"Jcarr" (jcarr)
03/18/2016 at 15:22 • Filed to: None
2
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"Jcarr" (jcarr)
03/18/2016 at 15:22 • Filed to: None | 2
| 28 |
I’m not out of the grey on The Garage so I’ll ask here: Why do the exhaust outlets for the 5 and 6 cylinders route the way that they do? Seems like it would make more sense for 5 to connect where 6 does and vice-versa.
64Mali
> Jcarr
03/18/2016 at 15:24 |
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I would hazard they are equal length headers so to make sure that 5/6 are the same length they jumped them.
Ike
> Jcarr
03/18/2016 at 15:25 |
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Timing?
jimz
> Jcarr
03/18/2016 at 15:25 |
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they’re trying to get equal-length header tubes. Also it has a pair of collectors with three runners each, so they can separate out the cylinders which fire sequentially to reduce backpressure. Meaning, if you were to number the visible cylinders front-to-back as 1-2-3-4-5-6 (treat this as an I6 for this example) and the firing order is 1-5-3-6-2-4, then the header tubes for cylinders 1, 3, and 2 would dump into one collector while 5, 6, and 4 would dump into the other.
edited to add: crossplane V8s actually have a bigger problem with this. each bank has an uneven firing interval of (typically) 180-270-180-90. That “90" means one bank has two cylinders firing in sequence, which makes optimizing the exhaust system of a crossplane V8 a bit of an art.
crowmolly
> 64Mali
03/18/2016 at 15:26 |
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Yup, either that or fitment. Maybe.
ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
> Jcarr
03/18/2016 at 15:27 |
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Equal-length headers.
By making them cross over like that, they become equal in length with the other 4, and that does something important and stuff.
Otherwise, ze Germans would not have done it that way.
Rusty Vandura - www.tinyurl.com/keepoppo
> 64Mali
03/18/2016 at 15:27 |
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Uniform length.
uofime-2
> jimz
03/18/2016 at 15:27 |
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this^
reduce back pressure and improve scavenging
EL_ULY
> Jcarr
03/18/2016 at 15:30 |
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equal flow length management technique :]
Party-vi
> ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
03/18/2016 at 15:31 |
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I don’t think ze Germans did it that way, as that looks like an aftermarket header. Ze Germans did it this way.
SnapUndersteer, Italian Spiderman
> Jcarr
03/18/2016 at 15:31 |
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Equal length headers to harmonize the exhaust pulse (bear in mind exhaust comes in pulses, not a continuous stream) such that when exhaust from a cylinder gets to the junction with 2 other cylinders and into the next section, it’s not fighting for its turn to enter the next section. Think of 2 people entering a doorway together, it’s better for efficiency’s sake them to go in succession as opposed to adjacent
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SnapUndersteer, Italian Spiderman
> Party-vi
03/18/2016 at 15:32 |
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Because they prioritze cat performance (emissions) over exhaust performance
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ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
> Party-vi
03/18/2016 at 15:33 |
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VWhat! A lazy German!!! This cannot be. VWe must find zem and destroy zem.
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Jcarr
> SnapUndersteer, Italian Spiderman
03/18/2016 at 15:34 |
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That’s the best analogy I’ve heard yet. Makes perfect sense now.
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jimz
> Jcarr
03/18/2016 at 15:37 |
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when diesels used injection pumps, the high pressure fuel lines going from the pump to the injectors were also equal length, which minimized injection timing variation. That’s why they looped around so much:
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Party-vi
> SnapUndersteer, Italian Spiderman
03/18/2016 at 15:37 |
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That is why, yes. They want those cats good and hot very quickly.
McMike
> SnapUndersteer, Italian Spiderman
03/18/2016 at 15:42 |
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I like that door analogy.
Imagine 100 idiots trying to run out a single door at once.
Now try to image 100 people single file trying to run out.
BiTurbo228 - Dr Frankenstein of Spitfires
> jimz
03/18/2016 at 15:43 |
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What I want to know is why it’s so popular in the BMW crowd to go with a 6-2-1 manifold design, and spend quite a lot of money often, when it’s known to be inferior to a proper 6-3-1.
