My father-in-law Lou Terra and his partner, Bruce Shaw, opened Shaw's speed shop. My father-in-law did all the machine shop work, Bruce ran the store. My father-in-law has built (or had a hand in building) nine NHRA record holding cars. For example, if you look at any cars Kenny Montgomery drove, or "Project Six Pack", both had work done on them by my father-in-law. You can see Shaw's logo on them.
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There is more to it than the price of the part.
My father-in-law is still in business, although starting to ramp down. He still gets people who bought parts from Summit, Jegs, etc. and then come in wanting free expertise on how to install them or make them work. He simply says to go back to where you bought the part for help.
What speed shops provided was not only the parts, but the expertise to install them and make them work well.