BOVs on Supercharged engines .. is it possible

[carsaregoodfun]

hi, i drive a supercharged v6 and i was just wondering if it was possible to add bov's (blow off valves) to the engine to a) blow off the excess pressure when the throttle body shuts during gear changes and b) make that PSSSHHHH noise?

i was just wondering if it was possible to do so, as i couldnt really find any information out on the net about it. I know the eaton sc i have does get rid of the excess via a line but it does this silently.

also, has anyone out there ever herd of a 'rebreather' for your car engine air intake, i think there illegal because they can catch fire or something. anyway just curious.

later.
j.

[swedish]

not really neccesary, as the t/b is closed for a short time, and as revs drop, so does psi.

[carsaregoodfun]

so basicaly your saying that BOV's will add no extra performance gains to my cars current setup (even tho i am going to pump up the PSI on my SC very soon to about 10psi+), thats fair enough. but what about the pure wank factor that is the PSSSSH noise during gear changes, you dont see many blown v6's or v8s (unless there turbo) making that noise around town, so it is possible to add BOVs if i wanted to tho?



j.

[Cbass]

Well, yes, you could add one if you wanted... kinda ricey if ya ask me, which you haven't

[TerminalVelocity]

Originally posted by Cbass
Well, yes, you could add one if you wanted... kinda ricey if ya ask me, which you haven't

agreed

[Layla's Keeper]

Yep, no performance gain for a lot of fabrication work. It doesn't even factor in reliabilty since a super charger shuts down as revs drop. A BOV on a turbo is to deal with the spin caused by the turbine's inertia. When you let off in a turbo car, the turbo keeps spinning and producing boost when the car doesn't need it. This can do serious damage (just ask 1962 Buick Jetfire F-85 owners) to the intake system from the throttle body onwards to the valves and pistons. So the blow-off valve deals with the excess by opening when the throttle isn't open. By contrast, a supercharger is driven by the crankshaft and its revs are determined by the overdrive or underdrive of the pulley or gear size. When the crank slows down, the supercharcger slows down. Thus, less boost is produced and the engine is saved. If you are having a problem with your supercharger not slowing down, tighten your belt.

As for making the, ahem, "cool PSHHH sound", I agree with TV and Cbass. You'd be mistaken for a total wanker/poser/ricer if you got found out. Not a good thing, since you appear to have your head screwed on straight.

[carsaregoodfun]

thanks for the replys, its all good i wont be putting on any blow off valves for the pure noise factor, as you probably wouldnt even be able to hear it at all over my SC's whine and the sports exhaust i already have.

[96 LT1-Z]

Carsaregoodfun, by your post i'm guessing you drive either a supercharged t-bird, Regal GS or GTP. If you are, then you have a roots type supercharger and have no need for a BOV. On a roots type application the supercharger is downwind of the throttle-body, therfore there is no way it can be forcing air against the trottle blade/s if it suddenly closes and hence no need to bleed off pressure. On a centrifigul(sp) supercharger(paxton,vortech,ATI) however one can be added, and many come with some sort of pressure release valve, though they aren't as loud as those you see offered for many turbo applications. On a centrifigul application they are needed because the supercharger is upwind of the throttle body. Besides a centrifugul charger is basically the compressor side of a turbo driven by a belt.

[carsaregoodfun]

ahh yes its a roots type charger. thanks for your post, very informative! the charger does have a bleed-off line so it must be there for something.

[DSMJim]

Yes you can use a blow off valve with a sc kit and yes there are performance gains, but it depends on your car etc. You didn't say what type of car you have. So here is a real answer.

First a blow off valve is not for noise, that's an after thought. If you want it for that, then delete this topic right now. On turbo cars a blow off valve is to release pressurised air from your intake piping so when your shifting (is your car auto?) and your throttle plate closes the air is not forced back on the turbo causing it to slow down, hense making your spool up inbetween gear shifts longer.

