With it’s front mounted supercharger, the 4½ became one of Walter Owen Bentley’s quintessential British sportscars. It was the vision and product of Sir Henry Ralph Stanley ‘Tim’ Birkin who persuaded Bentley to produce 50 examples of the model.

The company Birkin & Couper Ltd. was established to specifically supercharge Bentley’s four-cylinder cars despite W.O. Bentley’s apprehension. W.O. Bentley said “They would lack in their preparation all the experience we had built up in (our own) racing department over 10 years. I feared the worst and looked forward to their first appearance with anxiety.”

Unblown, the standard 4.5 liter engine put out 110HP. The use of a Roots supercharger offered an increase in power to 240 bhp. The power was necessary due to the massive size and weight of the car.

The overall design of the car was heavy and large. The tall engine forced the driver to sit with his head 63 inches above ground. To help compensate for a high center of gravity & weight Bentley employed the use of stiff and heavily damped springs. Excessive weight of the car due to huge brakes, large chassis & a bulky rear axle put the car in a class of its own. The Blower is one of the heaviest cars to compete in racing.

The first single seat prototype was initially built in 1929 with racer Mike Couper. The finished product was a 1½-seat Bentley with a massive 10 feet 10 inch wheelbase. It had a fabric skin stretched over steel structure that barely covered the components. It became the fastest racecar in Britain by holding the Brooklands Outer Circuit Lap Record in 1930.

Birkin established Birkin & Couper Ltd. where the first Blowers were made. W.O. Bentley said “They would lack in their preparation all the experience we had built up in (our own) racing department over 10 years. I feared the worst and looked forward to their first appearance with anxiety…”

The first appearance of the car was the Brooklands 6-Hour race on June 29,1929. Later in the year more 4½ Litre Bentleys were fitted with Birkin’s blower system and one placed second in the RAC Tourist Trophy, but they never won a race.

Bentley described the model at the end of the season “The supercharged 4½ never won a race, suffered a never-ending series of mechanical failures, brought the marque Bentley disrepute and incidentally cost Dorothy Paget a large sum before she decided to withdraw her support in October 1930…”

Despite the lack of results, Wolf Barnato allowed Birkin to try the cars again at Le Mans but none of them endured to finish. This meant Bentley was obliged to produce 50 road going examples of the model which they did.

The number 9 car featured above, UU5872, is an original Birkin car known as ‘Birkin Blower No.2’. This car made an appearance at the 1930 Lemans. The first car featured above is an unrestored 1928 works blower.

In Detail

tags	4.5
submitted by	Richard Owen
type	Series Production Car
production years	1929 – 1931
engineers	Walter Owen Bentley
production	55
engine	Inline-4
aspiration	Amhurst Villiers Roots-Type Supercharger
block material	Cast Iron
valvetrain	SOHC 4 Valves / Cyl
fuel feed	2 SU HVG5 Carburettors
displacement	4398 cc / 268.38 in³
bore	100 mm / 3.94 in
stroke	140 mm / 5.51 in
power	130.5 kw / 175 bhp @ 3500 rpm
specific output	39.79 bhp per litre
bhp/weight	90.67 bhp per tonne
body / frame	Fabric Body over Steel Ladder-Type Chassis
driven wheels	RWD
front tires	Dunlop 6×21
rear tires	Dunlop 6×21
front brakes	Mechnical Drums
rear brakes	Mechnical Drums
front wheels	F 53.3 x 15.2 cm / 21.0 x 6.0 in
rear wheels	R 53.3 x 15.2 cm / 21.0 x 6.0 in
steering	Worm & Wheel
f suspension	Solid Axle w/Semi-Elliptic Leaf Springs, Friction Dampers
r suspension	Live Axle w/Semi-Elliptic Leaf Springs, Friction Dampers
curb weight	1930 kg / 4250 lbs
wheelbase	3302 mm / 130.0 in
front track	1384 mm / 54.5 in
rear track	1384 mm / 54.5 in
length	4369 mm / 172.0 in
width	1740 mm / 68.5 in
height	1600 mm / 63.0 in
transmission	4-Speed Manual
top speed	~209.2 kph / 130.0 mph

Auction Sales History

1931 Bentley 4½-Litre Supercharged Two-Seater Sports MS3929 –

One of the 50 original supercharged “Blower” Bentleys. Original chassis, engine, D-type gearbox, and supercharger. The last “W.O.” Bentley bodied in period by Vanden Plas; offered with its original coachwork. Available after nearly 30 years of continuous family ownership. Documented and authenticated by Bentley historian Dr. Clare Hay. A veteran of the Mille Miglia Storica; complete with FIA paperwork. Exceptional purity; one of the finest ever offered.

Auction Source: RM Sotheby’s Monterey 2015

1931 Bentley 4½ Liter Supercharged Le Mans MS 3944 –

One of three Original Le Mans Specification production Supercharged cars built. Owned by the Noble family for more than 55 years. Original components and numbers as delivered new from Bentley. Documented by Bentley Expert Dr. Clare Hay. Eligible for Mille Miglia and more.

Auction Source: 2013 Quail Lodge Auction by Bonhams

1931 Bentley 4½ Litre ‘Blower’ Sports 2/3 Seater Boattail SM3916 – did not sell for $7,100,000 Among the Finest of the 50 Factory Blowers. Exceedingly Rare, Matching-Numbers, Original-Bodied Example. Exceptional Provenance Including a 54-Year Stewardship. One of Very Few Vintage Bentleys with Original Fabric Covering. Ideal International Driving and Concours Event Entrant. One of the Most Significant Bentleys in Existence
Auction Source: 2012 Pebble Beach Auctions by Gooding & Company