Credit goes to Ken on NEO again for this article…
As I’m building my turbo project and selecting components, I’ve wondered about BOVs (blow off valves): do I vent to atmosphere and get a “turbo” pshhhhhhhhht sound or do I recirculate the vent back to the compressor inlet? Many people would get a BOV for the sound, whether it is needed or not. In fact, I’ve heard of kits that fake a BOV sound when you release the throttle. Personally, that’s pathetic. I’m more of a function over form type of guy so I’ll see what it does for me before considering the cool factor but some people love the sound. Anyway. Today I talk about BOVs: why they exist, somewhat how they work and should you choose one that makes noise (or a sound for those who like it and consider noise to be derogative) or not.
- Why?
First of all, I hope everybody knows by now that I’m talking about a turbo setup here, nothing NA about this puppy…
For those who know little, a turbocharger (or supercharger) works by compressing air going into the intake manifold, thereby increasing its pressure and the amount of air you can fit in the cylinders. Compounded with more fuel, this leads to more power. Which is good. The problem is how does it do it? The turbo compressor (and some superchargers) is an axial compressor that draws air (at roughly ambient pressure) though an inlet and expels it throug the outlet. (see image, source is http://www.magoo.co.nz/turbos.htm)Get to the point! OK. It all works fine as long as the throttle is open. Air flows from the compressor outlet through the throttle body and into the cylinders (provided the valves are open!). But waht happens when you release the throttle (say to shift gears)? The throtle plate closes and the compressed air has no place to go! At this point, the turbo is still spooling (it has inertia) and keeps pumping air but it has now where to go. So pressure builds, flow drops and the compressor goes into a range of operation knwon as a surge.
courtesy of Turbonetics (www.turbonetics.com)
This is the compressor map for a T04E - 57 trim compressor. It shows the efficiency of the turbo given a flow and pressure ratio. The pressure ratio is the ratio of pressure at the compressor inlet and outlet. The dashed line on the left is the surge line. If you happen to have a given pressure ratio and a flow so low that you are in this range, you will get very odd things such as flow reversal (yes, the air will go from the outlet to the inlet) and other bizarre and non-linear things that wreak havock on a compressor by exposing it to very high stresses. So surge is bad :twisted: . Without a BOV, when you release the throttle, you are likely to go into this region and decrease the life expectancy of your turbo.
First a word about how they work and designs. Another turbo pressure controlling device is the wastegate. The wastegate opens when a given amount of boost relative to atmo is attained to prevent more build-up. The BOV is similar, but opens when a certain amount of vacuum is attained relative to atmo (an engine with a closed or partially open throttle creates a vacuum). You cannot rate a BOV by the pressure at which it opens, but you can sometimes set at what intake vacuum (when throttle is closed) level you want to open. Also there are various designs that can leak because of high boost levels (which is why some BOV are rated for low pressure or low hp, like only! 300hp), even when there is insufficient vacuum. This is usually resolved by using a twin-chamber design. Faulty diaphragms are also a common problem with BOVs. This is resolved by going with a piston-type design.
Atmospheric BOV (also known as dump valve):
This is the simplest type. When you release the throttle, this valve opens to the atmosphere and dumps the air to releive the pressure between the compressor outlet and throttle plate. This make a cool Pshhhhhhhhht sound that is often associated with a turbo. Unfortunately, for some cars that use a MAF (mass air flow sensor), this causes air metering problems. The ECU uses the MAF to determine how much air is going into the cylinders in order to know how much fuel to give. The MAF is commonly located before the compressor inlet (can be after in a blow-through design, more later…) which means all the air vented by the BOV is “missing”. This causes a lean condition on idle or when you step on it again (the ECU thinks there’s more air than there is, thus putting more fuel than necessary). This often leads to back-firing problems that are quite common on turbo cars (unburnt fuel gets dumped into the exhaust and is ignited their instead of in the cylinder). This condition can be fixed at the ECU level if it knows it’s going to occur (ECU programed for turbo with atm. BOV) or with piggy back fuel computer (tricks ECU and tells it there is in fact less air than measured).
Pros:
. solves the surge problem.Cons:
. doesn’t always work well with MAF designs
. if too small, still won’t flow enough to prevent surge
. if too big, releases all the (precious) pressure that you want when you open the throttle again. Boost must be built back up…Pro/Con?
. makes a soundRecirculating BOV:
A recirculating BOV is a solution to most of the problems of the atmo BOV: instead of dumping the compressed air into the atmosphere, it diverts it back into the compressor inlet (after the MAF), thereby recirculating it. This is a huge benefit for several reasons: although the flow is small, it causes a pressure increase at the compressor inlet, thereby reducing the pressure ratio (say 14.7+10 psi and dropping on outlet and 14.7 psi and raising on inlet). As seen on the compressor map, by reducing the pressure ratio, you decrease the fow below winch the compressor will surge, so you can have less flow than with an atmo BOV and still prevent surge. Another advantage is that this method actually keeps the compressor wheel turning longer so when you open the throttle again, you get quicker spool up (or reduced lag). Because the air is recirculated after the MAF, no metered air is lost and you shouldn’t have backfiring problems (unless the recirc is too close to the MAF and some air leaks back out through the MAF).Pros:
. solves the surge problem even better
. solves the air metering (and back-firing) problem
. keeps the compressor spooling longer for improved throttle responseCons:
. none really, except for very high boost and full off-throttle condition, where lots of air needs to be vented instead of infinitely recycled.Pros/Cons?:
. doesn’t make any sound (or much less)Best of both worlds:
There are also more elaborate ones that have both functions: recirculating when small amounts of air need to be vented and atmo. vent when large amount of air need to be vented (high boost conditions only). More expensive, worth it? I don’y know.That’s it for now. Questions and comments are welcome.
Ken
