Engine Autopsy

Elegantly simple two-stroke with German ancestry

Inside the beating, two-stroke heart of the Saab 92.

The Saab 92 that entered production in December 1949 was individualistic and groundbreaking. Gunnar Ljungstrom’s stiff unitary construction body and compact torsion bar suspension was cloaked in Sixten Sason’s aerodynamic styling, which allowed 65mph from initially

just 25bhp. The 92 pioneered the drivetrain layout that’s become today’s norm: a transverse engine mounted alongside a transverse frontwheel drive transaxle.

That engine, however, owed a great deal to DKW, the German firm that found itself in the Democratic Republic after the Second World War and reformed in West Germany in 1949. Its design bears more than a passing resemblance to the DKW F8 and later F89 units. The 76mm stroke is identical, but Saab increased the bore to give 764cc rather than 692cc. DKW cars were very popular in Sweden before the war, so this choice was a logical one for Saab to make.

When Saab teamed the compact, lightweight and reliable two-stroke – easier to start in freezing winters than a grumpy oil-bound four-stroke – with its own revolutionary layout and styling, it built a car like no other.

[A] CYLINDERHEAD

The head is light alloy, with a water jacket around the hemispherical combustion chambers. Note the extensive water transfer ports to allow the coolant to pass through from the block as it’s heated; the 92 has no water pump so the coolant circulates only by hot liquid rising towards the head and pulling cooler liquid in from the radiator. This thermosiphon cooling can leave 92s a little hot and bothered in city traffic unless the radiator is in top form.

[B] CRANKCASE TOP

The crankcase is also light alloy and fits together like a sandwich, with the split halfway up the crankshaft. The four studs you can see hold the two halves together and help to attain a good gas-tight seal, passing through the lower half to be fixed by nuts. The six large holes are for bolts that secure the cylinder block and clamp the three crankshaft bearings firmly in place.

[C] CRANKCASE BOTTOM

Note the immense cutouts for the three main bearings. In both the upper and lower crankcase halves you can see what look like oil supply holes, especially obvious in the smaller central bearing. But two-stroke engines have no oil supply except what’s mixed with the petrol and certainly no oil pump to supply oil under pressure. What these holes actually do is locate little dowels in the outer races of each one of the main bearings – two roller, one ball.

[D] CYLINDER BLOCK

The compact cast iron block is seen here from its exhaust side. The two large round ports are where the exhaust manifold attaches, while the smaller, squarer holes in the middle are windows into the transfer ports. There are two more of these windows, one at either end. On the other side of the block are two inlets for water coming from the bottom of the radiator, plus the inlet for the engine’s single downdraught Solex 32 BI carburettor.

[E] COVER PLATES

These small cast alloy items are more than just covers. Each has a shaped face that projects further at the top than the bottom. Their job is to create the ideal shape to promote the gas flow required in the transfer ports as the fuel/oil/air mixture is drawn up from beneath each piston to the combustion chamber. Manufacturing windows and covers would presumably have been simpler than attempting to cast a series of fine contours inside the block, and it allows for changes to be made if required – plus removal for easier decoking.

[F] PISTONS AND PINS

The pistons appear quite large, especially for the era – the engine’s dimensions are oversquare so the bore is greater than the stroke. There are three compression rings but no oil control ring; there’s no oil splashing up from a sump that needs controlling. Instead, oil arrives in the fuel mixture, passing through the windows in the piston skirts that open and close the transfer ports. The gudgeon pins, below, are secured in the pistons with circlips.

[G] EXHAUST MANIFOLD

The large, finned exhaust manifold connects to a single big-bore exhaust pipe, allowing good gas flow while shedding rather a lot of heat under the bonnet – open the lid of a 92 and the hefty transverse exhaust dominates the front of the engine bay.

[H] WATER ELBOWS

The two small castings fit onto the hidden side of the cylinder block and receive cooled water from the radiator. The larger one in the centre of the page attaches to the top of the cylinderhead and allows hot coolant to rise up to the top of the radiator, situated high up at the rear of the engine bay, and also houses the temperature gauge sender.

[I] CRANK ASSEMBLY

Like most two-strokes, the crankshaft assembly is pressed together as a preassembled unit. It’s the heart of the engine and gathers all the complicated and expensive parts in one place: the rearmost bearing (left) is a ballrace, while the much smaller centre bearing is a roller unit. The large front bearing behind the distributor assembly is also a roller bearing, as are the two big ends.

[J] DISTRIBUTOR

The DKW and Saab approach to arranging the sparks is rather motorcycle-esque. The small item in the centre of this ensemble is driven from the end of the crankshaft and has a cam that opens two sets of points in the baseplate, right, which sits in the housing on the front (right, as we see it) of the crank assembly. Each cylinder has its own set of points, its own condenser and its own coil. The points are timed individually – a relatively complicated and often misunderstood process that has a huge effect on the smoothness and sweetness of the engine. The two little hook-shaped items on the left are bob weights that control the ignition advance as the rpm rises. n

THANKS TO Graham Macdonald of Macdonald Classic Cars & The Saab Shop for expert assistance (macdonaldclassiccars.co.uk).