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Old 28th July 2008, 18:42   #1
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Default About backpressure and free flow exhaust systems

Exhaust Basics - Team Integra

Please for once and for all, anyone saying that its the loss of back-pressure that we get a less low-end torque, please go through this.

I remember asking the forum ppl how could backpressure actually help in increasing the low end torque.

Since no one could provide a satisfactory answer, I searched over internet, and came across this article.

The reason is the exhaust pulses, and has got nothing to do with the backpressure.
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Old 29th July 2008, 03:05   #2
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Very nice stuff , nice site - cheers man.
Peace

Last edited by cyneverdie : 29th July 2008 at 03:13.
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Old 29th July 2008, 03:35   #3
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That was very informative Alpha1, Thank for the material.
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Old 29th July 2008, 11:32   #4
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PPl could still debate about whats written on that site, and come up with their own explanations about backpressure.

But if that's the case, I want hows and whys.

The exhaust pulse explanation given there answers that and sounds convincing.
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Old 29th July 2008, 11:50   #5
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nice read alpha1 quite informative
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Old 29th July 2008, 12:32   #6
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Nice find alpha1 and the link is informative for nerds like me
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Old 29th July 2008, 14:17   #7
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Quote:
Please for once and for all, anyone saying that its the loss of back-pressure that we get a less low-end torque, please go through this.
The websites does say the same thing.

""" We see here a restrictive stock exhaust system. In the low RPMs, this tight formation of pulses becomes an advantage as it keeps the them close together so they can pull each other through the exhaust rather than lingering around. However as the RPMs rise, the pulses become closer and closer together, raising backpressure to restrict the piston in pushing out more exhaust gasses. """

They've mentioned that the stock exhaust setup with max back pressure gives an advantage at low rpms. That is why you feel that punch at low rpms on a restrictive exhaust system.

An engine produces max torque when exhaust flow rate reaches around 240-260ft/sec. When you have a restrictive exhaust system, this rate of exhaust flow can be achieved at lower revvs.

Ofcourse, max torque is the highest when there is minimal restriction, but it wont be achieved at low revvs. When you fit a 2" performance exhaust pipe, your engine needs to revv way more than it used to in order to achieve and exhaust flow rate of 240-260ft/sec. That is why a performance setup generally reduces low end response but the upside is that you get way better perfomance higher up the revvs (making the cars perfomance better, overall).

Shan2nu
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Old 29th July 2008, 14:41   #8
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Generally when we say back pressure, it usually means restriction. So i think instead of saying backpressure, we need to use the word "restrictive exhaust system".

The more restrictive the exhaust system, the higher the exhaut flow rate achieved at low revvs.

Shan2nu
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Old 29th July 2008, 23:55   #9
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Quote:
Originally Posted by Shan2nu View Post

The more restrictive the exhaust system, the higher the exhaut flow rate achieved at low revvs.

Shan2nu

I think the more precise term would be "higher exhaust velocity" . I believe that the overall flow rate would remain the same regardless of how restrictive.

But I would also think that if the exhaust system was very restrictive, the flow rate and velocity would decrease.

Do you agree?
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Old 30th July 2008, 00:18   #10
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I wonder how a catcon fits into all this
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Old 30th July 2008, 00:35   #11
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Quote:
I think the more precise term would be "higher exhaust velocity" . I believe that the overall flow rate would remain the same regardless of how restrictive.

But I would also think that if the exhaust system was very restrictive, the flow rate and velocity would decrease.

Do you agree?
Yeah, i meant flow velocity.

Well, the moment you start restricting the exhaust the flow rate (as in quantity over a given time) will def reduce, but flow velocity will still reach 240-260ft/sec. The max flow velocity might reduce though.

---

Another thing that is bothering me is....when manufacturers provide bhp and torque figures, are these figures with the stock intake/exhaust or are they the highest possible figures for that particular engine under ideal testing conditions.

Is the production engine really as powerful as it is on paper, after it has been fitted with the stock intake and exhaust system?

Shan2nu

Last edited by Rehaan : 30th July 2008 at 00:55. Reason: Merged. Please use the EDIT button to add to an existing post within 20 minutes instead of postnig another consecutive post.
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Old 31st July 2008, 03:01   #12
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Quote:
Originally Posted by Shan2nu View Post
Yeah, i meant flow velocity.

Well, the moment you start restricting the exhaust the flow rate (as in quantity over a given time) will def reduce, but flow velocity will still reach 240-260ft/sec. The max flow velocity might reduce though.

---

Another thing that is bothering me is....when manufacturers provide bhp and torque figures, are these figures with the stock intake/exhaust or are they the highest possible figures for that particular engine under ideal testing conditions.

Is the production engine really as powerful as it is on paper, after it has been fitted with the stock intake and exhaust system?

Shan2nu
As far as I know, the net horsepower is with stock intake, exhaust, all accessory drives. It is without the gbox, and at the crankshaft.

In the U.S., the standard used is SAE net HP. Europe is DIN, Japan I think is JIS. Dont know what standard is used in India. These standards should lay out all test conditons, etc.

But there is probably some amount of error on the part of manufacturers in how they rate the max net hp. Not sure if it is on purpose.
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Old 31st July 2008, 16:03   #13
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So is there anything that an aftermarket exhaust/muffler upgrade would do to improve performance across the entire RPM range?

The way I see it, by modifying the exhaust geometry you're only changing the characteristics of the engine, increasing power at high RPM while sacrificing power at low RPM (or vice versa if you opt for a more restrictive exhaust pipe than stock).

Of course, by throwing out the cat-con you probably gain 1%-2% due to lesser obstructions in the flow. Similarly a less effective muffler (I mean less effective from a sound viewpoint, i.e. louder) would also reduce obstructions in the flow. So are these the only "across the RPM range" improvements that you can implement?

Suddenly I'm much less interested in getting a free-flow for my car.
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Old 31st July 2008, 16:37   #14
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Very informative. Thanks for putting it up.
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Old 31st July 2008, 21:01   #15
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Quote:
So is there anything that an aftermarket exhaust/muffler upgrade would do to improve performance across the entire RPM range?

The way I see it, by modifying the exhaust geometry you're only changing the characteristics of the engine, increasing power at high RPM while sacrificing power at low RPM (or vice versa if you opt for a more restrictive exhaust pipe than stock).
There is some production car (not getting the name) that uses dual exhaust pipes. At low revvs the exhaust exits through one pipe designed to provide torque at low revvs but when the engine revvs increase, the exhaust is diverted to another pipe designed for high rpm torque and power.

Shan2nu

Last edited by Shan2nu : 31st July 2008 at 21:05.
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