It could be that the howling you were getting before was due to the stock plastic BOV leaking just enough to cause a squeal/howl. The whistle that you're hearing now is just the air being fed back into the intake when the BOV releases the built up pressure. You can get rid of that tube from the BOV that feeds back across the valve cover and into the intake if you want. You just block the steel plumb-back tube off with a section of rubber hose and a brass barb/bung. The holes on the side of the BOV are there to vent the air to atmosphere, so it might not be best to block them off. Unless, of course, you're intending for it to be a plumb-back valve that feeds into the intake again? The only issue here is that you're essentially feeding recycled hot air back through the intake and possibly losing a little bit of performance. One of the best things for your turbo is nice, cold air! Colder = denser = plenty of these >
Ok, well, I was given some quite specific turbo advice by welz in Germany. He said the BOV should feed back into the air intake, not just blow into the atmosphere. Was it something about the turbo not being subjected to such sudden changes in pressure, or something like that? I think he even said regulations don't allow blowing off into the air in a lot of places. Anyway, I quite like the noise it makes now; it's kind of like a bit of a dog bark when you let your foot off, not unpleasant at all, and not head-turning volume, either. It's a lot more like what I imagine a turbo should sound like. It would certainly make the installation a lot easier if it's not feeding back into the intake. Maybe I'll try that tomorrow.
The BOV you've purchased and the one that welz had linked earlier in the thread are 'Vent to Atmosphere' style valves, which aren't intended to be modified into being a 'Plumb-Back' BOV. This especially would apply to the valve that you've got, which has the side vents in it. A lot of the reason behind having plumb back BOVs fitted to factory vehicles is to meet noise and emissions requirements. Welz is right in that it's frowned upon to have a vent to atmosphere BOV, as it doesn't meet emissions requirements in some places. However, a lot of people just position the BOV in a way that the outlet faces down into the engine bay or hide the BOV completely. The benefit of a plumb back BOV (usually) is that they release the waste air back into the intake, which keeps the turbo spinning, so it spools back onto boost quicker. The detriment would be that it's hot air. HOWEVER, the Copen engine is *not* plumbed in a way that is conducive to a plumb back BOV being of any real benefit, due to the way in which the factory pipe routes the BOV outlet back to the intake. It's pointed in such a way that it essentially blows it back out through the intake and creates too much turbulence for the turbo to draw the air in as smoothly as it could. With a vent to atmosphere type BOV, you're releasing all that warm air and allowing your intake to bring in a fresh gulp of cooler, outside air. I'll draw up a diagram shortly to show you what I mean.
Here you go (attached) What you can see here in a plumb back configuration during a closed throttle condition, just before you move into the next gear, is that the air comes through the recirculation tube and is released into the intake in a direction that doesn't follow the direction of the intake air path, thus creating turbulence. What I think Welz might be concerned about is that, on some turbo vehicles with vent to atmosphere BOVs, it tricks the ECU into thinking there should be more air coming through the intake. This isn't the case with the Copen and therefore, isn't an issue.
It's illegal to vent to the atmosphere in Australia. In any case the amount of air that goes through the BOV is so small you'd never be able to tell the difference with performance anyway. When you think about it the BOV only operates in closed throttle conditions so the tiny amount of warm air recycling in means nothing. If you copen was making 500hp my opinion would be different.
That's true, but... How many people on the forum that have changed their wheels have ensured that they have the same rolling diameter as stock? How many people are using pod filters not enclosed in an airbox? I'd suggest that on a 500hp car, you'd see a lot less of a difference than on a low hp motor like these. They respond really well to any 'respiratory' improvements, as I've found from the last two I've owned.
Most tyre/wheel shops in OZ will not sell you a combination that is outside the 15% (I think) change in diameter that is legally allowed. You are right about the pod filters though. Remember the BOV only operates under closed throttle conditions, it has no effect on power when opperating correctly. On a highly stessed car pumping hot air into the intake may have an effect mainly with heat soak. The copen motor isn't stressed at all so would have basically no effect on performance at all.
Thanks to both of you for significantly increasing my knowledge of this component. I think I might now be somewhere in between, as the tape seems to have lifted a bit, so uncovering one or more of the side vent holes. The result is (to me) a rather pleasing but moderate sound that isn't objectionable at all, and sounds (to me) more like a BOV should sound. Also, my internal picture of the engine breathing process suggests that since the throttle is closed at the time of the valve opening, there is nowhere for the extra air to go in in any case. But what the "plumb back" does do is muffles the sound somewhat. What I can't do is picture what the pod filter would look like if it were encased in a "box" as suggested. There certainly isn't much room. One more question: What accounts for the hundreds of dollars difference between Welz' HKS Super SQV valve and my generic $15 one?
That filter is a totally different shape to the original, you can see the original airbox and filter on this video (I couldn't find any pictures and it's raining)
