DP Question: Reason for cast bellmouth

[sirsimon]

As I search for a DP, I was wondering if anyone can tell me why some have a cast bellmouth...is it for better heat resistance/improved strength?

[rao]

It is better for marketing

[Waxiboy]

i think it is has to do with the shape of the flange.

[robinlsb]

As I search for a DP, I was wondering if anyone can tell me why some have a cast bellmouth...is it for better heat resistance/improved strength?

 

 

Just marketing.

[sirsimon]

So, I would likely be OK with an Autospeed as opposed to say a Cobb?

[mickeyd2005]

There are other things to consider that are more important.

 

Fit and finish are the most important things to consider when looking for a downpipe. I have owned both the Cobb and Autospeed and they are both very nice.

 

Some other minor things to consider are wideband O2 bung location, catalytic converter type (metal or ceramic), cat location (some people don't like the cat on the bottom where it can be damaged), flange connection at CBE (2.5" or 3.0"), etc...

 

Forgot to add... price is also something to consider.

[sirsimon]

What are the pros/cons of metal vs. ceramic cats?

[SC GT]

What are the pros/cons of metal vs. ceramic cats?

Metal substrate cats flow better. All of the high flow cats are metal substrate. Ceramics (as in the stock downpipe and midpipe) are more restrictive and not as strong as metal.

[biz77]

Three reasons for the cast bellmouth.

Flow

Strength

Noise

 

By casting the bellmouth you have complete control over the shape from the flange back to the tubing. When you stretch a pipe in the shape of a bellmouth you get a uniform shape that is even side-to-side from center. The best example of this is the Cobb D/P vs an ebay downpipe.

 

http://www.cobbtuning.com/images_products/L_2558.jpg

 

http://images.channeladvisor.com/Sell/SSProfiles/10056191/Images/ImprezaWRX0206Logo.jpg

 

Notice on the Cobb piece that exhaust exiting the turbine wheel takes a straight shot down the bell-mouth and into the pipe, resulting in the straightest, least turbulent, most efficient flow when on-boost. Now take a look at the ebay piece and notice that air exiting the turbine wheel will endure a slight jog or turn down the bellmouth before hitting the pipe, resulting in a more turbulent, less efficient flow.

The increase in strength is a result of no weld joint between the flange at the turbo and the bellmouth. This area endures temperatures that constantly cycle by more than a thousand degrees. I personally wouldn't want two pieces of metal joined with a weld in this high heat, high stress area.

Noise is reduced on a cast piece by the simple fact of the material being significantly thicker. 3" tubing is pretty thin to begin with. Now stretch that into the shape of a bellmouth and see what happens. Take a look at the material thickness on that Cobb piece.

 

Or I suppose it could all be marketing.

[sirsimon]

^ Wow, good info. Perhaps I'll just save me pennies for the Cobb or an AVO.

 

Thanks for your help!

[rao]

How about a picture of the cheap downpipe's bellmouth?

[mickeyd2005]

When I was looking for a downpipe, I read Cobb's and PDE's explanation of their bellmouth design and it sounded like it made sense. However, when you look at the dyno charts and data logs, the real world performance difference doesn't pan out.

 

The really surprising fact is how well a gutted stock downpipe works. The blank plate connection seems like an inherent flaw in the design but it works well nonetheless. Lots of people have made very good power with gutted stock downpipes.

 

Fewer welds is better but the temperature at the bellmouth is significantly lower than at the manifold and uppipe so I don't think it's an issue. I've heard of uppipes cracking but I've never heard of a downpipe cracking.

[sirsimon]

Hmmm. Now you've got me spinning!

[DSMsuby]

have 17 years experience with DSMs and this is what I have seen.

 

The bellmouth is similar to our 02 housings. Stock o2 housings are made of cast iron and are not known to crack readily. In fact, I had several that saw over 200k of hard driving with no cracks to atmosphere.

 

Most people will upgrade to bigger cast iron units with very little hp gains except that bigger units may control boost creep better.

 

A few years ago I upgraded to a tubular stainless steel unit with excellent reviews, beautiful welds and thick walls. The problem is the unit is prone to cracks, especially at the welds closest to the turbo. It seems I can get about a dozen runs on it before it will start to crack. The cracks aren't huge at first and really only noticeable when the motor is extremely cold.

 

So my opinion is Cobb's unit would be better and the claims about its reliability makes sense. Not to mention, if a crack does form before the O2 sensor then not only will a loss of power be realized but the O2 sensor will be tricked and the motor will run richer and crappier and make less power.

[biz77]

When I was looking for a downpipe, I read Cobb's and PDE's explanation of their bellmouth design and it sounded like it made sense. However, when you look at the dyno charts and data logs, the real world performance difference doesn't pan out.

 

The really surprising fact is how well a gutted stock downpipe works. The blank plate connection seems like an inherent flaw in the design but it works well nonetheless. Lots of people have made very good power with gutted stock downpipes.

 

Fewer welds is better but the temperature at the bellmouth is significantly lower than at the manifold and uppipe so I don't think it's an issue. I've heard of uppipes cracking but I've never heard of a downpipe cracking.

 

In all honesty I doubt you will see much of a power difference between the two different bellmouth approaches. My explanation is based around the theory of the design, not the actual performance.

 

rao,

Sorry, but I couldn't find a picture that was similar to the Cobb otherwise I would have definately posted it instead.

[NewScooby]

After being convinced of benifits of a cast belmouth myself. I went with Invidia for my pipes, great quality without paying for the Cobb name.

 

I used the money I saved buying Invidia, and had my up/down pipes professionally ceramic coated for $96. With my car running, you can hold the downpipe, while the catback is too hot to hold. Should provide a slight reduction in the dreaded summer intercooler heat soak.

[biz77]

+1 for the Invidia. A quality piece for sure. I wish they had a WB02 bung.

[dtmenace]

reputable vendors for the invidia pieces??

[biz77]

JSC Speed

[Infamous1]

have 17 years experience with DSMs and this is what I have seen.

 

The bellmouth is similar to our 02 housings. Stock o2 housings are made of cast iron and are not known to crack readily. In fact, I had several that saw over 200k of hard driving with no cracks to atmosphere.

 

Most people will upgrade to bigger cast iron units with very little hp gains except that bigger units may control boost creep better.

 

A few years ago I upgraded to a tubular stainless steel unit with excellent reviews, beautiful welds and thick walls. The problem is the unit is prone to cracks, especially at the welds closest to the turbo. It seems I can get about a dozen runs on it before it will start to crack. The cracks aren't huge at first and really only noticeable when the motor is extremely cold.

 

So my opinion is Cobb's unit would be better and the claims about its reliability makes sense. Not to mention, if a crack does form before the O2 sensor then not only will a loss of power be realized but the O2 sensor will be tricked and the motor will run richer and crappier and make less power.

 

Not as much of an issue since Subie's turbo are quite a bit further away from the exhaust ports than the 4G63. Also the rear O2 sensor is not a concern anyway, the important one is in the header.