The Ford CVH Engine: A Deep Dive into Tuning

For enthusiasts and mechanics alike, the Ford CVH engine holds a unique place in automotive history. Renowned for its presence in iconic models such as the Escort, Fiesta, and Orion, this 'Compound Valve angle Hemispherical chamber' powerplant became a staple of British motoring. While often associated with its tuning potential, understanding the intricate details of its design, common pitfalls, and specific modifications is paramount for both its maintenance and unlocking its true performance. This comprehensive guide delves into the core components and characteristics of the CVH, providing insights akin to those found in a detailed repair and tuning manual, essential for anyone looking to keep these engines running optimally or push them to their limits.

The CVH engine, introduced by Ford in 1980, was initially conceived as a potential successor to the venerable Kent Crossflow. It was produced in various capacities for the UK market, including 1.3 litres (1,296cc), 1.4 litres (1,392cc), and the highly popular 1.6 litres (1,597cc). A lesser-known 1.1-litre version was sold overseas, and a 1.8-litre 'tall block' variant was built in the USA, which some specialists import for 'big' CVH builds. While the 1.8-litre engine exists, it is generally not considered suitable for tuning due to its cylinder head design, which prioritises swirl and economy over outright power, and its distinct roller follower camshaft.

Why the 1.6L CVH Remains a Tuner's Favourite

The 1600cc CVH engine stands out as a particular favourite for tuning, and for good reason. Its inherent simplicity in design and relatively light weight, especially when compared to older Ford engines like the Pinto and Crossflow, make it an appealing platform. Crucially, the cylinder head of the 1.6L CVH boasts a remarkable capability for high flow figures when correctly modified. This potential for significant power output, combined with its straightforward architecture, contributes to its enduring popularity among performance enthusiasts. However, like any engine, it has its Achilles' heel, primarily residing in its valvetrain design.

The Notorious Cam Wear Problem

One of the most widely discussed and common issues with the CVH engine is its tendency for cam wear. This isn't an urban myth; it's a fundamental design characteristic. The CVH camshaft relies on a limited oil supply, dripping through small brass bushes near the rockers. This already restricted lubrication can be further compromised if these bushes become clogged with carbon, often due to infrequent oil changes or the use of sub-standard engine oil. Adding to this challenge is the exceptionally stiff valve springs, which generate a load of 220 lbs at full lift – significantly higher than many other engine designs (e.g., 125 lbs for a Pinto). This high spring load is necessary to control the heavy rocker and lifter assembly, yet it still limits valve float to around 6,700 rpm. To compound matters, Ford didn't utilise the most durable materials for the camshafts, opting for induction-hardened material over the more resilient 'chill cast' alternative.

Consequently, cam wear is an unavoidable reality for the CVH. Unlike many engines where wear is a sign of impending failure, standard CVH cams exhibit steady wear from the moment they are installed. It's rare to find a medium-mileage CVH road engine without considerable wear on its lobes and lifters, with cylinder 4 often being the worst affected. This isn't due to oil pump distance, but rather a design flaw where the cam lobes on cylinder 4 are improperly positioned relative to the lifter bores.

When cam wear occurs, it typically starts halfway up the lobe, leading to dishing of the lifter base and material loss over the cam nose. If the lifters dish significantly, a sharp lip can form, preventing them from being withdrawn through the lifter bore. In such cases, never force them out with pliers, as this can score the lifter bores and compromise oil pressure. Instead, raise the lifters clear of the cam lobes, remove the camshaft, then push the lifters into the head and extract them via the distributor housing bore. Always check every lifter bore for wear, as heavily worn bores can render the cylinder head scrap.

It is absolutely critical to understand that fitting new lifters to a worn camshaft is futile; the new lifters will be destroyed rapidly. The proper solution involves replacing both the camshaft and all lifters. Furthermore, ensure the brass lubrication bushes are thoroughly cleaned, and if the head is severely carbon-clogged, chemical cleaning is advised to clear all oilways. When fitting new components, use ample cam lube on the lobes and lifter bases, and avoid idling the engine for the first 15-20 minutes after startup, maintaining around 2000 rpm to allow the cam to bed in and work harden. Always change the oil and filter, and consider flushing the engine to remove any worn material from the sump that could otherwise damage crankshaft bearings and the oil pump.

