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Okay, I'm bored here at work so I decided to start up this conversation again A post by vrg3 got me wondering:

Even with proper fueling, I would maintain that full boost (at the manifold) at part throttle is a bad thing, and would cause high EGTs regardless.

If you have full boost pressure at the manifold with the throttle plate partially closed, you are very inefficiently creating that boost. The pressure before the throttle (which is in this case a huge restriction) would be very high (much more than full boost), so intake air temperatures would be very high. High intake temperatures mean high exhaust temperatures.

The throttle is normally supposed to control how much air the engine is allowed to ingest. In naturally aspirated engines, it very directly controls manifold vacuum. In a supercharged condition, then, it controls how much of the boost pressure is allowed into the manifold.

If you make your boost control system try to maintain full boost in the manifold at all times, you basically change the function of the throttle from controlling manifold pressure to controlling how efficiently the manifold pressure is maintained.

I was talking with one of my engineering buddies who owns a turboed VR6 Jetta. I told him about the whole partial throttle, full boost EGT thing and he gave some of his input. His explanation was that although the air MIGHT be heated by forcing air through a small area at the throttle body, the turbo won't have to work as hard to create x amount of boost. His reasoning being that the wastegate is tied into the system via the MBC which takes pressure from before the throttle body. Therefore, when the throttle's not wide-open and the turbo starts to boost, it doesn't take long before the pressure within the intake tract reaches full boost . . . this results in a slower spinning turbo producing max boost within the intake tract (but not full boost within the cylinder). The result of which should be lower intake temps.

He's still convinced that the cause of high EGTs is due to improper fueling. The ecu's still in closed loop mode and not compensating enough for the increased airflow (if you've ever seen an a/f gauge displaying closed loop operation you'll see that the gauge jumps from stoich to lean, stoich to lean). That being said, once I hooked up my a/f gauge, I noticed that the ecu goes into closed loop mode at anything over 1/10 throttle position, when the car is at full operating temps. I know this because at 1/10 or more on the throttle, a/f goes to rich and stays there. Might be due to my fuel pump, but I doubt it.

I tried a couple full boost partial throttle runs (for VERY short periods of time, of course), and noticed that the a/f ratios stayed at rich. Now if an egt gauge were hooked up, wouldn't it give me the same reading (rich)? Yes, increases in cylinder temps could cause increases in EGTs, and increases in intake temps could cause increases in cylinder temps . . . but what happens in the summer. Doesn't the ecu compensate for increased cylinder temps by: 1, dumping more fuel, or 2, pulling timing? Either of those methods should keep the EGTs down, correct?

Sorry if this explanation is all jumbled. My friend and I were talking for a while last night about this and I'm trying to throw every idea out there.

Please discuss

Jason

If you connect an MBC as your friend described, between the wastegate actuator and the compressor outlet, then it's not possible to get full boost in the manifold at part throttle, so the whole issue wouldn't come up anyway.

...Right?

Hmmm, I think I might have mis-reported something. Lemme take a look at what I wrote again.

WHOOPS!! Yes, you are correct. But I did mis-quote that.

Lemme try and have him write something up here. He just seemed so adamant about it being due to fueling . . .

Hmmm, give me a couple hours

It's quite possible that it's a fueling issue. Under boost you need to run rich to keep things cool.

I thought that, because of that, our cars were supposed to go open-loop above a certain boost threshold.

Do you think the ecu is monitoring boost through the MAP sensor and accomodates for full boost? I dunno, under PTFB, the ecu definitely is in open-loop. *shrug*

I am pretty sure that the ECU does use the MAP sensor readings to know when to make the mixture richer than usual, as well as retard the timing.

If it is richening the mixture, I don't think it can be closed-loop, since the oxygen sensor is so nonlinear.

I think one of the problems with the MBC's are that you take the MAP sensor out of the loop.....or so I think...correct me if wrong.

The ECU uses the MAP sensor not only to enforce boost limit, but also adjust other sensors, fuel, timing, etc.

So with the MBC in there, the ECU can't adjust things properly. Which is why an EBC seems to be so much better......

jason, what kind of egt's are you seeing.

Properly installing an MBC shouldn't result in a nonfunctioning pressure sensor. You really do want to keep the pressure sensor in there, especially because you're messing with the boost and want to retain some kind of emergency failsafe (i.e. the fuel cut).

