NefMoto

I have a 2008 Audi A4 with a built motor , custom tubular manifold, 3 inch exhaust no catalytic converter, equipped with an external wastegate(14psi Spring). When I first started driving the car and attempting to tune it, I noticed that I didn’t have much control over the boost (overboosting, jerky, inconsistent, temperature based inconsistancy, etc). One day I added a catalytic converter and I noticed the boost was much lower, the spool was slower and the boost never went over 22 psi. At that time I was upset about how much slower the turbo spooled and I couldn’t wait to remove the catalytic converter. I adjusted the duty cycle and got the turbo boosting over 25 psi again.

Last month I removed the catalytic converter and now the car overboosts again. I believe I came up with a reason of why the car is overboosting. On Turbosmart’s website they mention that the wastegate cracking pressure is higher on the bench then on a car with backpressure. My theory is since I don’t have a cat, my downpipe, and exhaust are 3 inches, there isn’t much backpressure acting on the valve to assist with opening the wastegate(the car is totally dependent on diaphragm pressure only to open the valve). It is my opinion that lowering the spring pressure will  allow the N75 to control the boost better. Does this sound accurate?

Hi,

I've noticed the same. After changing exhaust to 3" it is really hard to control boost (with stock turbo on).

Absolutely. Less backpressure means you need significantly less spring. At some point, though, you may lose spool rate if you go low enough.

I dropped from a 15 psi spring to a 10psi spring, the boost still spikes but it's controllable.

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Is the reason for overboosting the same on stock turbo (internal wastegate)? I changed exhaust to 3" and at the same time did the clutch (single mass by ecs). Now it doesn't seem to follow the LDRXN at all. Before I was running 1.1 bar pressure, now it goes up to 1.5 and stright to limp mode.

Maybe. No point in guessing. Log. Try with N75 disconnected to make sure WG lines are intact. etc.

Tried to log, but as here is winter and -10c outside / very icey roads (making it impossible/useless to log wot pull), but something I see from the log... are my injectors too small to take it all / can it be the cause of overboosting (I mean that 3" is sucking it try) and having no control of it?

Also the MAF readings are maxed out...

Please post the original log.

Or learn how to use ecuxplot properly. Preferably both.

Here it is...

Not sure why you think injectors have anything to do with it. Did you disconnect the N75 like i asked you to?

From that log you just need to pull back on IMX significantly

Thanks nyet, pulled back IMX and it´s all good and contrallable, need to play with it some more to get it perfect. Didn´t know about that WGDC stuff before since I was almost stock before. Did also new injectors (530cc 0280158117) and SAI off...I feel super great, thanks again.

Now I need to figure out why there is boost spike after I let off the throttle...

Out of interest, what size is the valve in the external wastegate.

O.T.

Bad DV or N249..

you should tune PID for a tighter wg and this aint doin with a cat. wiki covering thats pvdksdk things and with proper tune even on a small turbo (btw is it stock one?) and 14psi wg you can achieve great results with wot and partial throttle
and check your fueling. idc 100% @3k wtf?

Forge splitter valve and N249 pluged/bypassed

Yes, stock turbo (going to change for gt2871r soon).
I think 100% idc is because of icy roads and a "lot" of wheel spin when accelerating (stock injectors, now they´re changed)

No.

100% IDC has nothing to do with icy roads.

OK, need to log some more, this tune was written on summer and then the injectors were near to 95% during wot. I had these 550cc already waiting, so I didn`t care about that much (since the afr readings stayd on rich side).