Low Down on Downpipes Backpressure and Torque

[pa-powerstroke]

Posted this on the Nation also:
So I was picking up some pipe clamps and adapters today to redo my exhaust with my new 4" DP, the local exhaust shop owner was telling me all about how going bigger was a bad idea because I loose low end torque and back pressure. Im not schooled enough about turbos to really argue much but I told him I was more worried about egt's and spool-time. Which brings us to my question-
On a turbo charged engine the back pressure is measured before the turbo correct?
How does exhaust gasses escaping more quickly from the turbo make my truck loose torque?
Anyone have any actual numbers or facts to prove this?
Powerstroke veterans, school me on turbos, back pressure, and downpipes!

 

[4.0l sahara]

The exhaust size weather it 4" or 5" won't cause any problems. You could run a open turbo or hood stack and have no noticeable power difference. That guy is full of ****.

 

[907DAVE]

I noticed a small loss of low end power on my stock 7.3 when I went to a 4" down pipe. No real data to back it up, just kinda felt different.

Otherwise, I agree!

 

[SSpeeDEMONSS]

I remember reading about differences between a 3.5 and 4" on tdg back in the day but it doesn't make sense on a turbocharged engine.

Garrett

probably getting paid at work to post this from my bionic

 

[BlueOvalDiesel]

It makes a small differance on naturally aspirated gas motors but not on turbocharged diesel.

 

[Ross@Flyindiesel]

your correct there is no backpressure really after the turbo its measured before the turbo and thats where its at. anyone who says that really doesnt need to be working on diesels.

 

[Johnboy3]

I think the loss of low end power some folks feel after switching to a bigger down pipe or exhaust is due to a drop in exhaust gas temps when running in the lower rpms. The cooler exhaust temps cause the turbo to spool slightly slower when taking off from a stop.

 

[pa-powerstroke]

Yup, thats exactly what some pretty knowledgeable guys are saying on the nation. Basically, the rapid cooling of exhaust gas is calling for the exhaust to condense and slow since it rapidly becomes a lower temp. this is ultimately causing a change in the powerband. A raise really, losing low end torque but gaining power higher up.

 

[Johnboy3]

You don't lose low end torque. If you knew what the egt was when the stock exhaust was in place and then raised it to that temp before taking off there would be no difference in power. I have noticed this very thing on the dyno when someone wants a before and after dyno run when getting an exhaust.