Many people run 3" downpipe and exhaust, but the outlet of their turbo elbow is less than 3".

Also some people may run intercooler pipes which are larger than the outlet of the turbo compressor outlet.

What is the deal here? Can someone explain the physics of airflow a bit and explain what happens in the above situations?

 

for evo owners, it comes down to packability....if that is a word. shivnu offered a true 3" dp, it was a nice peice, however they chose to limit their market, as they only sold it/made it to fit their complete exhaust. so they fisted their own **** on that 1.

I have a 3" downpipe / custom 3" o2 housing & I know it makes more power than the ebay o2 w/3" dp....but i never dyno'd to confirm. truely butt dyno, but there was a difference. I know this mod will shine even brighter when i install my fp-grn, as i will have less pr due to my 2.880" i.d. exhaust

WOT = 2.880" i.d. = just 3" tube straight out of the turbine housing.
http://www.socalevo.net/gallery/albu...ormal_side.JPG

works = 2.740" i.d. +/- 0.020 casting variances. i measured 2 different models.
invidia = ? i have never measured
ebay = 2.385" +/- 0.010" i.d. i measured 4 units.

it all comes down to minimizing your pr (backpressure) within your exhaust. yeah, you want to minimize your intake side of pressure drop to get the most out of your oem turbo if thats your situation, as is mine.

it all comes down to minimizing your pr (backpressure) within your exhaust.

IT'S ALL ADDITIVE, meaning any backpressure / pressure drops you encounter along teh way is a loss. there is no loosing it! no if's ands or buts.

i worked with a guy who helped develope the roush gto programs 2.4L 4 cyl. engine from the early 90's, a 4" exhaust pipe made the most power, straight out of the turbo, because they reduced their pr & was able to screw in more timing for the gasoline they had to use.

see above

see above

 

It's just the basic law of optimal flow with gases. Hotter gasses require a smaller ducting than cold. A lot of times you can optimize and change the pressures and flow to your benifit. The OEM ducting wasn't designed the way it is due to the availabilty of parts, some real smart Japanese dudes actually had performance in mind

 

So having an exhaust that progressively goes from 2.5" to 3" might perform just as well as a full 3"?

 

yes & mostly no.

if this was the case with exhaust systems then everyone would be building 2.5" exhaust systems or even smaller to get a quieter exhaust note. Buschur would only be selling muffler delete pipes because of the superior power that the oem exhaust offers over his current 3" exhaust sytems.

i wonder if the ams drag car is running a 2,.5" exhaust....maybe they are taking a bit of advice from above & running a 2" system...............i doubt it.

 

I was mainly referring to bleed air ducting on aircraft - the plumbing right off the compressor stages of a jet engine is about 5" diameter, as the air is cooled through heat exchange the diameter increases. It's just a standard in pneumatic design thats been around for years. The engineering statement was referring to the IC plumbing