Boost Dropping Off

[89CAL]

Hi Guys

I have a 1994 R33 GTST with stock turbo, Tomei Fuel Pump, GKTech intercooler, dual stage boost controller, power fc.....

Anyways, car has been tuned and is set to 12psi at the moment. The other day it was put on a dyno for a power run and slight touch up of the tune.

Now for the problem. The boost graph showed that it boosted up to 12psi as normal, spiked to about 12.5psi (fair enough) then as it got to around say 4500rpm it slowly dropped to about 9.5psi or so nearing redline.

Im just after some peoples opinions on what they think it could be. Have been suggested by tuner that it could be an exhaust restriction and i will test this theory when i get back home by pulling the cat out and going for a drive. It was also suggested that it could be my intake pipe from afm to turbo closing under higher boost levels then designed for.

So tell me what you guys think if you have any new ideas

btw plugs are ngk coppers gapped to 0.8, probably could do with a change soon but should still be ok

Cheers guys

Callan

[R33GOD]

. start with the obvious. Perform a boost leak test. Could the turbo be at its end?, actuator spring weak?? intake rubber sucking slightly closed??

plenty of things it can be. Your mechanic should know whats goin on....

[89CAL]

I dont have a mechanic, all work has been done by myself and father lol

yeh Im going home to try exhaust (away for christmas at the moment so im just really after ideas to try when i get back)

Intake pipe is a possibility to

had thought about tubo or actuator but i think they are both ok

actuator maybe? any advice on how to test that?

Any ideas are appreciated

[toffy]

It wouldn't be the actuator mate (well extremely highly unlikely) , if it was that it would be spiking , not bleeding off, change your afm to turbo inlet pipe (theres a diy on it on sau somewhere) and your cat then if that doesnt fix it ask more questions

[89CAL]

lol sounds like a plan plus i want to hear how it sounds with only the dump pipe on it

pobably sound like shit but oh well lol

yeah ive been told that the series 2 R33 rubber intake pipes have a steel spring in them to prevent i from collapsing so im trying to source one of them to test that theory, if so ill just use one of them till i put my front facing plenum on then get my pipe from turbo to intercooler changed from factory to stainless and a stainless intake pipe

[cmoney]

Could just be an old turbo. I have a similar setup on my RB20 R32 with a stock turbo. It starts at 13psi at 3000rpm, & slowly works its way down to 10psi at around 6500rpm. Very smooth linear line, no surging or anything, just dont think it can handle the heat from the boost. From memory, i think the 8psi run on low boost held it all the way pretty much to 7000rpm. I have a rather large front mount, but prob just the ceramic wheel & the fact that its old. 155,000kms or so & still kicking on strong lol.

Try a bolt on turbo with steel turbine wheel & see if it holds boost all the way if the budget will allow it. Im going for a KKR480, bit laggy on the RB20, but in the next year or 2 i should have an RB25DET NEO motor sitting in there instead, then il pull the turbo off & stick it on that sucker.

[toffy]

lol sounds like a plan plus i want to hear how it sounds with only the dump pipe on it

pobably sound like shit but oh well lol

yeah ive been told that the series 2 R33 rubber intake pipes have a steel spring in them to prevent i from collapsing so im trying to source one of them to test that theory, if so ill just use one of them till i put my front facing plenum on then get my pipe from turbo to intercooler changed from factory to stainless and a stainless intake pipe

im pretty sure the don't have a steel spring in them mate , i'd haveto check (i have a s1 and 2 laying about) , do the pipe mod or buy a silicone one off ebay

Could just be an old turbo. I have a similar setup on my RB20 R32 with a stock turbo. It starts at 13psi at 3000rpm, & slowly works its way down to 10psi at around 6500rpm. Very smooth linear line, no surging or anything, just dont think it can handle the heat from the boost. From memory, i think the 8psi run on low boost held it all the way pretty much to 7000rpm. I have a rather large front mount, but prob just the ceramic wheel & the fact that its old. 155,000kms or so & still kicking on strong lol.

Try a bolt on turbo with steel turbine wheel & see if it holds boost all the way if the budget will allow it. Im going for a KKR480, bit laggy on the RB20, but in the next year or 2 i should have an RB25DET NEO motor sitting in there instead, then il pull the turbo off & stick it on that sucker.

do some searching on this forum before you make that turbo choice mate

[89CAL]

Been a while since i posted anything about this

I've since got an Apexi PFC boost control kit which has helped the boost a little, and yesturday put a bit of 3 inch pipe in my rubber pipe to stop it collapsing. Boost is still dropping 1psi or so in 4th, not till somewhere around 5-6000rpm

so i might get a new pod filter and if that fails maybe drop the exhaust off from the cat to see if that helps. Otherwise i might just get the tune fixed up on 11psi or whatever it decides to run on the day

If anyone has further advice then it would be much appreciated

Cheers

[89CAL]

Another update... Did the cheap mod of putting a bit of steel pipe in the rubber intake hose which i think may have helped a little but boost drops to about 11psi in 4th.

So im guessing cat or maybe pod filter now...

Still open to advice

Cheers

[Turbz RB-25]

Been a while since i posted anything about this

I've since got an Apexi PFC boost control kit which has helped the boost a little, and yesturday put a bit of 3 inch pipe in my rubber pipe to stop it collapsing. Boost is still dropping 1psi or so in 4th, not till somewhere around 5-6000rpm

so i might get a new pod filter and if that fails maybe drop the exhaust off from the cat to see if that helps. Otherwise i might just get the tune fixed up on 11psi or whatever it decides to run on the day

If anyone has further advice then it would be much appreciated

Cheers

Drops 1 PSI? Geez mate you're very picky, on my previous application I was running 24 PSI and it was dropping down to 17 PSI (due to the weak actuator)

1 PSI ain't shit, if you're that worried, wind up the boost to 13 PSI so it drops t 12 if 12 PSI is what you relaly want to run.

Boost ALMOST always drops in the top end no matter what your application (assuming you don't have boost creep)

[89CAL]

yeah i know lol i thought it was dropping more then it actually was. 1PSI i can live with and probably will. I get slack to pull the exhaust off thats why i havent done it yet.

I dont reall want to lose my cat or resonator so might just leave it how it is. Might still change the pod filter but, its seen better days