Why does OEM BPV make noise after CAI or SRI installation?

[nickows]

My first ever mod to my car was the Mazdaspeed CAI, and I've always wondered why you could hear the BOV after this modification. In a stock car does the BPV even release any air? If so is the amount so small that we don't hear it?

Do you hear the BPV after a CAI/SRI install simply because there is more air being sucked in compared to the standard air box, therefore when you come off the throttle that excess air has to go somewhere? (ie released through BPV back into the intake) If that is the case wouldn't a CAI/SRI change the fuel/air ratio if it was sucking more air?

If anyone could explain that would be great

[DISIMS3]

We don't here it because the stock airbox is designed to reduce / muffle induction noise. Also the stock BPV is plastic and leaks under high pressure so it doesn't make a great deal of noise to start with. With regard to A/F ratios I can only suggest that the ECU adjusts this accordingly once it's recognises the upgrade?

[Doug_MPS6]

NICKOWS, Hi. A CAI or SRI does not provide the same sound deadening qualities as a stock air-filter, which, along with the resonator, is deliberately designed to hush up induction.

If you listen to a stock engine, and know what you are listening for, you can hear the BOV working, as it must with all turbo engines if you are to avoid the one issue it is put in place to deal with and that is compressor stall on throttle closure.

With either of the new fitments, you will not only get the induction hiss that you never heard with a stock filter, but also the sound of the BOV letting go to bypass as intended. And if you listen carefully, you can sometimes also hear your turbo responding to the increased bypass pressure.

You will not be getting so much more air with a CAI or SRI that your BOV/turbo setup is going to have any problem handling it on throttle closure, with respect to the stock setup.

Once you have driven with a more open system, your ears will become attuned to all the little sounds related to the induction system, such that if you revert to stock, you will still be far more aware of them than you are now.

BTW, the A/F ratio is dealt with by the MAF which is measuring the air temp and volume after the SRI/or CAI. It would only be an issue if you had a BOV which vents to atmosphere (VTA) and this means that your ECU would be metering fuel for air the engine never gets - and hence produces a rich mixture and consequent backfires*. Not a problem in your case with your 100% bypass BOV, as the air volume in the system remains the same throttle on to throttle off.

More BTW, the stock BOV is often said to be set to "leak" at around 15psi as a protective measure. This might be more apocryphal than reality, because under boost the top and bottom sides of the piston are at equal pressure when the throttle body valve is open, so there is no pressure exerted on the piston to open it under normal boost range. Other BOV's without the top line (off the intake manifold - post throttle plate) may open under excess pressure. There is a lot of S$#^ talked about BOV's leaking and this gives rise to a lot more C*&% about how screwing down a BOV with an adjustable piston will increase boost. It's tripe. All that will do is make it harder for the BOV to dump air and eliminate the compressor stall you might be experiencing. But if you have it set too light you might VTA too much and induce the enrichment I referred to above*, and incidentally wash your cylinder walls with raw fuel and mess up your cat.

Cheers

[pHiLix]

What's the difference between a BPV and BOV?

Sorry a bit off topic..

[Doug_MPS6]

PHILIX, none. I guess BPV is someones interpretation of "Bypass Valve" which in reality is a BOV that is set to Bypass 100% or some lesser amount according to how it can be adjusted.

The stock Mazda valve is a 100% bypass valve with no option to vent any to atmosphere. On the other hand, a Turbosmart BOV, for example, allows either 100% Bypass, OR 100% VTA, OR nominally 50:50 to each.

The usual distinctions are a true Dump Valve and a BOV (Blow Off Valve). A dump valve as its name implies is intended to dump excess boost. It is a less sophisticated engine management device than a BOV in that it is not dealing with eliminating compressor stall.

It is more typically used in engines that do not have to endure frequent full throttle closures (which would induce compressor stall), such as in aircraft. Often they would be adjustable so that additional boost could be dialled in as the aircraft climbed, so as to compensate for reduced air pressure (and hence loss of power). Full throttle altitude in a naturally aspirated aircraft engine is around 7,000 - 8,000'. This means that above this height, any further opening of the throttle is not accompanied by any increase in Manifold pressure. Above this height, turbo or supercharging is required to retain power.

Some DV's were electrically controlled from the cockpit, until aviation regulations mandated that engine control be isolated from the rest of the aircraft electrical systems. They then became manually controlled by a screw thread and cable or some similar setup, or by barometric feedback systems.

[kmh001]

What's the difference between a BPV and BOV?

Sorry a bit off topic..

The difference is largely in the name. Just like Mazda refers to the ECU as a PCM.

Just adding to Doug's point about the stock BPV, mine has 106,000km and doesn't leak.

Although it used to leak. When we pressure tested it, we found it was leaking, which we discovered was caused by someone forgetting to install the rubber gasket. Despite the absence of the gasket and the resulting leak the car never struggled to hold boost. But with a new gasket installed there's no leak.

[Nuliaj]

A BOV is designed to let air out of the system, a BPV is designed to have the air go back into the system - in our cars it goes back into the intake post MAF sensor. A BPV becomes a BOV if you stop the air going back into the system.

[pHiLix]

Some helpful posts there guys. Thanks for the help

[nickows]

Thanks everyone for the explanations; very much appreciated. So in summary: the addition of a CAI/SRI, with its reduced sound-deadening and greater air flow ability compared to the OEM airbox, makes the BOV louder when boost is dumped, because there is greater air flow (or more air to get rid of when you close the throttle)?

I must admit I've never heard the OEM BPV in my Dad's MPS, and his car is completely stock. However I can hear induction noise.

[Nuliaj]

No I don't think rate of airflow is significantly larger with an aftermarket intake. The plastic and rubber tubing plus the air box insulate sound in the stock setup. The metallic CAI and filter do not insulate sound.