Surge? Flutter? Is this off-throttle noise something I should worry about?
 

'94, new DIY turbo. T25 from a SR20DET. FMIC, a HKSalike BOV from siliconeintakes.com. Running on the regular wastegate for now, 5-6 PSI.



You can hear the BOV opening when I lift, but there's that additional, lower-pitched rattle/clatter noise that goes with it. My impression is that it's coming from the driver's side of the engine bay and not from the BOV, but I'm not 100% on that. If I lift the throttle a little more gradually, I don't get the additional noise, just the regular BOV vent sound.

Is that noise normal, or is it something I should be working to eliminate?

The BOV is mounted just ahead of the throttle body, sourcing signal from the port on top of the manifold where the carbon can used to connect. I tried changing the hose for something a bit wider, but that did not help.

So far, my working theory is that the BOV is a little slow to open and that's causing a little surge until there's enough flow through the valve to dissipate the pressure. Plausible? It would explain why I don't hear it if I lift more gradually.

If that's the case, what can I do about it? The BOV doesn't have an adjustment screw on the back, so I guess the only thing I could do is open it up and find a longer spring?

Though it might seem counterintuitive, perhaps running more boost would help? Once I have the EBC valve installed I anticipate running 10-12ish. More boost would mean more pressure to open the valve, which would mean it opens faster, right?

Totally normal. I ran like that for years. Surge is bad when the turbo is loaded-up and spinning, then it sounds like death.

This is the surge you worry about:

20psi at 4K and 300hp is too fast for poor turbo!

It sounds fine when I'm actually on the throttle. I would like to make the noise go away, though -- even if it's not actually hurting anything, it's kinda silly.

I would like for posts like this one to go away, but I don't always get what I want.

There is a balance between spring sound and performance, the alternative to the sound is sucking air in at cruise.

Fair enough. I'll :dealwithit:

I thought I found a solution for this shit by adding a small valve opens for low rpm to allow some of the flow to come out...

I take it you dont know what causes compressor surge while on throttle?

see antisurge housing

What's happening is basically at lower rpm shifts, your probably seeing a couple lbs of boost(1-2psi) and your bov spring is stronger than that pressure/vac trying to open it, therefore you get some surge from the pressurized air bouncing off the closed throttle plate and back toward the compressor. That's why it sounds like it's coming from the drivers side.

Or something like that.:2cents: If you're only planning to run 5-6lbs, you may want to experiment with a weaker spring, cutting your current spring, or removing any washers if you have any in there.

Hustler is right though, you can only get it so good.

just ditch the h k ass bov and problem solved

Oooh, a quiz! I love quizzes. I'VE READ MAXIMUM BOOST (the blurb on the back cover, anyway) I KNOW WHAT TIME IT IS

So, compressor surge happens when the pressure wave from the turbo goes nowhere, bounces back, and tries to spin the compressor wheel backwards, right? So if the throttle's open, it's not hitting the throttle plate, so it must be hitting some other roadblock in the intake. So I guess valves not open far enough/for long enough, overrestrictive IC, and/or possum climbed into intake tract and died.

Your suggestion only makes sense in the former case, though, so I guess that.

You failed the quiz. Read the question again.

If you still dont get it watch Hustlers vid.

Shit.

OK, to rephrase. Not enough airflow through the engine. The turbo tries to flow more air than the engine can handle. The inducer is too large relative to the exducer.

Or, again, there is a dead possum in the intake tract between the turbo and the BOV.

This is helpful, thank you. I'll get the EBC set up and see if it's still a problem.

Right so if you were to make the engine be able to flow more at that rpm what would happen with the compressor surge?

I guess that example doesnt work perfectly since more would go into the exhaust. I guess the example will be more as what if you can reduce the ratio or have the extra go elsewhere without going into the turbine.

Good things happen. Or fewer bad things happen.

So your suggestion is a sort of blow-off valve that opens at low RPM but high boost rather than low MAP and high boost. I can't help but wonder if it wouldn't just be better to resize the turbo instead.

How about a turbo with a variable inducer/exducer ratio? Sure, the engineering would be a nightmare, but it'd be cool.

Specifically RPM dependent. Up to say 4k or whatever it would be open, then at 4k it would close. The only solution so far is the variable vane turbos and our gas motors run too hot for those. Work lovely for diesel though. Porsche made one but it was worth an arm, both legs, and a testicle.

this thread is a trainwreck almost all parties.


you dont have compressor surge. you need to adjust your bov.


/thread.