26/12/2000
Building a Cosworth YB Cylinder Head: A Comprehensive Guide
The legendary Cosworth YB engine, a powerhouse found in iconic Ford Sierra RS Cosworth and Escort RS Cosworth models, remains a sought-after powerplant for performance enthusiasts. At the heart of any YB build lies the cylinder head, a critical component that dictates airflow, combustion efficiency, and ultimately, the engine's power output. Whether you're aiming for a spirited fast road car or a full-blown motorsport machine, understanding how to build and tune a YB cylinder head is paramount. This guide, drawing on the expertise of specialists like Smith & Jones Engineering, will delve into the intricacies of YB cylinder head preparation and modification, covering everything from basic upgrades to achieving significant horsepower figures.
Smith & Jones Engineering: Masters of the YB
Smith & Jones Engineering (S&J) have carved a niche for themselves in the world of YB performance, specialising in the manufacture of Cosworth YB cylinder heads and cylinder blocks. Their commitment to excellence is evident in their use of proprietary motorsport castings, sourced from the esteemed Grainger & Worrall foundry, ensuring the supply of first-class aluminium and premium iron castings. By working closely with top YB engine builders and engaging in in-house product development and engine testing, S&J consistently deliver products that are renowned for their strength, reliability, and proven performance. They offer a comprehensive range of supporting engine parts, catering to various build aspirations, from Big-power Turbo Cosworths to Historic or Naturally-Aspirated YBs.
Understanding the Standard YB Cylinder Head
The standard Cosworth YB cylinder head is a remarkably capable unit, a testament to Cosworth's engineering prowess. Even in its stock form, it can comfortably support up to 350bhp without any major modifications. This makes it an excellent starting point for many builds. However, as you push the boundaries of power, modifications become essential to optimise airflow and prevent bottlenecks.
Cylinder Head Tuning: A Step-by-Step Approach
The journey to a higher-horsepower YB cylinder head involves a series of carefully considered modifications. The following outlines a progression based on desired power outputs:
350-500bhp Modifications:
At this power level, the standard valves can often remain in place. The primary focus shifts to improving airflow through the ports. Enlarging the intake ports to 22.5mm and the exhaust ports to 23mm is a common and effective modification. This allows for a greater volume of air to enter and exit the combustion chamber, supporting increased power.
500bhp+ Modifications:
To achieve power figures exceeding 500bhp, further porting is required. The intake ports should be opened to 25mm, and the exhaust ports to 24mm. This level of porting is often paired with specific camshaft profiles to maximise efficiency. A popular combination includes a BD16 inlet cam and a BD15 exhaust cam, or a BD15 inlet with a BD14 exhaust. While the valves can still remain standard at this stage, their flow potential will eventually become a limiting factor.
600bhp+ Modifications:
Breaching the 600bhp barrier necessitates more aggressive cylinder head work. The intake ports need to be enlarged to 26.5mm, and the exhaust ports to 25.5mm. Crucially, at this power level, the standard valves will begin to restrict flow. Therefore, fitting larger valves becomes essential. This, in turn, often requires further machining of the head to accommodate bigger valves and the more aggressive camshaft profiles that are typically required to complement them.
Camshaft Selection: The Heartbeat of Performance
Camshafts play a pivotal role in determining an engine's powerband and character. For the YB, the range of available camshafts is extensive, catering to diverse applications:
- Fast Road Use (350-450bhp): A popular choice for this power range is a BD10 exhaust camshaft paired with a BD14 inlet camshaft.
- Higher Performance (450-500bhp): For those seeking a bit more, a BD14 exhaust and BD16 inlet camshaft combination works exceptionally well.
- Custom Profiles: For bespoke tuning requirements, companies like SCS offer custom camshaft profiles, often at a cost of around 560GBP, allowing for precise tailoring to specific engine builds.
Supporting Components: A Holistic Approach
While the cylinder head is a focal point, a truly potent YB engine requires attention to all its constituent parts. Here's a look at other key areas:
Crankshaft: The Foundation of Power
The standard Cosworth YB crankshaft is a robust unit, capable of supporting up to 500bhp without issue. They are also known for their excellent balancing. However, it's worth noting that crankshafts that have been ground are generally less strong. Original condition cranks can fetch around 500GBP. For those pushing beyond 500bhp, upgrading to a steel crankshaft from reputable manufacturers like Arrow Precision or Farndon is a wise investment.
