I read this on another board. I'm not saying he's wrong or right. Just wanted to hear you opinions on this because I myself am no tuning expert. Seems to make sense what is being said.

[["someone please tell me how an open wastegate is different from a pre-turbo / pre-o2 sensor exhaust leak.
this is hmt. is anybody else with me here?

a wategate opening is no different to a turbo than an exhaust leak. it's just that one is a controlled leak, and one isn't. a good wastegate only leaks out what you set it to leak out, to control power. an uncontrolled exhaust leak on the other hand, releases more exhaust than the wastegate already is, after AND before it's opened. therefore, exhaust leaks will slow turbo spoolup, and alter o2 sensor readings, further reducing power, right? please tell me why an open dumptube wouldn't do the same thing.

i can understand how you guys think that you want the o2 sensor to only read what the turbo has already seen, but guess what made those other gases. that same turbo pumping air into the cylinders. when you run an open dumptube, you're not reading everything that's getting dumped out of the exhaust ports. some of it is going out of your open dumptubes, and some of it is spooling up your turbo, and then going into your o2 sensored downpipe. a wastegate's only purpose is to let exhaust gases bypass the turbine housing, not the rest of the exhaust system entirely. an o2 sensor still wants to see those gases after the turbo, wideband OR narrowband.


physics: you can't have two things in the same place at the same time. sooooo... the exhaust gases coming out of the wastegate aren't the same gases coming out of the turbo.

i know open wastegates sound cool, and are easier to fabricate than a recirc setup, and i know they work. hell, we all do. that's what everyone else runs. but you're sacrificing tuning accuracy at high loads when the wastegate's open."]]

 

Someone correct me if I'm wrong but you are still getting a sample of exhaust gas going past the O2 sensor even if the wastegate is open, where as if you have a exhaust leak pre O2 sensor, you actually can suck fresh air in through the leak and then you get the bad reading.

 

if no fresh air is getting inside then the o2 sample should be unaffected. im gonna bank on the wastegate letting all exhaust out and no fresh air in.

 

The 02 sensor is still reading off of the exhaust gas that doesn't bypass through the wastegate, but the most important factor is that the 02 doesn't read anything at full throttle anyway....so it doesn't really matter

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Schister66 &raquo;</TD></TR><TR><TD CLASS="quote">but the most important factor is that the 02 doesn't read anything at full throttle anyway....so it doesn't really matter</TD></TR></TABLE>'

Narrowband... yes. Wideband? Absolutely not!

As long as the dump isn't bleeding off of one cylinder, the readings should be very similar.

 

An O2 isn't reading everything that comes out of the cylinder. It's taking a sampling. Look at the physical size of the O2 head, do you think it's reading everything that goes past it anyway? You don't need EVERYTHING to pass through one pipe in order for it to be accurate. If you take a small chunk of exhaust gas out of a big clump of exhaust gas, and it's a well-mixed gas throughout, it'll have the same a/f ratio as the rest of the clump It doesn't matter if you chop off 1/2 of the clump and toss it out an open wastegate dump.

 

Since the exhaust to intake PR is something like 2:1 and 3:1, that means 2-3x more pressure in the exhaust than the intake. An exhaust leak before the turbo on one runner could cause outside air to be sucked in at a certain point (right after the exh valve closes), but a wastegate is usually put at the highest pressure area of the manifold. Since you usually get a good mixing as its shoved into the turbo, you usually will get have WG gasses with the same AFR as the the turbine gasses.

Technically speaking, given the speed of exhaust at high rpm's, you could get worse readings with a re-circ, as the gasses from each cylinder may take more/less time to move through the turbine than the WG - meaning overlapping samples. But the way it works, giving you multiple samples for each datalogging cell, means we are concerned with a group of samples only.

 

Thanks for all your replies guys. Lots of good, interesting info

 

Yea, that was my post from HMT. HiProfile, you totally answered all of my questions/pwned me.

I was going to recirc pre-02 sensors, but now I'm not because of your posts. You make perfect sense.