Exhaust Upgrade Theory

chiefmeat68 · 09-28-2022, 01:57 PM

It seems everybody claims a significant boost in power by upgrading their exhaust. I believe these claims. I understand a higher flow less restrictive exhaust will give a performance boost.

What is not obvious is why many of the aftermarket exhaust setups I see suggested would be any less restrictive than the stock exhaust?

Like this one I am considering... suggested by moto taco... for the Hawk 250 as an example...
https://www.amazon.com/dp/B07GYSLJMD...roduct_details

It apears to be the same tube diameter and still has a cat... how does it make such a difference?

Is it actually bigger ID diameter? Smother on the inside? Less bends? Is the cat less restrictive somehow? Is the baffle/muffler the key?

Would it be better to keep the stock exhaust and remove the cat or would an aftermarket one with a cat still be better? Any suggestions for an aftermarket that comes without a cat?

Maybe it exists... but if not it seems someone could make some money selling just a straight pipe "header" that mates with the stock mufflers!!

**JerryHawk250** · 09-28-2022, 02:12 PM

You can remove the cat on the stock exhaust and achieve the same results. The aftermarket exhaust doesn't have the cat which makes it less restrictive. There is a perfect balance you need to maintain when doing exhaust. You want to have enough back pressure to keep the low end power. Less restrictive exhaust will move the power to the higher rpms and loose some low end power. I found my decat stock exhaust on my stock Hawk engine performed better. Once i did the porting and decking of the head the freer flowing muffler work better.

Megadan · 09-28-2022, 03:48 PM

75% of the power increase is the result of the catalyst being deleted, at least on the Hawk. The cat on the Hawk is horrendously restrictive. The other 25% would be the muffler.

I did the exhaust because I like having a stainless steel system.

CheapThrills · 09-28-2022, 05:14 PM

I felt a little more acceleration with an exhaust upgrade, but real benefit for me was better top end with ability to gear higher. Oh, I have a TBR7 btw.

chiefmeat68 · 09-28-2022, 05:58 PM

So maybe I've been making an incorrect assumption about the can looking thingy on the header. I have been assuming that is a cat.

If it's not a cat then what is it? Maybe an expansion chamber of some sort?

**JerryHawk250** · 09-28-2022, 08:08 PM

On the stock exhaust it is the cat. On the after market exhaust it just there for looks. Fake cat.

Thumper · 09-28-2022, 09:58 PM

Quote:

Originally Posted by JerryHawk250

On the stock exhaust it is the cat. On the after market exhaust it just there for looks. Fake cat.

Leap of faith is that the cat blimp on aftermarket exhaust is not filled with a honeycomb catalytic converter. They don't state this (gee, I wonder why!?). So which ones are EPA compliant (cat), and which ones are not?

The ambiguous nature of these aftermarket pipes makes it impossible to confidently buy an exhaust system.

The best way to avoid the confusion is to just gut the cat:
https://www.chinariders.net/showthread.php?t=30235

Crude, yes. But in this case, I have created a power bomb ("resonator?").

Megadan · 09-29-2022, 12:57 AM

Quote:

Originally Posted by jeffrey

The pipe that you linked is a copy of the FMF Power bomb header. It is like a resonator but I dont know its real purpose. They tell about it on thier website.

It's a resonance wave tuner. Simple terms, exhaust pulse hits the open chamber and the pressure differential acts as an anode and causes a wave reflection and once it reflects back off the exhaust valve right before it opens it can help scavenge the cylinder. Similar idea to an expansion chamber on a 2 stroke, but in reverse. Depending on the length of the head pipe before the chamber determines the frequency it is tuned for and the ideal rpm range that creates the greatest scavenging effect.

BrokeAss · 10-03-2022, 10:22 AM

THE BACKPRESSURE MYTH DEBUNKED:

https://motordyneengineering.com/sca...heir%20results.

http://www.automotiveforums.com/t943..._s_wrong_.html

Zapkin · 05-09-2023, 06:31 PM

Is there any performance benefit to removing the baffle from the carbed Hawk 250 stock exhaust?

XLsior · 05-09-2023, 07:46 PM

The engine can't get much past 8000rpm so other than making a lot of additional noise the removal of the baffle doesn't really equate to noticeable additional power.

Though I don't have a stock hawk muffler...

