How is primary length measured?

[rbogh901]

Guys, hey, can someone explain a basic concept I've been hunting: How is the primary length of headers measured? I assume it would be down the middle of the tube and be an average of the shortest and longest distances within a primary.


Does it end at the merge or at the mid confluence of the gases?

In other words, when primary length is specified how is it measured?

Maybe it's been addressed here but I haven't found it. Some great discussions overall but this one eludes me.

[rbogh901]

So here is an example. The difference between the blue and the white arrows is not insignificant.
Which one determines actual primary length for the purposes of header design?

[winders]

You want to measure from the exhaust valve all the way to the where the blue arrow points.

[Costa P]

Originally Posted by winders

You want to measure from the exhaust valve all the way to the where the blue arrow points.

Just like winders said, measuring the length along the centre point of the tube as you move along.

[rbogh901]

ok. So the blue arrow is where there appears to be total termination of the primary and it defines the end.

I hadn't thought to include the distance to the exhaust valve. What's that, 1-2 inches on top of the apparent primary length in the headers themselves?

[Flieger]

The resonance and restriction is going to be affected by all the bends and transitions so it's not quite as simple as a mean path length but I'd say that should get you in the ballpark (red line).

[rbogh901]

And a rough Target for 911 Motors is 29 - 31 inches?

[faapgar]

Actually primary length varies from 2.0 to 3.0 to 3.5 and your intake manifold height affects it as well.Turbos like short primaries to spool up the turbo quicker as they are force fed.

[PFM]

The header length calculations are just not that accurate. PipeMax can get you very close but still not 100%. Build it as short as possible with a few pipe diameters straight before the collector. Then build 3" extensions for testing on the dyno.



I built this for a 914-6 project some time ago, collectors are by Burns. The 3" extension clipped off 400 RPM at the top and was not a good trade off for the build.

Good luck.

PFM

[rbogh901]

Originally Posted by faapgar

Actually primary length varies from 2.0 to 3.0 to 3.5 and your intake manifold height affects it as well.Turbos like short primaries to spool up the turbo quicker as they are force fed.

I see how that would benefit turbo apps.

There are a ton of variables of course but most of the time it seems to be oversimplified by just designating a tubing diameter.

[rbogh901]

Originally Posted by PFM

The header length calculations are just not that accurate. PipeMax can get you very close but still not 100%. Build it as short as possible with a few pipe diameters straight before the collector. Then build 3" extensions for testing on the dyno.



I built this for a 914-6 project some time ago, collectors are by Burns. The 3" extension clipped off 400 RPM at the top and was not a good trade off for the build.

Good luck.

PFM

Just eyeballing those collectors it looks like a very smooth confluence. Does you get the flanges from them also?

[winders]

I like these guys better than Burns:

Exhaust Components - SPD Home Page B

They offer a 12 degree merge collector which I don't think Burns does. They have everything you need to make 911 headers.

I would not build primaries as short as possible. That will get you a peaky engine which is not usually what you want even for a race car.

After the merge collector, which should have an output smaller than what I see in the photo, you want to transition to the final outlet size. A 7 degree transition is good. The transition gives some restriction which is good.

[rbogh901]

Good info, I'll look into it and see what they have.


Well, now here are some headers for a 3.6, different primary lengths and collectors.

The headers in the first picture are being replaced by the headers in the second. I can't really say how long the primaries really are because the collectors are much further aft in the first set. I do not know what diameter they are in either case.

The craftsmanship in the first set is very nice.


http://forums.pelicanparts.com/porsche-911-technical-forum/930334-finishing-pelican-parts-911-rsr-tribute-project.html

[winders]

The headers in the first picture have a much better merge collector....but the primaries look little short.

[winders]

Here is a photo of great headers and mediocre headers:

[rbogh901]

Originally Posted by winders

The headers in the first picture have a much better merge collector....but the primaries look little short.

yes, agreed, but it's not clear since the overall configuration leaves the collectors so much further back.

[rbogh901]

Originally Posted by winders

Here is a photo of great headers and mediocre headers:

Much nicer collectors and construction, interestingly the primaries look shorter in length from here in the cheap seats. This would prefer a higher power band I assume. Was there any dyno or seat of the pants information comparing these? Might be hard to get objective data given the pregnant guppy tail pipes on the one set.

[winders]

Originally Posted by rbogh901

Much nicer collectors and construction, interestingly the primaries look shorter in length from here in the cheap seats. This would prefer a higher power band I assume. Was there any dyno or seat of the pants information comparing these? Might be hard to get objective data given the pregnant guppy tail pipes on the one set.

The primaries are longer when you consider the difference in the collector.

No dyno comparison as the engine setup changed quite a bit from when the old header setup was used to the new header setup. By the way, those "pregnant guppy" mufflers lose no power compared to a straight through reverse cone setup with no muffling. In other words, they are awesome!

Note the collector to exit transition on the S-Car-Go headers. The whole setup is a work of art!

[KTL]

I agree the "RSR" header primaries are exceptionally short. Has to be a reason why they made them so short.

I used to have a really nicely fabricated set of MODE headers on my racecar. They curved the piping on a rather large radius, relative to what i've seen on other headers, to create the length they were shooting for.

[rbogh901]

Those Mufflers are phase 9 correct? It seems they have become less common. What has superseded them?

How important is the short segment megaphone after the collector?