I have two Go Tech engines. A 5 port .21 and a 7 port .21. They are really good engines and they produce a lot of power. I have to agree with NitroOB4You that the Go Tech engine are extremely tight. You will need to use the extra head shim during the break-in procedure which lowers the compression and helps the engine turn over/start a little easier. The recommended pipes for the Go tech engine are the 2047, or the Dynamite 086 for the 5 port and Dynamite 053 for the 7 port.