BOV issues
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Joined: May 2022
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From: Los Angeles
BOV issues
Forgive my ignorance, I am not an expert. A and A super charger, slightly smaller pulley, 8psi of boost in last Dyno. Changed the intake tube from 4" to 3" LS2 MAF to all 4" and LS3 style MAF, down to 3.5" right at the throttle body opening. Changed BOV for larger A and A labeled 50mm race port. Same vacuum line and location.
On Dyno day it seems this new BOV is opening at approx 6psi, and then holding at 4psi. It is opening while still under boost and not allowing my 8psi or so. This BOV is not adjustable except for swapping springs. He got the same result many times in a row, very consistent opening at 6psi. To me, the non expert, it seems the spring in the BOV isn't strong enough to keep it closed?
A and A said the BOV can't open if there is sufficient pressure behind it - not sure what to do. I emailed race port and also asked them for guidance.
Rescheduled Dyno in 2 weeks.
On Dyno day it seems this new BOV is opening at approx 6psi, and then holding at 4psi. It is opening while still under boost and not allowing my 8psi or so. This BOV is not adjustable except for swapping springs. He got the same result many times in a row, very consistent opening at 6psi. To me, the non expert, it seems the spring in the BOV isn't strong enough to keep it closed?
A and A said the BOV can't open if there is sufficient pressure behind it - not sure what to do. I emailed race port and also asked them for guidance.
Rescheduled Dyno in 2 weeks.
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From: Los Angeles
I am open to it being some other issue - I really don't understand. I can fairly easily replace all the vacuum lines (BOV, boost gauge, hobs switches for aux pump, meth, etc) If I had some vacuum leak from some other lines off the vacuum manifold I would expect to never make boost and it would never work. It's really weird it starts venting at 6psi. I also have no indication in normal driving of a vacuum leak from my boost gauge. Operating in range as it always has. I also can't go ripping on it now to test since the tuning basically stopped at 5700rpm.
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I put my mighty vac on the BOV and it starts up open at -10 and basically impossible to push open at +4psi. So that all seems very normal. So somehow my car is making vacuum when I get to 6psi of boost and the BOV opens? I really don't get it.
Drifting
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where is the boost reference/vacuum reference line on the BOV hat routed to? Basically, the BOV will NEVER open under boost if its routed to the line its supposed to be routed to.
The BOV hat will see the same boost as the intake manifold, so both sides will be equal pressure + the spring will push it shut. Changing springs will do nothing other than make it harder for the vacuum to open leading to other tuning issues.
Is the line pinched? is it properly going to the large vac tube going to the brake booster? Is the brake booster check valve working properly? You can try and disconnect the brake booster and plug the lines and see if it will hold shut...
The BOV hat will see the same boost as the intake manifold, so both sides will be equal pressure + the spring will push it shut. Changing springs will do nothing other than make it harder for the vacuum to open leading to other tuning issues.
Is the line pinched? is it properly going to the large vac tube going to the brake booster? Is the brake booster check valve working properly? You can try and disconnect the brake booster and plug the lines and see if it will hold shut...
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From: Los Angeles
where is the boost reference/vacuum reference line on the BOV hat routed to? Basically, the BOV will NEVER open under boost if its routed to the line its supposed to be routed to.
The BOV hat will see the same boost as the intake manifold, so both sides will be equal pressure + the spring will push it shut. Changing springs will do nothing other than make it harder for the vacuum to open leading to other tuning issues.
Is the line pinched? is it properly going to the large vac tube going to the brake booster? Is the brake booster check valve working properly? You can try and disconnect the brake booster and plug the lines and see if it will hold shut...
The BOV hat will see the same boost as the intake manifold, so both sides will be equal pressure + the spring will push it shut. Changing springs will do nothing other than make it harder for the vacuum to open leading to other tuning issues.
Is the line pinched? is it properly going to the large vac tube going to the brake booster? Is the brake booster check valve working properly? You can try and disconnect the brake booster and plug the lines and see if it will hold shut...
There is a vacuum manifold on the brake booster line, just prior to the check valve that goes into the brake booster. 5 or 6 lines branch off from it.
Line to BOV does not seem pinched. I could easily blow through it and it's connected to the vacuum manifold. Vacuum in car on my way home seemed normal, no leaks or anything. 18-19 at idle and varying at RPM otherwise. Car doesn't stall out at idle or under load.
