Prototype Lotus 907

Following from my last article I wrote, about the Viva GT Lotus Prototype; Andy has now got everything out of storage and a good morning was spent looking at the engine and noting some of the unique castings and internals used by Lotus.

To remind you the Lotus 907 engine as used in the Jensen Healy and some of the Lotus saloons was developed in the front of a Viva GT. The Lotus 907 engine is a 4-cylinder slant four engine but unlike the cast iron Vauxhall Unit is cast alloy and twin cam not single cam. An eternal debate ensues about why the Lotus engine is the same layout and size as the Vauxhall engine, some say it was a co incidence others that one copied the other. Either way it’s a popular route for Viva owners these days to extra BHP.

Andy the owner of RAH 713F is also the owner of one of the development engines its engine number 907/007. The number is stamped on the block and is also marked as a 030” (assumed) oversized engine.

Here is the passenger side of the block, ignore the yellowing its only grease not corrosion, as its been in storage at least 20 years.

Here is the engine number as stamped on the block:

The main obvious differences for those that know their Lotus 907 engines are the mounts. Normally there are three holes in a triangular layout for each mount on each side of the block. In the case of this casting there are two engine mounts positions on each side of the block, both being four bolt layouts in a square fashion. Below is the driver’s side of the block showing clearly the two mount points.

The waterways are laid out differently although the water pump looks like a 907 production unit (so that part made it through development unchanged). The other striking difference is the oil pump and distributor layout, being much larger than the production units. The oil gallery running vertically to the head is also positions differently to both Production 907’s and Vauxhall slant fours, so the head and block are totally unique to each other and no cross swapping parts is possible.

The distributor is in the right horizontal plane, but the oil filter is 90 degrees different to the production engine. In the following image I have laid a 907 production casting next the development oil pump assembly to show the size differences.

The carbs are a set of 45 DCOE’s on a home made manifold; this design also appears in the production engine (albeit with more metal cast into it) and the addition of a water heater tube.

Here are the carbs next to a production 907 manifold

The air-box for the carbs is a heavy metal unit but its design and shape are almost 100% the same as the Lotus Sunbeam (albeit several years older). here is the setup with the air-box lid removed, note the loverly stainless trumpets.

The head again is covered in grease so ignore what looks like rust and corrosions as its not, one first looks it appears to be more Vauxhall shaped than Lotus shaped, the outer edges of the casting is angled in, (the production unit is very rectangular).

the oil gallery feed hole is different to that of a production 907 head, it’s closer the Cast Iron Vauxhall layout, but not exactly the same.

The valves are the same size as are the ports.

The other major difference is the thermostat location in the head at the front; the thermostat is not in the head on the production engines.

Here is the head next to a production 907 head, note the waterways are very different as is the oil gallery hole location. Its likely that this head was cast from the earlier designs, that were fitted to cast iron Vauxhall blocks during earlier development work before the block had been cast.

The inlet and exhaust ports look the same as the production unit.

the cam towers have a different design of oil filler, which appears to of been welded onto the lid as an after thought, with a breather vent to the side of the filler cap.

The cam wheels are cast differently to the production units and they are a vernier design using pegs in the back to adjust the individual timing of the camshafts as needed.

Here are the production cam wheels as a comparison to teh prototype ones, first prototype second production.

The camshafts have part numbers cast, I am yet to find out if these are different from production.

The pistons and liners have been removed from the engine block, the liners look stock, but noting the 030″ mark on the block next to the engine number, one has to assume (until they are measured) that the liners and pistons are 30 thou oversized? The pistons are not stamped oversized on the tops.

The skirts on the pistons have been cut and some customisation of the skirt shape is also noted. The inside skirt of each piston also has some engraving, this set of pistons is marked (set 18) and 0.388 which one assumes its either a tolerance or weight number relevant to Lotus when building up this engine?

The tops of the pistons are flush (bar the valve recesses) so its assumed these are High compression other Lotus 907 pistons inspected were dished.

The sump was front mounted in this instance and a lot of cutting was done on the alloy sump (I assume from an earlier engine?) in order to get this to fit into the GT for testing. Here it is mounted onto the engine block and sat on a Vauxhall cross member. You can also clearly see in this image the thermostat in the head.

The last item I didn’t picture was the crank, which looked totally stock compared to the Jensen one. Again all these are visual checks for now.
So there you go, a very rare engine indeed. The owner does wish to get the engine rebuilt but this is way beyond my ability, a specialist is going to be needed, we know very little of tolerances other than those we can glean from production 907 engines. Gaskets will all need to be custom made. It will be a very interesting build and I promise to bring you more on this when I know more.