I did try a baffle/non baffle test on my Aussie Muffler and the non baffle was unpleasantly noisy and no power bonus to speak of. The DIY baffle mod did help a little with maybe 100-200rpm but that might just be me wishing it to justify the work done to 'improve' that original configuration. I have noticed the muffler section does not appear to be pushing off the head pipe slip coupling, which to me suggests an improved flow rate and less back pressure.

If you're using an unported head then a de-cat would be more than adequate to help the engine breath better...

Bill Hilly · 05-09-2023, 08:31 PM

I , without a doubt can say that the Hawk benefits from just simply removing the cat, because all I have done to mine has been removing it. I had already removed the baffle/spark arrestor from the muffler,so I know the cat was holding it back. I have a TBR7 with full aftermarket exhaust, and I am pretty sure both my Hawk, and TBR7(both have 46/17 sprockets) are both really close to each other in power, although the TBR7'S smaller smoother tires gives it an advantage, but the cat removal from the Hawk did help the Hawk, in the same way that the full aftermarket exhaust helped the TBR7, which to me seems like they are not only happier, and less stressed at high RPM, but they pick up, and hold RPM better past mid range. There may be a little more to be had by working on the Hawk's muffler, because it's not really loud, but I would hate to go through the trouble, and not gain anything, and I like having my muffler guard, so I don't particularly want an aftermarket muffler.

Zapkin · 05-09-2023, 10:04 PM

Okay so then unless I just want to annoy folks, I should just leave it in there. Thx fellas.

chiefmeat68 · 05-09-2023, 10:25 PM

I hate loud annoying bikes... unless it's mine!!!

Bill Hilly · 05-09-2023, 11:02 PM

http://

At the bare minimum,you should remove the cat. If you do just that, it will be just barely louder than it would be with the cat, but the bike will be healthier. The small diameter cats are VERY restricted, and probably why similar bikes in countries that don't have strict emission standards produce more HP , than the lean jetted, and restricted exhaust bikes we get. I am posting a couple pictures of the Hawk cat, that I removed. On top of it is a quarter for reference. The diameter of the cat is small, and the pipe is thick walled to boot.http://

09-28-2022, 01:57 PM	#1
chiefmeat68 Join Date: Mar 2022 Location: Eastern WA Posts: 20	Exhaust Upgrade Theory It seems everybody claims a significant boost in power by upgrading their exhaust. I believe these claims. I understand a higher flow less restrictive exhaust will give a performance boost. What is not obvious is why many of the aftermarket exhaust setups I see suggested would be any less restrictive than the stock exhaust? Like this one I am considering... suggested by moto taco... for the Hawk 250 as an example... https://www.amazon.com/dp/B07GYSLJMD...roduct_details It apears to be the same tube diameter and still has a cat... how does it make such a difference? Is it actually bigger ID diameter? Smother on the inside? Less bends? Is the cat less restrictive somehow? Is the baffle/muffler the key? Would it be better to keep the stock exhaust and remove the cat or would an aftermarket one with a cat still be better? Any suggestions for an aftermarket that comes without a cat? Maybe it exists... but if not it seems someone could make some money selling just a straight pipe "header" that mates with the stock mufflers!!
	0

09-28-2022, 02:12 PM	#2
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,561	You can remove the cat on the stock exhaust and achieve the same results. The aftermarket exhaust doesn't have the cat which makes it less restrictive. There is a perfect balance you need to maintain when doing exhaust. You want to have enough back pressure to keep the low end power. Less restrictive exhaust will move the power to the higher rpms and loose some low end power. I found my decat stock exhaust on my stock Hawk engine performed better. Once i did the porting and decking of the head the freer flowing muffler work better. __________________ 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S 2004 Honda ST 1300 2016 Black Hawk 250 (sold) Keihin PE30 carb,125 main,38 slow.Pod filter,ported & decked head 10:1 CR,Direct Ignition Coil,15/40Sprockets,NGK DPR8EIX-9,De-Cat,Dual Oil Cooler,Digital Cluster 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
	0

09-28-2022, 03:48 PM	#3
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 8,020	75% of the power increase is the result of the catalyst being deleted, at least on the Hawk. The cat on the Hawk is horrendously restrictive. The other 25% would be the muffler. I did the exhaust because I like having a stainless steel system. __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	2