On a supercharged car it will do the same thing, however a supercharged like yours which is eatons roots based blower is positive displacement which means it forces air like a pump and it will be blowing no matter what. A turbo is centrifical which means its like a hair dryer where you can cup the end and the fan will spin but no air will come out the end. Eatons s/c again being positive displacement MUST AND WILL move that air. If you cover the end up, pressure will build until it blows, where a turbo wont.

Now that we understand that, how does a BOV work on a S/C car? Well it will release the pressure, but that doens't slow down a s/c because it's belt driven. It does however create added stress on the s/c when your boosting it higher like 10+ psi. It also adds pressure onto the system which creates heat. Heat is created when you compress air. Because a supercharger like yours will force air into the pipes regardless of weather the throttle plate is open or not it super heats the air, it is split second, but long enough for you to get hot air causing detonation for a split second inbetween shifts. A blow off valve will releive that pressure so when your throttle plate opens again it gets cool un heated air. Because the air that you won't be using is gone it's not being super heated by adding extra pressure on it.

Here is why I asked you what type of car you have. You need to know if your car has a MAF (mass air flow sensor) or MAP (manifold air pressure sensor) and here is why. A blow off valve releases air from your intake system. A maf and map both measure the air comming into the engine so you car knows what kind of fuel to add to keep a proper air fuel ratio. If you have a honda then you have a MAP sensor where the reading for how much fuel to add based on the air is done inside the intake manifold. You can add a blow off valve there with no problems at all. If your car has an MAF sensor then it's a different story. A MAF is right behind your filter and measures the air comming through your intake while you drive. If your blow off valve releases some of that air your car won't know that it's gone and it adds fuel for air that is not there causing an overly rich condition which is not good for making HP.

If you ahve a MAP then your cool. If you have a MAF then you have 2 choices. 1 you can vent to the atmosphere anyways and make your car run like crap, or you can re-route the blown off air back into your intake system before your supercharger after your MAF sensor so it still knwos its there.

This is why I need to know what type of car you have. If you have a Type-R with a jacksons charger on it, then you can't add a BOV anyways because your supercharger is in your intake manifold. If you have a Vortec type charger like for a Civic SI then you could because you could put a BOV after your super charger before your throttle body with no ill effects because it uses a MAP sensor. Lets say you ahve a stillen S/C on a maxima. You have a MAF so you would have to re-route it back into the system, but could use a BOV because the S/c is not part of your intake manifold.

Hope that helps. Thats a long explination but that is how it works. If you give me your specific application I can tell you if and how you can make it work. So the long and short of it is yes it will help you, but it depends on your application.

Werd yo...

[street_racer_00]

I ALMOST stayed awake for that entire explaination.

[carsaregoodfun]

well my car is a holden vx commodore with a 3.8l supercharged v6. of course this means im australian. the only other mods ive done so far to it is a full sports exhaust (extractors, mandrell bent 2.5inch dual stainless steal exhaust with high flow cats, a magnaflow mufler, 3inch tip and bullet resonators).

i was asking about the bov's because as i said i am going to boost my charger up very soon (just saving up for the motec ems - as the stock one is of course mega crap) and i am already thinking about putting water injection or an intercooler in and was just wondering if a bov would help as well.

holdens use mass airflow sensors (mafs). if bovs do indeed add cooler air into the intake then this of course would be of some help. like i also said that PSSSHHH noise wouldnt really be a factor with the engine cranking and the exhaust growling.

wurd back.
j.

[96 LT1-Z]

carsaregodfun, if I send you my address, could you mail me an HSV GTR? Damn, I want one of those.

[carsaregoodfun]

ummm ... no.



monaro gto(255kw) and gts(300kw) (grand touring sedans) even tho there dam coupes! there is no gtr hsvs :| there just camaro clones or something arnt they? (pretty much). from my understanding of chevy.

[96 LT1-Z]

Actually, I was referring to the sedan, the 400hp version. It is built on the same platform as our Caddilac Catera and now the CTS. We however only get a 225 hp n/a v6. We are slated however to get a CTSi in late 2003/early 2004 with the Z06's 405hp LS6 and six speed tranny.