90Mph? are you sure? That seems way too fast, thats like 145 Km/h. I would have to see evidence before I believed that. Even though that engine is a HPI 4.6 it would be heavy and would loose speed. The drive train would tear apart from the sheer pressure the engine would put in the diffs at that speed. It may produce 2.9HP over the claimed 2.28 the TZ does but the RPM would be a lot different. I'm not doubting you at all, I may put a BB in mine later but if not I will throw in a brushless system

What gearing are you running?