AudiWorld Forums - BOV's...a little tip

<center><img src="http://pictureposter.audiworld.com/1510/mvc-376s.1.jpg"></center><p>For those of you who have tried to make BOV's work and have been unable to, here is a little information.

Why use a BPV or BOV valve at all??? Well, if you are under boost and you come off of the accelerator, the throttle closes. The boost pressure backs up in the intake plumbing and will dramatically slow the spinning of the turbo compressor wheel....even if you instantaneously come off the throttle and go back on. Not only do you slow the turbo, but you have lost the inertia (like the train effect) and you have created one of the situations that is called "turbo lag". The BPV (or BOV) lets the boosted pressure out of the intake tract and reduces the slowdown of the wheel. There are those that feel that a BPV dumping air back into the compressor wheel keeps it turning at a high rate....actually, if you look at the design of most turbo compressor wheels on the intake side, you will see the fins somewhat overlapping...if you blow into it, it creates very little spin effect. They are designed to catch air and transfer it (suck in air). It is like blowing into the middle of a pinwheel as opposed to blowing at the tips. I think the manufacturer uses BPVs for several reason, one would possibly be emissions, another would be ease and noise restrictions...but certainly not for total performance. The BOVs let the turbo spin and slow on its own through friction, not through any induced pressure. I have been around many turbo race engines...Indy cars, Porsche 935/956/962, etc....and they all use BOVs.

We now use them exclusively in the Grand-Am Cup MTM S4 and have done so for about one full season of racing. We started doing so when special hoses that had been made for the BPVs failed at Mid-Ohio last year and blew as we were dumping the boost back into the intake hoses. These weren't stock hoses, but special parts that our sponsor had made for us that were supposed to be stronger than stock. I thought...wow, that is a lot of pressure buildup!! Under heavy boost, like with K04's, the stock valve cannot get rid of enough air quickly enough!

We then got adjustable valves from TURBOXS...first we got H29s. Solid anodized aluminum hosing with brass pistons...no internal rubber boot to blow.

We would shim up the underside of the spring with the enclosed washers so that the valves would just stay closed at idle....one more shim installed and they would have opened at idle, which is what you don't want...in fact, this is probably what is happening to those of you that are trying BOVs and the engines run very rich at idle and very poorly, but smooth out when driving at higher rpm....the piston is probably open at idle. The spring tension on a non adjustable valve isn't strong enough to keep the piston closed. You don't notice it when the valve is plumbed back in as a BPV because it is more closed loop and doesn't create a vacuum leak...I say vacuum leak, but it is actual a pressure leak because even though you are not boosting the turbos, there is still a small amount of pressurized air coming out of the open valve...and this is metered air that the engine does not get, hence lending to a richer running engine.

The key is to run valves (whatever kind they may be) that have an adjustable piston spring pressure if they are small valves. The smaller the piston, the more spring pressure you will have to run because the engine produces a lot of vacuum in the intake manifold at idle which will open smaller valves quite easy.

We later went to H39 (left in the picture) and now use TURBOXS H (right in the picture) valves which are very large...but they blow the excess boost off when the throttle valve is closed very efficiently. With the large pistons, it takes more vacuum to open them, but they dump the boost better. We have no lag between gear changes...instant boost when modulating the trhrottle in corners. Our idle is perfectly smooth...and yes, all of our intake system by design is the same as your street engines (we may have different hardware but the principles are the same). Our engines do not run rich at idle, nor do they belch out smoke from being too rich.

If someone says they won't work, either they haven't tried them or they have not applied proper mechanical expertise to the style they are using. A key is using a larger than stock valve size...which means using a larger diameter hose.

We certainly don't do it for the noise, which can become a bit too much (fans along the track like it)...we use them because they make our engines produce the horsepower quicker in between shifts.

Champion racing also uses them on their SCCA GT S4s...actually, they use one centered mounted BOV that is located where the power steering pump used to be(they use an electric pump).

Best wishes to all

Don Istook