How to extract more performance out of a BMC race engine.
Some things apply to a lot of types engines.
By Guest Writer
Nov 15, 2025 |
Story and Photography by Bill Holland
Without a doubt one of the most popular powerplants in the history of sports car racing is the venerable A-series engine. Throughout its nearly 50-year lifespan it appeared in such vintage racing icons as the Austin-Healey Sprite, Mini Cooper and MG Midget–plus limited-production cars like the Turner 950 Sports.
Despite those racing chops, the A-series has more humble origins. It made its debut in 1952-’56 Austin A30 and Morris Minor sedans, basic workhorses designed to transport postwar British families. The engine’s 803cc displacement churned out 30 horsepower.
The A-series soon got sportier, though, as the factory upped displacement to 948cc when placing it in the 1958 Austin-Healey Sprite. Mini and high-performance Mini Cooper variants soon joined the A-series ranks, and displacement eventually maxed out with the 1275cc version found in the 1966-’71 Austin-Healey Sprite, 1966-’74 MG Midget and 1969-’80 Mini. A revised engine, known as the A-Plus, would remain in production through 2000.
Various racing associations have allowed earlier Sprites and Midgets to run the 1275cc engine, making them a ubiquitous combination–except, of course, in more stringent programs and classes that require the original-sized engines.
Racing Means Breaking
One of the realities of vintage racing is that the engines require periodic rebuilding. The more aggressive the setup, the more frequently that rebuilding will take place. We’ve all heard tales of A-series engines producing 150-plus horsepower–but barely making it through the weekend.
Getting 100-plus horsepower per liter is certainly an admirable goal for an engine with restricted port configurations, but service life must also be part of the equation, especially if you’re racing on a budget. And while there are certainly many highly qualified engine shops that have extensive experience with the BMC A-series engine, there are also those enthusiasts who seek the challenge of going through the engine themselves. That’s our aim with this particular project.
Our A-series engine was professionally built in 2010 and enjoyed limited success, albeit more through consistency than the ability to outpower the field. But a couple of DNFs due to a blown head gasket and some internal carnage dictated a rebuild. So here we are.
Aiming for a 150-horsepower grenade was not part of the equation. We wanted to improve power through some basic changes in the intake system and camshaft. An updated valvetrain should expand the engine’s power curve. Our realistic goal: 10 to 12 percent more power.
Lighter Is Faster
In recent years there has been a trend of aggressively lightening the valvetrain. This allows the engine to spin up quicker and sustain a higher powerband while placing less stress on the components.
The original A-series valves feature 7mm stems, but some of the more prolific competitors are using valves with 5.5mm stems. A bit of research shows that certain Honda valves are ideally sized, requiring only modifications to the length and keeper groove.
Manley Performance, a major supplier of motorsports valvetrain components, has a solution with their custom Gen II stainless-steel valves. They keep valve blanks on hand that they can quickly finish to the customer’s specifications. In most cases, they say, the job takes 10 working days.
Manley also offers valve stems that narrow near the valve head itself, thus offering less restriction into the combustion chamber. Many of their Gen II custom valves offer this Pro Flo technology, and it’s said to significantly improve flow, especially at lower valve lifts. So, in short, Manley simply modified a set of Honda valves for our BMC engine.
Manley also manufactures valve springs, retainers and locks, facilitating a matched assembly. Their NexTek springs for our Honda valves, along with svelte titanium retainers and machined steel keepers, dropped mass by about 20 percent. The original configuration weighed 120.4 grams per cylinder, while the new ones check in at 99.9 grams.