Exaust: back pressure and velocity
 

Hello, I’m new to the car scene, and trying to best my knowledge. So I bought my 2013 FRS not too long ago. I’m new to this website so my apologies in advance for anything i may be doing wrong and/or nooby terminology.

First things first, I’ve recently started playing with my exaust, currently I have a high flow resonator with split y pipe and twin cans. I really want to go catless but my friend told me I would lose too much back pressure and higher velocity considering I already have a high flow resonator and no actual muffler. Now I looked into this as anyone would and to my understanding, I want a balance of back pressure and velocity, preferably low. Now here’s my question/‘s, if I were to take Out my cats would I lose power? If I bought high flow cats would I lose power? And if not what can I do to get louder without giving up some power? Or even just get a higher pitched sound?
Thank you

Keep in mind taking out your cats will do you little good unless you get tuned for it. You car's exhaust will also stink, which turns a lot of people off from going catless.

I'd recommend a high-end performance cat. Newer cat technology is W-A-Y better than it used to be with minimal performance losses. You won't have to deal with a check engine light either.

Installing an aftermarket primary cat is no guarantee that you won't get a CEL. A tune is always recommended. "Usually" the tuner will disable the P0420 function. Now a days, I not sure of the legality of that action.

There is a profound difference between a $150 cat and a $800 cat. The latter will work (at converting hydrocarbons) at least as good as OEM, if not better. We've never had a customer of our 911 cats get a CEL or fail emissions, even in Cali.

It has always been illegal, but with the modding popularity over the last several years, it's got to the point where some manufacturers are low hanging fruit for prosecution.

My experience is that you shouldn't overthink it. There are plenty of good headers you can buy and two good tuning solutions available. The development work and theory have already been taken care of.

Will the exhaust stink with just the header CAT removed but the front pipe still stock? would it still be bearable

From my experience a catless header will produce more exhaust smell but only a little more sound. The secondary cat does pretty good on its own. I do run e85 though so my exhaust smells like corn lol.

As far as the cat, as others have said here, just use either the stock front pipe, or get a high flow aftermarket cat. The stock front pipes are actually quite good and swapping them doesn't net much of a gain. The cat also keeps rasp down.

As far as "back pressure", what you really want in an NA exhaust is the ability for the exhaust gas to exit as freely and quickly as possible. This isn't just going with the largest pipe available.

Typically, stock exhausts are a hair small, but they're generally close to the right size because manufactures are looking for efficiency to balance power, fuel economy, etc. Usually speaking, going a bit larger helps power. Let's say from 2.25in inner diameter to 2.5".

The idea behind reducing "back pressure" as people say is just fluid dynamics. You want that exhaust pipe "the right size" to allow for the gasses to exit quickly. Not too big, not too small. And that is highly dependent on the engine size (IE V8 vs a boxer four cylinder like these), forced induction or NA, etc.

And age of the engine design! Many old engines with parts just sort of floating around in them needed back pressure to help things open and close. Modern engines with overhead cams, hydraulic lifters, etc just don't have the same requirements.

The Twins already come pretty much optimized for power in stock form. What traps people is thinking they are like Mom's old Civic with it's cast exhaust manifold, tiny exhaust and restrictive muffler that can see huge improvement by opening them up. Not to say that there can not be gains by increasing flow but they just simply will not be a great as those to be had on a car not designed for performance in the first place.

After I had my Tomei EL Catless headers installed (stock OP/FP/Perrin Catback), I noticed that there was reduced initial throttle response and (potentially due to placebo) lower power in the low rpm range (idle-2500rpm). The first half inch of the throttle seems more or less dead whereas before the header install, it felt very sensitive which helped clutch engagement.

Always wondered if this is because I reduced backpressure by going catless or if this problem points to something such as a leak. Love to hear if any other owners who installed cat-less headers felt this change.

Is it tuned? And if so with what/who?

That type of stuf is normal if you had it tuned after installing such parts. The ECU has to relearn the throttle position-power delivery. If you're still experiencing this, it could be a bad tune.

Don't know where to begin, but here it goes:

High RPM engines have very stiff valve springs. Or else, the valve springs set themselves into resonance within the operating ranges of speed. But when you use very stiff springs to over the resonance issues, you end up losing a lot of efficiency at mid-range. At full throttle, there is plenty of power to overcome the mechanical losses, but at partial and low throttles, where the engine makes significantly lower power, the mechanical losses due to valve spring compression become dominant. For this reason, the exhaust backpressure during mid- and low-range RPM creates enough negative pressure to help ease the effort it takes to compress the springs. The motorcycle industry relies on ex-up valves that can be adjusted on the go. I'm no longer involved with the exhaust mechanisms in the auto world, so I don't know if the current automotive technology uses this kind of valves, or still the exhaust pressure from other cylinders.

TLDR: You need backpressure in low-and mid-range to minimize mechanical losses from the valvetrain.