Is going all catless on a na fa20 motor such a good idea? - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

Astroboy · 09-09-2013, 12:34 AM

I heard some motors need restriction in order to keep its power consistent. And eliminating some of these restrictions may even cause a decrease in power... Im no car genius but I'm assuming that the only way to get the best out of it is to switch to e85.. so what are your thoughts on the fa20 motor with nothing but 2.5 straight pipes.

I would also like to know if maybe even a tune for 93 oct would help...

And like I said before, I'm no car genius, that's why I'm asking this.

campy · 09-09-2013, 12:47 AM

I thought the backpressure myth was busted.

Fast_Freddy · 09-09-2013, 02:02 AM

Back pressure is never desireable but neither is having an exhaust pipe so big that the gasses lose too much velocity. Maintaining exhaust gas velocity as temp drops from entry to exit is what we really want to accomplish. You'll see that many oem exhaust systems use decreasing diameter pipe from front to back.

IMO 2.5" would work well enough although 2.25" would work equally well. Anything larger than 2.5" is strictly for FI. The biggest restrictions in the oem exhaust are the cats. I gained 25whp with just a Borla UEL, Tsudo hi-flo cat front pipe and a Visconti stage 2 tune. Replacing the rest of the exhaust probably wouldn't add much more than 5 more whp, if that.

d1ck · 09-09-2013, 07:47 AM

Quote:

Originally Posted by Fast_Freddy

Back pressure is never desireable but neither is having an exhaust pipe so big that the gasses lose too much velocity. Maintaining exhaust gas velocity as temp drops from entry to exit is what we really want to accomplish. You'll see that many oem exhaust systems use decreasing diameter pipe from front to back.

IMO 2.5" would work well enough although 2.25" would work equally well. Anything larger than 2.5" is strictly for FI. The biggest restrictions in the oem exhaust are the cats. I gained 25whp with just a Borla UEL, Tsudo hi-flo cat front pipe and a Visconti stage 2 tune. Replacing the rest of the exhaust probably wouldn't add much more than 5 more whp, if that.

Bingo. The ideal exhaust system would in theory have a constantly decreasing diameter with a few bends as possible. It would also have no restrictions such as cats present; which slow down the exhaust gas flow.

Guillaume · 09-09-2013, 08:21 AM

The ideal exhaust manifold has two diameters with an abrupt variation. It controls the pressure waves in the manifold and thus the acoustic effects affecting the engine's intake.

Now, on a street car, with an engine designed to run with cats, the valve timing has been set up to run with back pressure.
Losing the cats would help, but running with absolutely zero back pressure (i.e. too large decat pipes) on stock cam shafts and ECU would not give you better performance.

Ryephile · 09-09-2013, 08:25 AM

As a rule of thumb, modern catalytic converters lose very little power. The FA20 has lots more on left on the table than trading 1-ish HP for smelling like open ass every time you get out of the car.

FR-S Matt · 09-09-2013, 08:50 AM

For the sake of my own and everyone else's ears, please keep your cats on.

No one wants to hear a god awful raspy fartcan.

Honestly, the difference between cat and catless is negligible at best. One runs cleaner than the other and sounds MUCH better. Don't be a cheapass.

Jive Turkey · 09-09-2013, 02:21 PM

full catless will not only not net a benefit, but it will sound so raspy.

Bonks86 · 09-09-2013, 02:28 PM

I am running a JDL Header, overpipe, Invidia catless midpipe and N1 and I have very little to no rasp at all. I know @nelsmar can attest to this as he was cruising next to me for a while on Saturday.

I thought for sure after the header install I would nee to go back to a catted frontpipe, but to me it sounds great without any cats.

I am going to try and get some good audio of it but I need something better than my phone

ShadowSeraph · 09-09-2013, 03:04 PM

I am running Full catless with the Borla UEL and Perrin 3" catback exhaust... no rasp.. no fartcan.... all growl. Power seems fine... if i could only find where a dyno is around my area.

Dephective · 09-09-2013, 04:37 PM

Rasp isn't necessarily caused by lack of cats guys. Add another resonator and the same effect of reducing rasp can be found. It has a lot to do with the frequencies that vibrate the materials. Also, shitty made mufflers can rasp with or without cats, and may only present the sound AFTER you go catless, i.e changing the frequency of the exhaust.

If you ever plan to autocross your car, you definitely want to keep a cat.

SkAsphalt · 09-09-2013, 04:43 PM

For environmental reasons you should keep your cats
Do you like the paint on your car? enough people remove their cats and we will have acid rain again

Kelbyat07 · 09-09-2013, 05:01 PM

Quote:

Originally Posted by ShadowSeraph

I am running Full catless with the Borla UEL and Perrin 3" catback exhaust... no rasp.. no fartcan.... all growl. Power seems fine... if i could only find where a dyno is around my area.

I heard that that setup was nice and very loud. Is yours really loud?

StormTrooper · 09-09-2013, 07:17 PM

Quote:

Originally Posted by SkAsphalt

For environmental reasons you should keep your cats
Do you like the paint on your car? enough people remove their cats and we will have acid rain again

False.....

It does stink and it won't gain any power though, so no real good reason.

