Too much boost
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From: Missouri
Too much boost Ok heres the deal.....I have a Precision GTS 74mm sitting on a homemade log manifold. I am running a Tial 38mm wastegate with a 5lb spring and a manual boost controller. This is on a motor that was a bone stock LS1 til about a week ago when I did a head swap. It was making 10psi with no issues prior to the head swap.
The main reason for doing the head swap was because my valvesprings were shot and it wouldnt rev past about 5000rpm.
I swapped on a set of 317 heads with new springs and a very mild home port job. The chambers cc'ed at 72cc so the new compression ratio should be somewhere around 9.3 to 9.5 depending on the math. The only other thing I changed was I sent the manifolds to Jet Hot to be ceramic coated.
Now it is making 15psi on the gauge by 5000rpm and it runs out if fuel right about then so I cant stay in the throttle. I unhooked the manual boost controller so it would just run on the spring and it still made 15psi. Tonite I took the gate off and checked it over and it is fine-not sticking, moves freely and the diaphram holds fine. What is causing it to now make 15 lbs??
Does it have something to do with the weak valve springs causing it not to be able to build boost? Or is the ceramic coating keeping more heat in the exhaust now and spinning the turbo harder? Obviously the way it sits now (running on the 5lb spring and making 15psi) the gate cant get rid of enough exhaust/isnt physically big enough. But why did this happen after the head swap? Like I said it was making exactly 10 psi before......
The main reason for doing the head swap was because my valvesprings were shot and it wouldnt rev past about 5000rpm.
I swapped on a set of 317 heads with new springs and a very mild home port job. The chambers cc'ed at 72cc so the new compression ratio should be somewhere around 9.3 to 9.5 depending on the math. The only other thing I changed was I sent the manifolds to Jet Hot to be ceramic coated.
Now it is making 15psi on the gauge by 5000rpm and it runs out if fuel right about then so I cant stay in the throttle. I unhooked the manual boost controller so it would just run on the spring and it still made 15psi. Tonite I took the gate off and checked it over and it is fine-not sticking, moves freely and the diaphram holds fine. What is causing it to now make 15 lbs??
Does it have something to do with the weak valve springs causing it not to be able to build boost? Or is the ceramic coating keeping more heat in the exhaust now and spinning the turbo harder? Obviously the way it sits now (running on the 5lb spring and making 15psi) the gate cant get rid of enough exhaust/isnt physically big enough. But why did this happen after the head swap? Like I said it was making exactly 10 psi before......
Thread Starter
11 Second Truck Club
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From: Missouri
Ok I fixed the fueling issue so at least I can keep it WOT a little longer. I have been reading up on boost creep and I think thats whats going on. I read where cold weather can cause or aggrevate a boost creep condition. The ambient temp around here has been in the 20's.
I am going to lower my shift points to around 5500 and try to tune it for 15 psi and see what happens. I was hoping to get away with 12 or 13 but hey, more boost equals more power right??
I am going to lower my shift points to around 5500 and try to tune it for 15 psi and see what happens. I was hoping to get away with 12 or 13 but hey, more boost equals more power right??
Pull the spring out completely, and see how boost responds. If it still makes stupid boost, you need a larger gate, because the OPEN no spring'ed gate simply cannot flow enough exhaust to decrease drive pressure and keep boost in check.
Thread Starter
11 Second Truck Club
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From: Missouri
I never thought to try that. I will check it out, probably be Tuesday before I have time. I suspect it will still make the stupid boost. It bugs me that it was working fine before the head swap though. The weather wasn't near this cold then.
Thread Starter
11 Second Truck Club
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Joined: Jan 2002
Posts: 1,412
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From: Missouri
The only things different now are the new heads and valve springs, the ceramic coating on the manifolds, and the cold weather.
I made a couple logs on HPT today and it shows 208kpa, I guess thats about maxed out on the 2 bar setup?
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