TT Exhaust Review + Dyno Results

[Kor]

I do not have much knowledge about whats available on the VAG-COM but I am starting to think that I should spend some time looking at it...

[Kor]

I am re-formulating my plan after realizing that I might blow up my engine using just the regulator. Working on a better system.

[1.8Trip]

Do you feel that the surging is a direct result of the exhaust upgrade or do you think that something has recently failed?

[Boy Racer]

Originally posted by Kor


. My own theory is just that reduced backpressure is increasing the flow through the turbo creating a higher initial "spike". Surge is caused by boost spiking too high and the ECU deciding that it needs to take countermeasures.

Kor,

This makes perfect sense to me. Control loops (speaking more from Electronic experience such as phase lock loops but the theory is exactly the same) are extremely sensative to all aspects within the loop. The response time of the device you are controlling (turbo pressure) has to be matched to the response time of the control loop. By decreasing the time of one aspect (spool time), the system will lag and won't apply the correction in time, it therfore has to overcorrect to bring things back under control but because we are now reacting quicker to given input the turbo is now starting to produce much less output than expected so the control system overcorrects again...etc.etc...

So in theory, what you need to do is to alter the gain of the N75 (or find one that matches you mods) as this really seems to detemine the control loop response time. Have you got one that has the adjusting screw? This might enable you to get acheive critically damped condition as opposed to the underdamped one you have.

Also Vag Com can definately be used to see if your injectors are running at too high a duty cycle.

[Kor]

I have been reading

The surging is a direct result of the exhaust upgrade.


Exhaust differential pressure is the pressure difference between pressure before and after the turbo's turbine. A different exhaust system can lower the downstream pressure (that’s the whole idea). This means greater pressure differential is realized. Exhaust pressure differential exerts pressure on the wastegate which can push the wastegate open, counteracting the spring pressure. This is sometimes called mechanical regulation.

In a stock car, they make the spring firm enough that mechanical regulation is compensated for. So if the exhaust differential pressure is exerting 4 psi and you want the wastegate to open at 10 psi then they make sure that the wastegate spring requires 14 (10+4) psi.

Mechanical regulation in my car is now probably incorrect. This means that the wastegate is opening too hard and fast far under boost. The ECU can detect this sudden venting and thinks there is an emergency situation, so it cuts the boost out. This is my surge feeling.

My solutions are:

1. Harder wastegate spring. Not really an option here.

2. Bleeder valve, regulator or restrictor going to the wastegate. This will reduce the pressure opening the wastegate. Using my example from before, if the exhaust differential is an equivalent of 4psi and I want the wastegate to open with 10 psi then I need to "bleed off" 4psi from the signal. So the result will be 4 + (10 - 4) = 10 psi.


I am also considering installing a pressure relief valve going to the wastegate. Again on a stock car, the wastegate spring has a predefined stiffness so that under maximum boost it opens to redirect some of the air. On our cars we are running more than stock boost. This means that when we hit stock boost levels (say 10 psi) the wastegate is at least partially open. Our chips continue to request more boost (14-15 psi) and boost is delivered, but I think some of it is leaking out of the wastegate even as it builds. So it takes longer to hit the higher chipped boost level.

If I installed a pressure relief valve before the wastegate and set it to 15 psi then I could be sure that I was not losing any boost until I hit at least that level. This would be a major improvement in my opinion. You may even be able to hold more boost to redline because less air is being vented.

So I am gonna get started on this as soon as I can figure out where to get the parts.

[Boy Racer]

Ahhh yes, I forgot the mechanical boost control on the exhaust side that these have in addition to the electronically controlled system on the intake side. A trip to the local aquarium store for some hardware just like the good old days of turbo tuning!

[Kor]

Think an aquarium store can really hook me up? I've never owned an aquarium

[Boy Racer]

You can go with a mechanical bleed using an aquarium valve or

http://www.dawesdevices.com/

[Kor]

The dawes device they sell isn't really the right thing as I understand it (correct me if I am wrong).

Its basically a pressure relief valve as I discussed in the last part of my long post (above). So when I hit a specific pressure to the wastegate, it lets the signal through. Until that point, the wastegate stays closed.

However it would not allow me to reduce the overall pressure delivered like a bleed would. So when the dawes device or relief valve was triggered, the pressure that went through would still be (possibly) too high and I would get surge.

Yes?

So I could probably get the dawes device AND a bleed and it would be just fine.

However, the dawes is $60 canadian and I am thinking that I can get a pressure relief valve somewhere in town for less than that... probably at some kind of gas equipment store or something.

[Simon]

When are you going back to get the straightened out Kor? I'm really interested to see if you can get the exhaust looking right (shouldnt be a problem). I sorta dont wanna go with Brullen as I havent seen any really great gains with it on Vortex dyno's.

[Kor]

Curtis said 1 to 2 weeks because they needed to get the materials. I am sure they will get it looking tip-top for me after that. I hope.

[Boy Racer]

Bleeding off Exhaust gas is not going to be easy (plumming, heat etc would make this very tough), I was thinking about the control vacuum which according to your description would not make sense.

Can't you get to the wastegate spring?

[Simon]

Originally posted by Kor
Curtis said 1 to 2 weeks because they needed to get the materials. I am sure they will get it looking tip-top for me after that. I hope.

have they taken up the misfitting issue with TT? what do they have to do to make it fit? i'd assume weld new hangers on? or are they replacing pipes

[Kor]

I would not be bleeding exhaust gas, it would be air from the turbocharger. I would be bleeding it before it goes to the wastegate, between the wastegate and N75 in fact. It doesn't matter WHAT gets bled as long as there is less air to the wastegate itself. Its kind of risky because if I am bleeding the turbo pressure, if I bleed too much I think the DV won't open and I will just continue to deliver boost until the boost level becomes too high and.... poof. However there is limp mode to protect from that situation.

This is how the wastegate works:



If you look, you see it is controlled both by exhaust and by charge via the N75.

Getting to the wastegate spring.... no one has ever done it from what I understand. And I wouldn't know what kind of spring to put in there to make it stiffer. I could shim it somehow but honestly I have never seen a wastegate and do not want to experiment with taking it apart on my own.

I am very confident right now that the best approach is a relief valve plus a bleed valve. However, in the short run I could test it with just the bleed to eliminate surging - the relief valve is just to prevent wastegate creep and is not required.

The problem is finding the hardware. Sean F says he has had a hard time finding high quality fittings and hardware that are accurate enough and heat resistant for this purpose... for the bleed valve I need it to be VERY ACCURATE so that I am not bleeding too much. On Vortex they refer to this type of bleeder as a "needle valve".

As for the TT exhaust... Curtis just said they would fix it for free and that TT was going to send them some new parts. The hangers I think are just fine. Honestly I think maybe it might just be that they did not install it properly (don't tell the guys at C1... if they're reading this sorry guys). But when I go back I can discuss it with them. I think the current idea is just to move the exhaust pipe coming out of the muffler so that it points up (through the cutout in the valence) instead of down (making sparks fly as it drags behind me).

[Simon]

does the TT exhaust come with tips? or do you have to put your own on