Upgrading Your Lotus 907: Crankshaft Choices

The Lotus 907 engine, initially delivering a respectable 140 horsepower in its Jensen Healey incarnation, was a revelation in its time, boasting a 16-valve design years ahead of its contemporaries. However, for those seeking to truly unleash its capabilities and keep pace with modern motoring, the journey often begins with a fundamental question: what crankshaft do I need for a Lotus 907? While the original engine offered exciting performance, an additional 40 to 50 horsepower can utterly transform the driving experience, making it a worthwhile investment for engines undergoing a rebuild or those already in robust condition. This article delves into the crucial crankshaft upgrade and explores the complementary modifications that will elevate your Lotus 907 engine to new heights, ensuring it's not just a classic, but a formidable road machine.

It's a common misconception that rebuilding the Lotus 907 engine is prohibitively expensive. In reality, a sensible, stock rebuild can be quite manageable. The true value comes from strategic enhancements that harness the engine's inherent potential. Remember, these upgrades yield the most significant benefits when applied to an engine that is fundamentally sound, or as part of a comprehensive rebuild.

The Heart of the Beast: The 2.2 Litre Crankshaft

For enthusiasts aiming to create a fast street motor that can genuinely compete with contemporary vehicles, upgrading to a stroker crank is an absolute necessity. More torque translates directly into more power, and that's something every driver appreciates. The most recommended path for the Lotus 907 involves upgrading to the later 2.2-litre configuration. This isn't just a simple swap; it's a foundational step that, in conjunction with a complete upgrade package, can result in a very pleasant, eminently streetable engine delivering over 200 horsepower.

The recommended option is the reworked 2.2 cast, Vauxhall/Bedford crankshaft. These units are highly regarded, capable of handling revs up to 7000 rpm, and are manufactured to the same rigorous standards as the original crank found in the 907 engine. A 2.2-litre crank in your Lotus 907 engine can truly make your Jensen Healey or Jensen GT the smooth, fast road car it was always destined to be.

Complementary Engine Enhancements

Achieving that 200+ horsepower target isn't solely about the crankshaft; it requires a symphony of carefully chosen components working in harmony. Here are the key modifications that complement a stroker crank upgrade:

Higher Compression Pistons

While you could opt for new Hepolite piston & liner assemblies, a more cost-effective and performance-oriented solution is available. Forged pistons, such as those from JE Pistons, are highly recommended. These can be sourced .035 oversized, allowing your worn liners to be bored to match. Expect to pay a reasonable fee for liner boring. Unlike the cast Hepolites, forged pistons offer superior strength and their expansion characteristics are remarkably similar. For a daily-driven Jensen Healey, a 9.5:1 compression ratio is a popular choice. A set of four forged pistons, including wrist pins, offers excellent value and performance.

Camshafts: Finding the Right Profile

One of the 907 engine's design advantages is its ability to run fairly high-lift cams without becoming undrivable, thanks to its inherent architecture. However, prudence is advised unless your vehicle is strictly a weekend or track car, as aggressive cams can significantly impact fuel economy and increase hydrocarbon emissions.

• Lotus 107 Camshafts: These are excellent, peppy camshafts for general street use. They are kind to your stock valvetrain and perform admirably up to around 5,500 RPM. While they will pull to 7,000 RPM, their edge begins to diminish past 5,500 RPM, especially in a 2.2-litre setup.
• Lotus 104 Camshafts: For those seeking a high-horsepower, yet streetable 907 engine, the '104' cams are the preferred choice. These require new springs. According to experts like JH Guru, Garry Kemps, the 104 cam (272 degrees at 10 thou lash and 0.415-inch lift) is particularly effective in a 2.2-litre engine with decent compression (10+). It truly comes alive between 3,000-4,000 RPM, pulling cleanly and strongly to 7,000 RPM, yet remains docile enough for city traffic, ticking over smoothly at 900 RPM.

