3" dp?

[87boostedbuick]

Just wondering is a 3" down pipe really makes the much of a diffrince as it is I'm running 2.5" exhaust no mufflers and hollow cat and I think I need more back pressure with that set up so I guess I'm wondering with that set up is a 3" dp really nessary

 

[TURBOPOWERED68]

an exhaust guy told me
Try to use The biggest DP off of the turbo that you can fit- the gasses are hot and expanded- thats why bigger is better. and as the gasses cool down they shrink reducing the need for the bigger pipes.
i am sure some one with more knowledge than me will post up.

 

[RmvBfrFlght]

You don't really want more back pressure, less is better. One direct item with less back pressure is that the turbo will spool faster.

As for down pipe (DP) diameter, larger is better. The main reason is that as the gases exit the turbo they are spinning in a circle. With a smooth transition to the same diameter pipe, they continue to spin, and don't want to move down the pipe. Just spin around the inside of the DP.

Put a larger DP on and now there is an abrupt transition from the turbo to the DP. This causes turbulence which breaks up the spinning exhaust gases. This allows the exhaust gas to move down the pipe and out the tail pipes.

RemoveBeforeFlight

 

[Bescurred]

My car LOVED a larger dp. Go 3" or bigger.

 

[87boostedbuick]

Ah makes sence maybe I will have to get one then the oh reason I thought I might need more back pressure is well becuase after I changed my exhaust I got a lean o2 code only after about 5 min of idel then as soon as you hit the gas it's gone and I know 100% the car is not running lean some one told me that becuase of the free flowing exhaust the o2 isn't able to get a proper read but I'm not sure how true that is and the o2 is only about a summer old

 

[earlbrown]

Has the crack in the drivers side header been welded yet? That can let fresh air in before the manifolds get pressurized and give a false lean reading.


I cringe every time I hear the word 'back pressure'.


On a turbo engine you NEVER want any kind of exhaust restriction. The turbo is driven by a pressure differential on the exhaust wheel. For every pound of exhaust restriction you have, that's 1 pound of pressure that's holding the turbo from spooling. And another pound of pressure keeping burnt exhaust in the cylinder instead of oxygen.


The "I need more backpressure." wives tale comes from millions of hottrodders with too much overlap on their 3/4 cams. When you over scavenge a N/A engine with a cam that's too big you can pick up power by corking up the exhaust and keeping the fuel/air in the cylinder where it can get burned. (not as much power as getting the right cam though)

 

[Buick Mark]

3" downpipes look cool too

 

[Bescurred]

Just do it!!!! You'll love it!!!

 

[87boostedbuick]

Well thanks for all your help guys looks like ill be getting a 3" dp and thanks for clearing up the back pressure thing for me I think I may have a small crack in one of the headers so I will have to pull it out and check thanks again guys lots of good info