i pulled my UIM.and i noticed that my secondary intake runners are a light brown to gold,while my primary runners are 100% clean.why is that?

 

I am not completely sure but here is my best guess. The primary runners don't have any fuel running through them. The secondary runners have injectors about half way down. The fuel may have some gunk in it that builds up over time. The injectors can clog from debris and crap in the gas so it is only logical that some of this could get stuck on the inside of the runners too. The fuel will also get back up the manifold a little from returning pressure pulses. This will leave the runners a little sticky and dirty inside. I could be wrong though.

 

Well cant say for sure about 13B cars. But on 12A 1st gens the primaries have a huge anti-reversion dam at the mainifold/center plate interface. The sces have only a small dam so consequently its common to see the secondary runners with more crap in them from the reversion pulse that rotarygod alluded to.

 

what does a anti-reversion dam look like?

 

Its a mismatch of the manifold to engine interface. The outlets of the manifold should be smaller (even if slightly) than the engine side. This hurts any reverse flow you get.