Boost Drop Off, Possible Wastegate

[Reese69]

Hi

I am struggling for help/ answers, hoping someone with your knowledge can help.

My car is an RB25det runnning Garret 35/40 turbo on high mount exhaust manifold, greedy intake plenum, 740 injectors, 500hp fuel pump, now currently running haltech platinum pro.

I had my tune last week everything was great until he started the psi testing which was ok until he tried to set the boost -

He could set the boost at 15psi and 18psi, car would hit the boost level set (tested on elec control through haltech and manual turbosmart T) once set woiuld do his pull in 4th gear boost was hit, then boost would drop off back to stock boost for remainder of rev range..

Tuner has tuned car on base boost so i can get it registered ect then i will return the car, but i was hoping to try sort it out before i return it to try save some time/ money.

My tuner has said it could be watesgate spring or faulty wastegate, he also said could be exhaust gas pressure. he will determain for sure when i take it back, but if i can save some time and money on the dyno i will.

In your opinion what could possible causing this? And what could be the fault,? Is there anything i can do to try and determine what is faulty.

For example if i can determine its a wastegate i will up grade it and get it organised before i go back to the tuner.

current wastegate size is 38mm or 40mm and brand name is a cheap universal and i think it is running a 6psi spring,

My tuner said if it is the gate he would recommend a turbosmart 50mm gate

any help oe advice would be good..

all vac lines are ok same as pips and clamps,

car runs beautiful on stock psi..

[mad082]

i had a similar problem after doing a turbo install on a mates 33. put it on the dyno and the boost would drop from 16psi down to about 12psi by redline. we even disconnected the boost controller and it still did it. turned out it was the cat converter had collapsed. it simply couldn't flow enough exhaust gases to keep the turbo making enough boost

[Reese69]

oohh im assuming thats easy to pull off and test..

[mad082]

yeah just drop the exhaust off and shine a torch in the cat.. sometimes tapping them can tell if the cat has collapsed too.

[Hanaldo]

Has the boost controller got a 'vacuum' source plumbed into the top of it, above the diaphragm? If not, a 6 pound spring is very soft, without the boost controller helping it stay shut, it's going to struggle to hold boost.

[OEM_as_f*ck]

Has the boost controller got a 'vacuum' source plumbed into the top of it, above the diaphragm? If not, a 6 pound spring is very soft, without the boost controller helping it stay shut, it's going to struggle to hold boost.

This ^^

Make sure the vacuum/boost control arrangement to the gate is plumbed up correctly. If still no win, get a stronger spring closer to the pressure you want to run. If still no win, buy a decent wastegate, I don't care what anyone says, cheap gates are nothing but trouble.

[Reese69]

yeah i am gonna upgrade the spring and see if this helps, my runer did say that could be the cause..

i agree about the waste gate, i want a good one but i think if i can spend 50 on a new spring and last me a while to save some money then i will.

my tuner tried 3 set ups 2 with the electronic controller and one with the manual T peice contoller.

he had know issues hitting what ever psi he wanted it just wouldnt hold.. which brings us to the wastegate again.