I've just finished welding up my manifold and now need to tee off the collector for an external, I'm going to run either a 38/40mm, so would I be right in saying the tube size needs to be the same? Space is tight on my collector so 44mm would be a struggle.

Thanks,
Ben.

 

it depends on your boost level/turbo etc-ideally yes to avoid boost creep-but i would think this unlikely with an external wastegate..you can but try and see

 

tube size same as what ?

Same with any valve sealing a hole....the valve itself is a huge restriction to any diameter.

So valve size inside a housing will flow less less than same diameter of open tube

 

Sorry I should have explained a bit better!

Basically, if I'm to run a 38mm gate, I'd need at least 38mm tube to tee off from the collector, correct?

 

I'd buy the external wastegate you are going to use first, then measure the OD of the v-band flange usually supplied with it to get the ID of the tube size you need to use.