Back-pressure
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Member
Joined: Jun 2001
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From: Orlando, FL
Back-pressure
Ok guys i am sorta new to the list. I am learning and grasping as much of this as i can. I have a question though. What creates back-pressure in the R's exhaust system - is it the cat or the muffler? I was contemplating on taking either one of them off to let air flow and car to breathe easier. Someone give me a good explanation of this please. Thanks in advance.
PS - has anyone done this to their R?
Mike
PS - has anyone done this to their R?
Mike
Member
Joined: Feb 2001
Posts: 6,131
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From: Santa Rosa
Re: Back-pressure (talkie25)
I run a testpipe, and I have Dynoed it with the B-pipe and muffler. When I removed the muffler I gaind only .3 (3/10) WHP. When I removed the B pipe and muffler, I gained 4 WHP. For some reason the R just keeps making more power when you remove the exhaust.
I run straight out the testpipe when I race.
HTH
I run straight out the testpipe when I race.
HTH
Trial User
Joined: Apr 2000
Posts: 1,345
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From: Toronto, On, Canada
Re: Back-pressure (EuroITR1689)
I posted this up in our tech archive at SHO (no snide remarks please):
please excuse the lecture tone as this was addressed to beginners but the back pressure aspect was the part I wanted to raise to your attention.
cheers
[Modified by Michael Delaney, 10:26 AM 7/7/2001]
generically there are 3 designs: straight thru vs chambered vs 2 pass design .
Most important: pay attention to the B pipe and in muffler tube diameters for your engine combination:
2-1/4" up to 210HP @ the crank
I find that most of you have engines with bolt-ons only so look for a 2.25 in. exhaust not a brand. If you begin to
think about big cams, 2.5 in. collector headers, and pistons then go up to the next level in diameter.
2-3/8" (60mm) up to 235HP @ the crank
2-1/2" up to 265HP @ the crank
2-3/4" up to 325HP @ the crank
3" big big HP (FI)
Some people with >210 hp step the diameter after the cat to 3 in. anyway to get an antireversion effect.
Pay attention to what is used as a sound absorbing insulator. Stay away from fiberglass or what's called a glass pack. Fiberglass melts with heat over time and guess what? you become loud as **** and it's not due to more power..it's due to a failed muffler.
Pay attention to materials if you live in a snowbelt area that uses road salt: get aluminised coated stainless steel.
Pay attention to fitment and hardware provided like gaskets and areas for the hangers to hook on to.
I agree with the reply above: please don't obsess over an exhaust because it makes around 2-4 whp in bolt-on only < 190 whp combinations.
Loudness does not equate to power gain...loudness depends on these: muffler length, presence of a resonator, absorption material, whether the muffler has louvers or holes , tip size/length , and the design : straight thru with resonator vs chambered vs 2 pass (number of passes...the 2 pass Mugen exhaust without a resonator is quiet). If you want a non-coffee can quiet throaty sound, look for the characteristics I listed..a longer muffler is a good start.Power depends on how the exhaust works with the engine combination you have in terms of compatibility to help maintain a high exhaust gas velocity out of the head for the volume of air you are dumping in...most experts agree that the exhaust flow should be at least 80-85% of the intake flow (if it's more than this...even better). So for exhausts as related to power?: please pay attention to diameter, diameter, diameter that will suit your combo. A big rice newbie misconception: My exhaust is loud so it must be great! wrong.
Big huge diameter tips are better: another newbie misconception. You design the tip size to fascilitate where you want the bulk of your power to be along the rpm band.
If the exhaust has the characteristics that work for you and is cheaper, get it. There's not much separating exhausts these days in terms of 2.25 in. straight thru exhausts. Be forewarned: the Tanabe G-Power and Racing Medallion have a 3 in. long bottleneck that is less than 2.25 in. diameter at the B pipe just behind the cat flange...not good. I just found this out on mine and will be chopping that part off.
<U>The backpressure myth</U>: As you get more advanced in your engine mod combination you want less to no backpressure at all. Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 3-5 psi backpressure.
