1965 Cobra Daytona Coupe: Chassis test fits

Model Factory Hiro kits are well known for needing lots of test fitting. Still some adjustments to make to get it all to fit under the resin body.

Stay tuned! 

Why British engines have traditionally been small-bore designs

An enduring mystery to me has been why Jaguar stuck with typical pre-war engine design, typified by a long stroke and a small bore, until well into the '60s. At Le Mans in 1957, the 3.8 litre Jaguar D-Type arguably beat the much more powerful Ferrari 315 S, also with a 3.8 litre motor, mainly due to having disc brakes -- once Ferrari adopted discs, it was all over for the classic Jag engine. (Some references also credit the Jag with better aerodynamics.) Let's look at some numbers. 

The Jag 6 and the Ferrari V12 are within a couple of cc's of each other. But the similarities end there. First is bore to stroke ratio. The Jag, at 87 x 106 mm, is undersquare at a bore:stroke ratio of 0.82, while the Ferrari, at 76 x 69.5, is oversquare at 1.09. Power is also quite different: the Ferrari manages 360 horsepower at 7800 RPM, versus 306 at 5500 RPM from the Jag engine.

But the real interesting concept is taxable horsepower, how it was calculated in different jurisdictions, and how it affected engine design.

In France, the approach that generated the famous 2 CV appellation is based on displacement. (Note actual 2CV power output, in the 602 cc version, was 29 hp). There is also a fiddle factor and a rather bizarre RPM calculation. The formula in place from 1956 was CV = 0.00015*n*D^2*L*R, where n is number of cylinders, D is bore in cm, L is stroke in cm, and R=20 is another fiddle factor. So far so good; this is essentially displacement but replacing π/4 with 20 x 0.00015. (For details of the truly bizarre calculation for R, see here, in French). Both the Ferrari and the Jag come out at about 14.5 taxable hp. 

In most other European jurisdictions the calculation was displacement-based. (Today it is based on CO2 emissions). But in Britain, the RAC taxable horsepower calculation only took bore into account; big bore motors were thus taxed more heavily than small bore motors at the same displacement. The formula is P = n*D^2/2.5, where n is number of cylinders and D is bore in inches. The Jag is thus rated, for tax purposes, at 28 hp, but the Ferrari comes in at a whopping 43 hp. 

British manufacturers often provided both taxable and actual horsepower in the model name, hence the Wolseley 14/60 with 60 real hp but taxed at 14 hp; the Alvis 12/70 managed the trick of more actual power (70 hp) at a lower tax bracket (12 hp), presumably via a smaller bore.

This also acted as a non-tariff barrier to imports: the Ford Model T was rated at 22 hp and was thus quite a bit more expensive than an equivalent British car of similar displacement or performance.

So obviously British manufacturers built tall, narrow, small-bore motors to minimise the tax hit. And this persisted well into the '60s, when the classic Jag 4.2 litre had bore and stroke of 92 x 106; the 4.0 litre AJ6 motor that replaced it wasn't much better at 91 x 102. The 5.3 litre V12 was far more modern, at 90 x 70; even the 7.0 litre version was still oversquare at 94 x 84.

So here you have it: unintended (or maybe not) consequences of a poorly thought-out regulation.

1965 Cobra Daytona Coupe: Engine

The 289 is well modeled and went together well. I especially like the little machined venturis with the central pipe cut from nickel-steel tube. But the cylinder heads only go on it once the floor pan is in place, because the exhaust pipes need to thread over the floor pan and through chassis tubes and the steering column. 

I ran a brass fuel line inside the carburettor box. The plug wires, alternator and belt will all get finalised at some point later on, once it's all in the frame, which is started.

So far it's all typical Model Factory Hiro: very careful reading of the instructions and lots of test fitting will possibly lead to different assembly sequences. In particular I cut off the roll bar which was part of the chassis tubing, because the floor pan has to slide through it at an angle after it's installed. I tried a few times but I frankly don't see it happening without a lot of pain; so the roll bar will go in once the floor pan is in and no one will be the wiser. 

Stay tuned!

1965 Cobra Daytona Coupe: Overview

Getting started on the Model Factory Hiro kit (the old kit in 1/24 scale, not the new monster in 1/12) of the 1965 Cobra Daytona Coupe.

 

   

Sexiest car on the planet in my view, just edging out the GTO.

 

I'm going for CSX 2601, the 7th place finisher (first in the 3.0+ class) at Nurburgring, driven by Bob Bondurant and Jochen Neerpasch. 

 

Neither CSX 2601 or CSX 2286 managed to finish at Le Mans that year, although Jack Sears and Dick Thompson managed 8th overall (first in the 5-litre class) in the AC Cars entry. Of the Shelby American cars, one suffered a broken crankshaft, the other a blown head gasket -- sounds like someone got a little over-enthusiastic on the compression ratio. 

 

 

This was also the year that Graham Hill and Jackie Stewart finished 10th in the Rover-BRM turbine car, a car that I would dearly love to build a model of, if there were such a thing, which there isn't. I once owned a Rover 2000 TC, the piston-engined version of one of the later Rover turbine car prototypes; and McGill Mechanical Engineering had a functioning Rover turbine engine upon which students such as myself ran a heat and mass balance as part of the undergraduate thermodynamics labs. But I digress. 

1963 Lola Mark 6 GT: Complete (#3 for 2023)

Another one done. 

Struggles with the vacuum-formed glass are most evident in the headlamp covers. And the tires seem a bit large to me. But otherwise this simple kit provides a very nice build of a unique and interesting car.

What's next? Something with some more detail, I think. Stay tuned!

1963 Lola Mark 6 GT

Work in Progress: 1963 Lola Mark 6 GT. 

 

 

This was a predecessor to the Ford GT40, built by British specialist Lola to evaluate the potential of a mid-engined racer running the new 289. The car ran at Le Mans in 1963, ranking as high as 8th in the hands of David Hobbs and Richard Attwood before succumbing to a gearbox failure at the 15-hour mark, but the results were solid enough for Ford to press on with the GT40 program, and the rest is history. 

 

 

Paint is on and the white stripe looks good with no green paint (TS-9 British Green) bleeding through under the tape. Most of the rest of the kit is painted and ready to go, including the iconic Cortina tail-lights. 

 

 

The interior is complete, just needing some filing to get the dash to fit under the cowl. Decals are on and final assembly looms large.

 

This resin kit by DDP Models is nice but sadly has no engine or suspension detail, something I'll forgive in this instance as it is an historically interesting car. And given the simplicity of the kit, it will be a quick build (barring any disasters of course). 

 

 

Stay tuned!