How British Cars Managed To Get Leaks In Their Gas Pedals

How is your throttle hooked up to your accelerator pedal? It’s a cable, right? Or in some cases it might be electronic. It’s almost certainly one or the other, unless you drive an old school bus or a strange old British car — in that case, it might be that your foot is squishing air to help propel your car down the road.

For a while there, the vast majority of gasoline vehicles used largely similar throttle bodies, whether those cars were big, small, brown, or Italian. A cable hooked up the accelerator pedal to the linkage on the throttle, and a flex of the foot opened the throttle blade to allow more air to pass. More recently, e-throttles have become the norm. Computers were inserted between the accelerator pedal and the throttle, with electronic actuators in charge of sensing pedal motion and translating that into throttle motion.

Jump back to the 1960s, though, and Hillman had a weird idea. Why bother with fussy cables when a pneumatic system of pistons and seals could do the job? Today, we’ll explore how a forgotten British automobile ended up with an oddball air throttle seldom seen before or since.

Impish Ways

In the mid-1950s, Rootes Group was working hard to develop new vehicles. It was a key moment for the British automaker, which had weathered World War II and was now looking to the future. At the time, Europe had just tangled with fuel rationing amidst the turmoil of the Suez Crisis. The challenges of the decade pushed several automakers to pursue designs for lightweight, fuel efficient transport. German and Italian automakers were tinkering with tiny bubble cars, while BMC was pursuing what would eventually become the Mini.

Meanwhile, at Rootes, engineers were hard at work on the APEX project, having kicked off work on a new vehicle in 1955. Once the Suez Crisis hit in 1956, the project’s goals were refined. Rootes’ traditional heavy vehicles were suffering in the marketplace, and something new was desperately needed to suit the moment. The APEX project aimed to develop a lightweight, fuel-sipping automobile that resembled a more traditional vehicle, rather than the strange “bubble” microcars that were coming out of Europe.

The result of this work was the Hillman Imp. Slated for release in 1962, it wouldn’t actually reach the market until mid-1963 The two-door compact made waves on launch, noted for its all-aluminum engine and sleek, compact design. It debuted with a single engine option derived from the Coventry Climax, an 875cc inline-four good for just 42 horsepower. It wasn’t much, but with a kerb weight under 1600 pounds, the diminutive Hillman didn’t have too much trouble getting down the road. Top speed was a healthy 75 miles an hour.

Unfortunately for Rootes, the Imp was late. By the time it hit the market, Britain had already been falling in love with the Mini for four long years. Indeed, that car would serve as the Hillman’s main competitor, having been built for largely the same reasons. Each vehicle had tried to maximize space and practicality in a compact form factor, while using a minimum of fuel. They’d just gone very different ways about it—where the Mini had a front-engine front-wheel-drive, configuration, the Hillman Imp was exactly the opposite.

It was the rear-engine layout that is believed to have inspired the Hillman’s most unique feature—the pneumatic throttle. Precise engineering justification for the decision was never published, but general consensus is that the engine’s position dictated the choice. Located at the back of the vehicle, quite some ways from the driver and the pedals, a traditional throttle cable would have had to take a rather long and winding route to the carburetor. Perhaps due to fears around slack or cable binding, Hillman’s engineers went a different route.

The result was a relatively simple pneumatic circuit, which was used to control the carburetor. The accelerator pedal was set to push on a piston, mounted in the front footwell. This piston would send air pressure down a flexible plastic tube to an actuator mounted on the carburetor. This actuator would move in response to the air pressure, opening and closing the throttle blade in turn.

The concept has one major benefit, as a simple air hose can be more flexibly routed than a cable in a sheath. In a press release in 1963, Rootes Group also claimed other plus points that are perhaps more spurious:

Another interesting feature of this new car is the use of a completely new accelerator system. This involves the use of air pressure to replace the conventional rod or cable, linking the motion of the accelerator pedal with the carburettor. This unit is simple, economical and, above all, aids driving by increasing sensitivity of control.

It seems hard to imagine that a pneumatic throttle would come in cheaper than some stamped steel linkages and a cable, but Rootes claimed it was  simple and economical nonetheless. As for its ability to “aid driving by increasing sensitivity of control”? One might call that into question—somehow, it’s hard to imagine a squishy pneumatic link providing finer control than a taut steel cable.

A pneumatic accelerator assembly from a Hillman Imp. 

In any case, I tried to hunt down contemporary reviews that covered this topic, but they’re hard to come by. The best I could find was a road test from the December 1963 issue of The Practical Car Driver. Amidst the sexism typical of the era, the author apparently casually mentions crashing the vehicle in a field. However, I’m not sure this can be put down to the throttle control so much as the driver’s choice of beverages over lunch:

Since it is early days, my experience with the construction of the Imp should perhaps be discounted. Over the test drive the ash-tray tumbled free, the passenger seat worked itself from its fastenings and there was a most remarkable cheeping noise from the steering column. The rear window came dislodged. This was partly my fault as, at one point after a visit to the Capendonich Distillery, I failed to correctly judge a corner and proceeded to enter a ploughed field.

