Unleashing the MGA: Power & Performance Guide

The MGA, with its strikingly beautiful lines and back-to-basics driving experience, stands as one of the most beloved British sports cars ever created. It ushered MG into a new era, moving beyond the T-Series with a more spacious, refined, and aerodynamically styled bodywork. Supported by a brand-new chassis, the MGA boasted a low centre of gravity, promising a dynamic ride. But how much power did these charming roadsters and coupés truly offer, and what are the possibilities for those seeking a bit more punch from their classic?

Original BMC MGA Power Figures

When the MGA first rolled off the production line, it was equipped with a robust and reliable engine from the British Motor Corporation (BMC) parts bin. Understanding these original specifications is crucial before considering any modifications.

The MGA 1500: The Beginning

The initial MGA model, introduced to replace the T-Series, was powered by a 1489cc BMC B-Series engine. This powerplant delivered a respectable 68 brake horsepower (bhp). Paired with a four-speed gearbox, also sourced from BMC, it offered a significant improvement in acceleration, top speed, handling, and refinement over its predecessors. This marked a significant step forward for MG, bringing modern performance and comfort to their sports car lineup.

The MGA Twin Cam: A High-Performance Leap

In 1958, MG truly showcased the MGA's potential with the introduction of a high-performance variant: the MGA Twin Cam. This model featured a cutting-edge 1588cc version of the B-Series engine, distinguished by an all-new aluminium alloy twin-cam cylinder head and a raised compression ratio. The result was a formidable 108 bhp. This significant power boost enabled the Twin Cam to achieve a top speed of 113 mph and a 0-60 mph time of just 9.1 seconds, outperforming even more expensive rivals like the Austin-Healey 100/6. Beyond the engine, the Twin Cam also boasted four-wheel Dunlop disc brakes, a substantial upgrade from the drum brakes found on standard MGAs, and unique Dunlop knock-off ventilated steel wheels.

The MGA 1600: Standard Power Evolution

From May 1959, the standard MGA model received an upgrade, adopting the same 1588cc block as the Twin Cam, though without the specialised twin-cam head. This enhancement increased the power output to 79 bhp. Concurrently, front disc brakes became a standard fitment, improving stopping power and safety. The MGA 1600 also gained updated side and tail lamp units with separate indicator lamps, distinguishing it visually.

The MGA Mk2: Final Production Enhancements

The final iteration, the MGA Mk2, was introduced in the spring of 1961 for its last year of production. This model saw the engine enlarged once more, to 1622cc. While a specific horsepower figure for the 1622cc engine isn't explicitly provided as a standard, this displacement increase typically implies a modest bump in power and torque over the 1600. Visually, the Mk2 is identifiable by its inset grille with vertical bars and combined indicator/tail/brake lamp units at the rear, horizontally mounted from an early Mini.

Here's a quick overview of the factory power specifications:

Boosting Your MGA's Performance: The Quest for More Horses

While the MGA's original power figures were adequate for its era, modern driving conditions and the desire for a more exhilarating experience often lead owners to seek performance upgrades. Fortunately, the MGA's mechanical simplicity and shared B-Series engine lineage with the MGB offer a wealth of tuning possibilities.

The 5-Main Bearing Engine Conversion: A Foundation for Power

One of the most significant upgrades for an MGA seeking more power and durability is the installation of a 5-main bearing engine. These engines, notably the MGB 18GB (1964-1967), offer improved crankshaft support compared to the MGA's original 3-main bearing design, allowing for higher revs and greater longevity under stress. However, this isn't a direct bolt-in procedure and requires specific components and considerations:

• Flywheel and Rear Plate: You will need the smaller flywheel and rear engine plate from the 1964-1967 MGB 18GB engine. Later 1800 MGB engines (from 1968 onwards) used larger flywheels and were mated to 4-syncro gearboxes with larger bellhousings, which are not directly compatible.
• Tachometer Drive: The 5-main engines do not feature a mechanical tachometer drive. Therefore, you'll need to source an electric tachometer, typically from a 1964-1967 MGB, to ensure your instrument panel remains functional.
• Gearbox Compatibility: While some internal parts of the MGB's 3-syncro gearbox are interchangeable with MGA mainshaft components, MGB gearboxes are not a direct bolt-in for the MGA due to differences in the rear mount. Many owners opt for a later MGB gearbox (often with overdrive) or even a Ford Sierra or Mazda MX-5 five-speed unit, which requires custom fitting.

