Author Topic: Increase in Exhaust Diameter will bhp increase ?  (Read 8493 times)

Offline C J R

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Increase in Exhaust Diameter will bhp increase ?
« on: January 10, 2009, 04:17:27 PM »
Still undecided regarding my exhaust and whether to change it.
Replies to my other thread related to the screemer pipe suggest I may not need to pipe it in to the exhaust to pass a track noise test.
 
So I have a different exhaust related question;
 
If I increase my full exhaust size from 75mm diameter to 90mm diameter can I expect an increase in bhp ?
 
I currently run a full 75mm diameter system from the single turbo to a cat by pass, to a rear silencer, (no middle silencer).
 
Thanks
Chris

Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #1 on: January 10, 2009, 04:31:43 PM »
Its all down to the quality of the back box then. When we Db tested mine with 75mm straight through was 112db with a plumbed in screamer pipe.

I can\'t see any way you could pass a sound test on track with a screamer pipe. Possibly at Oulton park because they only test once and don\'t have any on track measurements. But you would still be pushing it.

I don\'t know if swapping to say 2 x 70mm diameter will reduce the volume level with maybe a crossover pipe in the middle ??

Shaun
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Offline batman

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Increase in Exhaust Diameter will bhp increase ?
« Reply #2 on: January 10, 2009, 04:47:43 PM »
i think if you increase the size of the exhaust you ll lose bhp less back presure
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Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #3 on: January 10, 2009, 07:37:45 PM »
Thats only with a NA engine where there is resonance issue with the amount of back pressure.

A turbo engine wants the least path of resistance, although there is a limit. Thats why we see drag cars running stumpy 5in exhaust systems.

I think there is a good balance point though.

Shaun
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Offline rx jamie

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Increase in Exhaust Diameter will bhp increase ?
« Reply #4 on: January 12, 2009, 01:05:41 AM »
yeah the more back pressure you have on a turbo car the less power you make as the turbo has a lot to push against.

Offline AtomicRex

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Increase in Exhaust Diameter will bhp increase ?
« Reply #5 on: January 12, 2009, 09:06:46 AM »
Plus this engine is probably one of the most volumetric engines going!  It passes more air than a herd of cows eating vindaloo\'s
 
4" is a good size, although finding someone to bend that big a pipe reduces the choice some what.
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Offline clive

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Increase in Exhaust Diameter will bhp increase ?
« Reply #6 on: January 12, 2009, 09:45:39 AM »
Hi Chris, please remember that if you do this you really need to check the map as a more free flowing exhaust could alter the map your currantly using.
All the best Clive

Offline BlitzBoy

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Increase in Exhaust Diameter will bhp increase ?
« Reply #7 on: January 12, 2009, 10:07:30 AM »
Quote from: batman;573248
i think if you increase the size of the exhaust you ll lose bhp less back presure


Not with turbo cars, only NA, the less back pressure (hence bigger exhaust from the Turbo back) will result in more power on a turbo car. you get pressure build up in the exhaust manifold of a good few psi ,the exhaust turbine wheel causing the build up as it is the restriction.

Therefore putting a bigger exhaust on the otherside of the wheel helps to relieve the pressure. (Im sure a physics person can explain this bit)
This is why rear AR sizing of a turbo changes the characteristics of the power delivery, too small an AR will cause the engine to choke and limit top end power but spool is better, to big and lag is introduced.

For ultimate power you will see drag cars with only a short fat pipe coming off the turbo exiting the wing.

So the answer to the question will increasing exhaust size increase power , Yes :D

but it will be noiser unless you have good 4 inch silencers with good baffles
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Offline C J R

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Increase in Exhaust Diameter will bhp increase ?
« Reply #8 on: January 13, 2009, 12:21:29 PM »
Does anyone know where I could get a 4" exhaust for the FD3 from ?
I\'m running a single Garret GT40, so I would need the down pipe, (no cat), middle silencer and rear silencer ?
 
Thanks
Chris

Offline trotter

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Increase in Exhaust Diameter will bhp increase ?
« Reply #9 on: January 13, 2009, 12:40:41 PM »
Quote from: BlitzBoy;573828
Not with turbo cars, only NA, the less back pressure (hence bigger exhaust from the Turbo back) will result in more power on a turbo car. you get pressure build up in the exhaust manifold of a good few psi ,the exhaust turbine wheel causing the build up as it is the restriction.

Therefore putting a bigger exhaust on the otherside of the wheel helps to relieve the pressure. (Im sure a physics person can explain this bit)
This is why rear AR sizing of a turbo changes the characteristics of the power delivery, too small an AR will cause the engine to choke and limit top end power but spool is better, to big and lag is introduced.

For ultimate power you will see drag cars with only a short fat pipe coming off the turbo exiting the wing.

So the answer to the question will increasing exhaust size increase power , Yes :D

but it will be noiser unless you have good 4 inch silencers with good baffles


Deal with this problem at work all the time (chemical engineer).