Many historical resources note the troublesome throttle, like this report from Driven to Write, though none elucidate any specific flaws beyond hints around “sensitivity” and that it took some getting used to. The best information I’ve found is a one-off forum post from a mechanic named Gordon Streeter, who noted working on the cars back in the day. “I remember instead of a throttle cable they had a pneumatic coupling between the pedal and the carburettor,” stated Street. “Although innovative, it used to leak and the car would slow down until you pumped the throttle again.” That’s one problem that a cable-based throttle wouldn’t have.

The air throttle wasn’t the only problem with the Imp, either. The pioneering automatic choke also never worked quite as well as Rootes’ engineers might have hoped it would. Cooling was a major pain point on the original car. The rear-engined layout proved challenging, with the Imp suffering from a feeble water pump and limited air flowing over the rear-mounted radiator. The lack of a temperature gauge didn’t help things, with overheating—and resulting damage to the all-aluminum engine—rather a common occurrence.

Come 1965, with sales falling and the Imp’s reputation rapidly sinking, Rootes Group took drastic action. The company attempted to reintroduce the Imp as an “Mk II” model, having made major updates to quell some of the original car’s foibles. The pneumatic throttle was, ultimately, deemed unfit for service, replaced with a simple cable unit. The automatic choke was also tossed, while the water pump, cooling fan, and head gasket were all upgraded.

Despite the changes, the Imp never reached the sales that Rootes might have hoped for. With the company’s crucial small car failing to succeed in the marketplace, the company ran into financial problems that saw it sold to Chrysler Europe in 1967. With minor further tweaks, Chrysler kept the Hillman Imp in production until 1976 with limited further success. 440,000 units were eventually produced, but half of that production occurred in the first three years. Unlike its rival over at BMC, the Imp would never go on to have the longevity, legacy or fanbase that surrounded the legendary Mini.

More Air

Rootes Group had failed to make a good case for the pneumatic throttle actuator, but it wasn’t the only one to experiment in this space. Notably, an obscure company called Williams Controls also once specialized in what it called “air throttles.” More specifically, the company developed pneumatic throttle solutions for heavy vehicles—tractors, trucks, buses, and the like.

In the Rootes design, pressing the foot on the accelerator pedal presses down on a piston, which creates a burst of air pressure which moves the throttle actuator. The Williams parts seem to work in a different way to the Rootes design. The driver pushing on the accelerator opens a valve, which directs air pressure from a remote source to an actuator on the throttle or fuel rail on the engine itself, depending on whether they’re designed for a gasoline or diesel application.

The Williams parts I’ve found look much more stout than the Hillman design. That’s exactly what you’d expect for valves intended to operate with a compressed air supply at 60 psi.

It’s easy to understand the value of an air throttle in these applications. In many trucks, buses, and motorhomes, the driver’s position can be a long way from the engine itself. Cable controls can become problematic on longer runs, particularly if there are a lot of twists and turns. In these cases, a pneumatic throttle solution may be more desirable. Plus, in heavy vehicles, precise and instant throttle response isn’t so important as it might be on a lightweight commuter vehicle.

There aren’t a whole lot of details out there as to which vehicles used air throttles from Williams Controls. However, I’ve found anecdotal evidence that they showed up on vehicles from Newell via forum posts, and Blue Bird, too. They were often found on vehicles with big diesel engines, including school buses from the latter manufacturer. Of course, diesel engines don’t have throttle plates like gas engines. On these applications, the pedal-controlled actuator in the engine bay is set up to control the fuel flow to the engine, as is the way in diesels.

A pneumatic throttle from Williams Controls, complete with pedal. Credit: Curtiss-Wright

Williams air throttles are often found on larger vehicles where the pedals are a long way from the engine.

Death of the Air Throttle

It’s easy enough to understand why the pneumatic throttle died in the car world. Just about every automaker except Rootes Group found a way to make simple cable throttles work just fine. Even in weird rear-engined and mid-engined cars, they honestly didn’t have any real problems. Indeed, even Rootes was able to make the Hillman Imp work with a regular cable throttle once they actually bothered to try. At the same time, the example from Williams Controls shows that pneumatic throttles can do the job just fine when they’re built right and better suited to the intended application.

In any case, don’t expect pneumatic throttles to come back any time soon. These days, it’s far easier to run a couple of wires to an e-throttle hooked up to an electronic accelerator pedal. This avoids the risk of leaks that you’d get with a pneumatic setup, as well as the mechanical issues that abound with long cable-actuated setups. It’s quite easy to send electronic signals over longer distances that you might find in a larger vehicle, and wires can be routed far more flexibly than pneumatic hoses or sheathed cables. There’s also additional flexibility in how the engine control unit uses the e-throttle; it can eliminate the need for additional idle controls on the engine, as well as provide varying throttle curves for use in different conditions.

Hillman’s failure is an oft-forgotten piece of automotive obscura. It’s a lesson that sometimes, doing something a little bit difficult is better than inventing a completely new solution that is largely untested. If you’re going off-book, you’d better have a really good reason for doing so—and you’d better make sure your oddball setup works, to boot.

Image credits: Hillman, via eBay, Williams Controls