67mm single turbo - losing boost in upper rpms
#1
67mm single turbo - losing boost in upper rpms
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.
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.
#2
TECH Enthusiast
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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
#6
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.
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.
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#8
FormerVendor
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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.
#9
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.
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#11
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.
#15
Pathological Modifier
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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#.
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#.
#16
I dont think this is the issue as I only have about 12 inches of exhaust pipe, lol.
#18
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.