I have been working with them the last couple of months and they work well. They have 3 different lengths available which will extend the header length by 13.5mm, 16.5mm and 19mm. The length difference between the 41021 and 20 is about 12.5mm to give you an idea of what lenghts you may want to try. I am also working maching new ones in lengths of 6,8 and 10mm to give us middle ground lengths to tune with. If you put the 16.5 and 19mm length's with the 21 header, the header acts too long and only has low end performance and falls flat in the midrange so if you are looking for a tractor pulling setup, there you go. The longer lengths will work better with really short headers and that is why I want to make middle of road lengths for those who want more than the 21 in torque but not quite what the 20 has for torque and loss of rpm. I will be working on those very soon so stay tuned