Bolt-ons only engine combinations use exhausts with some backpressure since there is this myth that it prevents the intake charge from shooting into the header at cam overlap during low to midrange rpms. No backpressure was thought to result in a midrange power drop due to loss of intake charge into the header much like our turbo tegs using all motor long duration cams. This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to reversion where the exhaust travels backwards into the combustion chamber and dilutes the fresh intake charge at overlap. So please ignore the "you should have at least some backpressure" sales pitch for an inferior incompatible exhaust. It's all about the energy of the exhaust pulse leaving the exhaust port and maintaining that velocity.
as eloquently stated by Larry at Endyn on the Backpressure issue:
quote:
Just changing back pressure is a bogus way of trying to create the "ideal" pressure in the system. The
exhaust system should work like a correctly conceived header. It should <U>extract</U> the exhaust from
the header, to minimize pumping pressures.
The only way to create a system that will serve as an extractor is to properly size the tubing <U>to
allow the flow velocity to create a sort of "vacuum" behind it</U>.
Just as with headers, creating a system that will provide the best of all worlds at all throttle positions
and rpm ranges is impossible. It's all going to be a trade-off.
You can tune for the throttle positions and rpm ranges where you desire the greatest
performance, but you'll sacrifice performance at the other end of the scale.
Know where and how wide your power band will be and choose a system that facilitates that goal. You may get more midrange but give something up at the top or want more power up top with some compromises at the midrange...this is what we are talking about when it comes to the old saying you want some backpressure. Yes you get more midrange with some backpressure in a bolt-ons only teg but at what cost at the top end? will this cost be liveable for your tastes?
Most important: pay attention to the B pipe and in muffler tube diameters for your engine combination:
2-1/4" up to 210HP @ the crank
I find that most of you have engines with bolt-ons only so look for a 2.25 in. exhaust not a brand. If you begin to
think about big cams, 2.5 in. collector headers, and pistons then go up to the next level in diameter.
2-3/8" (60mm) up to 235HP @ the crank
2-1/2" up to 265HP @ the crank
2-3/4" up to 325HP @ the crank
3" big big HP (FI)
Some people with >210 hp step the diameter after the cat to 3 in. anyway to get an antireversion effect.
Pay attention to what is used as a sound absorbing insulator. Stay away from fiberglass or what's called a glass pack. Fiberglass melts with heat over time and guess what? you become loud as **** and it's not due to more power..it's due to a failed muffler.
Pay attention to materials if you live in a snowbelt area that uses road salt: get aluminised coated stainless steel.
Pay attention to fitment and hardware provided like gaskets and areas for the hangers to hook on to.
I agree with the reply above: please don't obsess over an exhaust because it makes around 2-4 whp in bolt-on only < 190 whp combinations.
Loudness does not equate to power gain...loudness depends on these: muffler length, presence of a resonator, absorption material, whether the muffler has louvers or holes , tip size/length , and the design : straight thru with resonator vs chambered vs 2 pass (number of passes...the 2 pass Mugen exhaust without a resonator is quiet). If you want a non-coffee can quiet throaty sound, look for the characteristics I listed..a longer muffler is a good start.Power depends on how the exhaust works with the engine combination you have in terms of compatibility to help maintain a high exhaust gas velocity out of the head for the volume of air you are dumping in...most experts agree that the exhaust flow should be at least 80-85% of the intake flow (if it's more than this...even better). So for exhausts as related to power?: please pay attention to diameter, diameter, diameter that will suit your combo. A big rice newbie misconception: My exhaust is loud so it must be great! wrong.
Big huge diameter tips are better: another newbie misconception. You design the tip size to fascilitate where you want the bulk of your power to be along the rpm band.
If the exhaust has the characteristics that work for you and is cheaper, get it. There's not much separating exhausts these days in terms of 2.25 in. straight thru exhausts. Be forewarned: the Tanabe G-Power and Racing Medallion have a 3 in. long bottleneck that is less than 2.25 in. diameter at the B pipe just behind the cat flange...not good. I just found this out on mine and will be chopping that part off.
<U>The backpressure myth</U>: As you get more advanced in your engine mod combination you want less to no backpressure at all. Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 3-5 psi backpressure.
