Cat vs De-cat

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Jan 14, 2020 | 08:39 AM

#21

turbotom1052

Road Master

Joined: May 2012

Posts: 867

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From: Vermont

All of the freed up airflow you your getting will surely be of benefit when your at wide open throttle, with fuel injectors mapped to accommodate the increase. My bet would be that the lack of restriction will wind up hurting in the lower rpm ranges even if you mapped out the flow in them cells. I don't know too many people that ride their Harleys at wide open throttle for very long. For the sake of noise you would probable be hurting your performance at the RPMs we like to ride at, EVEN with adjustments to fuel delivery.

Jan 14, 2020 | 09:45 AM

#22

Gary1

Road Master

Joined: Dec 2017

Posts: 835

Likes: 161

From: Texas

I decatted my header last week and I really enjoy the less heat that coming from that area. I read from vance and hines that you would experience a loss of torque from 2000-2500 and this might be true, but I can't feel it. Now the exhaust tone does seem more raspy and loss some of the deep tone, but in general it's louder and has more of a high pitch to it. I have a fp3 so I selected the header without the cat from their canned tunes and flashed the ecm and it runs great. I also have Cobra Neighbor Hater slipons.

Last edited by Gary1; Jan 14, 2020 at 09:47 AM.

Jan 14, 2020 | 10:31 AM

#23

Cossack84

Tourer

Joined: Mar 2015

Posts: 476

Likes: 133

From: BlueGrass

Quote:

Originally Posted by CWOFOR

For the last 3 years I have been reading most of the threads about cat vs de-cat; in each of the threads is a common theme of non-agreement as to weather or not the cat provides a restriction of exhaust flow to warrant re-tuning or not. It is also a common theme that none of these threads offers any official proof as to what the flow is with the cat and what is without the cat.So, I am in the process of changing my exhaust from cat to de-cat; I have a 2018 SG with a 107; I purchased a stock head pipe from a 2018 SG 107 and de-catted it. I took both head pipes to my local speed shop and hooked them up to a flow bench.

Cat in/ 28 inches of vacuum/ flowed 280 CFM.
De-cat/ 28 inches of vacuum/ flowed 385 CFM

Both tests were done with O2 sensers in and the cross over blocked.

The speed shop tech said this is likened to the difference between a 2 barrel carb vs a 4 barrel carb and It is significant.

Not drawing any conclusions, just providing information/results from my testing session. Hopefully this also provides info to further the discussion.

Where does the 28 inches come from?

Jan 14, 2020 | 12:55 PM

#24

IBE

Seasoned HDF Member

Joined: Dec 2019

Posts: 5,276

Likes: 4,055

From: central Fl

Quote:

Originally Posted by Oldskewl

In Commiefornia there are whispers of a new bill requiring motorcycles to get smogged. They would start with all bikes 2017 and later and then gradually (say 2026) move to earlier years. Just another way for this state to make money.
With that said, I've been thinking about buying and stockpiling stock head pipes for all Harley Davidsons 2017 and later. Everyone who destroyed the stock headpipe would need to come to me or someone else for a replacement because Harley ain't gonna do it.

I have multiple stock headers boxed and bubble wrapped now. The minute the bill is introduced I will start purchasing stock headers in bulk. Other states will follow suit and perhaps I can retire early.

that would mean all the aftermarket headers are or will be no good. and everyone would have to go back to factory or the after market headers would have to start building headers with cats in them. man whats wrong with CA. and what about the bikes that were built with no cat??

Jan 14, 2020 | 04:42 PM

#25

PoCoBob

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Joined: Nov 2006

Posts: 1,164

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From: Port Coquitlam B.C.

It's pretty simple math, a 107 cu in motor turning 6000 rpm is moving 185 cfm air. That's if it exchanges all the air inside it per cycle which is impossible. Getting a 80% air exchange would be on the high end so in the OP's numbers the cat shouldn't be restricting anything.

Jan 14, 2020 | 05:44 PM

#26

miner999r

Road Captain

Joined: May 2014

Posts: 675

Likes: 252

From: Alberta

Got an inexpensive set of stock head-pipes that were decatted and the bike runs noticeably cooler. This hit the benefit bar for me. I still have the stock headpipes with the CAT in case of requirement for whatever reason. Dyno tune was not required at the change but engine was dyno tuned later because it had to be rebuilt.

Sound? I have SE fatshots, I can't tell the difference, too deaf from too much loud in my youth. (Now a days, loud pipes just let me know the engine is running)

Jan 19, 2020 | 02:10 AM

#27

turbotom1052

Road Master

Joined: May 2012

Posts: 867

Likes: 160

From: Vermont

Quote:

Originally Posted by PoCoBob

Actually you can reach volumetric efficiencies that exceed 100% in a normally aspirated engine, and it doesn't even require any internal work. A quality full exhaust with the right collector can create a negative pressure wave in the intake that actually can fill a cylinder more that the cylinders actual displacement. Its not even new technology, guys have been doing it for decades.

Jan 19, 2020 | 03:44 AM

#28

Wizardofaus

Road Warrior

Joined: Apr 2017

Posts: 1,451

Likes: 230

From: United States

Quote:

Originally Posted by golfblues

you don't need a retune. If you have the stock air cleaner and stock Mufflers the decat is not going to affect it. The ECM will adjust on its own. It will probably even run better. I have a 19 107 with four and a half inch Rinehart slip-ons and decatted header. Stock air cleaner and stock ECM. Rides like a champ.

With a de catted pipe that flows better, Even with stock air filter and pipes, the flow is increased and that in its self with cause effects on cylinder scavenging / Fuel charging.

I doubt that the 17+ ECM’s will have enough band width compensate for such a change being there run by the mothership so freaking lean out the door.

Wiz 🍻