Dellorto Carburettors: Unleashing Fuel Delivery

While the U.S. spec Strombergs are reliable and fuel-efficient, the true performance enthusiast often gravitates towards Dellorto carburettors. Not only do they offer a cleaner aesthetic, but they also run cleaner and provide a distinct performance advantage over the Federal fuel system. The inherent superiority of port-on-port carburetion is undeniable. While the 40mm pairs found on non-Federal Jensen Healeys perform well, the 45mm Dellortos offer an even greater performance edge. It's worth noting that while Weber kits are available, they often produce an almost unresolvable mid-range flat spot on the 907 engine, making Dellortos the superior choice for this application.

Engine Bearings: Sourcing Quality Replacements

The original supplier for Lotus 907 engine bearings, Vandervell, no longer produces them, and remaining stock is astronomically priced. A viable and high-quality alternative comes from engine bearings designed for a Holden 2 or 2.2 litre motor. These are high-quality trimetal copper-lead bearings, comparable to or even better than the originals. They do require minor modification for fitting, as the nips on the main bearings are slightly wider than the old Vandervell units, necessitating some minor filing on the block. The centre #3 main bearing also has its nip on the opposite side, requiring a small indent to be filed. These are straightforward modifications that most home mechanics can accomplish relatively quickly.

Adjustable Camshaft Gears: Precision and Durability

For any serious performance engine, the newer style half-moon tooth belt and gear set is a must-have. While the initial investment might seem significant, the peace of mind offered by this heavy-duty belt is invaluable. It provides a positive grip, preventing belt jumping and enhancing torque output directly from the crankshaft – dyno tests have even indicated an extra two bhp. An added bonus is their extended lifespan, often lasting up to 40,000 miles, with quality US-made replacements being readily available and reasonably priced.

Lightened Flywheel: Sharpening Responsiveness

Reducing the rotating mass of the flywheel significantly improves acceleration, particularly from a standstill. While the legendary Tilton aluminium 7-pounder is a rarity today, other lightweight options exist. An aluminium flywheel will make the pedal feel a little more sensitive, but it's generally within most owners' tolerance for weekend and general use. A less costly alternative is to have your original steel flywheel lightened by a local machine shop. However, it's crucial not to reduce its weight below 19 pounds to mitigate the risk of failure during operation. It's important to remember that the stock flywheel is heavy for a reason: it contributes to smoother engine operation at high RPM. If you opt for a lightened flywheel, a precision balance of the bottom end is strongly recommended, a practice advisable for any comprehensive engine rebuild.

Ignition System Upgrades: Sparking Performance

Optimising your ignition system can yield significant benefits:

• Multiple Spark Discharge (MSD) Ignition Control Unit: The MSD unit is relatively simple to install, plugging into extended coil adaptors. It offers instant starts, improved fuel burning for better power, and cleaner running throughout the RPM range. Crucially, it saves your engine, as over 80% of engine wear occurs during cold start cranking. While it may render your electric Smiths tachometer non-functional (requiring a tach adapter or an internal circuit board update), the overall benefits in performance and longevity are substantial.
• Pertronix Ignitor: Favoured by many racers, the Pertronix Ignitor is a robust and well-constructed unit designed specifically for the 25D Lucas distributor found in the Jensen Healey (Model LU142). It replaces the traditional points and condenser with a reliable magnetic trigger, eliminating the need for frequent point changes. It fits cleanly within the distributor housing, avoiding unsightly external components, and is immune to dirt, oil, heat, and moisture, switching twice the energy of conventional systems. Even without an MSD unit, the Ignitor offers noticeable improvement over stock points.

Other Vital Enhancements

• Oversized Stainless Steel Exhaust Valves: The 907 engine is notorious for burning exhaust valves. Upgrading to a set of eight oversized stainless steel exhaust valves, often originally built for motorcycles and then modified for the 907, is a sound investment. They are significantly more durable than the original Lotus parts, which were made from a less robust stainless alloy.
• Reusable Camshaft Cover Gaskets: A simple yet effective fix for the common 'burning-oil-on-the-exhaust-manifold' syndrome. These gaskets are relatively easy to install and address a significant design flaw, provided the threaded bolt housings are thoroughly cleaned beforehand.

