Cant fix overboost since new turbo install


Turbo Spooled by CORN
May 25, 2001
I posted this in tech, but want to try here.

This weekend I installed 42.5 lb injectors and a TA-49 turbo on my friends stock Buick. Install went good without a problem. We replaced stock stuff with, turbo, wastegate(not HD accuator), GM boost line with "y" fitting, intercooler hoses. I set the wastegate rod to the longest setting and went for a test drive and the boost would go to a easy 18 psi (let off due to pump gas).

I have since swapped everything that has to do with boost control with new parts from my Buick. Here is what we did....

1. lengthend wastegate rod all the way out during all tests
2. changed new wastegate back to original stock....since changed back to new, thinking maybe old one was luck
3. Checked new "y" fitting for correct size hole.....ok, put old boost line back on with "y" fitting, put known good "y" fitting from my car on no luck.
4. replaced wastegate solinoid with new luck
5 by-passed solinoid and "y" with vac line going from comp housing to luck 10psi in 1st gear 18 in 2nd......
6. If we undo the wastegate from the puck the car will not make boost just like it should. If I slip the rod back set to the length that the flapper is barley open it still makes no less that 16 psi
7. Puck on downpipe is not stuck moves freely

After all this the only thing i can think of is the hole in the exhaust housing needs to be enlarged for the exhaust to bled out easier alowing less boost. But its just a stock motor with a little 49
But why cant we get boost down to say 10psi. We dont want a bleeder/grainger valve or a bandaid fix.
------------Current set-up on overboosting car

Stock original 15k motor
K&N with hard Maf pipe
stock intercooler
new adjustable wastegate
adjustable FPR
walbro 340 hotwired
009 inj
93 octane JL chip
precision 3000 stall
new TA-49
For what it's worth...

It seems like the TA-49 and the THDP flow exhaust so well, that it's hard to contain the boost. I had similar problems with my set-up. Things were OK, until I installed an ATR exhaust system behind my THDP. That seemed to open the exhaust enough so that the 49 would just spool and spool.

Assuming that you have checked everything for broken stuff, there are two things to consider:

1. Enlarge the wastegate hole. With my set-up (THDP and ATR 2.5 Exhaust), I had to open up the hole to about 1-3/16" diameter and fully bellmouth the inlet side of the hole to get the boost to stay stable at low levels (15-16psi). Note: the first time I ported my wastegate hole, I went to just over 1", and that wasn't enough. Another note: the bigger the hole, the lower the boost, with the same actuator rod length.
2. If boost is stable, but just higher than you want, you can get your chip re-burned to run the wastegate solenoid less. Most aftermarket chips run the wastegate solenoid at over 90% DC (duty cycle). You could have your chip burned to run the wastegate solenoid at a lower number (say 60% DC), and that would significantly lower boost. A very rough estimate is that every 5% reduction in wastegate solenoid DC will reduce boost by about 1psi.

Knowing what I know now, I would port the wastegate hole to the largest size that will fit under the puck with at least 1/16" all around for sealing. I would suspect that would fix your problem.

Good Luck,
Well I pulled the exhaust housing off the new TA49 and ported it to about 1/8th an inch from the puck size, and really hogged out the inside diameter off the hole.

We got the boost to drop a good 5 psi at WOT. Now with the wastegate solinoid and "y" back in the boost stays at 14-15 psi.
Livable but would have liked it a little lower. I guess it is time for alky on the street

thanks for the help