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Rob's Garage

Turbocharged Civic tuned for maximum impact Add to ...

Rob, the other day I was pulling away from a four-way stop and this little Honda Civic blew past me coming from the opposite direction. My window was down and as the Civic went by the side of my car, the driver shifted and the noise that came from his car scared the #@%& out of me. I almost drove into a curb!

I couldn't believe how loud it was. It sounded like a pop-whiz-hiss!

After the shift, the car was gone and I mean GONE! It just blew up the road and disappeared behind me.

What would have made such a noise and how is it that such a little car could have accelerated that fast?

Thank you, Steve

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The sound you heard was probably an unfiltered blowoff valve, or BOV. The fact that you mentioned the rate of acceleration makes this all the more plausible because the little Honda was probably equipped with a turbocharger, and these days, hot-rodders are pairing turbos with BOVs.

Steve, your observation of the noise made during a shift is dead on because what you heard was the BOV releasing, or "blowing off," intake manifold pressures to the atmosphere which is timed to occur during a shift using manifold "vacuum," or more accurately; manifold low pressures.

BOVs are placed in the intake ducting between the turbocharger and the throttle plate. During acceleration, high boost pressures are created in the intake manifold by the turbo.

Turbochargers are nothing more than an air compressor that force feeds an engine with a high pressure intake charge.

During a shift, the throttle plate closes, blocking the high pressure flow from the turbo. When the throttle closes, it acts like a dam against the air that's being pumped out of the compressor which creates a shock wave backwards to the turbo. If this shock wave is of a high enough magnitude, the resulting reversal of the air flow can damage the turbo fins and possibly bend or even shatter the turbo compressor shaft. For this reason BOVs are considered a safety feature.

Another problem with this shock wave is the resistance it causes to the turbo compressor. If resistance is felt, the compressor will slow down rapidly causing a "lag" when the time comes to supply more boost. It takes time to re-spin, or "spool up" the turbo and if the rotation has slowed too much, the engine will not make the power it was designed for. It could be argued that this creates potential problems in an emergency avoidance situation...

That said, many young hot-rodders install them simply because of the noise that comes out of the intake system during a shift - subjecting everyone outside their vehicle with your aptly described pop-whizz-hiss.

Type "BOV sounds" into the search bar at You Tube.

This noise is not only annoying, but is dangerous as you have experienced Steve. I have seen some for sale that have sound magnifying trumpets mounted to them. This sound is so loud and sudden that it can cause another driver to become startled and react by unintentionally twitching the steering wheel. I can say this because it has happened to me. But this does not have to be the case because some BOVs come with filters, or mufflers.

Don't get me wrong, I like cool as much (or more) than the next guy, but our cities are noisy enough - can we not slap a filter on our BOVs?


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