Because of this juxtaposition of the ports, there is no need to redirect airflow to ‘activate’ the near side, as this is accomplished by the opposite port, so there is no benefit with a tuliped valve in this application.