I've been told by several rebuilders, and fellow RX-7 owners that the FD manifolds (LIM/UIM) are unevenly matched for the flow of the rotors. If this is the case, one rotor would be receiving a greater flow and making the rotor more susceptible to blowing a seal. Why the hell would it be designed this way anyway.

If anyone has any factual evidence, or experience with this... please post =)

THanks,
-Darren-

 

IT is true...they are unequal length and probably flow slightly differently due to changes at the primary runners near the block. These are emissions design requirements...the manifold has to sit further back to allow room for the air pump, and there are other emissions passages in the intake that obstruct the clear and straight path that we'd have liked to see the runners take.

This is the reason the GZ LIM was developed...for no emissions people. IT has straight and equal length runners, but it moves the lower and upper manifolds foward a couple of inches and has none of the additional castings and pathways for emissions components.

 

What Kevin said^.
To add the rear rotor has the better air flow because the 2 runners are more straight than the curved runners on the front rotor. This is one of the reasons the rear rotor usually blows first during a lean condition because it already runs leaner than the front. Also more heat in the rear half of the engine also contributes to the rear rotor running more lean since the water pump pumps the water from front to rear. By the time the water reaches the rear rotor, it has heated up substantially. By the way it's just the LIM that's uneven in flow the UIM is balanced.

On a side note, my 20b's LIM is unbalanced also. The center runners are straight while the outer front and rear runners are curved and longer. The 20b will usually blow the center rotor first durning a lean condition. However that's not just to say that it's because of the runner design. The center rotor also gets more heat from both sides of the engine compared to the front and rear rotors that also cause it to run more lean.

So what does all this mean? On a 13b to perfectly tune it you will need to install 2 exhaust temp probes to monitor exhaust temps while tuning. This way you can balance the egt by adding or subtracting additional fuel from the injectors in that chamber.

 

Ok thanks guys, that's all I needed =)

 

If you want to talk to someone quite knowledgeable about this, call Rob Golden at Pineapple Racing.

 

Yeah I'd imagine since PInapple had a hand in the design of the GZ Manifold and oil pan =P

They make good stuff.

 

Yep I sat an listened to him talk about this same subject at sevenstock 2004.