Will disconnect the check valve and blow through it and make sure that's working
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Just talked with the Race Port tech support line, and he is very confident I must have a leak in my boost reference / vacuum somewhere. He also advised taking it from the brake booster is super sketchy and advised I put the brake booster line back to dedicated and use my distribution block attached elsewhere. I am going to do that, and check / replace all fittings and lines coming from the vacuum manifold block and replace my brake booster hose assembly back to stock.
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If there is a leak, it's only manifesting after 6psi. Car idles fine, brakes fine, boost gauge works, car drives fine EXCEPT above 6psi. I am assuming there is some small leak that only manifests at +6psi.
Putting brake booster line back to stock, replacing the brake booster checkvalve, and running the boost reference / vacuum from the nipple right behind the throttle body on the intake manifold.
Putting brake booster line back to stock, replacing the brake booster checkvalve, and running the boost reference / vacuum from the nipple right behind the throttle body on the intake manifold.
Last edited by Benonmsn; Apr 27, 2026 at 02:35 PM.
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From: Los Angeles
I vent to atmosphere, no PCV. One of those two ports just behind the throttle body on the Inake, passenger side can be used for my vacuum manifold to run gauges and BOV, right? They are both blocked off, and have been for some time
Last edited by Benonmsn; Apr 27, 2026 at 02:36 PM.
Drifting
Joined: Dec 2016
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I believe those lines should work. One is for the fuel tank EVAP and the other is for the crankcase ventilation. Both should be pulling intake manifold vac.
Boost from the brake booster shouldnt be an issue, but i think as a general rule of thumb since brakes are important it should be a direct line to the intake. A&A kit pulls from the brake booster line but its probably because there isnt a better location available (C7 pulls from a different location).
Are you sure the vacuum manifold does not have a check valve? If it only allows vacuum and not boost, you are basically running the BOV at spring pressure (which would really explain the consistent 6 psi or whatever youre seeing). the line needs to see boost so it supplies pressure to the hat side of the BOV.
Boost from the brake booster shouldnt be an issue, but i think as a general rule of thumb since brakes are important it should be a direct line to the intake. A&A kit pulls from the brake booster line but its probably because there isnt a better location available (C7 pulls from a different location).
Are you sure the vacuum manifold does not have a check valve? If it only allows vacuum and not boost, you are basically running the BOV at spring pressure (which would really explain the consistent 6 psi or whatever youre seeing). the line needs to see boost so it supplies pressure to the hat side of the BOV.
Safety Car
Joined: Oct 2007
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From: South Florida
Before you even drive a forced inducted car it needs to be pressure tested.
Fill the entire plumbing from compressor to intake with boost pressure and investigate all leaking.
The bypass valve can open due to insufficient spring pressure. upgrade the spring or change the bypass.
It can also leak due to bad vacuum line situations, like too small of a line, too long of a line, or too slow of a response in the line, or leaking,
the diaphragm of the bypass could be leaking for example but you won't find that without a pressure test or careful inspection inside the bypass
I made a pressure test video was sick of hearing 'its not possible' after doing it for decades
The bypass is necessary in a supercharger app to protect the compressor from damage, it needs to open suddenly as you lift and make a large release of boost without any evidence of compressor surge. If you hear the compressor chopping at the air "flutter / surging" its chewing up the compressor blades and slamming the shaft in the bearing housing around causing damage over time. If it leaks in a turbo app it also causes high EGT and EGP and IAT and leads to engine damage but if it leaks in supercharged apps It can only cause loss of power.
Fill the entire plumbing from compressor to intake with boost pressure and investigate all leaking.
The bypass valve can open due to insufficient spring pressure. upgrade the spring or change the bypass.
It can also leak due to bad vacuum line situations, like too small of a line, too long of a line, or too slow of a response in the line, or leaking,
the diaphragm of the bypass could be leaking for example but you won't find that without a pressure test or careful inspection inside the bypass
I made a pressure test video was sick of hearing 'its not possible' after doing it for decades
The bypass is necessary in a supercharger app to protect the compressor from damage, it needs to open suddenly as you lift and make a large release of boost without any evidence of compressor surge. If you hear the compressor chopping at the air "flutter / surging" its chewing up the compressor blades and slamming the shaft in the bearing housing around causing damage over time. If it leaks in a turbo app it also causes high EGT and EGP and IAT and leads to engine damage but if it leaks in supercharged apps It can only cause loss of power.