Backpressure occurs as a result of trying to flow too much fluid (gas or liquid) down a pipe with too small a cross sectional area. As you try and flow more down this area the fluid velocity increases (Velocity = Volmetric flow / CSA). As a result of the increase in velocity, the friction acting on the fluid increases (pressure drop down a pipeline equation - google it!) As a result of this, the pipe becomes a restriction to fluid flow which sort of \'backs-up\' in the pipe causing backpressure. (Think of emptying a juice bottle through two different size holes - which would empty first?)

If you think of a NA car, the backpressure helps close the exhaust valves. If you increase the exhaust diameter and thus the cross sectional area, you reduce the restriction and cause less pressure \'back-up\'. This loss can lead to intake charge passing through the exhaust valves as they might not fully seal and thus a loss of power.

In an FI engine a smaller exhaust (bigger restriction to flow) will increase the backpressure. This increase will be more for the turbo to push against and thus strangle flow through it and cause a loss of power. Increasing the exhaust size again removes the restriction and allows the turbo to more easily throw exhaust gases out.


Hope that makes some sort of sense!!

Graeme
« Last Edit: January 13, 2009, 12:42:59 PM by trotter »
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Offline Nik da Greek

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Increase in Exhaust Diameter will bhp increase ?
« Reply #10 on: January 13, 2009, 12:52:16 PM »
Not completely accurate on the N/A exhaust dynamics there, I think...in an N/A engine thats been properly designed, the headers will be of a tuned length so as to employ resonance frequency return pulses to scavenge gas away from the exhaust valves and speed up the removal of gas along the exhaust. Short version; valve belches out a pulse of exhaust gas that wafts along the pipe until it hits a restriction, then returns along the pipe, just like a bum-gas ripple in yer bath when it hits the edge :Giggle.

This creates a high-pressure/low-pressure cycle at the valve area, high as the pulse returns, low as it recedes, just like waves on the beach (ahh, the poetry:)). If you tune the exhaust length, ie put the first restriction that causes the wave return in the exact spot you want it, it can time the low-pressure wave with the valve opening at a certain rpm, and thus scavenge the gasses away faster.

This is one of the major problems with N/A tuning, because along with cam profile efficiency, this effect is narrowing the rev range at which the engine is at its most efficient. Especially noticeable on bikes that rev to ridiculous cielings but are only efficient really for a couple of thousand rpm out of what can be a 17,000 rpm scale....

None of which really bothers turbo\'d engines, which push the exhaust gas out pretty much regardless of what restrictions may be in the way...up to a point. Obviously, the less restriction, the easier it is for the turbo to pump gas past it, which is why the less backpressure, the better.


None of this is especially interesting, but it is true :Giggle
« Last Edit: January 13, 2009, 12:54:38 PM by Nik da Greek »

Offline BlitzBoy

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Increase in Exhaust Diameter will bhp increase ?
« Reply #11 on: January 13, 2009, 01:12:12 PM »
Excellant info Graeme and Nik
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Offline Archbish

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Increase in Exhaust Diameter will bhp increase ?
« Reply #12 on: January 13, 2009, 02:06:51 PM »
Quote from: C J R;573236

If I increase my full exhaust size from 75mm diameter to 90mm diameter can I expect an increase in bhp ?

yes. :D :D
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Offline BlitzBoy

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Increase in Exhaust Diameter will bhp increase ?
« Reply #13 on: January 13, 2009, 02:14:44 PM »
Quote from: Archbish;574368
If I increase my full pipe size from 2 inches length to 6 inches length can I expect an increase in flange?


Yes Bish

hahahahah :rollin
"Piston Killa" 3rd Gen\'94 \'Oldone Racing Full Bridge Ported REW 13b engine with SCR seals\'. HKS T51 KAI turbo BBM mapped Motec ECU

JDS/HKS Street Class Championship Runner up 2011
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http://pistonkilla.blogspot.com/
old spec HB T51 470 @ wheels @ 1.2 BAR, 555 @ fly 10.46 @ 130 @ 1.25 Bar
new spec FB T51 482 @ wheels @ 1.15 BAR, 570 @ fly

New Project
"el fénix" RX2 by Mark & Mark

Offline trotter

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Increase in Exhaust Diameter will bhp increase ?
« Reply #14 on: January 13, 2009, 02:15:57 PM »
Quote from: Archbish;574368
yes. :D :D


:rollin :rollin

Agreed nik, I was looking at it from a fairly narrow angle admitedly!

Also something to bear in mind is that the same theory applies to removing cats from exhausts - these are essentially a restriction in the \'pipe\'. Its also why tuning NA is made slightly harder if you don\'t know whether this restriction was accounted for in the design of the system - you can actually do more harm than good for the reasons myself and Nik outlined above. :Thumbs-up
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