How stuff works: BLOW OFF VALVES EXPLAINED...
 

Ok another common misconception is that bov choice should be selected by the amount of boost you run. problem these days is that most blow off valves suck and they dont perform properly. For this explanation i'm going to use the TIAL 50mm bov because that bov works exactly how it should and to explain it I need to use that.

THESIS STATEMENT: HOW MUCH BOOST YOU RUN DOES NOT MATTER WHEN CHOOSING A BOV OR BOV SPRING!!! VACUUM DOES

Ok so your turbo is sucking in air through the compressor. Below is a picture of a turbo system, very plain and easy to understand. After the intercooler there is a throttle body with a plate. I'm sure everyone reading this knows how that works but its connected to a cable or a solenoid (drive by wire). once that plate closes when your foot releases the gas peddle, whatever compressed air (and some still be compressed by the still spinning compressor wheel) in the piping and intercooler has no where to go. With no BOV, it will go the path of least resistance. If all your clamps are tight the compressed air is going to backtrack backwards to the compressor wheel and you will hear it from your air filter. So now we put a tial bov on the car. Once we let off the gas all that air sucks into the motor and you see your boost gauge go from xx psi to xx vacuum immediately. SO everyone knows the vacuum line that goes to the TOP of the boost controller? Well once theres lets say
-22 inches of vacuum (remember there is no such thing as negative psi) the vacuum PULLS the bov open releasing that pressurized air from the piping/intercooler. TIAL offers 5 diff bov springs but 2 of them are for special applications. The main 3 are 7, 9, and 11. Each spring is good for double that in inches of vacuum. if your car has some heavy cams in and barely makes any vacuum, you need a 7 psi spring. That way when you let off the gas and your car drops to -13, -14 inches of vacuum, that soft spring will be pulled open. if you have too stiff of a spring then the bov wont open because the vacuum wont pull the stiffer spring. Lets say you have -22 inches of vacuum on a stock car and you put a 7 psi spring in there. WELL at idle, the bov is gonna be open because that spring opens at 14 inches and you have ANOTHER 6 inches pulling it open just ad idle.


http://static.howstuffworks.com/gif/turbo-plumbing.gif


So a bone stock STI needs an 11 psi spring? and a fully built GT42R STI running 45 psi with 280 cams needs a 7 psi spring? Answer is YES.


any questions? Post them




PS: some high HP drag cars with 2 steps that build boost and so on kinda tweak a few of these rules. Sometimes they need an ultra stiff spring or the car can fall on its face off the line. This was written up to give people a general understanding of how it works. I tried to make it as simple as possible to understand.

-Steve

This could be stickied as well if anyone wants it to be

hmm...interesting. so, stock car has about -22inches of vaccum and cammed car has less, like in -13 and -14 inches?
is this arbitrary numbers for the explanation? or is this true numbers?

so, what do you suggest for 272/272 cammed evos?

A thread like this already is out there.

Diverter Valve Tech:
https://www.evolutionm.net/forums/sh...d.php?t=209640

at

stock is around 21-22 inches and yea depending on your cams, (usually your intake cam) you'll have less. While every car is different most are within 2 inches of another with a similar setup. If you have 272's, you'll probaby need a 9 psi spring in a tial bov (which is good for 18 inches of vacuum). It might not open at low rpms where the vacuum doesnt pull as hard between shifts but it should suffice when WOT shifting at high rpms. best bet is to try to get as close as possible then experiment from there. Alot of people ave the wrong spring in their bov's or wastegates sometimes and they go through hell trying to decipher a problem

-Steve

OOO, didnt know that, thanks man!!1

-STeve

Have you guys noticed all the new people recently buying evo's? So strange hopefully they'll learn to read stuff like this and learn.

Not knocking this post though!

I agree reading is just something that seems to be hit/miss with some people here.

great information, i was looking to buy HKS bov but i guess i shouldn't.
272/272 intake, stock 9 turbo with injection, boosting at 27-8lbs of boost.
so, you suggest tial BOV with 9psi spring?

the tial bov and the stock ecu are not going to get along very well...the stock ecu is designed for use of a diverter valve (air being vented back into the intake)

releasing the air to the atmosphere is problematic

check out the forge rs diverter valve for your application

ya know I have to say for cars with maf sensors the evos do handle it pretty well. I mean yes what chaotic said is true, recirculating is the best for driveability, no cel's, and its the most efficient setup, but evos do handle it better then some other cars, especially evo 9's.

well i made 2 threads about how stuff works, hopefully people at least read them

to the top!!!

Good job Steve. Since we are playing Q?A what happens when you have two BOV say Tial 50mm. How does that effect the spring rate of each? Same concept could be asked about dual wastegates. Do you find your spring rate and divide by two and if not why? :D

First person gets a :beer: on me with the right answer.

Chris

^^ well you definitely dont devide it, as the pressure on each did not get reduced my 50%. I think you'd leave the same rate. The only thing you'd gain by doubling up bov's is having more air being able to exit the charge system. Lets say you had 100 feet of 5" diameter intercooler piping. One bov wouldnt be enough to vacate all of that pressurized air. But the pressure might be the same as the guy running 2.5" piping thats 5 ft long.