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  #21  
Unread 25th September 2018, 03:51 PM
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I have always thought that n/a motors need equal length headers for better power and torque, am I wrong ?
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  #22  
Unread 25th September 2018, 08:46 PM
Beachworm Beachworm is offline
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Quote:
Originally Posted by jf1sf5 View Post
I have always thought that n/a motors need equal length headers for better power and torque, am I wrong ?
I would agree with you, at least as far as the SH Forester goes. I've done some research and the upshot is that it's very difficult to improve on the factory system. In one case they put a stock car on the dyno then fitted aftermarket uel headers and there was a small loss of power. Even changing the system from cat back as well didn't do anything to help.
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  #23  
Unread 18th October 2018, 08:21 AM
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Quote:
Originally Posted by Rallyru View Post
Right bit of misinformation here. Even the cheapest ebay UEL headers will add power over the stock header unless you have a ej204 ej253 ie newer avcs motor. The stock headers are ridiculously restriced with tiny diameter short primaries and if you cut one open you will see sharp poorly formed joins that jut out obstructing flow.
While I agree on the joints that obstruct flow, I donít agree on the primary runners being too small. A mates dad thatís been in the exhaust building game longer than Iíve been on this earth tells me the runners are far too large for the size of the engine. To get better exhaust gas velocity the primaries need to be smaller, and those joints smoother.


Just swapping UELs on a factory system might give a small note change but not worth it in my opinion unless you add a cat back system with a sports muffler thatíll let more noise out

As for power you might lose a tiny amount but really youíre not going to notice the difference with a nice note to listen to...

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  #24  
Unread 18th October 2018, 03:04 PM
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It is worth for sure. Not small note but big note .

Last edited by scalman; 19th October 2018 at 04:01 PM.
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  #25  
Unread 18th October 2018, 09:12 PM
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Engineering Explained has a good video about exhausts & gas extraction.
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  #26  
Unread 21st October 2018, 10:16 AM
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Quote:
Originally Posted by El_Freddo View Post
While I agree on the joints that obstruct flow, I donít agree on the primary runners being too small. A mates dad thatís been in the exhaust building game longer than Iíve been on this earth tells me the runners are far too large for the size of the engine. To get better exhaust gas velocity the primaries need to be smaller, and those joints smoother.


Just swapping UELs on a factory system might give a small note change but not worth it in my opinion unless you add a cat back system with a sports muffler thatíll let more noise out

As for power you might lose a tiny amount but really youíre not going to notice the difference with a nice note to listen to...

Cheers

Bennie
I wouldn't advise putting headers alone on any car its pointless. To see gains you would need to replace the whole system including decat. The diameter of the primaries may be adequate for a road car but the length of the primaries are short like 6" short. Back pressure and gas velocity has been debated to death over the years and in tge US everyone swears by a exhaust bore of 2.25" but in the UK motorsport engine and exhaust builders say if its producing over 150bhp it needs to be 2.5". Likewise manifold primaries are often 1.75" diameter and anywhere from 12-18" in length depending on the compression,cams and where peak tourqe is found in the rev range. These guys regularly see 135bhp per litre on NA competition engines. There is a guy locally to me that consistently gets 320+bhp from 2.5 duratecs on throttle bodies. Surely some of the same practice can be transfered across.
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  #27  
Unread 21st October 2018, 10:32 AM
Rallyru Rallyru is offline
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Ive seen tests recently on uel headers for the brz engine and the are making marginally more everywhere in the rev range than aftermarket el headers.
When it comes to exhaust pulses clashing and causing reverberation back down the exhaust. Its true that can happen but is much more common on V8 engines where the exhaust pulses are extremely close together. For this to happen on a 4cyl the manifold would have to be unfortunately just the wrong length. Most equal length manifolds have primarily and secondarily length runners to within 3" of each other. Thats not exactly an exact science. Search rs25.com for header dyno comparisons and you will see proof of gains to be had from both el and uel headers.
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  #28  
Unread 23rd October 2018, 04:22 AM
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Quote:
Originally Posted by Rallyru View Post
I wouldn't advise putting headers alone on any car its pointless. To see gains you would need to replace the whole system including decat. The diameter of the primaries may be adequate for a road car but the length of the primaries are short like 6" short. Back pressure and gas velocity has been debated to death over the years and in tge US everyone swears by a exhaust bore of 2.25" but in the UK motorsport engine and exhaust builders say if its producing over 150bhp it needs to be 2.5". Likewise manifold primaries are often 1.75" diameter and anywhere from 12-18" in length depending on the compression,cams and where peak tourqe is found in the rev range. These guys regularly see 135bhp per litre on NA competition engines. There is a guy locally to me that consistently gets 320+bhp from 2.5 duratecs on throttle bodies. Surely some of the same practice can be transfered across.
Since the conversation is about road going vehicles thatís what Iíve stuck to in what Iíve said. Race builds are typically more complicated than an exhaust for a small gain or a nice sound if youíre that way inclined. Real performance builds usually have many more supporting mods including extensive internal engine mods. Things like that just arenít going to happen on a road going vehicle unless youíve got money to burn or some warped desire to fulfil...

So for the point of the conversation, a swap to UELs may result in a small loss of power but will give a ďbetterĒ subi note, especially with a cat back sports system or something of the like. I enjoy my modified system on the Ď22íd L series and the UELs with a catback system on my sisters Gen3 RX Liberty Thatís all I need...

Cheers

Bennie
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  #29  
Unread 23rd October 2018, 09:45 PM
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Reading I have done in the past suggests that in order to get an "extraction" effect from headers. the primaries must be equal length and have a volume at least equal to the cylinder (there is a formula that designers use). The theory is that the expelled gas from one cylinder's combustion needs to pass before the next exhaust valve opens. this prevents interference and if the timing is just right, there will be a lower pressure behind the first pulse that tends to draw the second pulse through.

There is no reason why uel headers couldn't be constructed to provide this effect as long as the volume of the pipes fits the formula. If interference is required to get the Subaru sound, it is theoretically impossible for such a system to improve gas flow.

The other thing to consider is that after exhaust gas leaves the combustion chamber it begins to expand so the diameter of the exhaust should increase as each primary joins.
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