Connecting Rods: Strength Under Pressure
The original YB connecting rods are made from cast steel and are remarkably strong. A minor drawback can be their slightly shorter length, which can increase lateral forces against the cylinder walls. Using longer rods with shorter pistons can alleviate this, potentially yielding an extra 8-10bhp. Conversions for this can be carried out by specialists for around 750GBP.
Pistons: Forging Ahead
Standard Mahle pistons are generally suitable up to 400bhp, with some tuners suggesting they can handle up to 500bhp. However, they can become a weak point when nitrous is employed or in high-revving engines. Upgrading to forged pistons from leading manufacturers is a common practice. Given that forged replacements typically cost around 600GBP, it often makes little sense to replace standard pistons with new standard items when forging is an option.
Compression Ratios: The Balancing Act
Compression ratios are a subject of varied opinion. For a 450-500bhp build, a compression ratio of 7.2:1 is considered a safe and reliable choice. For builds up to 350bhp, an 8:1 ratio is generally acceptable.
Engine Block: The Mighty Core
The 2WD Cosworths typically featured the 205 block, while the 200 block is considered the stronger variant. The 200 block also offers the advantage of accepting longer head studs more readily. While fitting longer studs to the 205 block can be inconsistent due to casting variations, machining work for longer studs is available from specialists for approximately 300GBP. For power outputs up to 500bhp, six studs are usually sufficient. However, for 500bhp+, the use of ten longer studs is highly recommended. In some high-performance applications, up to 700bhp can be achieved with six studs, provided the outer studs on the exhaust side are extended by 5mm. Ten-stud configurations are often paired with Nikasil coated steel liners, a modification typically reserved for serious motorsport applications.
Liners: Enhancing Durability
Liners, particularly Nikasil coated steel liners, offer increased strength and allow for higher boost levels. However, they require different piston rings. A 10-stud and Nikasil wet liner conversion can cost in the region of 2000GBP, depending on the specific requirements.
Turbocharging: The Breath of Life
The standard T3 turbocharger is adequate for up to 300bhp. For increased power, upgrades are essential:
- GT30 Turbo: Capable of supporting around 450bhp.
- GT35 Turbo: Suitable for 600bhp and beyond.
It's important to note that GT series turbos typically require an external wastegate. A 44mm external wastegate will cost approximately 350-400GBP, and modifications to the manifold and downpipe will be necessary for fitment.
Engine Management and Injectors: The Brains and Fuel
Engine management systems offer a vast array of options, and the best advice is to consult with your chosen tuner for a recommendation tailored to your build. For fuel delivery, Bosch injectors are a popular choice:
- Dark Greens/803s: Suitable for 300-330bhp, costing around 70GBP each.
- Light Blues/400s: For power outputs up to 400bhp.
- Greys/403s: Recommended for 400-450bhp.
For those aiming for over 450bhp, an eight-injector setup is a viable option, offering the benefits of sequential injection. Eight Green injectors can typically support up to approximately 500bhp. Siemens has also introduced 'Black' injectors, which can negate the need for an eight-injector setup and fit directly into the original fuel rail.
Sump Modifications: Reliability for the Long Haul
The oil sump, often overlooked, plays a crucial role in engine reliability, especially under track conditions or when fitted with wider, grippier tyres. Two primary solutions exist: a dry sump system, which can be costly, or baffling the original sump with a trap-door style baffle. Julian Godfrey Engineering offers sump baffling services for around 250GBP.
Frequently Asked Questions
How much bhp can a Cosworth YB crank support?
The standard Cosworth YB crankshaft can reliably support up to 500bhp. For higher power outputs, upgrading to a steel crankshaft is recommended.
What are the key modifications for a 500bhp YB cylinder head?
For 500bhp+, you'll typically need to enlarge the intake ports to 25mm and exhaust ports to 24mm, and consider pairing this with suitable camshafts like a BD16 inlet and BD15 exhaust.
What is the difference between the 205 and 200 YB blocks?
The 200 block is generally considered stronger and offers better compatibility with longer head studs compared to the 205 block.
Building a Cosworth YB cylinder head is a rewarding process that, with careful planning and execution, can transform your engine into a formidable performance unit. By understanding the capabilities of the standard components and the benefits of targeted modifications, you can create a YB that meets your specific power and reliability goals.
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