TR6 Power Loss: Fuel Injection vs. Carburetors

The Triumph TR6 is a beloved classic sports car, known for its distinctive styling and engaging driving experience. However, a common point of discussion among enthusiasts revolves around the perceived difference in power output between models equipped with Lucas Fuel Injection (PI - Petrol Injection) and those fitted with twin Stromberg carburetors. Specifically, the question arises: why might a TR6 appear to lose around 50 bhp when comparing these two systems, and what are the underlying technical reasons for this discrepancy?

Understanding the TR6 Power Claims

When the TR6 was introduced, Triumph advertised different power outputs for various markets. The 'CP' series, often associated with the European market and featuring Lucas fuel injection, was typically rated at around 150 bhp. In contrast, the 'CR' series, predominantly sold in North America and equipped with twin Stromberg carburetors, was rated lower, often in the region of 105-110 bhp. This significant difference of approximately 40-45 bhp is what fuels the debate about fuel injection versus carburetion.

The Real Source of the Power Difference

As highlighted by forum discussions, the 50 bhp difference isn't solely attributable to the fuel injection system itself. Several key factors contribute to the higher power output of the PI models:

1. Camshaft Profiles

The 'CP' PI models often featured a more aggressive camshaft profile. For instance, a common camshaft used in these engines was the 280-degree grind. This longer duration camshaft allows the engine's valves to stay open for longer, facilitating better cylinder filling at higher RPMs and thus increasing power. In comparison, the North American market 'CR' models typically used a milder camshaft, such as a 240-degree or later a 256-degree grind. Milder cams are generally better suited for lower RPM torque and emissions compliance, but they limit peak horsepower.

2. Compression Ratios

Compression ratio plays a crucial role in an engine's power output. The PI-equipped TR6s often ran a higher compression ratio, typically around 9.5:1. This higher ratio squeezes the air-fuel mixture more tightly, leading to a more powerful explosion and greater efficiency. The North American market cars, due to fuel availability and emissions regulations at the time, generally had lower compression ratios, often starting at 8.5:1 and later dropping to 7.75:1. This reduction in compression directly impacts horsepower.

3. Cylinder Head Design

While not always a drastic difference, later PI models sometimes benefited from cylinder heads with improved port designs. These enhancements could lead to better airflow into and out of the combustion chamber, further contributing to increased power. However, the primary differences in power are more strongly linked to the camshaft and compression ratio.

4. SAE Horsepower Rating System

It's also important to consider the horsepower rating system used at the time. The industry standard switched from SAE gross to SAE net horsepower. SAE gross ratings were often measured on an engine dynamometer without accessories like the exhaust system, fan, and alternator, and without accounting for parasitic losses. SAE net ratings, introduced later, were a more realistic measurement of power delivered to the flywheel under more typical operating conditions. While Triumph was generally considered honest in their ratings, this shift in methodology could account for some perceived differences in power across different eras of vehicle production, though it's less of a factor in the direct PI vs. Carb comparison within the TR6 range.

The Role of Fuel Injection

While the camshaft and compression are the primary drivers of the power difference, fuel injection itself offers advantages:

• Precise Fuel Delivery: Mechanical fuel injection systems like Lucas PI deliver fuel directly into the intake manifold or cylinders in precise amounts based on engine speed and load. This can lead to more efficient combustion compared to carburetors, especially under varying conditions.
• Drivability and Economy: Modern fuel injection systems, in particular, offer significant improvements in drivability, fuel economy, and emissions. While the Lucas PI system is mechanical and less sophisticated than modern electronic systems, it still provides a more consistent fuel delivery than carburetors, which can be sensitive to temperature and altitude changes.
• Performance Tuning: For those looking to extract maximum performance, fuel injection systems, especially modern electronic ones, offer greater tuning flexibility.

Carburetor vs. Fuel Injection: A Comparison

Let's look at a comparative overview:

Can a Carburetted TR6 Be Upgraded?

Many owners of North American spec TR6s seek to replicate the performance of the PI models. This can be achieved through several modifications:

• Engine Rebuild with Upgrades: The most effective way to increase power is to rebuild the engine with components that mimic the PI models. This includes installing a higher compression piston set (e.g., 9.5:1), a more aggressive camshaft (like a 280-degree profile), and potentially a ported cylinder head.
• Carburetor Tuning: While not adding significant horsepower on its own, ensuring the Stromberg carburetors are properly rebuilt, tuned, and synchronized is essential for optimal performance.
• Modern EFI Conversion: Some enthusiasts opt for modern electronic fuel injection systems. These can offer superior drivability, fuel economy, and tuning capabilities compared to both Lucas PI and Strombergs, but come at a higher cost and complexity.

Frequently Asked Questions

Q: Did the TR6 fuel injection system actually add 50 bhp?
A: No, the fuel injection system itself did not add 50 bhp. The power difference was primarily due to a combination of factors, including a more aggressive camshaft, higher compression ratio, and potentially minor cylinder head improvements, all of which were typically found in the PI-equipped models.

Q: Is the Lucas PI system reliable?
A: The Lucas PI system is generally considered reliable when properly maintained and tuned. However, being a mechanical system, it can be complex to diagnose and repair for those unfamiliar with it. Many issues stem from worn components, vacuum leaks, or improper adjustments.

Q: Should I convert my carbureted TR6 to fuel injection?
A: This is a personal choice. If you are looking for enhanced performance and drivability, a carefully planned upgrade can be rewarding. However, for a cruiser, a well-maintained carbureted TR6 with appropriate engine upgrades (cam, compression) can still provide a fantastic driving experience.

Q: Why do some American market TR6s seem to be in better condition?
A: This can be due to various factors, including less exposure to harsh weather and road salt in some regions of the US, leading to less rust. Differences in original specifications and market demand also play a role.

Q: What are the benefits of modern electronic fuel injection (EFI) over Lucas PI?
A: Modern EFI offers significantly better fuel economy, easier starting, smoother idle, improved emissions control, and much greater tuning flexibility through programmable ECUs. It is generally more reliable and easier to diagnose than the older mechanical PI systems.

Conclusion

The perceived 50 bhp loss in TR6 models equipped with twin carburetors compared to their fuel-injected counterparts is a result of a holistic engine package rather than just the fuel delivery method. The combination of a more aggressive camshaft and higher compression ratio in the PI models accounts for the majority of the power advantage. While Lucas PI offered a more advanced system for its time, focusing on increasing compression and upgrading the camshaft are key to unlocking more power in carbureted TR6s, ensuring these classic Triumphs continue to thrill their owners.