09-09-2013, 08:25 AM	#6
Ryephile Hot Dog Join Date: Apr 2011 Drives: quicker than arghx7 Location: Metro Detroit Posts: 1,316 Thanks: 103 Thanked 173 Times in 83 Posts Mentioned: 6 Post(s) Tagged: 0 Thread(s)	As a rule of thumb, modern catalytic converters lose very little power. The FA20 has lots more on left on the table than trading 1-ish HP for smelling like open ass every time you get out of the car. __________________ "Wisdom is a not a function of age, but a function of experience." Just Say No to unqualified aftermarket products.

09-09-2013, 04:37 PM	#11
Dephective Senior Member Join Date: Mar 2013 Drives: '13 Whiteout FRS Location: Bay Area, Florida Posts: 233 Thanks: 110 Thanked 79 Times in 58 Posts Mentioned: 2 Post(s) Tagged: 0 Thread(s)	Rasp isn't necessarily caused by lack of cats guys. Add another resonator and the same effect of reducing rasp can be found. It has a lot to do with the frequencies that vibrate the materials. Also, shitty made mufflers can rasp with or without cats, and may only present the sound AFTER you go catless, i.e changing the frequency of the exhaust. If you ever plan to autocross your car, you definitely want to keep a cat. __________________ Previous: '01 Accord-daily '93 Rx7-fire breathing track monster 545hp 397tq 2300lb. '06 Mazdaspeed6 one helluva love-hate relationship

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
FA20 + Boring holes = bad idea?	gt8613	Scion FR-S / Toyota 86 GT86 General Forum	19	08-20-2013 01:36 PM
j2 catless pipe	sosick	Engine, Exhaust, Transmission	28	07-31-2013 10:32 PM
Catless Vs HFC?	Koolkiid	Engine, Exhaust, Transmission	14	05-28-2013 08:03 AM
Going to track with stock setup good or bad idea?	evasquez7	BRZ First-Gen (2012+) -- General Topics	11	08-21-2012 02:33 PM
Considering selling my BMW 335i to get FR-S... Good or Bad idea?	iBurnTires	Scion FR-S / Toyota 86 GT86 General Forum	112	08-02-2012 08:46 PM

09-09-2013, 12:47 AM	#2
campy No rice, no life Join Date: Dec 2012 Drives: 2013 Asphalt FR-S Location: Jersey Shore Posts: 1,143 Thanks: 266 Thanked 1,019 Times in 388 Posts Mentioned: 7 Post(s) Tagged: 0 Thread(s)	I thought the backpressure myth was busted.

09-09-2013, 02:02 AM	#3
Fast_Freddy Senior Member Join Date: Jul 2012 Drives: Passat Location: East Coast Posts: 626 Thanks: 331 Thanked 188 Times in 130 Posts Mentioned: 6 Post(s) Tagged: 0 Thread(s)	Back pressure is never desireable but neither is having an exhaust pipe so big that the gasses lose too much velocity. Maintaining exhaust gas velocity as temp drops from entry to exit is what we really want to accomplish. You'll see that many oem exhaust systems use decreasing diameter pipe from front to back. IMO 2.5" would work well enough although 2.25" would work equally well. Anything larger than 2.5" is strictly for FI. The biggest restrictions in the oem exhaust are the cats. I gained 25whp with just a Borla UEL, Tsudo hi-flo cat front pipe and a Visconti stage 2 tune. Replacing the rest of the exhaust probably wouldn't add much more than 5 more whp, if that.

09-09-2013, 08:21 AM	#5
Guillaume Senior Member Join Date: Jul 2012 Drives: Exige S 240 Location: Belgium Posts: 732 Thanks: 319 Thanked 602 Times in 287 Posts Mentioned: 4 Post(s) Tagged: 0 Thread(s)	The ideal exhaust manifold has two diameters with an abrupt variation. It controls the pressure waves in the manifold and thus the acoustic effects affecting the engine's intake. Now, on a street car, with an engine designed to run with cats, the valve timing has been set up to run with back pressure. Losing the cats would help, but running with absolutely zero back pressure (i.e. too large decat pipes) on stock cam shafts and ECU would not give you better performance.

09-09-2013, 02:21 PM	#8
Jive Turkey the tuna, no crust. Join Date: Oct 2012 Drives: 2014 Hachiroku of the year Location: jersey Posts: 2,298 Thanks: 874 Thanked 1,770 Times in 761 Posts Mentioned: 78 Post(s) Tagged: 0 Thread(s)	full catless will not only not net a benefit, but it will sound so raspy.

09-09-2013, 02:28 PM	#9
Bonks86 Senior Member Join Date: Feb 2013 Drives: Gt-86 Location: AZ Posts: 222 Thanks: 48 Thanked 72 Times in 53 Posts Mentioned: 10 Post(s) Tagged: 1 Thread(s)	I am running a JDL Header, overpipe, Invidia catless midpipe and N1 and I have very little to no rasp at all. I know @nelsmar can attest to this as he was cruising next to me for a while on Saturday. I thought for sure after the header install I would nee to go back to a catted frontpipe, but to me it sounds great without any cats. I am going to try and get some good audio of it but I need something better than my phone

User Name		Remember Me?
Password