For Jaguar enthusiasts looking to enhance the performance of their classic Xj6 or E-Type, cylinder head modifications are a key area to consider. Differences between cylinder heads from various Jaguar models, particularly those from fuel-injected engines (1978-1987) compared to earlier Series 2 XKE heads, offer both opportunities and challenges for upgrades. This expert insight, originating from Dick Maury of Coventry West, Inc., a renowned Jaguar rebuild specialist, delves into the nuances of XJ6 cylinder heads and how to optimize them for improved performance while maintaining compatibility.
Key Differences Between XJ6 Cylinder Heads
Jaguar cylinder heads evolved over time, with notable changes introduced in the fuel-injected models. Understanding these distinctions is crucial for any upgrade project.
Valve and Port Size Enhancements
The fuel-injected cylinder heads, produced from 1978 to 1987, boast larger valves and slightly enlarged inlet ports compared to the Series 2 XKE heads. This increased size allows for improved airflow into the combustion chamber, a critical factor in engine performance. However, dimensionally, they remain largely similar to the Series 2 XKE heads, making them a potential upgrade option.
Coolant Passage Variations and Compatibility
A key difference lies in the rear of the fuel-injected heads. They are slightly longer to accommodate two additional coolant holes. When retrofitting these heads onto a Series 1 XKE, these extra coolant holes must be plugged. Failure to do so will result in the head overhanging the engine block, potentially causing leaks and other issues.
Camshaft Specifications and Evolution
While the camshafts in later models maintain the same lift and duration, Jaguar introduced quieter lobe designs in 1969. These later camshafts, identifiable by their 4-bolt front fastening, were implemented in Series 2 XKE cars. According to Coventry West’s expertise, there is no significant performance difference between these camshaft variants when valves are correctly adjusted to specification for each cam type.
Air Injection Port Considerations
Cylinder heads manufactured between 1978 and 1981 offer a further variation: they lack air injection holes. This absence simplifies installation in applications where air injection is not required, eliminating the need to plug these ports, as might be necessary with other later heads.
Upgrading Your XJ6 Cylinder Head: Options and Considerations
Several paths exist for upgrading XJ6 cylinder heads, each with its own set of advantages and considerations.
Utilizing Later Fuel-Injected Heads
Employing the later fuel-injected heads presents a direct route to increased valve and port size. However, as highlighted, compatibility with earlier models like the Series 1 XKE necessitates plugging the additional coolant holes. For those seeking readily available performance gains, these heads offer a viable starting point, provided these modifications are correctly addressed.
Modifying Original Heads for Enhanced Performance
For purists aiming to maintain the originality of a matching numbers XKE, modifying the original cylinder head is a compelling alternative. This approach involves installing larger valve seats and valves and porting the head to optimize airflow. The external appearance remains unchanged, preserving the car’s original aesthetic, while internally, the head benefits from performance enhancements comparable to the later fuel-injected versions.
Distributor and Carburetor Adjustments Post-Upgrade
When considering cylinder head upgrades, the distributor often requires no modifications, particularly when retaining stock camshafts. However, carburetor needles might need fine-tuning. Coventry West advises caution against unnecessarily rich needle setups, as these can paradoxically reduce engine power. Gas analyzer testing under load is recommended to ensure optimal air-fuel ratios after modifications.
Manifold Compatibility: A Critical Check
A crucial warning from Coventry West concerns the compatibility of intake manifolds. Triple SU manifolds are slightly longer at the front compared to fuel injection manifolds. Therefore, it is essential to verify that the chosen manifold sits flush against the modified or upgraded head. Minor grinding may be required to achieve a proper fit, a lesson learned firsthand by experienced Jaguar technicians.
Conclusion: Balancing Performance and Originality in XJ6 Cylinder Head Upgrades
Upgrading Jaguar XJ6 cylinder heads offers tangible performance improvements, primarily through enhanced airflow facilitated by larger valves and ports. Whether opting for later fuel-injected heads or modifying original units, careful attention to compatibility, particularly coolant passages and manifold fitment, is paramount. Consulting with Jaguar engine specialists like Coventry West ensures that upgrades are performed correctly, maximizing performance gains while respecting the unique characteristics of these classic engines. For those seeking truly exceptional performance, further modifications like extensive porting and competition camshafts, as utilized in Coventry West’s race engines, represent the pinnacle of XJ6 engine tuning.