Cylinder Head Variations and Tuning Potential

The CVH engine's cylinder head is a cornerstone of its performance potential, but understanding its various iterations is key. The engine broadly splits into two types: pre-1986 and post-1986 designs. It's important to note that EFi (Electronic Fuel Injection) engines largely retained the hemi design, while carburettor engines transitioned to lean-burn after 1986.

Pre-1986 Hemi Head (81SM6090)

Early CVH engines feature a hemispherical combustion chamber, which is the origin of the 'CVH' name itself – 'Compound Valve angle Hemispherical chamber'. This head design, when properly modified, can flow exceptionally well, even surpassing the flow of a big valve Pinto engine. However, achieving optimal flow requires precise reshaping of the ports, a skill often lacking in less experienced tuners. There are two variants of this head: the standard casting with a plain, flat external finish, suitable for standard or 43.5mm valves, and the rarer cross-hatch casting, identifiable by a ribbed pattern on its external sides. The cross-hatch variant boasts thicker port walls and superior port shapes, making it the only suitable choice for 45mm valves.

Post-1986 Lean Burn Head (86SM6090)

The post-1986 lean-burn engine incorporated 'heart'-shaped combustion chambers, akin to those found in a Mini engine, along with different pistons and a slightly altered oil pump design. While fundamentally similar to the earlier engines, the lean-burn head suffers from some valve shrouding by the chamber walls, resulting in slightly lower flow potential compared to the hemi head. A tuned road engine with a lean-burn head might produce around 5 bhp less than an equivalent hemi engine. Modifying this head for good flow requires more extensive metal removal in both the chamber and ports, making it a more time-consuming and costly process. For critical high-power applications, converting late engines to a hemi design is often preferred if suitable cylinder heads are available.

EFi Hemi Head (89SM6090)

The EFi (Bosch L) hemi head, found in models like the Fiesta XR2i and XR3i from 1989 onwards, also features injector cutouts in the inlet ports, making them unsuitable for DCOE manifolds without welding. Despite this, these heads are excellent for big valves due to their very thick port walls and superb port shapes, offering significant flow potential.

Valve Selection and Porting

The CVH engine responds exceptionally well to larger inlet valves. While standard heads come with 42mm inlet valves, true performance gains are seen with 43.5mm and 45mm options. It's crucial to distinguish genuine 'big valves' from smaller, less effective alternatives (e.g., 42.6mm or 43mm). However, fitting 45mm valves is challenging, as it often risks breaking into a waterway unless specific castings are used, such as the 89SM6090 XR2i heads or rare cross-hatch 81SM6090 variants.

Conversely, the standard 37mm exhaust valves are already oversized for a 1600cc engine. Under no circumstances should larger exhaust valves be fitted; any tuning firm recommending this demonstrates a lack of understanding of proper CVH modification.

Achieving the CVH head's full flow potential absolutely requires specific port modifications. The area around the valve guide, where the port bends, is critical and must be straightened and shaped to blend smoothly into the valve throat. This necessitates the removal of the valve guides, meaning any head that has merely been polished without constructive port work will yield minimal improvement. Every properly modified head should have the guides removed first to allow for correct port shaping.

Unleaded Fuel Compatibility

All UK cars sold after 1988 were designed to run on unleaded fuel, meaning 88SM and later heads incorporated specific seat inserts for this purpose. However, the inserts in earlier CVH heads, like many aluminium cylinder-headed engines, have proven quite robust on unleaded fuel for road use. Despite many tuners offering expensive 'unleaded conversions', these are often unnecessary for road cars. For race applications, where extreme conditions are met, changing exhaust seat inserts on pre-1988 heads might be advisable, although many pre-1988 heads have also performed faultlessly in race environments.

Crankshafts, Blocks, and Capacity Increases

Beyond cylinder head work, increasing engine capacity is a popular, albeit complex, avenue for CVH tuning. A common conversion for the 1600cc unit is to enlarge it to 1905cc, utilising crankshafts and connecting rods from the 1800 CVH engine in conjunction with special pistons. This is far from a simple 'drop-in' modification.

While some tuners market this 1905cc conversion with a standard head and cam as a 130 bhp engine, real-world figures typically hover around 105 to 110 bhp. This illustrates a crucial point: simply increasing capacity yields relatively little power gain unless accompanied by proportional improvements in cylinder head flow and camshaft lift. Achieving a mere 10-15 horsepower increase for a £2,000 engine makes it a costly upgrade, especially when similar power can be achieved more economically through dedicated head and cam work on a standard 1600cc unit. Such engines also tend to produce peak power at a low 5,000 rpm, leading to a less engaging driving experience despite improved mid-range torque.

