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?

 

i decided to not be lazy and do a search first but i didn't find anything. What the hell does a wastegate do. god it's such a noob question i hate to ask.

 

http://www.1300cc.com/howto/how2/turbo.htm

 

The wastegate 'short-circuits' the turbo so it makes less boost. Just think of it as a controlled exhuast leak around the turbine section of a turbocharger. If the exhuast goes around the turbo instead of through it then the turbo spins slower and consequently makes less boost.

 

"The most popular method of controlling turbocharger speed" Hugh MacInnes quote.

 

so that's why i only have 8psi? So is basically keeps boost spiking down and for me to get more boost i will need a boost controller?

 

In a nutshell, yes

Now for reality:

1) If you're running a stock turbo you're already falling off the peak efficiency islands on the compressor map at 8 psi. This means higher discharge temps and lower flow rates. The result is higher pressure but less power and more danger of detonation

2) The factory intercooler is at best a restriction and more like an interheater. When you try and push even more air mass through this thing which will be much hotter than the stock boost level it was designed to >try< and cool you'll lose boost and continue to deliver air that is much too hot to the engine

3) If you're running the stock ECU it will fuel cut before you get any more boost. If you fool the factory ECU with a fuel cut defenser (FCD) then you will ultimatley destroy your engine as it leans out because the factory ECU had no idea how much boost was actually in the manifold. This is because the FCD fools the computer into thinking there is never more than about 5psi boost.

Now's a good time to do a search on fuel systems, turbo upgrades, intercoolers, and second mortgages!

 

Yeah, since you haev an external wastegate, they use a spring to set the tension. With the reference line hooked to the manifold, the most you will get is 8 psi with the particular spring you have. You won't get more without a boost controller.

But you need the ECU and injectors for big boost now, then throw the boost controller at it and you'll have a damn fast car.

 

Anyone have any idea how low a ported stock S4 wastegate can keep boost down to? Just ported as best as it can be, not adding a different flapper or anything. Thanks.

 

I had one with a stock flapper, but a clipped tubine wheel. It would do about 6 psi, and creep to about 8-9 in 5th. I imagine a stock wheel would be a psi or two higher.

 

Where did you find the stock maps? I would really like to see them.

 

Well its not the stock turbine wheel, its a P-trim one I guess. Has a T04B compressor housing and compressor wheel and a 360 dgree thrust bearing.

 

Sorry to lead you on like that, I don't have them. I am making an educated guess (which I think you will find most turbo shops will agree with). The stock turbo was definately optimized to give better part throttle response and an early peak in the torque curve. If you look at a plot of boost vs. RPM on the stock car it always falls off after about 5000. This could be just as much a turbine related effect but I think the general consensus is that the stock compressor section is pretty lousy at doing anything more than about 8 psi. This is why I'm interested in the T04B hybrid stuff that many people offer. Stock turbine, much improved compressor section!

 

Man there is really no telling, you have to machine out the turbine housing for a P-trim, and it depends on who does it and how much they do it. i'd think with that kind of mods one would want to put a better flapper door on though.

 

The ECU cuts fuel at 8.6psi boost. That's a wee bit dramatic. For starters the pressure sensor does not affect fuelling, it affects ignition timing. As manifold pressure increases, the ignition is retarded to avoid detonation. But like most factory tuning there's a significant safety factor built in, so running slightly more ignition advance due to the FCD is not going to destroy your engine. Mild boost increases with an FCD are fine, and many people happily run 10psi boost (with the right amount of fuel of course) without problems. Any more than this and you're asking for trouble though, especially with the stock intercooler, because intake temps will be starting to get pretty high due to the turbo running well past its efficiency peak. Again, not quite right. Decent FCD's allow the pressure sensor signal to be sent to the ECU unaltered until just before fuel cut, and then clamp the voltage. So the ECU thinks it's running just over 8psi, not 5psi.

 

In my opinion all you need is a

phase 1 linear chip
phase 2 adj boost contoller
these two items were designed just for the TII you will not need fuel pump,injectors,FCD or wastegate mods. All you need is a TII then exhaust system and change your panel filter then the above two mods.

10psi no problems

 

NZ Vert: Very interesting!

I'd like to know how you came up with "For starters the pressure sensor does not affect fuelling". I'm only skeptical because of what I see in the FSM.

If this is correct, then base fueling is provided by the air flow meter. The bummer is that the air flow meter will also ultimately be a restriction as it will only be able to report a volumetric flow up to it's mechincal stop. Once your air demand exceeds the measureable flow capacity of the AFM you'll be just as hosed.

On my 89 S5 T2 I see the AFM reporting high volts at idle and low volts at high flow. At key-on with no engine rotation I see 3.850 volts, which would be the least amount of flow. At the lowest voltage (6000RPM 6psi boost) I see about 0.320 volts. If the sensor can report all the way to zero volts (doubt it) then we would at best be allowed about 8.3% more flow. I suspect just a few PSI can do this without too much effort.

With respect to the FCD: I've got an HKS unit here that someone gave me that puts a 'knee' in the output at 2 volts changing the slope from that point on. My stock MAP sensor delivers kPa=Volts*43.21+3.566 so that puts me at about 90kPa which is still in vacuum when the thing kicks in. I definately don't like that!

 

Several people have posted results from dyno testing this by using a little hand vac pump attached to the pressure sensor to send fake pressure signals to the ECU. Changing the pressure seen by the sensor didn't alter fuel injector pulsewidths, but did change ignition timing. Correct. Have a read of this. Not all FCD's are created equal.

 

NZ vert,

I'm afraid I'm going to have to disagree on the MAP sensor only changing timing. I ran a little test tonight on the way home. I put the car in 5th gear on the freeway at about 70 miles an hour and floored it while I datalogged the ECU's flounderings. Remember this is a series 5 turbo II that is bone stock save for my data logging equipment.



As you can see there is a direct relationship between the MAP sensor and pulse width. The AFM volts hardly change during this plus 9 psi spike below the secondary injection turn-on point.

I suspect this is where most people 'pop' their motors as the boost rises quite high due to the load and much too soon before the secondary injectors can help do anything about it. In this particular episode my primary injectors were at 74.3% duty cycle (~15mS) at peak boost of 9.5 psi. Notice how the stock S5 wastegate control is very sluggish at beating the spike and also returning me to 8 psi of boost. The good news is that the A/F ratio is PIG rich anyway at 11:1

 

ummmmm how did you datalog a stock ECU??? and why havn't you told anyone about this before??

 

Scott 89T2,

I basically spliced into all the factory sensors at the ECU and I'm borrowing some equipment to monitor what the factory ECU does. It was a lot of work to get running and is what I'd consider way past the budget and skill level of the average person. I'm just lucky to be able to borrow this stuff! I'm also a long way from being 'done' so that's why I've not said anything. My goal is to reverse engineer the factory ECU enough to program a stand-alone fuel/igntion system to mimick it (for smog purposes) and also to improve upon it (for smiles-per-hour).

 

So if this is true then, and the preas. sensor is the main load sensor, why the hell would they have the AFM. Also why did my car run fine with the preas. sensor disconnected? my car is N/A, but i'm sure the ecu acts the same way as a TII though.

 

Cool ****, man!
Keep it up!

 

is there anyway i can turn my boost up by adjusting the wastegate?