Bolt-ons only engine combinations use exhausts with some backpressure since there is this myth that it prevents the intake charge from shooting into the header at cam overlap during low to midrange rpms. No backpressure was thought to result in a midrange power drop due to loss of intake charge into the header much like our turbo tegs using all motor long duration cams. This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to reversion where the exhaust travels backwards into the combustion chamber and dilutes the fresh intake charge at overlap. So please ignore the "you should have at least some backpressure" sales pitch for an inferior incompatible exhaust. It's all about the energy of the exhaust pulse leaving the exhaust port and maintaining that velocity.
as eloquently stated by Larry at Endyn on the Backpressure issue:
quote:
Just changing back pressure is a bogus way of trying to create the "ideal" pressure in the system. The
exhaust system should work like a correctly conceived header. It should <U>extract</U> the exhaust from
the header, to minimize pumping pressures.
The only way to create a system that will serve as an extractor is to properly size the tubing <U>to
allow the flow velocity to create a sort of "vacuum" behind it</U>.
Just as with headers, creating a system that will provide the best of all worlds at all throttle positions
and rpm ranges is impossible. It's all going to be a trade-off.
You can tune for the throttle positions and rpm ranges where you desire the greatest
performance, but you'll sacrifice performance at the other end of the scale.
Know where and how wide your power band will be and choose a system that facilitates that goal. You may get more midrange but give something up at the top or want more power up top with some compromises at the midrange...this is what we are talking about when it comes to the old saying you want some backpressure. Yes you get more midrange with some backpressure in a bolt-ons only teg but at what cost at the top end? will this cost be liveable for your tastes?
cheers
[Modified by Michael Delaney, 10:26 AM 7/7/2001]
Trial User
Joined: Apr 2000
Posts: 1,345
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From: Toronto, On, Canada
Re: Back-pressure (Michael Delaney)
If you have the following assumptions:
mean flow velocity in an engine is determined by:
- rpm range
- runner and port area
- displacement
- compression ratio
- rod to stroke ratio
- valve timing (amount of overlap)
(BTW notice you can alter any of these with the correct parts)
Peak torque occurs at peak volumetric efficiency and this occurs when mean flow velocity = 240-260
ft/sec. The build up of rpms until you get that magical flow number of 240-260 ft/sec. determines where the peak
occurs.
Control how fast you can get up to that mean flow velocity and you control where peak torque occurs.
Flow can be controlled at the TB-intake manifold, head port, headers, collector, and exhaust all working together. Now
let's focus on the exhaust end.
we can dictate where peak torque occurs along the rpm band based on our header-exhaust design:
- bigger diameters move the peak torque to a higher rpm
- changing length rocks the torque curve about the peak torque...a longer pipe will have a greater increase in torque early and then trail off torque faster at the upper rpms.
- having a crossover points in exhausts and collectors (merge angles) influence the rate at which torque rises up to peak torque.
- steps in the header and exhaust combine the individual torque curves of each tube's diameter to gain a wider power band and prevent reversion.
The header-exhaust builders know how to manipulate these to the desired effect.
cheers
mean flow velocity in an engine is determined by:
- rpm range
- runner and port area
- displacement
- compression ratio
- rod to stroke ratio
- valve timing (amount of overlap)
(BTW notice you can alter any of these with the correct parts)
Peak torque occurs at peak volumetric efficiency and this occurs when mean flow velocity = 240-260
ft/sec. The build up of rpms until you get that magical flow number of 240-260 ft/sec. determines where the peak
occurs.
Control how fast you can get up to that mean flow velocity and you control where peak torque occurs.
Flow can be controlled at the TB-intake manifold, head port, headers, collector, and exhaust all working together. Now
let's focus on the exhaust end.
we can dictate where peak torque occurs along the rpm band based on our header-exhaust design:
- bigger diameters move the peak torque to a higher rpm
- changing length rocks the torque curve about the peak torque...a longer pipe will have a greater increase in torque early and then trail off torque faster at the upper rpms.
- having a crossover points in exhausts and collectors (merge angles) influence the rate at which torque rises up to peak torque.
- steps in the header and exhaust combine the individual torque curves of each tube's diameter to gain a wider power band and prevent reversion.
The header-exhaust builders know how to manipulate these to the desired effect.
cheers
New User
Joined: Jul 2001
Posts: 1,820
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From: Hickory Hills, IL, USA
Re: Back-pressure (Michael Delaney)
OK, I feel we're all in agreement that Michael is too smart for his own good. 