For a truly potent large-capacity CVH, high-flowing heads, aggressive camshafts, and ample carburetion are essential. A well-built 1905cc engine should realistically achieve around 150 genuine bhp. Specialist builders can offer further capacity increases up to 2.1 litres, though these 'plus-2-litre' engines incur substantial build costs, often exceeding £4,000. It's also worth noting that Ford produced several block variations over the years, and selecting the correct type is critical for optimal performance and durability, particularly given the limited metal between the bores and water jacket on the 1.6L block when boring out.

Fitting the Oil Pump Correctly

Despite the CVH's relatively simple assembly, one common error that leads to premature engine failure is incorrect oil pump fitting. The oil pump must be centrally aligned on the crankshaft nose and should never be forced to align with the sump flange. Forcing the pump can take up all the clearance in the pump gears, leading to their destruction by crankshaft vibration and potential casing rupture. The correct procedure involves fitting the pump loosely, then gently moving it side-to-side and up-and-down to feel the free play between the pump and crank. Once centred as accurately as possible, the bolts can be nipped up. Any minor misalignment with the sump flange can be effectively sealed with silicone on the sump gasket. Incorrect oil pump fitting is a leading cause of failure soon after an engine rebuild.

Power Outputs and Performance Tiers

Understanding the realistic power outputs of the CVH engine, both in standard and modified forms, is crucial for setting expectations. Ford's claimed figures often proved optimistic, with a healthy standard hemi head engine typically producing around 75 bhp at the wheels, equating to roughly 90-92 bhp at the flywheel. The leanburn engine, with its smaller carburettor and less efficient head, generally yields a couple of bhp less.

Significant gains are achievable through focused modifications:

• A properly modified standard valve size head combined with a Stage 1 camshaft can see a 1600cc engine produce around 115 flywheel bhp (98 bhp at the wheels) even with the standard carburettor.
• Upgrading to a pair of Weber DCOEs and a Stage 2 camshaft can push figures to an easily attainable 130 flywheel bhp (110 wheel bhp).
• In 1700cc form, with a modified head, CVH33 cam, and DCOEs, 140 bhp plus is a solid target.
• Adding a big valve head to any of these specifications can yield an additional 10 bhp, though this may necessitate valve cutouts in the pistons, making it an ideal modification to plan alongside a full engine build.
• At serious power levels, a high-quality exhaust manifold and system can add another 5 bhp or more, particularly on larger capacity engines. Beware of excessively large pipe diameters, which can actually hinder power.
• The standard air filter casing is restrictive; a K&N or similar aftermarket filter can provide a few extra bhp, though caution is advised in winter as they can cause carburettor icing due to the lack of warm air pickup.

Ultimate power from larger engines with big valve heads and throttle body fuel injection can exceed 170 bhp, rivaling even Cosworth performance. The following table provides a clear overview of achievable power figures, based on the hemi engine (deduct 5 bhp for a lean burn in the same state of tune):

It's vital to be realistic about power figures from rolling roads. Transmission losses are typically around 15% of the flywheel figure. A wheel bhp reading of 90, for instance, does not equate to 150 flywheel bhp, despite what some might claim. Always request the wheel figure and then divide by 0.85 for a more accurate approximation of flywheel bhp. Be wary of rolling road printouts that show inflated flywheel figures based on unrealistic transmission loss estimations.

Injection Engines: Bosch K and Bosch L Types

Ford introduced the CVH with two primary types of injection systems: the early Bosch K-Jetronic and the later Bosch L-Type EFi.

Bosch K-Jetronic Engines (Early XR3i)

Found in the original XR3i, the Bosch K-Jetronic is a mechanical injection system where injectors continuously flow fuel. The base engine components (head, cam, compression ratio) are identical to the carburettor version. Ford claimed a 9 bhp increase to 105 PS (103 BHP) solely from this system, but this was often optimistic. A good XR3i typically produces around 80 bhp at the wheels, about 5 bhp more than a carburettor engine, putting flywheel figures in the high 90s. While the K-Jetronic system won't match the power of twin sidedraft carburettors, tuning recommendations for other components remain similar to carburetted cars. Expect around 120-125 flywheel bhp with a properly ported standard valve head and a 274 cam.

