Exhausts with good flow velocity - Page 3 - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

Dirty Harry · 10-29-2021, 01:30 AM

Quote:

Originally Posted by nikitopo

Yes. This part number is the STI sports muffler.

Well at least I know my butt dyno is accurate. Still a good part I guess and I got it pretty cheap but would have been good if it was the performance version.

CrowsFeast · 10-29-2021, 11:15 AM

Quote:

Originally Posted by Dirty Harry

Well at least I know my butt dyno is accurate. Still a good part I guess and I got it pretty cheap but would have been good if it was the performance version.

Always important to keep the butt dyno calibrated

Goingnowherefast · 10-29-2021, 12:54 PM

Whoa whoa whoa. Let's back this train up... The reason that in a few rare cases a smaller diameter exhaust pipe can make more power than a larger one is NOT due to higher exhaust velocity, it is due to exhaust scavenging. Let's unpack the science behind this.

Firstly, we need to understand that fluid dynamics IS pressure diffentials. The lower you can reduce the exhaust backpressure, you will create a higher pressure differential which will increase the volume of flow. This results in an overall increase in cylinder volumetric efficiency.

Secondly, I believe the phenomenon you are actually referring to is exhaust scavenging, not velocity. See, as exhausts waves created by the combustion events over through the exhaust into a collector, a wave is sent backwards through the runner. The more exhaust valve overlap you have, the more important this effect becomes. If timed correctly, you can use the low pressure zone created by the pulsation to your advantage which again, results in increased volumetric efficiency.

So, to conclude, we have two different ways that exhausts can be tuned to produce a more efficient (and more powerful) engine. The reality is more complex though. While less backpressure is always beneficial, a properly tuned (!!!) exhaust that matches the intake, exhaust and cylinder heads correctly could produce higher numbers in rare cases. The problem is to do this you need an advanced combustion analysis system that literally costs OE's tens of thousands of dollars including custom machining cylinder heads to take in-cylinder dynamic pressure traces. Sorry, no aftermarket company is doing that realistically.

TLDR:
-You are incorrect about exhaust velocity
-You are correct about exhaust scavenging potentially producing higher results in rare cases
-In reality, if an aftermarket company claims to be doing this, they probably just got lucky
-Just get an exhaust with less backpressure and thank me later

Goingnowherefast · 10-29-2021, 12:57 PM

Yes, I realize this is a bit dense for internet car forum lol. Let me know (anyone) if you have any questions .

Ultramaroon · 10-29-2021, 01:16 PM

Quote:

Originally Posted by Goingnowherefast

Yes, I realize this is a bit dense for internet car forum lol. Let me know (anyone) if you have any questions .

Crushing dreams. How dare you!

PulsarBeeerz · 10-29-2021, 01:55 PM

Quote:

Originally Posted by Goingnowherefast

Yes, I realize this is a bit dense for internet car forum lol. Let me know (anyone) if you have any questions .

So will a $1800 axleback exhaust make 1 or 3 more horse power? Asking for a friend.

Ashikabi · 10-29-2021, 01:56 PM

Quote:

Originally Posted by PulsarBeeerz

So will a $1800 axleback exhaust make 1 or 3 more horse power? Asking for a friend.

Is that axleback for a Lambo? Does that bolt up to stock mid pipe?

Sent from my ONEPLUS A5010 using Tapatalk

PulsarBeeerz · 10-29-2021, 01:58 PM

Quote:

Originally Posted by Ashikabi

Is that axleback for a Lambo? Does that bolt up to stock mid pipe?

Sent from my ONEPLUS A5010 using Tapatalk

You would think it was. lol

https://www.vividracing.com/sti-stai...150749122.html

NoHaveMSG · 10-29-2021, 02:10 PM

Quote:

Originally Posted by Goingnowherefast

Yes, I realize this is a bit dense for internet car forum lol. Let me know (anyone) if you have any questions .

I remember reading books on exhaust scavenging 20 years ago thinking I could build my own 2 stroke exhaust but math is hard.

