Singer Owner July - August 2008

Twin Carburettors or high compression ratio

David Freeth

Ashley's Roadster article in the March/April magazine was, as usual, very interesting and informative and the section on twin carbs was clearly designed to get a good debate going.

In principle under normal driving conditions there is not a great deal to be gained and synchronising twin carbs to achieve low-speed running is far more difficult than a single 30FAI. However, if you want to indulge in fast driving, the twin carb set-up comes into its own, particularly above 4,000rpm when the single carb will run out of puff.

Raising the compression ratio from 7.1 to 7.47 is unlikely to raise the power output by more than 3 or 4 BHP. Singers claim 10bhp more with the full 2-carb set-up, so I think the carbs have a significant effect in the order of 10% not the 2% claimed and helped to raise the top speed by 5 to 6mph according to the 'Autocar' road test with a best one-way time of 81 mph. Since wind resistance played an important part at these speeds, this is quite a gain for a car having poor aerodynamics.

However, I think that the figures generated in the 'Autocar' road test dated 10th September 1954 are, quite frankly, suspect. Even the gear ratios and acceleration are quoted for a 4.875-1 differential for a single-card Roadster and, if we look at the 'Autocar' road test of the SM 1500 twin-carb Saloon, the 0-30 time of 5.6 seconds is quicker than the 6.3 seconds of the Roadster. Since the saloon as tested weighed 1 ton 7cwt as against the Roadster of under 1 ton I suspect the Roadster was way off time or the figures are wrong. Incidentally the best top speed of the SM 1500 saloon was 79.5mph which was pretty good for a car with a significant frontal area and poor aerodynamics.

Whilst looking through the Club's archives, I unearthed the `Road and Track' American road test published in July 1953 for the twin-carb car. This car was submitted by Vaughn Singer Motors in Hollywood and had been modified by skimming the cylinder head by .062" which would no doubt have liberated a few extra BHP. However, the road test times are significantly different to the 'Autocar' in acceleration, if not in top speed and I think the `Road and Track' times are more representative of a good twin-carb set-up Roadster.

Of further interest, the last 12 HRG 1500cc cars produced were fitted with a standard Roadster engine except that they had twin SU's and their own exhaust system. A top speed of 86mph was claimed.

Basically, if you want your 4AD to perform under modern road conditions and have the ability to cruise on the main roads and motorways at 70mph+ you need the 2-carb set-up and the essential 4.4-1 r or 4.1 final drive ratio. Having converted my car 15 years ago I can confirm the benefits. However, if you are happy to cruise the byways at much more modest speeds, the single-carb set-up with the standard 4.8-1 diff. ratio allows you to potter around in top gear all day long using the significant torque of the 1500cc engine in such a light car. Therefore it's horses for courses!