I am making a Y pipe for my hedman shorty headers with a 3" collector. I don't know wether to make the whole thing 3", or if I should make the pipe out of two 2.5" pipes that converge into a 3" pipe. What is the general opinion? Is 3" going to give me clearance problems?

 

2.5" to a 3" collector will be fine. Most people are running a 3" and have no problem, some are running 4" with no clearance issues so you should be fine.

 

Do I gain anything going to 3"? It seems to me the only thing I will gain is clearance problems!

 

Question? At the collector point of the headers is the hole opening 2 1/2 inches or 3 inches. That will tell you what size to make the pipes into the "Y" pipe. Usually they are 2 1/2 inches. Make sure the header opening is not closed in by sloppy welding or forming. If so grind it out or you will loose HP. Same where the header mounts to the head.

Saw a pair of headers over the weekend from a reputable company and the hole size at the collector was only 1 3/4 inches due to the way it was formed. That was barely bigger than one header by itself. Otherwise the headers were well made and looked good. The owner had them opened up to 2 1/2 inches.