A post by vrg3 got me wondering: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.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.
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
