1G Cheap Downpipes

[4gfun]

Want to replace the stock downpipe on my 93 6 bolt swapped TSI AWD. The car is a fuel eco project/DD/Fun car. Not opposed to Ebay/cheap parts for an eco project like this. If I want to spend, I can do that on my DSM.

Have 2 questions:

1) Can you teach me how to determine whether or not the downpipe replaces the cat when I look at pics? I guess I am wondering how long the pipe should be?

Does this replace the cat as an example:

http://www.ebay.com/itm/1989-1994-MITSUBISHI-ECLIPSE-GSX-AWD-BBP-STAINLESS-STEEL-DOWNPIPE-FLEXPIPE-90-92-/351061329067?pt=Motors_Car_Truck_Parts_Accessories&fits=Model:Talon&hash=item51bce2c4ab&vxp=mtr

2) Also, I notice that the majority of the ebay pipes (if not all) neck down to 2.5 inch at the O2 housing. Based on that, does anything matter afterwards? Is there any point in caring if any point past that bottle neck is greater than 2.5 inches? Or am I wasting money caring whether anything from that point to the muffler tip is greater than 2.5 inches? I don't understand how exhaust flow works when there is a bottle neck.

Thanks for any info!

 

[Chumpaumpalumpa]

That one appears to, since there is only a flex joint and no cat. At least for N/A cars, someone correct me if this does not apply to forced induction, but the bottle neck increases velocity of the gases and expels them quicker than fixed diameter.

 

[NordicSaab]

Exhaust gas hydrodynamics is extremely complicated and im sure you will get as many opinions as there are DSM owners.

Firstly, If you are interested in improved gas mileage increased exhaust volume will not help you. Additional back pressure, to a limit, actually increases fuel efficiency because it slows down the intake and exhaust flows. Thus, lowering your fuel consumption.

In regard to immediately after the turbo thing about your engine as a air pump. To a certain extent that is all an engine really is. The engine is producing compressed air which is released into the exhaust. High pressure air wants to move toward low pressure. The larger the VO3 of the area after the turbo (i.e. your downpipe)the greater the velocity of that air, which is why a larger downpipe improves spool. After that point the exhaust, to a certain extent, is only as efficient as the point of greatest resistance/obstruction. This is why exhaust that are uniform in diameter and generally straight flow better (all other things being equal).

General rule of thumb, if VO3 of gas is standard, a larger exhaust will flow at a slower velocity, a smaller exhaust will flow at a greater velocity.

The short answer is you will see some improvement with a downpipe in performance but probably not fuel economy.

Best of luck.

 

[RamenPride]

About 99.9% of the cheap downpipes on ebay eliminate the cat.

 

[4gfun]

Thanks all. I guess I can pretty much shop in confidence then.

As far as fuel economy goes, I appreciate that advice as well. While the overall theme of this car is fuel economy, performance is a side benefit. Trying to make the most that I can with the existing equipment. Building the car so that I have a fun commuter and go 70-80 on the highway while getting good fuel economy. Hard to explain the goal unless you commute here.

 

[Chumpaumpalumpa]

Thanks all. I guess I can pretty much shop in confidence then.

As far as fuel economy goes, I appreciate that advice as well. While the overall theme of this car is fuel economy, performance is a side benefit. Trying to make the most that I can with the existing equipment. Building the car so that I have a fun commuter and go 70-80 on the highway while getting good fuel economy. Hard to explain the goal unless you commute here.

Change your gearing then

 

[4gfun]

Change your gearing then

No kidding man!! Well then again hoping that the 215/65/16 LRR tires fit without rubbing and take care of some of the overall gearing.

 

[serviceguy]

The stock o2 housing determines the narrower size of the downpipe inlet. Even with a full 3" downpipe it would have to be notched at the o2 flange or it will not bolt to it. I would not trust the pictures but rather then guess I would just contact the seller and ask them, at least it gives you some ground to a refund if the item does not match its description or what they answer to your questions, in any case it's better than 'tuners said it would fit...

 

[4gfun]

The stock o2 housing determines the narrower size of the downpipe inlet. Even with a full 3" downpipe it would have to be notched at the o2 flange or it will not bolt to it. I would not trust the pictures but rather then guess I would just contact the seller and ask them, at least it gives you some ground to a refund if the item does not match its description or what they answer to your questions, in any case it's better than 'tuners said it would fit...

The good thing about ebay these days is that you generally don't have to pay for return shipping anymore when the seller sends you a virtual brick. So long (as you say) the seller advises it fits and it doesn't. In my viewpoint, this includes the auction itself advising that it fits when it doesn't. This can now be argued under significantly not as described. Sure, who wants to go through that....but sure beats the not too distant days of paying to ship a brick back. Figured I'd share this!