What is the '-C' suffix on the Hall pipe?

The "C" corresponds to a specific pipe drawing.

Example:

HPR 7 was a completely new pipe with a new set of dimensions and a corresponding drawing of those pipe dimensions labeled HPR 7

If HPR 7 is modified it now becomes HPR 7A and there is a HPR 7A drawing.  If HPR 7A is modified, it becomes HPR 7B.  The third modification to HPR 7 turns it into HPR 7C and so on. 

A modification may entail removing 5mm from the head pipe or it could mean one section of the pipe was removed and replaced with a different cone angle. 

The engine sees each pipe number or a pipe number with a different suffix as a different pipe with different dimensions. 

During pipe development, we try to modify a pipe as many times as possible before we start over with a completely new pipe.  It is common for each pipe number to have five or more modifications.  As you can see, pipe development is an expensive time consuming process, usually requiring 10 or more different pipes and sometimes testing over 100 different pipe dimensions before selecting one particular set of dimensions to use for the production pipe.

By the time we are ready to go into production we have a huge library of pipe dimensions, corresponding dyno and field-testing results and the specifications for all of the different engines used during the developmental process.  Having this huge database of pipe dimensions and corresponding dyno curves allows us to modify or analyze the dimensions from just about any pipe on the market and have a high level of confidence of how it will perform because we have “already been there”.