09-28-2022, 05:14 PM	#4
CheapThrills Join Date: Jan 2021 Location: Lehigh Valley Area Posts: 148	I felt a little more acceleration with an exhaust upgrade, but real benefit for me was better top end with ability to gear higher. Oh, I have a TBR7 btw. __________________ - My 2 Bikes: TaoTao TBR7 Boom Vader Gen 2 (BD125-10) Testing a YouTube Channel
	0

09-28-2022, 08:08 PM	#6
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,561	On the stock exhaust it is the cat. On the after market exhaust it just there for looks. Fake cat. __________________ 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S 2004 Honda ST 1300 2016 Black Hawk 250 (sold) Keihin PE30 carb,125 main,38 slow.Pod filter,ported & decked head 10:1 CR,Direct Ignition Coil,15/40Sprockets,NGK DPR8EIX-9,De-Cat,Dual Oil Cooler,Digital Cluster 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
	0

09-28-2022, 05:58 PM	#5
chiefmeat68 Join Date: Mar 2022 Location: Eastern WA Posts: 20	So maybe I've been making an incorrect assumption about the can looking thingy on the header. I have been assuming that is a cat. If it's not a cat then what is it? Maybe an expansion chamber of some sort?
	0

10-03-2022, 10:22 AM	#9
BrokeAss Join Date: Mar 2021 Location: Ohio Posts: 43	THE BACKPRESSURE MYTH DEBUNKED: https://motordyneengineering.com/sca...heir%20results. http://www.automotiveforums.com/t943..._s_wrong_.html
	0

05-09-2023, 06:31 PM	#10
Zapkin Join Date: Sep 2022 Location: Ohio Valley Posts: 686	Is there any performance benefit to removing the baffle from the carbed Hawk 250 stock exhaust? __________________ 2022 Hawk 250
	0

05-09-2023, 07:46 PM	#11
XLsior Join Date: Aug 2020 Location: Australia Posts: 1,290	The engine can't get much past 8000rpm so other than making a lot of additional noise the removal of the baffle doesn't really equate to noticeable additional power. Though I don't have a stock hawk muffler... I did try a baffle/non baffle test on my Aussie Muffler and the non baffle was unpleasantly noisy and no power bonus to speak of. The DIY baffle mod did help a little with maybe 100-200rpm but that might just be me wishing it to justify the work done to 'improve' that original configuration. I have noticed the muffler section does not appear to be pushing off the head pipe slip coupling, which to me suggests an improved flow rate and less back pressure. If you're using an unported head then a de-cat would be more than adequate to help the engine breath better...
	0

05-09-2023, 08:31 PM	#12
Bill Hilly Join Date: Feb 2021 Posts: 934	I , without a doubt can say that the Hawk benefits from just simply removing the cat, because all I have done to mine has been removing it. I had already removed the baffle/spark arrestor from the muffler,so I know the cat was holding it back. I have a TBR7 with full aftermarket exhaust, and I am pretty sure both my Hawk, and TBR7(both have 46/17 sprockets) are both really close to each other in power, although the TBR7'S smaller smoother tires gives it an advantage, but the cat removal from the Hawk did help the Hawk, in the same way that the full aftermarket exhaust helped the TBR7, which to me seems like they are not only happier, and less stressed at high RPM, but they pick up, and hold RPM better past mid range. There may be a little more to be had by working on the Hawk's muffler, because it's not really loud, but I would hate to go through the trouble, and not gain anything, and I like having my muffler guard, so I don't particularly want an aftermarket muffler.
	0

05-09-2023, 10:04 PM	#13
Zapkin Join Date: Sep 2022 Location: Ohio Valley Posts: 686	Okay so then unless I just want to annoy folks, I should just leave it in there. Thx fellas. __________________ 2022 Hawk 250
	0

05-09-2023, 10:25 PM	#14
chiefmeat68 Join Date: Mar 2022 Location: Eastern WA Posts: 20	I hate loud annoying bikes... unless it's mine!!!
	2

05-09-2023, 11:02 PM	#15
Bill Hilly Join Date: Feb 2021 Posts: 934	http://At the bare minimum,you should remove the cat. If you do just that, it will be just barely louder than it would be with the cat, but the bike will be healthier. The small diameter cats are VERY restricted, and probably why similar bikes in countries that don't have strict emission standards produce more HP , than the lean jetted, and restricted exhaust bikes we get. I am posting a couple pictures of the Hawk cat, that I removed. On top of it is a quarter for reference. The diameter of the cat is small, and the pipe is thick walled to boot.http://
	0