Build Overview

For many car enthusiasts, the shape of the Cobra is recognizable instantly.  In spite of this familiarity, there are a number of variations on the Cobra 'theme', even among the original cars (e.g., 289 FIA; 'slabside'; 427).  In a sense, then, the pure 'Cobra' design does not exist, although, for many, the 427 is the archetype.  One excellent history of the Cobra is ‘Cobra: The first 40 years,’ by Trevor Legate.; another is 'Shelby Cobra: Fifty Years’ by Colin Comer.  The web also provides many other sources of information on the Shelby Cobras.  A good summary of the design changes to the AC (i.e., ‘Cobra’) can be found in this Wikipedia article.  The ‘427’ was created in 1964 with a beefed-up chassis (4” main chases tubes and a coil spring suspension; most were fitted with Ford’s 428 c.i. engine) but was unable to get homologation for the 1965 racing season.

The Factory Five steel space frame chassis and aluminum sub-chassis are a clear descendent of the original AC chassis used in the first Cobras.  The 4” round tube chassis of the modified AC chassis for the 427 Ford engine is shown in the photo below.

427 Chassis, Shelby American


mk4frame-694x413

Factory Five Mk IV Chassis

The Factory Five 4” round tube chassis is shown immediately above.  As with the body shape, the frame went through a number of modifications as the Cobra evolved, from the original AC chassis modification to hold the 221 Ford V-8 to the 427 and suspension modifications.  So, too, with the original Cobras, the Mk IV has aluminum panels attach to the steel chassis to form a subframe, adding strength.  The only fiberglass used on the F5 Mk IV is the non-stressed body shell with the familiar Cobra shape.

As I begin my own build, over 9,000 Factory Five Roadsters have been purchased.  In addition to the excellent engineering reflected in the chassis design, the F5 is attractive to me for for other reasons.  Primarily among these is the opportunity that the F5 'kit' provides me to build a car taylored to what I want.  Additionally, a substantial 'aftermarket' existes for parts (and ideas) for the roadster.  

The main attributes of my Mk IV:

  •  EFI (I live in the mountainous West and will drive over a wide range of altitudes)
  •  HP: ~ 360 (goal: 0 to 60 in < 4 sec.)
  •  Guardsman Blue (perhaps a darker blue) w/ Wimbledon White 
  •  Competition dash
  •  Black powder coated rollbars, etc.
  •  15” wheels
  •  Solid rear axle (8.8”) with a 3.55:1 gear ratio with positraction and    ‘TruTrac’.  The rear suspension is the 3-link developed by Factory Five Racing with Koni coil over shocks.

My goal in the build is not to make a detailed replica of a 1965 Shelby 427 Cobra but to create a car that has the ‘character’ of one.  In fact, I don't really think of the Mk IV (or its predecessors at Factory Five) as a replica so much as a space frame race car with modern mechanicals that has a 'cobra' body shape.  I’m also taking a somewhat minimalist approach in that the completed car will have more of a race car look than not.  I plan to minimize the use of chrome in favor of black powdercoating.  

So, how much horsepower?  My goal is to build a roadster with a 0 to 60 time of less than 4 seconds-- basically, acceleration in the 'supercar' range.  The plot below shows data taken from Car and Driver (July 2013 in red) for a wide range of vehicles (see supplementary material page: Road and Track data) and plotted as  acceleration times as a function of power to weight ratio.

HP v wt

Note that given the wide range of cars sampled, a HP / wt. ratio of greater than 0.150 should yield my goal.  So, given that the Mark IV and driver weigh about 2500 lbs., the target HP is 360 HP.  Note that the F5 competition Roadster (F5 data) has a HP / wt. ratio of 0.228.  Obviously, for a given HP / wt. ratio, acceleration is also dependent on other variables such as the suspension, tires, etc.  Another interesting perspective is performance cost-efficiency.  The chart below plots acceleration times for the Road and Track data (minus the Veyron) as a function of purchase price.  The Factory 5 Competition roadster is shown by the red dot.

Grassroots Motorsports recently published an article (December 2014, Vol. 31, #8) on 'What Makes the Perfect Sports Car?', by Norman Garrett, the designer of the original Miata.  Below is a table from that article comparing various classic sports cars on a weight to power ratio (note that this is the reciprocal of the parameter in the HP/wt. plot).

Finally, there is the estimation of 1/4-mile trap speeds from the ratio of fly wheel horse power to weight (HP / wt.).  One such model is Hale’s:


The figure below shows 1/4 mile trap speeds as a function of HP / wt. ratio (data mostly from various Road and Track) issues.  The curve fit is a power function.   With a F5 MkIV HP / wt. ratio of 0.228, the regression provides an estimate of a trap speed of 138 mph.  We’ll see!