Addressing Oil Pressure Concerns for Performance

While typically not an issue for street-driven Jensen Healeys, oil pressure can become a significant concern for track racers. During long, hard, sweeping turns, a slight drop in pressure may be observed. Severe and extended high-RPM use can escalate this into an oil starvation problem, leading to unexpected and costly damage. The 907 engine, unlike the 912, lacks a beefed-up bottom end, which exacerbates these issues on the track. Fortunately, several inexpensive fixes exist:

• Restricting oil flow to the cylinder head is highly effective.
• Hinging the restrictor baffles in the sump can also provide assistance.
• The most effective preventative measure is installing a strengthening plate, which can make the 907 engine truly bulletproof for track use.

Engine Lifespan and Maintenance

The Lotus 907 engine has an unusually long lifespan, spanning nearly thirty years in various Lotus models and other vehicles. Its development, starting in the early 1970s, based on Vauxhall engine dimensions but featuring an alloy block, head, and sump with cast iron wet liners, set it apart. Inclined at 45 degrees, it allowed for a lower centre of gravity and bonnet line. Over the decades, power output evolved from 120bhp to 300hp in models like the Esprit Sport 300, all while adapting to changing emission requirements, even meeting California's stringent standards.

While early Jensen Healey owners experienced some reliability issues, the engine has significantly improved over time, largely due to advancements in oil technology. With diligent regular servicing, including crucial cam-belt changes, it's not uncommon to find Lotus vehicles with over 100,000 miles on their original, unrepaired 907 engines. Like all mechanical components, they will eventually wear, but proper care ensures a long and spirited life.

Key Lotus 907 Engine Modifications Comparison

Frequently Asked Questions (FAQs)

Is the 2.2 Litre crankshaft a direct bolt-in for the Lotus 907 engine?

The 2.2 Litre Vauxhall/Bedford crankshaft is recommended for the 907. While the crank itself integrates well, the accompanying Holden 2/2.2 litre engine bearings, which are necessary due to the scarcity of original Vandervell bearings, require minor filing to the block for proper fitment. This is a straightforward modification.

What kind of power increase can I expect from these upgrades?

When combining the 2.2 Litre crankshaft with higher compression pistons, performance camshafts (like the 104s), and 45mm Dellorto carburettors, you can realistically expect to achieve a very pleasant and streetable engine producing over 200 horsepower.

Are these modifications suitable for a daily-driven vehicle?

Yes, many of these modifications, particularly the recommended 2.2 Litre crank, 9.5:1 compression pistons, and 104/107 camshafts, are designed to create a powerful yet entirely streetable engine. While a lightened flywheel might make the pedal a bit more sensitive, it's generally well within the tolerance for daily and weekend use.

How do I address oil pressure issues if I plan on track use?

For track use, where oil pressure can become a problem, there are several fixes. Restricting oil flow to the cylinder head is highly effective. Additionally, hinging the restrictor baffles in the sump can help. The most robust solution is to install a strengthening plate, which significantly improves the 907 engine's durability under severe track conditions.

Are original Lotus 907 engine parts still available?

Many original parts, such as Vandervell engine bearings, are no longer produced and remaining stock is prohibitively expensive. This has led to the development and sourcing of high-quality aftermarket alternatives, such as the Holden 2/2.2 litre motor bearings, which often perform as well as or better than the originals.

Embarking on an engine upgrade for your Lotus 907 is a rewarding endeavour that truly unlocks the car's latent performance. By strategically selecting components, with the 2.2 Litre crankshaft at its core, you can transform your classic into a genuinely potent and reliable machine capable of exhilarating modern motoring. The meticulous attention to detail, from forged pistons and performance camshafts to improved fuel delivery and ignition, ensures a harmonious and powerful outcome. Investing in these enhancements not only boosts horsepower and torque but also significantly improves the overall driving experience, ensuring your Jensen Healey or Lotus remains a joy on the road for years to come.

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