Singer Owner March/April 2008

Roadster Repairs - Twin Carburettors Or Higher Compression Ratio?

Ashley Crossland

In the last Roadster Repairs you read that I was measuring up to get some sheet steel to make some panels. So just after Christmas I had some steel delivered and I have made a couple of panels - a rear inner wing from l.6mm steel, and a front inner wing flitch plate from 1.25mm steel. These are the most commonly used thicknesses for Singer Roadster "flat" panels, although some thicker metal is used in lesser quantity on brackets etc. However, I quickly realised that a jigsaw would come in handy for cutting these components, so I bought one whilst the January sales were on. The picture shows my efforts to make the front inner wing flitch plate, with which I must say that I'm rather pleased. There are still a few holes to drill in it at this stage - see the photo.

We restorers can sometimes be faced with a dilemma in choosing a thickness of sheet steel from which to produce a replacement part. As you saw from the last Roadster Repairs, sheet is now only available in metric thicknesses, and not unsurprisingly these do not match the Imperial sizes that Singers originally made the cars from. If it is a structural component, then the prudent advice should always be to choose a thicker gauge of metal, but sometimes there is a big gap to the next metric size. I suppose the metric sizes most usable to car restorers are 1.25. 1.6, 2.0, and 2.5mm, and these 5 sizes basically replace 7 Imperial thicknesses that were available to

Singers so often there is this conflict on choice. So could you use the "nearest" size, implying that if this was a bit thinner would it be OK? Well you could argue that modern steel is stronger than the original, and also that Singers would have allowed some "corrosion" allowance in their designs, in common with many other engineering practices. However, what tends to happen is that some of the parts (like the boot floor and the rear seat base) had strengthening ribs pressed in them, and these get conveniently forgotten about when a home replacement is made. Of all the mistakes that can be made, I would say the most serious one is to use what I will call "sheet iron" which is not as strong, and does not weld properly with the methods normally available to the restorers. Sheet iron sounds "dead", has no "resilience" and bends easy. I know restorers can be enterprising in acquiring raw materials, but I would be wary of using washing machine cases, or the backs of filing cabinets etc unless I was absolutely certain of the material. In all this, if in doubt, please err on the side of safety - which probably explains why I have bought some new stuff. Whilst there is no need to add unnecessary weight into the car (this obviously affects performance so read on) but I suppose by modern standards, these cars are not as crash resistant as their modern counterparts, and extra safety would be an asset if you have new parts to make.

It's a bit boring just filling up Roadster Repairs on how to cut out metal, so perhaps its time to catch up on an odd throw-away line from previous Roadster Repairs, like what's the significance of the thin cylinder head gasket that I mentioned a couple of issues back?

The Roadster Team will know by now that a 1500cc engine is going into the latest car, but will I be going the whole hog on power and fitting twin carburettors? After all the 1500cc engine was available with twin carburettors in the Roadster, the SM1500 and the Hunter saloons, so it should be an easy job to do if you can find a manifold and a few other parts. A good source of 1500 engine parts comes from the SM1500 and Hunter Range (the SM1500 - and the Roadster for that matter - had twin carbs as an option from Sept 1952), whilst the 9hp engine for Roadsters was never marketed as such, although HRG fitted twin carbs when they used the engine. I can't see many people scrapping off an HRG engine just to get at a twin carb manifold for a 9hp Roadster, but I suppose it's a possibility. But would it all be worth getting a twin carb set-up for the 1500cc Roadster? Well I'll be controversial and say that I don't think so. It is very nice if you already have them, but by my reckoning, simply fitting 2 carbs instead of one on the same engine will only raise the power by let's say 2%. And that's when the carbs are properly balanced, which is not necessarily always the case and you might end up with less power. Hence for this argument it's probably safer for me to stick with one carburettor.

Mind you, in the 1950's, twin carburettors had a good "go-faster" image, and certainly the marketing men milked this for all it was worth as it was a justification to charge more for a car! In the case of the 4AD Roadster, the twin carburettors came with a higher compression ratio courtesy of Singers using a) a thinner cylinder head, and b) a thinner cylinder head gasket. Plus a different back axle ratio, distributor with vacuum advance, and a few other bits which all made it difficult to strip out the true effect of adding that extra carburettor. These high compression cylinder heads are relatively easy to identify, as (in unadulterated form) they measure 4.19" deep against 4.25". (Incidentally, pundits please refer to the Hunter Workshop manual for the tolerances on these head and gasket figures.) High compression cylinder head gaskets are 0.036" against 0.075" (uncompressed thicknesses).

So the resulting compression ratios were: -

Low comp head, thick gasket, 48 bhp 7:1
High comp head, thick gasket 7.2:1
High comp head, thin gasket, 58 bhp 7.47:1

So I would claim that Singers got most of the power improvement from the increase in compression ratio, and not from having twin carburettors, and this is the way I would recommend anyone to go - strictly in the interests of better fuel economy of course. But as I said, it was good from a marketing point of view, so good in fact that this perception persists to the present day. What I would not implicitly trust would be the power outputs claimed by the motor companies. However, not to leave out the 9 Roadster figures, the reported power outputs were: -

9hp Roadster with SU carb 35 bhp
9hp Roadster with Solex carb 36 bhp
Super 10 Saloon SU carb 37 bhp

So you could argue that a 10 engine again was a useful "direct" spare to have in the garage, but a swap wasn't going to add sparkling performance to you car. Another misconception? 9hp to 10 hp should give you 11 % more power, not the 3% that Singer's figures suggest.

Just to go back to the later 1500cc high compression cylinder heads, Singers made them "universal" in the sense that they could take either 1 or 2 carburettors by simply changing the side cover plate and using the appropriate exhaust manifold. This cover plate fits over porting that is exposed on the actual head casting, and the beauty with this internal manifolding is you didn't need an external balancing pipe as you would generally have on other twin carb set-ups. The earlier cylinder heads were configured only for a single carb, but what I have never investigated is if there is enough metal on them to skim off and so increase the compression ratio. I must look into this some time. Or has anyone in the Roadster Team ever done this, and can save me the trouble? Generally there is some allowance for in-service re-facing, but I would not know if there is sufficient here to do the trick. However, if you have access to a real 1500cc high compression cylinder head, this would be the safer part to use. Of course even 7.47:1 is not a high compression ratio by today's standards.

So, there we have it, a thin cylinder head gasket, and a thin cylinder head and whilst twin carburettors are nice if you already have them, there's no need to bother with them otherwise. Trust this helps.