MAKE YOUR JAG FUN TO DRIVE!
The newly developed RG cylinder head has been specifically designed for the Jaguar connoisseur who desires truly modern performance and everyday drivability from his classic Jaguar without violating the authenticity of his vehicle. All of the original limitations have been overcome. For 4.2 liter engines, each combustion chamber is now properly centered over each bore. For all XK engines, the ports have been raised and reshaped to provide far better flow, the compression ratio and squish area are substantially increased and, if a Twin Spark model is ordered, two plugs improve power, drivability, efficiency and emissions. These features are all contained beneath the period appearance of the original head.
Each head is chill cast in a 3D sand mold to produce an aerospace quality component. Heads are poured as Single Spark models that can be machined to make a Twin Spark model. This means that a Single Spark model can be converted to a Twin Spark model at any time.
Performance increases of 100 hp are typical from 4.2 liter engines with stock 9:1 pistons, triple SU carbs and original cast iron headers. Even more power is made possible with Webber carbs and higher flow exhaust systems. Special CP‐Carrillo® pistons permit one to install the RG cylinder head on 3.8 liter engines while raising the compression ratio to 10:1.
RG Heads come with steel billet, seven bearing high performance camshafts that provide more power without affecting idle and low RPM performance. Considering that an E‐type weighs about 1,000 pounds less than a present‐day Corvette, increasing its power by 50% puts your E‐type in the same performance category. Just think what that would feel like from the driver’s seat!
People commonly ask why we did not design a four valve cylinder head. If the original valve angle is maintained, which is critical to maintaining the aesthetics of the original head, a four valve combustion chamber would be even larger than the original hemispherical one. Many attempts were made to make four valve designs for classic era engines – all of which failed. Even if we could overcome this obstacle, the XK engine’s extra‐long stroke limits high RPM, which prohibits it from enjoying a substantial benefit from a four valve design.
JAGUAR XK6 3.8 LITER CYLINDER HEAD UPGRADE KIT:
This kit is designed to unleash all of the potential from a 3.8 Jaguar XK6 engine for street performance driving. The 3.8 liter XK engines require higher domes on their pistons to achieve a given compression ratio as compared to the 4.2 liter engines. This makes them prime candidates for Twin Spark ignition because the higher dome acts as an obstruction to the flame front during combustion. Although RG CAN build the 3.8 liter head as a Single Spark model, we don’t recommend it because it is especially prone to pinging at 10:1 compression. Bear in mind that the highest compression that Jaguar offered was only 8.6:1 that they called, “9:1”.
Adding Twin Spark ignition reduces spark advance by 9 degrees allowing higher compression ratios when running on street grade fuel with a much reduced risk of pinging. CP-Carrillo® has developed a custom piston that adapts the RG cylinder head to the 3.8 liter bore spacing. This piston is universal to all bores – irrespective of bore offset. Power increases of 100hp have been documented during dyno 2 testing over the stock 3.8 liter engine with S.U. carburetion and cast iron exhaust manifolds and other configurations using popular induction and exhaust components.
- A fully assembled RG Twin Spark Performance cylinder head
- A set of forged CP‐Carrillo® 10:1 compression pistons
- A complete Twin Spark Ignition System
- Optional CP‐Carrillo® forged connecting rod set
JAGUAR XK6 4.2 LITER CYLINDER HEAD:
The RG 4.2 liter cylinder head can be built as a Single Spark or Twin Spark model. For those who want to maintain the original appearance of their engine compartments, RG can build your head with only six spark plugs. This will eliminate the added expense of the Twin Spark ignition system. A Single Spark head can be upgraded to a Twin Spark by machining the 2nd set of plug wells at a later date. A Single Spark head will require 9 degrees more spark advance, which may present pinging issues when used with moderate or low grade fuels due to the increase in compression ratio from 8.6:1 (what Jaguar called “9:1”) to 10:1. There will also be a slight loss in power, fuel consumption, drivability and emissions as compared with the Twin Spark configuration.
The Twin Spark model of the RG 4.2 liter cylinder head is designed to unleash all of the potential from a 4.2 Jaguar XK6 engine for street performance driving. Adding Twin Spark ignition reduces spark advance by 9 degrees allowing higher compression ratios when running on street grade fuel. Power increases of over 100hp have been documented during dyno testing with S.U. carburetion and cast iron exhaust manifolds and other configurations using popular induction and exhaust components. Click here to see actual dyno test results on our blueprinted 4.2 liter engine.
- A fully assembled RG Single or Twin Spark Street Performance cylinder head
- An optional complete Twin Spark Ignition System
- An optional CP‐Carrillo® high performance connecting rods & pistons
SINGLE SPARK HP
TWIN SPARK HP
OTHER RELATED ACCESSORY PARTS
JAGUAR XK6 RACING HEAD CASTING:
For those who wish to configure their own XK cylinder head for special purpose racing, RG offers raw cylinder head castings, which are machined to provide the basics for assembly. This permits one to develop custom porting configurations and to use custom valves. RG also sells camshaft blanks that can be custom ground to any profile. The RG cylinder head can support valve lift up to 0.600”.
RG 7 BEARING CAM COVERS:
RG offers cam covers that utilize all studs on the RG head’s seven cam bearing caps, as it has three more cam bearings than the standard XK head on each cam. These are designed to replace the Series I style cam covers that were polished aluminum. Utilizing all studs spreads the load over more points reducing point loads that cause cracking and also making oil sealing much improved.
TUBULAR HEADER OPTIONS:
RG offers a tubular header called a “Long Tube Header” (LTH). Tubular headers improve the engine’s breathing by confining each cylinder’s exhaust gasses to a singular tube that actually sucks the air out of the engine compared with a cast iron manifold that has very short primary passages. RG offers tubular headers because the exhaust ports on its heads are slightly higher than the original Jaguar head and because their shape is different. Although you can use a factory cast iron or an aftermarket header with the RG cylinder head, those will not be an exact match in terms of flange opening shape or performance. The RG exhaust components are tested with the RG cylinder head to optimize the torque curve for street driven cars. RG headers and standard downpipes are fitted with “bungs” for oxygen sensors.
HIGH PERFORMANCE EXHAUST:
RG has dyno engineered a 2‐1/4” stainless steel exhaust system that augments the performance of the RG XK6 Cylinder head – especially when tubular headers are used. If tubular headers are ordered, the system has adaptors that link them to the main silencer. If one is using the cast iron Jaguar exhaust manifolds, the system comes with downpipes that are properly sized to the application (i.e. 3.8 or 4.2 liter).
The RG HP exhaust system is unique, as it has a single chambered main silencer. RG’s dyno testing has found that this produces a better overall torque curve when used with the RG cylinder head. It also has four 1‐3/4” intermediate pipes leading to two twin pipe resonators with twin tips. This is similar to how a Ferrari exhaust system would be designed. The primary purpose of going from two inlets to four outlets has to do with getting sufficient clearance under the IRS while flowing sufficient exhaust gas. RG’s systems are complete with all brackets, hardware and fittings required to hang it on an E‐type.