Dialcaliper
Well-known member
Another vote for compressor surge - Are you still able to get full boost at other times, if you "nurse" the accelerator down instead of mashing it to the floor? Will it still build boost if you give it gas while running at 5000-6000 RPM?
Also, are you running an internal or external wastegate?
Any time you have an undersized turbine mated to a big compressor, the turbine spins up quickly at lower airflow, causing the compressor to stall. When it stalls, what happens is a rotating mass of air gets "trapped" in the blades, and spins with the compressor - meaning that it loses the ability to pump much air - new air cannot flow through the compressor. There is an added complication which is that once the wheel hits a stall condition, it won't revert as soon as it comes back into the region below where the stall happened - often it has to drop down to a much lower airflow level, almost as if the trapped air gets "stuck" in the blades.
Take a look at This Diagram. Look at the 69,500 RPM line. At 6-8 psi, you're at about 1.4 PR. What's probably happening is that you're flowing less than 12 lbs/min, and the turbine is trying to spin the compressor past that RPM line at low airflow levels - the compressor is trying to boost more than that 8psi, but it's being spun above the "surge line" (the air gets trapped instead of flowing)
If your BOV is actually fluttering, it might go something like this: When the stall happens, airflow and boost pressure drop in the manifold. Once pressure drops and the compressor starts pumping again, it creates a pressure pulse in the pipe, which the BOV "thinks" is a closed throttle plate, and pops open, venting pressure - which then causes it to close as the pressures equalize. Rinse, repeat. It's also possible that it's not fluttering at all, and what you're hearing is strictly the turbo.
Having the BOV open at idle is normal - any time there's less pressure in the manifold than in the IC piping (due to the throttle plate restricting flow), the BOV should be open, allowing air to bypass the turbo. Having the BOV open while the compressor is stalling of course makes it worse (moves left on the compressor map)
This is not quite the same kind of "compressor surge" that happens when you have a BOV that doesn't open quickly, or run no BOV, but it happens because the turbine is trying to operate in a similar range (high PR/low airflow)
If you are actually able to get boost by carefully applying the throttle, you're probably having surge problems. There aren't many easy solutions beyond replacing the turbo with a unit that's matched better.
Having the turbine wheel clipped might solve the problem, but that's a permanent change, and not guaranteed to solve it. (Clipping makes it less able to spin up at lower flows, but it reduces backpressure by letting a bit more air by the turbine).
Nursing the throttle up, and testing at high RPMs are the only tests I can think of that might tell you if surge/stall is indeed the problem. Try all the obvious things first related to the BOV (check for vacuum line leaks, replace the BOV spring, etc) If none of those work, start looking at the turbo. You might be able to find or borrow a similarly flanged turbo with a larger turbine/smaller compressor and see if the problem goes away.
Also, are you running an internal or external wastegate?
Any time you have an undersized turbine mated to a big compressor, the turbine spins up quickly at lower airflow, causing the compressor to stall. When it stalls, what happens is a rotating mass of air gets "trapped" in the blades, and spins with the compressor - meaning that it loses the ability to pump much air - new air cannot flow through the compressor. There is an added complication which is that once the wheel hits a stall condition, it won't revert as soon as it comes back into the region below where the stall happened - often it has to drop down to a much lower airflow level, almost as if the trapped air gets "stuck" in the blades.
Take a look at This Diagram. Look at the 69,500 RPM line. At 6-8 psi, you're at about 1.4 PR. What's probably happening is that you're flowing less than 12 lbs/min, and the turbine is trying to spin the compressor past that RPM line at low airflow levels - the compressor is trying to boost more than that 8psi, but it's being spun above the "surge line" (the air gets trapped instead of flowing)
If your BOV is actually fluttering, it might go something like this: When the stall happens, airflow and boost pressure drop in the manifold. Once pressure drops and the compressor starts pumping again, it creates a pressure pulse in the pipe, which the BOV "thinks" is a closed throttle plate, and pops open, venting pressure - which then causes it to close as the pressures equalize. Rinse, repeat. It's also possible that it's not fluttering at all, and what you're hearing is strictly the turbo.
Having the BOV open at idle is normal - any time there's less pressure in the manifold than in the IC piping (due to the throttle plate restricting flow), the BOV should be open, allowing air to bypass the turbo. Having the BOV open while the compressor is stalling of course makes it worse (moves left on the compressor map)
This is not quite the same kind of "compressor surge" that happens when you have a BOV that doesn't open quickly, or run no BOV, but it happens because the turbine is trying to operate in a similar range (high PR/low airflow)
If you are actually able to get boost by carefully applying the throttle, you're probably having surge problems. There aren't many easy solutions beyond replacing the turbo with a unit that's matched better.
Having the turbine wheel clipped might solve the problem, but that's a permanent change, and not guaranteed to solve it. (Clipping makes it less able to spin up at lower flows, but it reduces backpressure by letting a bit more air by the turbine).
Nursing the throttle up, and testing at high RPMs are the only tests I can think of that might tell you if surge/stall is indeed the problem. Try all the obvious things first related to the BOV (check for vacuum line leaks, replace the BOV spring, etc) If none of those work, start looking at the turbo. You might be able to find or borrow a similarly flanged turbo with a larger turbine/smaller compressor and see if the problem goes away.
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