I can agree somewhat with f rod, but also have my own opinion to offer. One main difference is price. Grubee kits sell for anywhere from 150 to 200 depending on sales and shipping prices. kits on ebay sell for 150 or less. replacement grubee engines sell for 150. ebay replacement engines sell for around 90 or less. i picked up a replacement engine for 90 from boygofast on ebay. i liked it better than my grubee engine as soon as i pulled it out of the box and noticed that the front engine mount would fit my wider down tube. no more rigging. and even before i got my x chamber, the ebay engine had a lot more power than i was used to with the grubee engine. at 22 mph top speed, i was scared to drive my bike the first couple times because i could feel that extra torque. after installing my expansion chamber, i gained ten mph and a significant amount of torque. i would say my engine probably has 3.5 to 4 hp. another big thing with grubee is terrible customer service. i speak from personal experience here. i lost $50 to grubee on a part i never received. i called them and they refused to send me the part because according to them, it was already sent. grubee has gotten a really bad rep because of their lack of customer service. i know when i order something on ebay, there is a 95 percent chance of me getting what i ordered. i havent ever once gotten a bad item from ebay. i cant say the same about grubee.