Correct, bigger diameter pipe and removing the secondary cats exactly accomplishes higher exhaust velocity at high rpms when the engine is producing max power. Definitely, ideal custom setup is the one you have in mid. Resonator and cat delete with a larger diameter X-pipe is the way to go.

 

Great feedback again.

I have opted for the custom route as the OP would follow more or less the same design as the usual suspect brands in market, the only difference is it's at a discounted price and not the ridiculous prices out there!!

I will start with X pipe and see where things take me, maybe stage two is an custom axle back correcting the steep narrowing bends.

 

Cool. Post videos when you have it installed.

 

There's a science problem with this statement. A bigger tube invariably has lower velocity for the same volume of gas (the volume is fixed by the engine's displacement). This is why primary header tube sizing (length and diameter) is so important to exactly where the engine makes best torque. Smaller/longer tubes have higher velocity and make better torque at lower rpm, bigger/shorter tubes have more backpressure (yes, this is counterintuitive, but it's true because backpressure is directly related to tube cross sectional area) and make better torque at higher rpm assuming WOT under both conditions. Part throttle throws a wrench into everything on engines with throttleplates - diesels don't have this problem.

Removing any restriction will increase velocity. Not all "restrictions" are intuitive or obvious, and anyone who has spent time with a flow bench will tell you this. I've seen horrible looking things flow shockingly well, and I've seen things that looked like they should flow like magic and absolutely do not.

Cats are not necessarily restrictive. GESi cats are well known for zero power loss, meeting emissions compliance, and the ability to tolerate racing conditions (high loads) for extended periods. So you don't have to spew out unburned hydrocarbons (yes, the standard Lexus tune is intentionally rich to ensure the engine survives warranty even on bad fuel and the engineers expect the cats to clean it up) and carbon monoxide just for a few horsepower. IMHO it is irresponsible to run a street vehicle without some kind of catalyst. Our kids have to breathe this crap!

So, if you arbitrarily make the tube larger, you might end up very unhappy the same as if you arbitrarily make it smaller. It's all about balance and where you want to make power. That said, the biggest influence here is the header itself, and the rest of the exhaust is a minor player in relative terms AND you're just rocking the power curve which changes the peak number, but the area under the curve stays fairly consistent.

 

Well said. Laws of physics win again.

 

Me again.

Still working through the decision between deleting cat + resonator or just resonator.

I have heard all the videos for res deletes but not for cat + resonator delete from both inside cabin (windows up/down) and outside of the car.

 

What did you end up doing?