Strategic Engine Modifications for Increased Output

Beyond the engine swap, various modifications can significantly increase your MGA's horsepower and torque, transforming its character on the road. Many of these principles apply to both the original MGA B-Series and a swapped-in MGB engine.

Cylinder Head Enhancements

The cylinder head is critical for engine breathing. Modifications can include:

• Shaving the Head: Reducing the cylinder head's height (e.g., by 0.047 inches) decreases chamber volume, raising the compression ratio for more power, especially when combined with a larger displacement.
• Hardened Valve Seats: Essential for durability when using modern unleaded fuels, preventing valve seat recession.
• Porting and Polishing: While radical porting can be expensive and is often reserved for full race applications, subtle grinding to match port diameters with valve seats can significantly improve airflow and breathing, almost like using larger valves.

Camshaft Upgrades

A performance camshaft is one of the most effective ways to increase power, particularly in the mid to upper RPM range. A popular choice is the "Fast Street Camshaft" (e.g., Moss Motors stock number 222-270 or Crane Cams part 342-0010). Key aspects of this upgrade include:

• Increased Lift: This cam offers greater valve lift (e.g., 0.399 inches intake, 0.419 inches exhaust compared to stock 0.355 inches), allowing more air/fuel mixture into and out of the cylinders.
• Improved Timing: It features a mild change in valve timing for more open time and later closing of the intake valve, improving high-speed breathing. This may lead to a slight drop in torque below 1500 rpm but maintains a good idle.
• New Forging vs. Regrind: Opting for a cam made from a new forging ensures slightly taller lobes for increased lift, rather than just a regrind of an old camshaft.
• Indexing: This particular cam often requires a 6-degree advance in timing, which can be achieved by skipping the timing chain by one tooth and using an offset cam key.

Addressing Valve-to-Block Clearance

With increased valve lift, especially combined with a shaved head, the clearance between the valves and the top of the engine block can become an issue. This requires adding "eyebrows" – small indentations – to the top of the engine block to ensure adequate clearance (at least 0.060 inches for intake, 0.080 inches for exhaust). This is a precise modification that can often be done by a competent DIY mechanic.

Managing Valve Train Inertia and Valve Float

Increased valve lift means valve train components move faster, increasing inertia and potentially causing valve float at lower RPMs (where valves fail to close completely). Solutions include:

• Heavy-Duty Valve Springs: A cost-effective upgrade (under £50 for a set) that allows the engine to rev higher before valve float occurs, typically up to 6600 rpm.
• Lightweight Components: Further reducing inertia can be achieved with alloy valve spring caps (about half the mass of steel parts) and by switching to shorter, hollow bucket tappets and longer pushrods from an 18V MGB engine. These combined modifications can allow revving to 7500 rpm or more without valve float.

Lighter Flywheel and Clutch Upgrades

While not directly increasing horsepower, a lighter flywheel (e.g., an 8-pound lighter MGB 3-main bearing flywheel) significantly reduces rotational inertia. This translates to quicker throttle response and improved acceleration in lower gears, making the car feel much more lively and fun to drive. Pairing this with an MGB diaphragm clutch provides a marginally stronger clutch and often cheaper replacement parts.

Exhaust System Improvements

Replacing the restrictive MGA exhaust manifold with an early MGB manifold (featuring a long centre branch) and the corresponding 2-into-1 exhaust head pipe can provide significant breathing improvements. This MGB manifold is often comparable to tubular headers up to around 6000 rpm, while being quieter and potentially cheaper. Be aware that the MGB manifold might interfere with the MGA steering column, requiring a slight angle finishing cut on the gasket face to reposition it.

Tyres and Wheels

While not an engine modification, upgrading to stickier road racing tyres (requiring 5.5-inch or 6-inch wide wheels) dramatically improves grip and handling, allowing you to effectively use any newfound power. However, these often have a shorter lifespan and may not be ideal for everyday street use.