Bosch L-Type EFi Engines (XR2i, Later Escorts)

From 1989 onwards, models like the Fiesta XR2i and injection Escorts featured a derivative of the Bosch L EFi system, which employs electronic injectors that pulse fuel only when the inlet valves are open. Ford also made improvements to the head and cam, and the compression ratio was slightly higher. The valve shape was enhanced for better flow, and the head casting included 'cutouts' in the inlet ports for the injectors. This EFi head flows considerably better than the non-EFi casting as standard, though it possesses the same ultimate flow potential when fully modified. The EFi cam also offers more lift and duration than standard XR cams, meaning performance cam upgrades might yield a smaller percentage gain compared to earlier engines.

Ford claimed 110 PS (108 BHP) for these engines, a much more realistic figure than the earlier XR3i claims. A good XR2i can show closer to 90 bhp at the wheels, indicating approximately 106 bhp at the flywheel. Power potential mirrors that of the earlier injection engine, with a genuine 120 bhp or slightly more being achievable with a good head and 274 cam. An additional 10 bhp can be gained with a big valve head. When tuning EFi engines, it's advisable not to use excessively wild cam durations, as this can confuse the airflow sensor. Alternatively, DCOEs or throttle bodies can be fitted, reverting to the standard injection system for resale.

A significant advantage of the EFi system is its ease of adjustment for engine modifications, thanks to a built-in adjustable fuel pressure regulator. By removing the cap, a 4mm allen key screw allows for fuel pressure adjustment. One full turn typically increases pressure by about 7 psi, often sufficient for most engine modifications to correct top-end fuelling. While this system raises fuel pressure across the entire RPM range, it offers a simple and cost-effective method for tuning. For ultimate performance, mappable ignition and injection with throttle bodies are the ideal route, with the injected engines providing an excellent base due to their existing high-pressure fuel pump and wiring.

It's important to remember that all CVH engines, whether a base 88 bhp Escort engine or a 108 bhp injected XR2i, possess the same ultimate power potential when brought to the same state of tune. Starting with a higher-spec engine merely means Ford has already realised some of the inherent power potential, potentially reducing the power increase per pound spent on further tuning. For an all-out build with twin DCOEs or throttle bodies, starting with a base model engine can be more cost-effective as many original higher-spec parts would be replaced anyway.

Valve Spring Systems: A Critical Upgrade

For high-performance CVH engines, especially those running high-lift camshafts, the valve spring system is paramount. Many aftermarket cam kits supply double spring systems that often fail to function correctly with standard caps and spring bases. These springs may not be properly located, leading to one spring coil-binding before the other, failing to generate the correct preload. This can result in valve bounce at lower RPMs than intended, sometimes even lower than with standard springs (e.g., 6,500 rpm).

Furthermore, many 'uprated' single valve springs sold with high-lift cams are, paradoxically, weaker than standard Ford springs. To accommodate higher lift without coil bind, these non-standard single springs are made from thinner, weaker wire, allowing greater compression but causing valve bounce long before the engine's desired rev limit. This is a common and misleading practice in the tuning industry.

The only proper solution for high-RPM CVH engines is a correctly designed double spring system. Such systems are engineered to solve these issues, enabling RPMs close to 7,500 on hydraulic lifters and even higher with solid lifters. They are typically designed for cams lifting more than 450 thou (the limit for standard valve springs) but can be used with any cam, including standard ones. A well-designed system will cope with lifts up to 550 thou, exceeding most currently available cams. Key components include machined spring caps for proper inner spring location, high-quality stem seals that resist hardening, and thick steel spring seat washers that correctly locate both inner and outer springs while setting the precise preload for high-RPM use. Investing in a quality spring system is often more cost-effective in the long run than purchasing a complete cam kit only to find it underperforms due to inadequate springs.

Fitting Instructions for Double Spring System

Before fitting, inspect the inside edge of the aluminium valve spring seat around the valve guide for burrs. Remove any burrs with a small file or scraper until the seat surface is completely flat. Position the 4mm thick seat washer with its recessed side facing upwards; this recess correctly locates the inner spring. Next, fit the stem seals, lubricating their interiors with oil and gently tapping them into place on the valve guide using a small socket or tube. Finally, fit the valve and springs in the normal manner, ensuring the inner spring seats properly into the washer recess.