A 6-2-1 will do a good job of exhausting gases (much better than the standard twin-log-like manifold), but the pulses end up interfering with each other at certain points in the rev-range, creating flat spots.
In a well designed 6-3-1, the exhaust pulses help to scavenge air from the cylinder improving combustion. A 6-2-1 does do this, but at fewer points in the rev range and often much higher up (usually out of the normal rev range of most engines).
Just seems odd to me. Perhaps building old OHV Triumph engines means you have to get each little bump up you can get...
Birddog
> McMike
03/18/2016 at 15:45 |
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Well, the single file people would never get out because of the 100 idiots blocking the door.
DoYouEvenShift
> Jcarr
03/18/2016 at 15:48 |
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You guys have it all wrong with your logic and facts.
The 8 year old building it in China screwed it up.
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SnapUndersteer, Italian Spiderman
> Jcarr
03/18/2016 at 15:51 |
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Hey, thanks! I do appreciate that
I’ve always heard that if you can’t explain something simply, then you don’t understand it well enough.
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SnapUndersteer, Italian Spiderman
> DoYouEvenShift
03/18/2016 at 15:53 |
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^this
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jimz
> BiTurbo228 - Dr Frankenstein of Spitfires
03/18/2016 at 15:57 |
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I can see how a 6-3-1 would be advantageous in theory so long as you paired up the cylinders appropriately. maybe the gains aren’t that significant and don’t justify the added cost.
Dave the car guy , still here
> SnapUndersteer, Italian Spiderman
03/18/2016 at 16:13 |
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Great analogy amd explaination. I couldn’t have said it any better. If he is running duals there is a secret to getting more power with an H or X pipe. The way my hot rodding friends have achieved the best gains is using spray paint. The dual exhaust gets completely assembled 1st and then the ends of the headers and part of the pipes under the center of car are spray painted cold. Afterwards you get to take the car out and run the crap out of it until the exhaust is very hot. The paint will burn back to some point along the exhaust pipes. Right where the paint burning stops is where you must add the H or X section for the exhaust gases to merge for top gains. Some people have said this is an old wives tail but it works somehow. I’ve been in a Fox body Mustang 5.0 that had duals and we used the same method to locate the H. It was definitely better and stronger running after we used this method.
Tom McParland
> SnapUndersteer, Italian Spiderman
03/18/2016 at 17:52 |
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Well I learned something today. That was a great explanation.
Mattbob
> Jcarr
03/18/2016 at 18:02 |
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153624
finn's arm
> McMike
03/18/2016 at 18:26 |
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But now imagine that the 100 idiots are on fire while trying to run out the door.
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BiTurbo228 - Dr Frankenstein of Spitfires
> jimz
03/18/2016 at 18:27 |
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I’d be very surprised if the gains weren’t significant. On the lowly little Triumph OHV 6 it usually means a fairly minimal bump in peak power (maybe 5-10bhp or so, which is still a big deal on these little engines) but a massive bump in torque (and thus power) all over the midrange. Something to the tune of 40lb-ft. This all on an engine that’s making 150-180bhp and 140-200lb-ft of torque. That’s a big jump.
I could see that maybe on the Triumph head it has more of an issue getting it’s exhaust gases out so the gain might be more minimal on a more modern head, but that doesn’t change the fact that a 6-2-1 just links the wrong cylinders together on an I6 so that you end up with pressure in the wrong place to get exhaust gases out efficiently in the midrange.
I can definitely see the cost argument though. 6-3-1 manifolds aren’t cheap! And that’s for non-optimal designs as well.
Still, I’ve seen really high spec E46 M3 builds that still use 6-2-1 manifolds and it strikes me as odd...
Diesel Panda
> Jcarr
03/18/2016 at 19:40 |
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The advantage of un-equal length is the sound and probably only sound but can make it sound goofy or unique as you want! I believe that’s “one” of the reason the VW VR6 has unique sounds is because the exhaust doesn’t account for the length of the exhaust runners
