I've come to terms with the fact my stock cast exhaust mani is going to be a real bottleneck at my goal power levels so I'm going to need to get the TIG plant rocking. Is there a formula to work out what diameter tube would be best suited for a specific application, or is there a general rule of thumb on xxx power requires xx tube?

Also, before I get the elbows ordered through work, is there a company that people use themselves for their elbows?

Ta muchly!