What to Avoid: Overkill for Street Use

It's crucial to match modifications to your driving intentions. For a primarily street-driven MGA, avoid:

• Full Race Camshafts: These cams provide terrible idle characteristics, drastically reduce low-end torque, and make the car very unwieldy for street driving.
• Larger Carburettors: Stock 1.5-inch SU carburettors are sufficient to produce 100 bhp. Larger carbs offer minimal power gains below 5000 rpm and can negatively impact idle quality and low-end torque. They are typically reserved for dedicated racing engines.

Building a Full Race Engine

For those genuinely looking for a full race MGA, the modifications are far more extensive and costly (potentially £6,000+). These include:

• Radically ported cylinder heads with larger valves.
• Aggressive, full-race camshafts.
• Larger carburettors.
• Large bore exhaust headers.
• Oil pump modifications.
• Lightening of connecting rods for sustained high-RPM use.
• Aluminium flywheel and competition clutch.
• Stronger, custom propshaft.
• Wide alloy wheels and sticky race tyres.
• Significantly upgraded brakes.
• Stronger sway bars and potentially lowered suspension.
• Possibly a non-slip differential.
• Extensive weight reduction, including gutting the interior and removing non-essential components.

Such a car is typically not street legal and would be uncomfortable and impractical for anything other than track use.

Frequently Asked Questions About MGA Power

How much power does a standard MGA 1500 have?

A standard MGA 1500 is equipped with a 1489cc BMC B-Series engine producing 68 bhp.

What is the most powerful factory MGA?

The MGA Twin Cam is the most powerful factory variant, featuring a 1588cc engine with an aluminium twin-cam cylinder head, delivering 108 bhp.

Can I install an MGB engine in my MGA?

Yes, it's a common modification. MGB engines, particularly the 5-main bearing variants from 1964-1967, can be installed. However, it requires specific MGB flywheel and rear plate components, and an electric tachometer, as well as addressing gearbox compatibility.

Are MGA and MGB engines interchangeable?

The MGA and MGB engines (from 1956 to 1980) are all from the B-Series family, meaning their blocks are largely interchangeable, and many cylinder heads can be swapped. This simplifies many performance upgrades.

What are the best performance upgrades for an MGA for street use?

For street use, highly recommended upgrades include cylinder head enhancements (shaving, hardened valve seats, mild porting), a 'Fast Street' camshaft, heavy-duty valve springs, lightweight valve train components, a lighter flywheel, and an upgraded exhaust manifold (like an early MGB unit). These provide noticeable power gains without compromising street drivability.

Is a 5-speed gearbox conversion worth it for an MGA?

Many MGA owners find a 5-speed conversion (often using Ford Sierra or Mazda MX-5 gearboxes) to be a welcome feature, especially for modern driving conditions. It can improve cruising refinement and fuel economy by offering a taller overdrive gear.

What are the potential downsides of extensive engine modifications?

Overly aggressive modifications, like full race camshafts or excessively large carburettors, can severely degrade idle quality, reduce low-end torque, and make the car difficult and unpleasant to drive on the street. They are generally only suitable for dedicated competition vehicles.

Our Verdict: The MGA's Enduring Appeal and Upgrade Potential

The MGA remains a fantastic choice for enthusiasts seeking to experience the best of 1950s sports car motoring, offering a blend of classic charm with surprising comfort and practicality beyond its T-Type predecessors. Its ride is remarkably comfortable, the steering delightfully direct, and a well-sorted MGA can deliver a turn of speed that truly puts a smile on your face without risking your driving licence. What makes the MGA particularly appealing is its ability to effortlessly bridge the gap between ancient and modern; even those unfamiliar with classic cars quickly adapt to its straightforward nature.

For those desiring more power, the MGA's BMC B-Series engine platform provides a robust foundation for sensible, effective upgrades. Whether you opt for a 5-main bearing engine conversion or judiciously apply a 'Fast Street' camshaft and other breathing improvements, you can significantly enhance your MGA's performance without sacrificing its civil street manners. A well-executed upgrade can result in a lively, enjoyable classic that confidently tackles modern roads. While full race builds exist, the true joy of an MGA often lies in its balance of classic feel and accessible performance, making it a truly rewarding car to own and drive.

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