This BMC A-Series “Mini” Engine Gets A New Lease On Life

While the ubiquitous small-block Chevy V8 engine is revered for its amazing 49-year production life span, across the pond a similar statistic was accumulated from 1951-2000 by the venerable BMC “A-series” 4-cylinder engine that powered many British sports cars and sedans over the years; most notably the Austin-Healey Sprite, MG Midget, and the fabled Mini Cooper.

Originally developed as a minuscule 803cc powerplant for the Austin A30 and Morris Minor sedans, the factory displacement increased through many stages, with 948cc, 1098cc, and 1275cc sizes being the most prevalent in sports cars. The engine featured here is of the 1275cc variety that’s fitted into an early 1959 Sprite nicknamed “Emma”, which competes in vintage racing circles (SVRA, VARA, etc.). It was time to take advantage of contemporary technology with the old girl.

Whiile the A-series might be a tiny powerplant, it does enjoy a solid aftermarket. An adjustable belt drive from England’s MED handles cam timing, with an SFI 18.1 balancer from BHJ replacing the OEM unit.

Building the Baby Engine

Performance and reliability go hand-in-hand in road racing, so the primary objective of the rebuild —handled by Penta Motorsports in Canoga Park, CA — was to amp up the British buzz bomb’s power but keep everything on the safe side. A number of American and British performance parts manufacturers added their expertise to the project.

The block was precision bored and honed by QMP in Chatsworth, CA, and fitted with steel main caps and ARP main bolts. A Moldex forged steel crank has been part of the engine’s combination for 20-plus years and was freshened up by Marine Crankshaft in Santa Ana, CA. Also coming along for the ride are a set of forged steel H-beam rods fitted with ARP rod bolts.

According to a number of experienced engine builders familiar with the BMC A-series, a 12:1 compression ratio is considered a safe limit, so a set of custom JE forged aluminum pistons were ordered to provide the necessary squeeze in the combustion chamber. They were fitted with a Total Seal piston ring set that features a gas-ported Gapless top ring. A billet steel SFI 18.1 spec damper from BHJ replaced the OEM unit with its deteriorating elastomer.

ARP head studs and a Cometic MLS gasket team up to provide a reliable seal of the combustion chamber. (left). The custom JE Pistons 12:1 compression, forged 2618 aluminum piston, wrist pin, and forged steel H-Beam connecting rod with ARP bolts tipped the scale at 852 grams (right).

Class rules require the use of a cast-iron 5-port head (two intake and one exhaust ports are Siamesed, making for a total of two intake and three exhaust ports) and this engine came with a highly regarded Longman version of the cylinder head. Valley Head Service in Northridge, CA performed the refresh and the Longman head was significantly enhanced through the addition of lightweight Manley valves built with 5.5mm valve stems instead of the stock 7.6mm stems, valvesprings, and titanium retainers. At 99 grams, it’s a 20-percent weight savings over stock. A Cometic MLS head gasket and ARP head studs (11 total instead of the nine originals) provide the necessary sealing.

Given the uneven flow characteristics that Siamese ports bring to the equation, compared to their more conventional isolated neighbors, a specially designed “Scatter” camshaft from APT in Riverside, CA was employed to balance the charges through individualized lobe designs. APT lifters and pushrods combine to actuate a set of Harland Sharp 1.5:1 ratio aluminum roller rocker arms. Cam timing adjustments are controlled by a belt drive manufactured by MED Engineering in the U.K.

Larry Ofria of Valley Head Service handled the rejuvenation of the cylinder head. Manley valve springs for a Honda application with titanium retainers are part of the valve train, which is 20-percent lighter than the original parts. A huge improvement came with replacing OEM valves with svelte 5.5mm-stem Pro Flo valves from Manley.

A unique lubrication system was developed that utilizes an Aviaid external wet sump pump with a pressure regulator and an external line feeding the main galley. A Moroso oil accumulator is part of the system, which utilizes Royal Purple synthetic as the lubricant of choice. A Peterson inline stainless steel mesh oil filter keeps the synthetic lifeblood clean and the hoses and fittings are from XRP.

Feeding the engine through a Cannon intake manifold is a Weber 45 DCOE carburetor, with fuel delivery handled by Holley and Aeromotive. This includes a low-pressure Holley “red” fuel pump and Aeromotive regulator set at a nominal 3.5 psi, along with a 100-micron filter tank-side and a 10-micron Aeromotive filter coming into the Weber. A K&N air filter adds protection while a Pertonix-converted stock distributor keeps the air/fuel mixture ignited through Taylor wires.

A unique lubrication system features an Aviaid external wet-sump pump and regulator with external lines feeding the galley.

While appearance doesn’t add any horsepower, it sure makes for better-looking photographs and approvals from race fans. “MG Maroon” engine paint from POR-15 was employed to give the block and head a distinctive look, with a contrasting cadmium plated oil pan and accessories. The finishing touch comes in the form of ARP polished stainless-steel bolts.

Power is not lacking, as a session on Penta’s DynoJet chassis dyno produced 120.3 RWHP, which equates to roughly 135-148 flywheel ponies by most accounts. Even taking the lower end of that range, the engine makes almost 106 horsepower-per-liter, or 1.74 horsepower per cube. The stamina of the potent little BMC A-series engine — the handiwork of Penta’s Doug Pearce — will soon be tested on the track, pending the lifting of pandemic restrictions in California.

The Sprite coaxed just over 120 ponies from Penta’s DynoJet Chassis dyno, which is said to equate 135-148 flywheel horsepower, exceeding the 100-horsepower-per-liter target.

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About the author

Bill Holland

Bill Holland has been involved in racing and the performance aftermarket since the 1960s in the capacities of racer, speed shop proprietor, journalist, street rodder, designer and advertising/PR/marketing professional. Along the way he’s raced Top Fuel and Funny Car, been editor of NHRA’s publication, National Dragster, was involved in off-roading as publisher of SCORE News, built a variety of Featured Vehicles for the SEMA Show, as well as a Track “T” that was a Contender for the AMBR award. He currently races vintage sports cars. Bill was inducted into NHRA’s California Hot Rod Reunion Hall of Fame in 2017.
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