Tell me if I'm an idiot for thinking this...
 

I had my BEGI recirculated downpipe chopped and re-welded to dump the wastegate under the car. Ever since, the car has been running bad. I bought the car tuned on AEM F/IC while the downpipe was recirulated. My wideband O2 sensor is after the recirulation point shown below:

https://fbcdn-sphotos-a.akamaihd.net..._4886697_n.jpg

Since then, when get on the throttle like 75%, my AFR's go down lower and lower which is good, but if I go WOT, the wastegate dumps and the AFR's shoot back up, way lean. Could this be because the was tuned expecting the wastegate from the wideband? And now it's not getting any?

The car is also idling really lean, and almost misfiring since this. Doesn't seem related though, because it didn't do it until a little while after. I've done a lot of work on the car lately, so I'm not sure if it's a fuel issue, spark issue, a vacuum leak, or an intake/intercooler piping leak.

Starting to get really fed up :crx:

That is a very unique DP setup. I have never seen anything like it, and to tell you the truth, I'm diggin it!
Aside from that, what kind of vacuum are you pulling at idle? Are you running a MAF and stock ECU? Give more details please...

Not sure on the vacuum, the boost gauge I have only shows from 0 psi to 15 psi. Still using the MAF with a recirculated BOV and the stock ECU with a AEM F/IC piggyback.

I'm thinking your exhaust should have absolutely nothing to do with your issue. Most likely a vacuum leak. Make sure recirculating BOV is hooked up and plumbed correctly.

I was going to do this at one point but pussied out.

I suspect it would effect the tune a bit simply because its going to flow better, but I dont think it would be that dramatic.
As for the AFR readings, if you have a leak between the wastegate tube and the DP it would make the wideband read wrong. Does it sound or feel like any exhaust is coming out of the wastegate dump tube when idling?

I can't seem to find a vacuum leak. I'm most concerned about why the car is going lean at WOT. The recirculated BOV is plumbed correctly.

Old pic, but BOV is still set up like this:
https://www.miataturbo.net/attachmen...ine=1332390644

How much boost are you running?

I can relate to the -----ing out. It's an expensive pipe to hack up, but I just said ---- it. To be honest, it's super loud at 10psi, and doesn't sound the greatest. I shoulda kept it super quiet, sleeper status.

But it's pretty dramatic, the AFR's jump way up, but are fine if I ease on it. I'm not sure where exactly you're talking about that I could have a possible leak. But yes, at idle it seems to come out a tad bit, but not much. I can hear the difference at idle since it'd be cut.

10 psi, sometimes I've seen it spike to 12 psi before though.

Can the fast-moving exhaust gas suck air through the wastegate tube and give you funky AFR readings?

Also what injectors are you running?

I'm not exactly sure what you mean. Like up the cut pipe? Not sure exactly how that'd happen, but I'd suspect if it were happening, it could have an effect.

305cc green Supra injectors with a BEGI RRFPR.

Im thinking that there is a leak at the downpipe flange between the turbo outlet and the wastegate outlet,
On my BEGi setup there was a peice of metal welded in the outlet of the turbo to separate them, but Ive seen other downpipes have this peice welded to the downpipe flange instead.

Im thinking that if the outside air is getting into the exhaust stream through the dump tube while the wastegate is closed, this would bugger up the wideband readings.
The only way to fix it would be to go back and make some further modifications to isolate the wastegate from the downpipe at the turbo.


As for the vacuum leak. Is the idle any higher than normal?
If not, I doubt thats whats going on.


Another thing.
Are you still using a MAF, or is the car running speed density now with the AEM?

Makes sense, I'll check that flange tomorrow when I wake up, but I don't hear anything that would indicate a leak there.

I think I'll just have the downpipe welded back up so it's recirculated, I don't like the sound that much anyways.

Yes, the idle is higher than normal at times, and my AFR's at idle are off the charts, literally. They read at "---" and it seems like it's almost misfiring sometimes, but other times the AFR's are in the 14-15 range. Not sure where a vacuum leak could be, I did recently switch out all the vacuum lines with new ones and replaced intercooler couplers, but I've double checked everything and don't hear any leaks. I'll investigate this tomorrow as well.

And yes, the car is still running on the MAF with the AEM F/IC.

Yah, it wouldnt idle if it were really that lean. Even with good tuning its difficult to make an engine idle leaner than 15.0L1, and you see a lot of people idling their cars in the 13s.
So that pretty much tells me that there must be some sort of exhaust leak coming through that pipe, or maybe around the flange.

Were you having boost control issues that inspired you to make this modification or was it just an experiment?

The car hasn't ran perfect for a while now, I did the downpipe modification while the car was running decent. Still some things that need to be sorted out, but it's so hard to track everything down. I mainly did it because I heard this video, and wanted to sound like it, no boost issues inspired this.

yes the crappy running can be attributed to the dump you created.
its tripping out your oxygen sensor from the air thats getting into your exhaust stream.


why do you think pretty much NO ONE EVER did something like this? You think people before you didn't think of it? lol

the only time it "might" work is if you made 120% sure that the iwg flapper is completely isolated from the downpipe opening and getting 100% of its flow through the port and not turbine or rest of dp

I not much help but from what the other guys are saying, take a step back n have it welded back. The only one way to tell for sure. no worries im a dumb ass too I recently deletec the muffler off my 335I and it soinds like an srt4. Lol. I like the back firing but not the in cabin rattle/drone :(. Good luck

[Hyper] Just because you don't understand it doesn't mean it can't work. Once I am finished perfecting my unique new system, I will share the results with everyone. Until then, stop talking ѕhit about things you don't understand. [/Hyper]

:D

But seriously...

I'm thinking that Vlad is probably on the money here.

Think about the concept of exhaust scavenging in general. A large quantity of gas flowing at high velocity through a closed path is going to tend to draw air into the channel through which it is flowing when a properly positioned orifice is available.

When you are at high load but less than WOT, the wastegate is open, dumping exhaust into the wastegate downpipe, and thus preventing atmospheric air from being drawn in.

When you nail the throttle hard, the wastegate will be fully closed, and the absence of its contribution into the wastegate downpipe will allow atmospheric air to be drawn upwards through the wastegate downpipe and into the main downpipe, thus diluting the exhaust gas and causing a false lean indication.

The same phenomenon would occur at idle and light load, when the intake tract is below atmospheric, and this the wastegate is again fully closed.

It may seem counter-intuative, but this is precisely how exhaust-driven crankcase evacuation systems work. They use the gas flowing through the exhaust pipe to actually create a negative pressure across an orifice which provides vacuum to the crankcase while at high load.

Examples:

http://prestoliteperformance.com/Por...ystem_6002.pdf

http://www.allstarperformance.com/sp...sheets/828.pdf

or maybe run a check valve on the end of it, not sure if they make a cutout small enough though.

Thought I could try it, didn't think it would give negative effects like this :facepalm:

Thank you, some great info here. Much easier to understand now, makes sense, which goes a long way. I'll get it welded back together this weekend and report back.