Would be interesting to know how much HP is gained by eliminating the secondary cats. The cats are the most restrictive part of any exhaust, so I'm sure it's worth something.

 

Probably around 5 - 7 whp max with cat delete alone. Full catback is around 18 - 20 whp (JoeZ before/after was 26 whp), but that factors in the larger diameter of exhaust and the better flowing muffler design etc.

 

Yea I was assuming a full catback was probably around 20whp. But I'm also willing to bet the cats are still the most restrictive part of the exhaust, its like a honeycomb in there. 10-12whp would be my guess from eliminating the secondary cats.

 

This is somewhat true. According to Paul from PPE engineering the greatest gain from the exhaust system is by the complete removal of cats or upgrading the headers. If you are to go the aftermarket header route best place the cat further away from headers is more beneficial for the power band. The factory places the cat higher up the manifold as the cat is more efficient with heat and works better for emissions. But doesn't help with HP gains.

 

OP, sounds great! Has anyone ever opened up the factory RC F muffler? I am curious whether Lexus used an internal valve/flap to allow straight-through flow at WOT, or aggressive driving. I have an RC350 with headers, and a previously installed full ARK exhaust. I hated how loud it was, and devised a plan for what I thought would be a happy medium between sound and flow. I procured a set of factory RC F mufflers, and mated them to an Apexi mid pipe with resonators. No cats whatsoever. The results were not what I expected. High-idle at start is loud, really loud, with a deep rumble. WOT is also incredibly loud, but normal driving is incredibly subdued, with no drone and just a bit of deep exhaust note (similar to the old G35s). I suspect there's a flap in the muffler that is changing the tone depending on exhaust flow, but I cannot find any info on the RC F muffler internals.