Question on high-end boost
02-18-02, 07:07 AM
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Question on high-end boost
First off, here are my mods: '88 TII, great condition 74k miles, good compression, custom 3" dp/mp, GT spec catback, racing beat FCD, ACT 6-puck w/ 17lbs flywheel, greddy boost and a/f guages. It has all new vaccum hoses after the TB.
On to the question. Right now my max boost is .9 bar, or around 13 psi, but it spikes this high at around 5500 rpms, and tapers off to around .5 bar (~7psi) by redline. It is VERY noticeable as far as the pull of the car. From 3-5500 there is a huge hit of torque, thereafter it slowly (not suddenly) tapers off and evens out at .5 bar... doesn't feel slow, but it does feel like it drones on at higher revs, instead of pulling as hard. I have a few theories but they seem kind of dumb. First, is the wastegate is opening late because of the sudden spike of boost, but I would imagine it would open sooner than it does. Second is the wastegate is not channeling ENOUGH gases away from the turbine, and at high rpms the backpressure on the engine is too high and it chokes itself. The least likely cause is the turbo itself, which I've inspected on both the intake and exhaust side and it is in great condition.
I plan on getting a electronic boost controller, upgrading the fuel, injecting H20 into the intake for added saftey in the future. I'm wondering if anyone has experience with this type of problem. If it's the first problem an EBC should solve it. If it's the second, then an EBC and some hefty wastegate/manifold porting will solve it. If it's neither (ie. something to due with fuel/ignition, that I'm somehow missing) then I'd like to know about it.
Edit: Another thing. The A/F mixture does get a bit lean at .9 bar and then richens well as boost tapers off, which leaves the secondaries out of the equation, I believe.
Thanks in advance for any replies.
On to the question. Right now my max boost is .9 bar, or around 13 psi, but it spikes this high at around 5500 rpms, and tapers off to around .5 bar (~7psi) by redline. It is VERY noticeable as far as the pull of the car. From 3-5500 there is a huge hit of torque, thereafter it slowly (not suddenly) tapers off and evens out at .5 bar... doesn't feel slow, but it does feel like it drones on at higher revs, instead of pulling as hard. I have a few theories but they seem kind of dumb. First, is the wastegate is opening late because of the sudden spike of boost, but I would imagine it would open sooner than it does. Second is the wastegate is not channeling ENOUGH gases away from the turbine, and at high rpms the backpressure on the engine is too high and it chokes itself. The least likely cause is the turbo itself, which I've inspected on both the intake and exhaust side and it is in great condition.
I plan on getting a electronic boost controller, upgrading the fuel, injecting H20 into the intake for added saftey in the future. I'm wondering if anyone has experience with this type of problem. If it's the first problem an EBC should solve it. If it's the second, then an EBC and some hefty wastegate/manifold porting will solve it. If it's neither (ie. something to due with fuel/ignition, that I'm somehow missing) then I'd like to know about it.
Edit: Another thing. The A/F mixture does get a bit lean at .9 bar and then richens well as boost tapers off, which leaves the secondaries out of the equation, I believe.
Thanks in advance for any replies.
02-18-02, 09:44 PM
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The spike in boost is called boost creep and is caused by the bigger exhaust. The best way to control creep is by porting the wastegate hole. It drops off up top is because your stock turbo runs out of steam. The only good answer to that is turbo upgrade.
J
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