How short is too short for open downpipe?

[GSLENK]

How short is too short for a downpipe? The best location I can find for an exhaust cutout is at the first bend of the downpipe, letting the cutout point straight down, (well mostly down, slightly towards the back of the car.

This location also prevents anything from hitting the ground if I lower the car (like speed bumps...) because the lowest point of the cutout will be close to the wheel base up front, and the part under the wheels almost never bottoms out even when lowered.
^^^ this is why I think it is the best spot.


BUT... I am not sure this is the best spot for flow, it seems like the exhaust will be too short with the cutout open. It will only have about 1 foot of 3" DP before it is dumped to the atmosphere. It seems like it has virtually no restriction and unlimited flow, but that has to be bad, even for a turbo motor. Would It ruin performance? Would it slow down exhaust velocity? Would it ruin my WB o2 readings (sensor in the front factory location)?
^^^ This is why I am hesitant to do it.

I have seen plenty of custon DPs with the cutout further down. I want right at the 90* bend. I have not seen one of these yet, and if I cannot find even one, there must be some reason, hence my hesitation, and question.

I have asked this a few times on other posts where this was related, but not the whole topic, and it just gets ignored... I also cannot find anything proof/solid to back up my idea, only other people with the same idea.

Some one give me the go ahead on this, or I might just have to find out for myself

 

[92tsiawdguy]

It won't hurt performance. Depending on your horsepower levels it could improve it greatly. I've ran 4" off the turbo and right out the bumper. That's like 3ft of piping.

 

[GSLENK]

ok so there zero chance of anything going wrong? even on a 14b? the cat+catback is factory. would this change my VE table at all since i am running speed density?

And dont worry, I will make sure the heat will not cause issue with the alternator or anything else, I will shield and route as necessary. I just want to make sure Im not going to shock anything having the exhaust too short.

 

[92tsiawdguy]

Running an open downpipe on a 14b is going to cause you to have boost creep. Make sure you port the o2 housing and wastegate hole and you will be set.

 

[Ppayer38]

Dont quote me on this as I cant remember and it has been a while since ive reasearched it...

I believe that it will drop the torque but raise the HP #'s and help spool out alot!

If you Cutout is before your WBo2 then it will not read correctly, you need to have the WBo2 atleast 6" before the cutout just so there is no chance of the air inbetween exhaust pulses messing with the readings... I know you are worried about the sensor going bad but I ran mine 5-6" from the turbo outlet and never had an issue for over a year with my AEM UEGO, Many subaru's do this also!

When I had my cutout I had it where the DP connected to the catback and loved it but ended up welding a resonator in to quite it down a tad cause it rattled everything!!! Im now in a 05 WRX with a straight 3" catless exhaust to a Buschur SD muffler(Pretty freaking LOUD haha)

 

[bmoha7321]

So when you idling... The heat of your exhaust goes straight down and the ground... And then rises right back up? Not so good. Plus as your driving exhaust gases go backwards towards passenger compartment? Not so good. I don't have a problem with short cutout... Just at least route the cutout out the front bumper/fender. Alot of people do it right in front of drivers side front wheel. Keep in mind it's need to be thermal wrapped so you don't cook powersteering pump.

 

[GSLENK]

Running an open downpipe on a 14b is going to cause you to have boost creep. Make sure you port the o2 housing and wastegate hole and you will be set.

Ill figure something out about that. (probably end up doing as you say, or dumping 02 to atm.) Ill get to that later. thanks for the reminder

So when you idling... The heat of your exhaust goes straight down and the ground... And then rises right back up? Not so good. Plus as your driving exhaust gases go backwards towards passenger compartment? Not so good. I don't have a problem with short cutout... Just at least route the cutout out the front bumper/fender. Alot of people do it right in front of drivers side front wheel. Keep in mind it's need to be thermal wrapped so you don't cook powersteering pump.

It would only be open while moving. Its an electric cutout valve. for now, Im gonna wire it to a physical switch, and only turn it on momentarily, or while moving. so theres no bouncing off of the ground and back up to bay or me. I know I will eventually want to route it out of the engine bay and through the side of the bumper or something.

UPTADE: Im going along with it. Heres the mock up, its a tad low, and I just ordered the valve. (I have a block plate for now). Here are some pics. I have to do more cutting for clearance, but thats about where it will be after the valve is attached (ill move the split higher, and the valve will sit at the lowest current point)