Need Help Confirming Proper Turbo Operation
 

I have turbocharged my high compression motor and after driving for over a month now have a few questions to confirm (im)proper operation of the turbo. I have divided the questions into 3 discrete sections. Thanks in advance for any help.

1. I am experiencing a sharp drop in boost when the boost reaches 8psi. From other posts I gather it is normal for the stock S5 turbo to be able to hold 9psi to redline. When the boost does hit 8psi at ~5.5-6k RPM it drops rapidly, in about ~250ms, to 5psi where it stays to redline.
Setup: large street-port, stock TID and AFM, full RB Rev-TII exhaust,
- wastegate mods: ported S5 wastegate, back plate milled down ~5mm to allow wastegate flapper to open further. Also the corner of the flapper door is taken down, if you have rebuilt a stock turbo you will notice it is worn from hitting the back plate when open. The compressor housing pressure feedback is connected directly to the wastegate.

My belief is the the stiction of the wastegate is preventing it from opening until 8psi is achieved. If I modulate the throttle and keep it at 7psi it holds past 6k RPM. If it is infact stiction a EBC could close the wastegate but I would hate to spend the money unless someone else has had experience here.

2. I have some oil in the compressor side of the turbo. The car does not smoke at idle, under boost or during shifts. I rebuilt the turbo using the G-Pop Shop T3 kit with 1 piece encapsulated carbon seal. Seeing as this is rebuilt I didn't expect to have oil in the intake. I have no PCV system, the crankcase and oil filler nipples are open to atmosphere. The oil supply is a mazdatrix old style aluminum block oil filter adapter. I feed the oil from the 3/8NPT fitting through a -4AN line to a -4AN fitting which is tapped in the oil supply side of the turbo. The drain is stock to the front cover. Any suggestions - ok or not?

3. When I shut the car off I hear a gurgling sound coming from the turbo. This occurs even if I drive the car for 5 minutes, idle it for 5 minutes and shut it off. Ambient air temp is 45F. My water supply are the BAC connections, one is the rear iron for the connection that would go to the LIM on a stock car, and the other is to the water pump. Just live with this or is it a problem?

Thanks again

the only one i can comment on is #1

if you are running a s4 car with a stock ecu at 5.5k rpm the ECU is mapped to drop the boost to 5.5 psi no matter the wastegate size. the only time that the wastegate becomes an issue is when u over boost. it sounds like your car is modded alot more then that but just for your info. on a stock motor or even street ported without stronger seals its not good to have boost higher then 6 psi past 5k rpm....plus if you have a high compression engine this is not a good setup for a turbo so you really dont want to push the boost beyond that

1. Is this on the stock ECU? If so I am surprised your motor hasn't gone bang yet. What fuel mods have you done?

3. This is water being pulled through the turbo to cool it.

The stock turbo just isn't efficient enough to maintain the pressure all the way to redline. Replace your stock TID and you'll be able to squeeze a little more out of it. But since you made no mention of fuel control/injector size etc, I wouldn't suggest doing anything but lowering the boost with high CR rotors.

The wastegate is not the problem. When you modulate the throttle you reduce the amount of air the engine is able to pull in, which changes the efficiency range the turbo will be operating in. At lower pressures and lower airflow, the stock turbo is more efficient.

Most of this is incorrect. The S4 ECU does not have a role in boost control (aside from fuel cut at 8.6psi). Only the wastegate (which has a 5.5psi spring) controls boost levels. And there's no reason you can't make more than 6 psi on OEM seals. Tons of people produce a lot more than that on various turbos at high RPM.

Thanks for the replies.

My car is S5 with S5 N370 electronics all round for the engine. As for fuel mods I have new 550cc injectors and a Walbro 255 with the rewire mod. I have a wire ran before the FCD to the an SAFC connecting to the throttle wire; fuel is being corrected on boost. I opted for 550cc over 720cc so I would not have to run negative correction. I was worried that since the SAFC is shaping the AFM signal to look as if there is less of a volume charge entering the engine the ECU may advance timing. Even with the 550cc's I see 10.5-11.5 AFRs in boost in 1st-3rd gear which I think will last for a while.

As for the turbo being out of its efficiency range, I think I'll just have to wait to get out of school to get a hybrid or larger.

I have the gurgling sound too. I've had it for a while, including when I did my 900 mile trip from FL to VA with no stops. My understanding is that its bad seals in the turbo causing it to create air bubbles. I have to bleed the system every coupe of months or so or else the buzzer will go on and off randomly. I wouldn't bother with it, the stock turbo is a pile of shit. Just wait till you can upgrade.

guess i was wrong. i know u can produce more power on stock seals. i would just be alittle nervous on a high comp engine.

Edit: on the s4 the car from factory was made to drop boost to 5.5psi or so past 5k rpm i wont quote the page# from the FSM im sure u can find it

So do you have the factory boost control solenoid hooked up or not?

Without a boost controller internal wastegates naturally taper boost up top.

What max pressure are you running? Hopefully you're controlling it fairly well around 3500-4500 RPM, where peak torque will be. Something to think about with 550cc primary injectors is that they can max out even with the stock turbo. The primaries will max out before 3800 RPM, if you're making say 10+ psi in that range. You may have a lean spot in that range that's hard notice when you mash the throttle, and adding fuel with the SAFC won't actually do anything here. Once the injectors top out, you'd just be fooling the ECU with no benefit.

Also keep in mind stock timing isn't at all geared towards higher compression ratios, so all you can do is throw fuel at it.

It's not so much the high RPM range you have to worry about. The midrange, where peak torque (and peak pressures) will be produced, is generally where detonation will occur. Especially before the injector staging point with a stock ECU.

The wastegate spring on S4 & S5 turbos restricts boost to around 5.5psi. S5 ECUs have a turbo duty solenoid to control, which is just a bleed-type boost controller, but that only allows boost to go up (that's where the S5 7.5psi rating comes from). Wastegates restrict boost, not ECUs. There is nowhere in the S4 FSM that says "above 5k RPM, boost is cut to 5.5psi". Due to the wastegate spring, boost is ALWAYS supposed to be limited to 5.5psi. Saying at X RPM it's limited to the same pressure would be redundant. I don't know what page you're talking about, but if it's 4B-56, re-read it. That page only talks about checking turbo function when revving the engine without load. It mentions 0.7 psi above 5000 RPM, which is about all you're going to get free revving.

Ive been boosting 12 psi at peak and around 8 or 9 until redline with a stock s4 turbo since I swapped another old worn out 13bt into my car which was around 4 years ago. Zero problems regarding that

MacGyver, I would sell the SAFC and FCD. Get an rtek 2.1 or a standalone if you plan on taking the car any further.

arghx - No, no duty solenoid. The pressure feedback from the compressor housing is connected directly to the wastegate actuator.

RotaryRocket88 - Boost does not exceed 8psi, I was aiming for 6-7psi for right now hence all the trouble I went through modifying the wastegate. Good point about torque & lean spot though. When I first got the turbo installed I was curious about the injector staging. I put it in 5th gear since its easy see the AFR/psi/RPM since acceleration is slow. Sure enough it does lean out even with correction.

I have injector bungs welded on my UIM for two additional injectors. In the near future I'll be building a two injector controller based off a PIC microcontroller, it will also delay the CAS pulse depending on boost to retard timing.
Most will tell me to stop right here, dump the piggybacks and get a real EMS. I have motivation not to do this because of other things being done with the car but will not post here until done, too many haters here on the club.