So I get this flutter/possible compressor surge in 4th and 5th gear, around 20ish psi, full or partial throttle. It works perfectly fine in 1st, 2nd, and 3rd gear. My tuner believes its the spring in the bov. Currently there is an unpainted spring inside. Judging by their chart, its a 10/11 psi spring. Vacuum is around -9hg, so I believe the spring I need is the Black one. Which makes me curious: Wouldnt it be flutter cause it cannot hold the boost in higher gears, with a lighter spring? In my mind, it would seem like I need a stiffer spring, hence its opening/closing rapidly under heavy load. What are your thoughts?

Setup: Stock block, FP Black set to 26 psi on E85. Speed density. Tial 50mm bov

 

9inhg iirc -4 psi. Is that what black is? Watch the vacuum as you decelerate from higher rpm. It will decrease the amount of vacuum pulled as you near lower rpms.

I'm not sure why people fret about a bov leak. A responsive bov yields greater drivability. If you're backing off the throttle slowly you should not be fluttering. You should be venting enough to equalize pressure. Which in some cases will be constant.

It is tough to set up a bov proper when they all have different philosophy's behind them.

 

You need the black 8psi spring

 

Black spring is 7psi (for 50mm, 6 for Q)

Its fluttering when at wot, not any other time. Boost creeps to 20-22psi around 4k rpm in 4th or 5th, then it starts fluttering. Therefore not holding boost (feels like compressor surge, even a mild fuel cut) We are absolutely positive its the bov as we have not found any leaks or turbo issues. The oddest part in 1st, 2nd and 3rd gear it works perfectly fine. No flutter and pulls hard. Get to top of 3rd, shift to 4th, flutter. If it affected after wot, I could live with that. SD & VTA is a harsh combo. Gotta expect some minor issues. Though this is far from minor as 4th and 5th cannot be boosted in.

lostsouls - I will order it and give it a try. Thanks

 

In 1-3 the motor can out run the turbo , in high gears it cannot. I would suggest that is true surge limit. Tial says wot flutter is not a bov issue. And that spring rate has no interaction with boost holding , it just controls opening .

 

This ---^

The BOV is pressurized on both sides by boost pressure. So when you have an 8 psi spring in it, one side sees boost pressure, and the spring side sees boost pressure + spring presssure.

 

What size line do you have running to the bov? and is it t'd to anything?

 

I don't know what size, but it's definitely bigger than the egr vacuum lines. Also my vacuum line for the BOV is NOT t'd off with anything else. It has it's own source and suggest you do the same

 

It was originally T'd between the MAP sensor and the Tial (not a 8/9, therefore dont have a MAP port on the IM). I changed it today to have a direct source. It was ran with 1/4" double ply tubing (same as before). I ordered the 7 psi spring just in case. Ill know in a few days if the issue has been cured.

Funny the previous owner of the car didnt have an issue running it T'd.