Anyone running a downpipe?

[Rolo]

You would sacrifice some low-end (RPM) performance for high-end performance. A larger exhaust offers too little or no resistance at the low-end, forfeiting the Bernoulli effect (resistance = velocity, cf. a garden hose) and the Venturi effect at the intake; however, with more exhaust moving at the high-end, you gain those effects back without the overbearing restrictiveness of a smaller pipe.

It is an either-or situation, and you choose based on your application, same concept as a 4-to-1 header versus a 4-2-2 (tri-Y) header.

Now, if someone could make an adaptable variable width exhaust...ka-ching!

[3BarBoost]

quote:Originally posted by Rolo

You would sacrifice some low-end (RPM) performance for high-end performance. A larger exhaust offers too little or no resistance at the low-end, forfeiting the Bernoulli effect (resistance = velocity, cf. a garden hose) and the Venturi effect at the intake; however, with more exhaust moving at the high-end, you gain those effects back without the overbearing restrictiveness of a smaller pipe.

It is an either-or situation, and you choose based on your application, same concept as a 4-to-1 header versus a 4-2-2 (tri-Y) header.

Now, if someone could make an adaptable variable width exhaust...ka-ching!

Actually, in the case of Turbos (and for everything fuel injected) - there is no such thing as too little backpressure. That is something held over from the carburated days. So go BIG, especially in a Turbo - because the turbo spools up faster with less restriction and needs less wastegate up top to make the same boost. Backpressure costs horsepower and response, which is why an exhaust is one of the first things you should change on a turbo car.

That's why my race pipe is only 2.25" and pulls like you wouldn't believe. Put the cat back in, and - blah. But clean air is the name of the game right?

[MEXICANPT]

So 3Bar, should I go for the 3" 02 housing, 3" downpipe/racepipe and (suggest what kind of exhaust) to get more out of my car while I wait for Stage II?

Please give me some pointers here.

[MEXICANPT]

One more thing 3Bar, if I remove the cat will my CEL come on even if I have Stage I?

[Rolo]

quote:Originally posted by 3BarBoost

Actually, in the case of Turbos (and for everything fuel injected) - there is no such thing as too little backpressure. That is something held over from the carburated days. So go BIG, especially in a Turbo - because the turbo spools up faster with less restriction and needs less wastegate up top to make the same boost. Backpressure costs horsepower and response, which is why an exhaust is one of the first things you should change on a turbo car.

That's why my race pipe is only 2.25" and pulls like you wouldn't believe. Put the cat back in, and - blah. But clean air is the name of the game right?

ooooOOOOOooooooh! Duh! I see. Yes, my little bit o' knowledge is outdated. Cool, that simplifies tuning quite a bit. Thanks!

Hmmm...this shifts my timetable around a bit, too. I was going to replace the exhaust with the Stage 2 installation. I guess I am in the market for an exhaust now.

Stipulating to Stage 2/280 HP, would there be an advantage to a 3" pipe over 2.5"?

I am also curious about MEX's question about deleting the cat, as well as any other ramifications of doing so.

[MEXICANPT]

I've read that 2.5" should be enough for 300 HP, but I want 3" just in case, am I wrong here?

Stage I is not supposed to be using the downstream O2 sensor, therefore there should be no CEL when there's not cat in the system, I just want confirmation from our good friend 3Bar, and more importantly, will Stage II use the downstream O2 sensor? or just behave like Stage I?

[Rolo]

I am guessing that the stage PCM's derive a lot of their power by throwing the emissions garbage under the juggernaut called "performance".

[SteelBlueGT]

I am just speculating here, but I would bet that the cat now on our cars is maxed out for probably 2.5" exhaust system. I would never get rid of my cat! So Stage 2 is probably my goal! I am a firm believer in taking care of the weakest link and not over sizing down stream which will effect mostly noise. However, I dont like to replace parts twice either!! If you want to max power and not noise think we have to start at the engine and work outward towards the tailpipes!! Of course, we have to have a game plan and know how many horses we want to have!

[Kirby]

The article posted by the turbo engineer early last year said virtually the same as 3Bar, with this adition: reduction of backpressure is best acomplished closer to the turbo. Ergo: a 3" downtube and high flow 3" cat should net more efficiency than our low restriction exhaust systems. I personally feel that a whole system would only help all aspects of engine operation.

[3BarBoost]

Actually, a CEL will come on if you delete the cat on any of the Stage kits (remember what I keep saying about passing the sniffer?). All the Stage kits run just fine with it in there, and cats don't degrade if you push more air through them. So doodling around town, the cat works just fine and keeps the tailpipe squeaky clean. Up top, the fuel goes open-loop anyhow and catalyst efficiency drops whether or not you have a Stage kit. Fortunately emissions isn't regulated up there - not that any car could stay squeaky clean at peak power, turbo or not.

If you were serious about power, and cared not about emissions compliance - a 2.25" straight through (race pipe) is more than fine with the stock turbo. However, a bigger turbo will want something bigger - the Stage 3 doesn't make any more power on the SRT with a 3". A PT installation might be different (because of the extra bends), but I think most 2.5" systems are fine. The O2 housing (what I call the downpipe) is not really that restrictive - it's something you'll change if you are after every last BHP, but your money is better spent upgrading to the Stage 3 turbo housing.