The C7 or C-7 ACERT? The C7 has the HEUI system and was used up until 2010. The ACERT line has what I believe to be the only way EGR should have been designed--clean exhaust from post-DPF is put through the EGR cooler rather than dirty fuel like other manufacturers use. My shop foreman, who worked on
cat engines for a good number of years told me the 3204 is very heavy, too heavy for what I want to use it for. He also said the 3100s aren't as bad to set the rack on as everyone makes them out to be as long as you're meticulous and have the tools (he can get them...). That line also has the HEUI system and the 3126 is the precursor to the C7.