The only difference between MBCs and EBCs is how they pass pressure through to the wastegate actuator. MBCs are mechanical and "dumb" while EBCs can be "smarter."

kelly, I don't have an egt gauge (I know, I know) but that's next on a long list of mods.

vrg3, I was thinking that the ecu is in open loop as well, since the reading on the a/f gauge reads rich and stays there. *shrug*

vrg3 wrote:It's quite possible that it's a fueling issue. Under boost you need to run rich to keep things cool.

I thought that, because of that, our cars were supposed to go open-loop above a certain boost threshold.

water injection would allow me to run relatively less fuel under boost yet still keep things cool, wouldnt it?

Yes water injection will cool things down. But it's a bandaid and should be used on drag cars only.

I have to agree with boostjunkie on this. It's a tuning problem. I have never even heard of this being talked about exept by newer Subaru types.

I can tell you why it is bad to do that on a WRX and it's because the ECU stays in closed loop much longer. If you take the factory boost control out (that limits boost at less that %75 throttle so it can run lean) for a MBC you can get full boost in closed loop where the ECU wants to keep the AFR stoic. Bad things happen.

-K- wrote:Yes water injection will cool things down. But it's a bandaid and should be used on drag cars only.

most high boost/HP rally cars run water injection. paul choinere lost STPR (or maine i cant remember right now) because he stopped to fill his water injection tank at the wrong time and got a uge penalty. it isnt just a bandaid. it can be very usefull. it keeps the internals of the engine VERY clean too. it can also be very helpfull when you are not able/allowed to run race gas.

advance the timing and adding water will keep detonation away and allow more boost.

vrg3 wrote:If you connect an MBC as your friend described, between the wastegate actuator and the compressor outlet, then it's not possible to get full boost in the manifold at part throttle, so the whole issue wouldn't come up anyway.

...Right?

it is possible, in fact i get it all the time. i can control it though. its about 60% throtle off the line in 2nd gear when it happens

OK sorry, only on RACE CARS. And that's just what I think, run it if you like. I just couldn't see me running it on a street car. I know it can kick ass but I don't see the plus being more than the negative on a street car.

-K- wrote:OK sorry, only on RACE CARS. And that's just what I think, run it if you like. I just couldn't see me running it on a street car. I know it can kick ass but I don't see the plus being more than the negative on a street car.

thats cool.

learned aboput it from my subaru tech at the dealership. he ran it on his road race BMW 2002. said the heads and piston looked like new every time the tore the engine down. it had a VERY high CR.

and this discussion has just led me to another idea for a new thread.

Well, since I have an EGT and a stock ECU:

Cruising sub-highway speeds: ~1200-1300* F

Cruising highway speeds: ~1300-1400*F

PTFB: ~1400-1550*F

WOT FB (@15 psi): ~1500* F


With a boost controller installed, the stock MAP sensor still must see manifold pressure. If you'll recall, this is why we have the VRG FCD - to only allow the ECU to see 12 - 12.5 psi. It's still hooked up - just not functioning (no control over wastegate)

With a boost controller hooked up reading the MANIFOLD pressure instead of the compressor pressure, partial throttle full boost is an issue - since the turbo is then working to keep the pressure inside the manifold at the preset level.

With a boost controller hooked up reading the compressor outlet's pressure directly, partial throttle full boost is not nearly as much of an issue, as the boost will still be limited at the compressor. With the throttle plate partially closed, the "full boost" that the boost control is set to is reached much quicker (pressurizes the pipes) and a portion of that is allowed to bleed into the manifold.

So I'm gonna say PTFB is not really that much of an issue as long as you're controlling the wastegate from the compressor outlet.

Of course, I'd still rather be able to limit myself to wastegate boost for any throttle position below say, 80 or 90%, for longevity reasons. I will work on this soon...

I don't think you want to do that.
My WRX does limit boost untill you floor it and it pisses me off to no end. It has no midrange unless you floor it then you get a big surge when the ECU decides you "get" full boost. But then sometimes it doesn't give full boost and it just bogs.
You really have to drive it hard to get full power out of it, then it's a blast to drive. But for a normal drive around town I never know when I get on it if the ECU is going to let me go fast.

True, it is fun to drive normally without the TPS-based boost limitation... 6 psi definately sucks. (blows? lol)