Because that's what the mills make and exactly what a couple of others said already and I did with my description. I have taught welding and metal fab for 20 years and before that welded in the avation world.. It is concept that allot of people struggle with.

Manufacturers will engineer around known and available materials..

The OD remains a static measurement in tubing as stated, the ID is variable depending on the type of material used. This is one reason it is important to consider the material that your exhaust products are made out of. An extreme example is the 321 stainless we use versus mild steel. A popular vendor uses mild steel with a wall thickness of 0.125" versus a wall thickness of 0.049" for the 321 we use in the exact same application. You can see how this greatly affects the ID of a 1.5" tube. It also has a huge affect on weight. Our system weighs half that of the mild steel counterpart and is stronger and more durable. Yes it also costs a lot more as well but as you can see you get what you pay for.


http://forums.pelicanparts.com/uploa...1539611943.JPG

http://forums.pelicanparts.com/uploa...1539612205.jpg

Now I must inquire...

Is there a different thought path if say someone (SauCy) recently installed the larger of the 2 sized headers in question on a stock 3.0...but that engine is a true Euro spec 930/10 with large intake runners and higher CR...?

With an M&K 911R muffler...of course :)

Brian sorry to disagree but type of material doesn't dictate wall thickness.. More common materials used in stainless are .065, .049, .036.. 304vs 321vs any other type doesn't matter its about what the Mill makes and how big that particular size has for a market..

I've seen 1 3/8"ID SSI used on everything from a 2.4 to a 3.6, The bigger the motor the more effect it has on top end power, on a 3.2 it only affects the very top of the rev range go to a 3.4 and the affected rev range moves down, on a 3.6 more so(down to ~4500 here). Thee factory 964 3.6 uses 41.275mm(~1 5/8")ID tubes but is a poor design due to the crossover and merge, a factory 993 3.6 uses 39mm(~1.535")ID a better design overall and used on the 3.8 993Cups as well as the 3.8RS

When too big is used a similar effect occurs but in reverse, It's less noticeable because you aren't usually going for max power at the bottom end of the rev range. What does happen is that the flow slows down, the bigger the pipe the more it slows. The issue is further complicated by the use of a muffler. each muffler affects the system in it's own way, adding more or less back pressure. The one sure thing that any muffler does is inhibit acoustic tuning.

Stainless is the preferred material for longevity but also adds a small amount of heat insulation. The hotter the gasses, all else being equal, the faster they move the more efficient the exhaust.

Some will add external mat insulation but this raises the issue of additional corrosion which will shorten service life.

Sure it does, how long do you think 0.049" thick mild steel headers would last? Point is the lesser materials have to be thicker to help improve longevity.

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911SauCy, are you asking a good question. From the information referenced here it looks like 1.5 is good.

The 930/10 is a common motor used in PCA E class racing. Some of those cars are REALLY expensive builds. I would assume they are using 1.5. I may ask on Renn where some of them post from time to time.

Now I'm questioning myself...digging deep into the grey matter, I think they are 1.5"...because I bought them from someone here who was busting me for asking him to measure ID:p

You are so right. I don’t know what that guy is talking about.

I think it is/was a matter of semantics. It is true that the type of material does not dictate the wall thickness of tubing from a manufacturing standpoint, it is the application that dictates the thickness.

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Here's a comparison of the exhaust gas flow into each pipe/Sec for various engines.
here Volumetric Efficiency is assumed to be 100% and the acoustic effects aren't considered
http://forums.pelicanparts.com/uploa...1539800492.gif

note that at say 15 l/s; the 2.7 is @ 4000, the 3.0@3800, the 3.2@3200, the 3.6@3000