Common Issues: Valve Seat Failure

One of the most common and potentially catastrophic problems associated with the CVH engine is the tendency for a valve seat to drop out of the cylinder head. This issue is particularly prevalent in the 2.0L CVH/SPI SOHC engine found in 2000-2004 Ford Focus models and also frequently observed in 1991-2002 Ford Escorts. Often, this failure occurs without warning, even in well-maintained engines. The valve seat on cylinder number 4 is most commonly affected, followed by cylinder number 2.

With factory valve seats, the typical lifespan of the 2.0L SPI in a Focus is approximately 100,000-120,000 miles, although failure can occur as early as 70,000 miles. When a valve seat detaches from the cylinder head, it falls into the cylinder, causing severe damage to both the piston and the cylinder head itself. In some instances, the detached valve seat can even be drawn through the intake manifold into another cylinder, where it is obliterated. This event can lead to scoring of the cylinder wall and bending of the piston connecting rods. When repairing an SPI engine that has experienced a dropped valve seat, it is absolutely essential to thoroughly remove and clean the intake manifold before bolting it to a new cylinder head. Any trapped particles from the failed valve seat in the manifold will inevitably enter the repaired engine, causing further damage to cylinder walls and potentially ruining the newly installed cylinder head. The most effective preventative measure against this valve seat failure is to install a new cylinder head that has been manufactured with a corrected valve seat design.

Frequently Asked Questions (FAQs)

Q1: Which Ford models commonly used the CVH engine?

The CVH engine was primarily fitted to transverse-engined Ford Escorts, Orions, and Fiestas. It was also used in a smaller number of Sierras, where it was oriented in-line, in both 1.6 and 1.8-litre capacities.

Q2: What are the main differences between pre-1986 and post-1986 CVH cylinder heads?

Pre-1986 engines typically feature a 'hemi' (hemispherical) combustion chamber, known for its excellent flow potential when modified. Post-1986 carburettor engines usually have 'lean burn' heads with heart-shaped combustion chambers, which offer slightly less flow and are more challenging to modify for high performance. EFi engines generally retained the hemi design throughout their production.

Q3: Is the 1.8L CVH engine good for tuning?

Generally, the 1.8L CVH engine is not considered suitable for tuning for power. Its cylinder head is designed for swirl and economy, not high flow, and it uses a different roller follower camshaft design compared to the smaller 1.3L, 1.4L, and 1.6L engines, making it less responsive to common performance modifications.

Q4: What is the biggest capacity a CVH engine can be built to?

While the 1.6L engine can be converted to 1905cc using 1.8L cranks and rods with special pistons, specialists can build CVH engines up to 2.1 litres. However, engines exceeding 2 litres are extremely costly to build, often requiring forged pistons and steel bottom ends.

Q5: What are the key steps to resolving the CVH cam wear problem?

The cam wear problem is best resolved by replacing both the worn camshaft and all lifters. It is crucial to clean the brass lubrication bushes and chemically clean the cylinder head if it's heavily carbon-clogged to ensure clear oilways. Proper run-in procedures, including using cam lube and avoiding extended idling, along with a fresh oil and filter change, are essential for the longevity of the new components.

Q6: Can standard CVH heads run on unleaded fuel?

Yes, for road use, early CVH heads (pre-1988) with their original inserts generally perform well on unleaded fuel. While some tuners offer 'unleaded conversions,' these are often unnecessary for typical road applications. For severe race use, changing exhaust seat inserts on pre-1988 heads might be advisable, but even then, many have run without issue.

Q7: Why is it important to fit the oil pump correctly on a CVH engine?

Incorrect oil pump fitting is a frequent cause of engine failure after a rebuild. The pump must be centrally aligned on the crankshaft nose, not forced to align with the sump flange. Forcing it can eliminate critical clearances within the pump gears, leading to their destruction by crank vibration and potential pump casing damage. Proper loose fitting, centring, and then tightening the bolts are crucial.

Q8: What is the 'black top' CVH conversion?

Beyond 300bhp, the original CVH block is prone to cracking, especially given its age. The 'black top' CVH conversion involves mating a CVH cylinder head to a later 'black top' Zetec bottom end, providing a more robust foundation for extreme power levels.

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