loss4u

I have a single turboed 346 CI car, with 317 heads, and a blower cam(223/231 610/617 on a 113lsa) with 918 springs.

Generally speaking I am running ~10-11 psi. Today I was doing some decently long winded pulls 3-4th and noticed that upon reachin 5500-6000+ rpm im dropping to 9 psi or as low as 7 or 8. The car pegs 11psi strong up to that point. upon shifting I'm right back to 11 psi until said rpm is reached again. I've been spinning the car to 6500ish.

Looking for some suggestions... I noticed the boost guage, kind of fluttering, like back and forth about 1/16th of an inch when its falling off. I've considered a boost leak issue, but i have had other turbo cars, and this issue isnt reminiscent of a leak... maybe valve springs? I'm completely lost here.

Ideas? Thanks in advance.

witry

iTrader: (8)

918's and that cam and boost most likely thats your problem i would deff. upgrade to some patriot duals they have a high spring seat pressure and they go to .650 lift 918 max out at .600 i think with the added boost that might cause an issue

52172

iTrader: (2)

the 918's are not adequate for boost. Upgrade your springs with a seat pressure of at lest 155; preferably 165-185. The springs should be your problem or it is the tinsy tiny turbo not able to keep up.

elias_799

iTrader: (8)

patriot duals should solve your problem.

SpraydZ

iTrader: (2)

Not to hijack, but should my 918's be ok boosting a stock 5.3 with a Z06 cam?

loss4u

SO far, i have assumed valve springs... which you all have backed up... as well as a possible boost controller issue... alot of people have recommended a hallman MBC.

I wouldnt assume a t67 would fall off at a mere 11 psi... I could be wrong though...

Anyone else?

Thanks to all who have replied thus far.

yessir

iTrader: (5)

turbo cant keep up, get something bigger

Shawn @ VA Speed

iTrader: (3)

your springs are def a problem,i wouldnt run those n/a with that cam let alone with boost.i wouldnt say that is all of your problem-that cam you are running is not correct for your application-alot of your intake charge is going straight out the exhaust.That turbo is really on the small side for a 346- i can def see it not supplying enough air at those rpms especially with that cam.I would reccommend replacing the cam and the valvesprings with more appropiate components for your application.

loss4u

Thanks to everyone for your help. I have ordered a set of PRC dual golds as of this morning. Can anyone suggest a good MBC? I have heard the hallman units were great, didnt know if there were any other alternatives.

fasstls1

no, your gonna have valve float with any sort of boost. I also recommend hardened pushrods when you put in the new springs.

JETmn

I am willing to bet the turbo is out of breath. It isn't really meant for an engine that big. That is a poplular 4 cyl turbo for the street, but on the big side. The JoeP boost controllers are good too, I have used those and Hallman for many years.

Rusted40

turbo cant keep up and/or the springs are getting tired.
did you say what the backpressure was?

SpraydZ

iTrader: (2)

Thanks.

elias_799

iTrader: (8)

i would also get a dual roller(you could snap the stock chain with those springs and harder pushrods) . and change the oil pump too a melling 296 while you are at it.

Ryan K

iTrader: (11)

Yeah, I was going to say that high back pressure in the exhaust could be a culprit.

High pressure before the turbo will act on the wastegate and cause the boost to drop off as it forces the wastegate open... I had similar issues with my old kit. but it was closer to 20#.

loss4u

I dont think this is the issue as I only have about 12 inches of exhaust pipe, lol.

BLOWNBLUEZ06

iTrader: (17)

Well, the A/R on the turbine housing will also determine your back pressure as it is inversely proportional to how quickly the turbo spools up.

DrTurbo

iTrader: (3)

Back pressure 1st, then the comp wheel is just to small for making any serious big power. Swap to a TC78.....cure the problem of back pressure, and still spool almost as fast.

topend

i would say u are overcoming your wastegate.

Ryan K

iTrader: (11)

No, Back pressure before the turbo... You know the pressure on the inlet to the WG.