Why are the rear two runners on each side completely siamesed while the fronts are not?

 

You must have the 1st style SLP runners. The 2nd design has a divider in both pairs.

By the way the 1st style SLP runners are a really bad design. However being the nice guy I'am, I will take them off your hands.

 

i have SLPs and they have the divider on both sides but are only siamesed for the first 2 inches or so.

 

1989GTA That is a very neighborly offer! ..... BUt I have to decline your offer. However it's all being polished and will be for sale shortly.
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So why did SLP do this and were there downfalls to this or did they actually help?

 

they dont revise a casting unless theres a problem... my guess is that the totally siamesed tubes are creating different turbulance than the others. a fully siamesed runner will flow more air but will lose the TUNED PORT effect and runner velocity which is what gives the TPI is brutal low end torque. by fully siamesing the runner u lose that effect but u gain increased air flow which will give u a higher RPM band. by them siamesing the first 2 inches or so of the runner divider they increase the effect plenum area and they make the runner shorter which allows an increase in the powerband/RPM capabilities. in your situation its an inbalance so the rear siamesed runners are doing diff things than the front runners aka its off balance.