Dump Difference

[bishir]

I have a TA-49, THDP, ATR 2.5" exhaust with testpipe, dump, with electric cutout. Anyways, I was doing a few WOT runs on the way home down the Interstate and noticed something funny. The car runs 760s, 24.5 lbs of boost, and .9 knock at WOT with the dump OPEN. THe car runs 690s, 21.5lbs of boost, and 4-5 degrees of knock with it closed. What's up

40lb injectors, 42lbs line off fuel pressure...

 

[mgmshar]

Something here is very weird. I have a very similar combo (TA-49, THDP, test-pipe with dump, ATR 2.5" duals), and until recently I also had red stripes.

With my car, there is very little difference between dump open and dump closed. I certainly don't get a 3 psi gain when I open the dump.

My best guess is that something else happened when you closed the dump. Maybe your fuel level in the tank was low, and your fuel pressure dropped a bit during your WOT blast. Maybe something else. Running more knock with less boost just doesn't make sense.

What chip are you running?

 

[bishir]

I just filled up with gas so that shouldn't be the issue. The chip is about 4 years old from Cotton's and is setup for 93 octane. That's all I know about it. I did let out once I saw the boost go up to 24lbs and knock started.. It probably would've creeped higher.

I'm assuming the knock figures at lower boost is because of the fuel issue, but why are the 02s so low with it closed yet higher with it open? I would assume more airflow would lean it out if anything...

Wierd...

I guess I also forgot to mention the SMC kit. Maybe the pump speed is just right for 24lbs and not 21?

 

[bishir]

I just took it for another spin today and it's still doing the same thing

 

[Turbo Brian]

hmm. weird. you should be leaner with the dump open. and that big of a jump in boost also seems excessive. on my car i noticed very little change in boost (maybe a #) but i did run leaner. why you'd be leaner with it closed doesnt make sense.

you could be seeing rich knock. if the alky is good at 24 it may be too much for 21. but then you'd also see high O2's at 21.

here's a thought, but its a stretch. with the dump open there is in theory less back pressure (although with the atr exh, you shouldnt have any BP). perhaps you have a small leak in the crossover before the O2 sensor that opens up when you're running though the exhaust with "more bp". that's really reaching though. im pretty much at a loss.

 

[mgmshar]

Another possibility...

When your dump is on, something is causing a cylinder (or several cylinders?) to mis-fire. Maybe too much alky? If a cylinder mis-fires, then it will shoot air (oxygen) and fuel out the exhaust (low O2 reading). In addition, it won't spool your turbo as hard (lower boost). The mis-fires may or may not trigger a knock sensor - not sure about that.

I would try this...

Turn off your alky system. Drop the boost to about 16psi so you won't knock. Run back-to-back with the dump open, and then dump closed. If there is no appreciable difference, then I would suspect that my theory above is correct.

Good Luck,