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"The BIG BOV thread" (Where all bov question threads get merged)

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#1 · (Edited by Moderator)
This is the big BOV thread. If you didn't find this thread, you didn't search.

This thread is where all BOV question threads get merged into.

The quote below in this first post is from our BOV info sticky and contains links to some very explanatory BOV information. Please take the time to explore the links before you ask repeated questions.

Thanks!


A stock WRX/STi comes with a BPV (Bypass valve) which is a vacuum-actuated valve designed to release pressure in the intake system when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake before the turbo.

An after market blowoff valve (VTA) does basically the same thing BUT unlike the stock unit it releases air to the atmosphere (giving you the loud sound). Some people see an after market BOV on a stock WRX as being in poor taste (probably due to the fact that it is basically a noise maker that really as no other benefit), while others simply find the sound very attractive.

Proponent opinions notwithstanding, there is a small mechanical downside to having an after market BOV (VTA) on a stock WRX. The problem comes when releasing air to the atmosphere that has already been metered by the mass airflow sensor. When the BOV releases (blows off), the ECU will be injecting the wrong amount of fuel into the cylinders. The engine temporarily runs too rich (meaning too much fuel is injected into the cylinders). This temporary rich condition isn’t typically THAT harmful, but it may cause bucking or hesitation on lift-throttle as well as some other concerns outlined in the links below.


Links for BOV info:

Sinister's BOV FAQ

Wikipedia

Unabomber on Nasioc

Davenow on Nasioc





 
#2,782 ·
I would disagree with this reasoning. OTHER than the fact that the STOCK STI exhaust has no cat in the uppipe, either turboback exhaust from either the WRX or the STI will be VERY SIMILAR in performance. (I know the STI has a higher flow Prodrive muffler, so maybe a moderate 5BHP gain there or so)

I think everyone would agree that any increased power in the STI is due to:
1. Larger engine displacement
2. Larger turbo
3. Variable valve timing
4. Better Intercooler, Injectors, Fuel Pump, etc
 
#2,783 ·
also ive heard some blow off valves people are losing hp on stis or call stalls. has anyone had this prob with the aps one on a sti?
I had a Forge BOV 100% on my STi, ran with no probs and sounded great.

BUT, after a race with an EVO which I dont think should have lost by that much(rolling start) some Clubwrx members said maybe my BOV could be costing power.

I took it off, and was performing the butt dyno with my Fiance in the car HER WORDS:

"WOAH your car feels faster, hey where is the pssshhh sound it was cute"

Well aside from the cute comment, her unbiased observation was that my car felt faster. FWIW
 
#2,790 · (Edited)
spark plug anti-fouler fix?

I never heard of that one. Could someone explain it?

For $300 i'd get things in this order:

1) boost gauge (even w/o any other mods it'll help you get more speed)
2) EGT gauge (necessary for ANY performance mods)
3) MBC (till you can afford real engine management)
4) used stock up-pipe to clean out the inside of all that metal
5) Axxis Ultimate brake pads (starting with) the front

I threw away the stock pads (@ 4-6 K miles) and then a set of all-round Porterfields (@ 12-15 K miles) before finding the Axxis Ulitmates stopped with a real bite. Going faster is dangerous unless you can stop faster. With the stock pads, i'd stomp on the brakes .... and wait ... and wait. Not good.
-----

UTEC, MRT turbo-back, Autospeed up-pipe, K&N squeeze-in, 4-nozzle intercooler sprayer, DBT drilled/slotted front rotors, Axxis Ultimate OEM replacements, NitrousExpress 50-shot w. WOT and RPM-window switches and heater and blanket, Autometer EGT and nitrous pressure gauges on pillar pod, fire extinguisher.
 
#2,792 · (Edited)
antifouler for up-pipe w/o an EGT bung?

OK. I found the posts about this mod.

My Autospeed up-pipe has no EGT bung. What would be the point of drilling a hole in it and putting that pipe fitting in there?The flow is more laminar without anything sticking into the pipe. Smooth pipe flows more air, everything else equal.

The Autospeed-supplied resistor is working fine to keep the CEL light from coming on. Knock on wood ... i've never thrown a code.

I keep an eye on EGTs by watching the gauge ... the way god intended for high-performance turbo cars. It gives more info about how the car is running and responding to mods than trying to log everything with the UTEC and be looking at all the logs and trying to remember what the car was doing when such and such happened.

I don't know how people can modify their car's intake and exhaust without boost and EGT gauges. Once you get an EGT gauge you'll know what i'm talking about.
 
#2,794 ·
I agree with everyone who said uppipe.
There are a lot of them to pick from, but personally I suggest a Godspeed or a JDM (import) STi uppipe. The Godspeed uppipe is a good piece. Non flex, has an EGT bung so no CEL, and as the guy who installed mine said... The thing has a near perfect fit compared to others he has installed. The JDM STi pipe is a Subaru part, so it's going to fit perfectly, has a flex (not sure about the bung), but is more expensive.

Personally, I'd say spend $190+shipping to get a used one from Dan at Godspeed w/ new gaskets, and use the rest to have it installed. It should be about a 2 hour job at a shop (after the car has cooled down) so you will need a little over $300 in the end. That's unless you can find somebody local who knows their stuff and can install at, or a garage day.

Uppipe key features -
Removes the first cat allowing faster turbo spool
Removes the first cat allowing for more power
Removes the first cat making sure your turbo is safe


Then do yourself the next best thing... Get a boost gauge.


If you want something fun and easy... Get a GFB Hybrid BOV. It's something almost anyone could install, and well... It's more of a fun mod to do the car than anything.

I highly suggest going through the first 1000 miles and breaking the car in though before modding it (if the car is new)
 
#2,795 ·
up-pipe

Another factor is that even though it means removing one of the three cats, the cat there is only needed for cold start emissions. But you need to warm up the car anyway before an emissions test.

My local emissions test program lets you pass the visual part of the test if they can see just one cat. They don't check to see how many the car comes with and look for them. The thing is almost impossible to see from the top, anyway.

It's not easy to put in. If you do it at home i'd recommend that you arrange in advance to be able to borrow another car if you need a part or something ... one reason to do it when the parts store is open.
 
#2,797 ·
Guys -
Maybe I wasn't clear enough...

I wasn't asking, "What makes the STi different?"

I was asking, If the regular WRX exhaust and the STi exhaust are the same diameter, is there anything else that makes the STi exhaust different.

This is the exhaust thread, after all. :D ;)
 
#2,798 ·
snvin,

I think I touched on the differences between WRX and STI exhausts in my last post. Basically, the diameter of the pipes is the same, except the WRX has a catted uppipe and a higher restrictive muffler than the Prodrive muffler the STI gets.

Hope this helps.
 
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