Ashikabi · 10-29-2021, 02:13 PM

Quote:

Originally Posted by PulsarBeeerz

You would think it was. lol

https://www.vividracing.com/sti-stai...150749122.html

STI shit is so fucking over priced

Sent from my ONEPLUS A5010 using Tapatalk

NoHaveMSG · 10-29-2021, 02:16 PM

Quote:

Originally Posted by Ashikabi

STI shit is so fucking over priced

Sent from my ONEPLUS A5010 using Tapatalk

Have you seen Porsche exhaust? I get that it is handmade blah blah but jeez.

https://www.dundonmotorsports.com/pr...-crack-pretzel

Ultramaroon · 10-29-2021, 02:19 PM

Quote:

Originally Posted by Ashikabi

STI shit is so fucking over priced

dat brand loyalty doe

Ultramaroon · 10-29-2021, 02:21 PM

Quote:

Originally Posted by NoHaveMSG

Have you seen Porsche exhaust? I get that it is handmade blah blah but jeez.

Oh, FFS... money>brainz

NoHaveMSG · 10-29-2021, 02:24 PM

Quote:

Originally Posted by Ultramaroon

Oh, FFS... money>brainz

You could almost buy a new GR86 for what a full exhaust cost from Dundon.

The Dundon guys are really cool though. Sometimes they bring their support trailer out to the Ridge and will help anyone out.

10-29-2021, 12:54 PM	#31
Goingnowherefast Senior Member Join Date: May 2019 Drives: AP2 S2000, 91 Miata 1.8L Rotrex 5MT Location: Detroit, MI Posts: 465 Thanks: 275 Thanked 299 Times in 187 Posts Mentioned: 5 Post(s) Tagged: 0 Thread(s)	Whoa whoa whoa. Let's back this train up... The reason that in a few rare cases a smaller diameter exhaust pipe can make more power than a larger one is NOT due to higher exhaust velocity, it is due to exhaust scavenging. Let's unpack the science behind this. Firstly, we need to understand that fluid dynamics IS pressure diffentials. The lower you can reduce the exhaust backpressure, you will create a higher pressure differential which will increase the volume of flow. This results in an overall increase in cylinder volumetric efficiency. Secondly, I believe the phenomenon you are actually referring to is exhaust scavenging, not velocity. See, as exhausts waves created by the combustion events over through the exhaust into a collector, a wave is sent backwards through the runner. The more exhaust valve overlap you have, the more important this effect becomes. If timed correctly, you can use the low pressure zone created by the pulsation to your advantage which again, results in increased volumetric efficiency. So, to conclude, we have two different ways that exhausts can be tuned to produce a more efficient (and more powerful) engine. The reality is more complex though. While less backpressure is always beneficial, a properly tuned (!!!) exhaust that matches the intake, exhaust and cylinder heads correctly could produce higher numbers in rare cases. The problem is to do this you need an advanced combustion analysis system that literally costs OE's tens of thousands of dollars including custom machining cylinder heads to take in-cylinder dynamic pressure traces. Sorry, no aftermarket company is doing that realistically. TLDR: -You are incorrect about exhaust velocity -You are correct about exhaust scavenging potentially producing higher results in rare cases -In reality, if an aftermarket company claims to be doing this, they probably just got lucky -Just get an exhaust with less backpressure and thank me later __________________ 2022 Hyundai Elantra N 6MT SCCA TT S3//Will be back in an 86 eventually

10-29-2021, 12:57 PM	#32
Goingnowherefast Senior Member Join Date: May 2019 Drives: AP2 S2000, 91 Miata 1.8L Rotrex 5MT Location: Detroit, MI Posts: 465 Thanks: 275 Thanked 299 Times in 187 Posts Mentioned: 5 Post(s) Tagged: 0 Thread(s)	Yes, I realize this is a bit dense for internet car forum lol. Let me know (anyone) if you have any questions . __________________ 2022 Hyundai Elantra N 6MT SCCA TT S3//Will be back in an 86 eventually

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