This is probably a REALLY newbie question. But, I don't know
I'm new to the whole performance scene. All performance exhausts don't have catalytic converters, correct?
This is probably a REALLY newbie question. But, I don't know
I'm new to the whole performance scene. All performance exhausts don't have catalytic converters, correct?
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Member
Joined: Jan 2001
Posts: 2,633
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From: Louisville, KY
Re: Back-pressure (io_burn)
All performance exhausts don't have catalytic converters, correct?
Track only cars don't have cats. (well, also the people that are willing to swap the cats out each VET test dont either)
Trial User
Joined: Jun 2000
Posts: 252
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From: North of Boston, MA, USA
Re: Back-pressure (io_burn)
Yes,
Turbo Miata is right. Cars used for street use are required to have catatlytic converters. Some choose to get high flow cats while others will go with a test pipe. Test pipes replace the cat and are just a hollow pipe that is the same length of the cat and therefore bridge the gap between the header and the catback exhaust system. Also, test pipes are very illegal and if caught can result in a very high fine.
Turbo Miata is right. Cars used for street use are required to have catatlytic converters. Some choose to get high flow cats while others will go with a test pipe. Test pipes replace the cat and are just a hollow pipe that is the same length of the cat and therefore bridge the gap between the header and the catback exhaust system. Also, test pipes are very illegal and if caught can result in a very high fine.
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Joined: Jun 2001
Posts: 108
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From: Homer the Don, usa
Re: Back-pressure (Hyper165)
Volumetric efficiency has everything to do with air flow velocity. Too small a tube or pipe makes too much backpressure and restriction. Too big, the air fills up that larger area, becomes lower pressure, and flows slower. It applies to both intake and exhaust manifolds/systems. Expert shops port (mill out larger volume) and polish (smooth and refine walls) heads and intake manifolds for optimal air flow velocity, so power is gained at almost all rpm. Finely crafted merge collectors by Spoon, TODA, or HiTech allow gradual and smooth exhaust merging, while not sacrificing power and improving overall torque. Of course, such exquisite quality has its price.
Trial User
Joined: Apr 2000
Posts: 1,345
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From: Toronto, On, Canada
Re: Back-pressure (Hybrid ctr)
okay wise guy....
if you believe the possibility that during the overlap period the exhaust gas can be pushed up the intake port as the piston is pushing upward for the exhaust stroke (P3 > P1) , you can believe the intake manifold can experience reversion. If burnt exhaust gas can travel up the intake port at overlap, it can surely go into the manifold plenum. yes? ...how far reversion occurs is a pure guess on my part.
So there you have contaminated air in the manifold plenum and it dilutes your fresh intake charge...can it flow back against TB flow? highly unlikely. You're right I should not have used the term antireversion at the TB portmatch.
However, try putting a larger diameter TB to a smaller port on the manifold and see what that does. It kills intake velocity. So is having a slightly bigger manifold plenum antireversionary in the sense that exhaust gas flow contaminates hi intake flow???? probably not...
but having a slightly bigger manifold port is better than having a slightly smaller one or an equivalent one (portmatch)....
this is what I did with my ITR TB...70mm at the intake end of the TB, 65mm at the manifold end, port the intake manifold plenum 20 thousandths over 65mm....not 65mm exactly to portmatch...
Dan Ponze said that the ecu doesn't like the bigger port on the taper TB's...[Modified by Michael Delaney, 7:56 PM 7/7/2001]
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Joined: Jul 2001
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From: Houston, TX
Re: Back-pressure (Michael Delaney)
Hi Michael,
I'm certainly glad you found our site at n2performance.com and are finding the information from Jim McFarland useful on this thread. Jim's materials are copyrighted, but we don't mind it being used in these situations. I just ask you give Jim his props. Mentioning where you got the material and/or including a link would be nice! Thanks. John
I'm certainly glad you found our site at n2performance.com and are finding the information from Jim McFarland useful on this thread. Jim's materials are copyrighted, but we don't mind it being used in these situations. I just ask you give Jim his props. Mentioning where you got the material and/or including a link would be nice! Thanks. John
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