The Yasuni R and Z are restricted E marked road legal exhausts that are not really any better than a Polini Funky, ZX, etc etc... They are just a bit more rare so more bragging rights. They are restricted in the down pipe and in the end can. There really isn't much point in buying a R or Z due to the fact there are plenty of other exhausts on the market that not only do they perform as good as the R and Z they also are chromed so they wont rust like Yasuni's do and are around the same price. Eg. The Laser X-Pro and Hebo NG. The Yasuni City 16, C16, C20, C21 and C30 are the main Yasuni exhausts. The C exhausts are for race use only and don't have any E marks or any restrictors. The C before the number stands for Carrera which is Race in Spanish. They have recently brought out the City 16 which is an all round better version of the C16 and as an extra bonus it's e-marked. There is more information on it here. Each exhaust in the C range is designed for a different stage of tuning. C16/City 16 MHR Rep, C20 MHR, C21 Ported MHR and the C30 is a better performing version of the C21. Although the C16 is actually designed for a slightly more powerful 70cc kit than the MHR rep if you have everything else including the all important 21mm carb you will be fine. There is no way in hell you could put a C30 on anything else other than a ported MHR 70cc kit or it wouldn't perform as well or probably at all. The C30 and C21 are designed to run at a very high RPM that no other 70cc kit will produce. You also can't just use the exhausts by slapping the 70cc kit and the exhaust into and onto a bike. You also need to do every other part like carb and transmission. This will maximize the RPM which will in turn maximize the performance. It also gets more complicated as there are lots of different MHR kits for LC bikes. Like the MHR, MHR Team, MHR Team Speed, MHR Cross, MHR Team Replica, etc. There are lots of other 70cc kits as well that all need different Yasuni exhausts like the 2fast 4race, Polini Evo 2, Metrakit Pro Race 2, Hebo Manston-Racing 2, etc. Note that the use of MHR kit's in this guide is just to make it a little easier to explain. If you need any specific advice on what Yasuni your 70cc kit will work best with feel free to ask on our Forum.