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Bov? Which Do You Perfer

sleek3

By sleek3,
in General Automotive Discussion

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Dani Boi Grand Master

Dani Boi

Posted
Posted
hmmm..

tell me more..

"Quote from Xspeed

"The 'flutter' noise reported by many excited enthusiasts may sound good but is actually very unhealthy for the turbo.

The 'flutter' occurs when, upon shutting the throttle, compressed air is caught between the free-spinning turbo and the closed throttle. The consequent back pressure forces the air back through the turbine blades. Thie process, more commonly known as a Compressor Surge or Cavitation, places enormous loads on the turbo and can lead to premature wear of the thrust bearings or, in extreme cases breakage of the compressor wheel. Another common side-effect of the compressor surge is the dreaded turbo lag between gear changes.

The compressor surge or 'flutter' can be fixed by either fitting a blow-off valve onto a system that hasnt got one, or making sure the blow-off valve is matched to the output of the turbo and is not working outside its flow capacity. It is also important to ensure that the blow-off valve is set up correctly."

But yeah you get people saying its not true then you get people saying it is, I find it pretty reasonable tbh.

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TTT Grand Master

TTT

Posted
Posted

I agree that having the stock bov on a car that came with it makes the car behave as intended.

but this theory above.

explain 300,000km+ stock engine stock turbo's VL turbo's.

Cordia GSR's

Nissan EXA or Pulsar ET turbos?

R31 GTST's

Cefiros

DR30 skylines with FJ20T's

they don't have broken turbo's etc.

millions..

that is the number of threads on all this.

it gets no-where.

Best thing to do is agree that I'm right about everything I say and go back to making fun of BTAK hunduhs

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Kinks Grand Master

Kinks

Posted
Posted

^^ remember that one aspect of long-lasting turbo engines with the standard BOV is that everything else is usually standard as well. People only break things when they under-maintain a car or start making modifications.

That said, I bought a car with an 89 RB20DET in it from a guy who was running 14psi on the standard turbo with no BOV. I continued to run it that way for 2.5 years, then changed the turbo for a later model one because I felt like it. When the old turbo came off it was still in great nick, and I continued to run the new turbo without a BOV and had no problems. Who has actually broken a turbo and can attribute it DIRECTLY to not having a BOV? Anyone? Didn't think so.

"Quote from Xspeed

"<rubbish>

All completely false.

in Part 2 of our interview, we speak to a leading industry expert Simon Gishus about Nissan engines, Holden Gen 3 V8s, and directions for modifications...

What are some areas where people often make errors modifying their turbo car?

"The classic blow-off valve.

"The blow-off valve is designed as an emissions control device for OE manufacturers. It came about when smaller engines made more and more power using larger turbochargers and bigger intercoolers. As you close the throttle, the build up of pressure and the larger volume inside the intake has to go somewhere; it can't go into the engine because the throttle is shut. Instead, it has to do a U-turn and it comes screaming out the airflow meter. That creates the 'gobble-goggle' sound.

"The gobble-gobble sound is something the public has grown to love.

"The airflow meter is not all that smart and does not realise the air is going in the wrong direction; it therefore measures the air twice (once going into the engine and again going out in the wrong direction). The computer now tips in twice as much fuel as what's required, making it run rich - making it not pass emissions.

"Therefore, manufacturers fit a blow-off valve - or a recirculation valve as they are actually called. A recirculation valve opens when it senses manifold vacuum, returning the air trapped at the throttle body to between the airflow meter and the turbocharger. As such, the airflow meter does not take a double reading - the car now passes emissions.

"Unfortunately, we've had people ringing up and wanting the "audible gear change alarm".

""What audible gear change alarm?" we ask. "You know, when the Sierras were running around and just when they went to change gear it used to go whoda-whoda-whoda" they tell us.

""No pal, that is the dump valve..."

"Some people do think that at the absolute upper extremes of boost levels - about 30-plus pounds - the blow-off valve does, somewhat, save the compressor wheel and shaft from trying to rotate backwards. It doesn't actually rotate backwards at all - all you're hearing is cavitation. What happens is, you've shut the throttle, the turbocharger is doing 100,000 rpm and now has a boost spike of 50 psi. Because it's working in a higher region than what it's designed for, it slips; it basically does a skid like a car tyre does when you dump the clutch. That's the noise you hear - the whoof-whoof-whoof is the air doing a skid."

Is there any performance gain to a blow-off valve?

"We've tested one on a manual gearbox performance car run at Winton Raceway. I think it was running 1 minute 40s back then, but it would lose 2 seconds a lap putting the gobble-gobble valve on. When you look at data acquisition, what you find is - as you change gear - the blow-off valve dumps all the pressure built up through the intercooler and pipes. It then goes back to zero manifold vacuum when you get back on the throttle, you have to build all that boost back up.

"The fact that people think that they keep the turbo spinning is a problem. The people that suggest this have never had an engine on the dyno and never had a turbo tacho in their hands. What people don't realise is, when you shut off the throttle, you shut off the air supply to the engine - this shuts off the exhaust gasses coming out of the engine. When there is no exhaust flow, there is no energy to keep the turbine spinning - the turbo slows down at an alarming rate.

"If you change gears at quite a good speed, you can actually get a boost spike on changes; if you're trying to hold a constant 30 pounds, when you do a racing change you'll get 32-33 pounds when you crack the throttle open again. If everything's working well, you've got a full head of stream waiting to go into the throttle as soon as it's opened.

"I've done this on a rally car and it was quicker through every timed section without a dump valve. You'll never hear a World Rally Car going pssshhht because they don't use a dump valve - you get the woof-woof-woof noise instead."

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floody Experienced

floody

Posted
Posted (edited)
hey Floody I hope your ready for a turbo rebuild if you run no BOV at all... ;)

Well then... Having had in my possession two 20 year old RB20DET ceramic turbos which had never been used with a BOV, I can fairly safely say unless Nissan turbos are something bloody special, the turbo rebuild thing is a myth. One was a spare and one was removed to make way for a new turbo, the one I removed had been running around 1 bar from 1996 to 2005. Neither showed any damage to the compressor or abnormal bearing wear, and in fact were both then used in other cars and as far as I know are still going over 2 years later.

Nissan in particular introduced BOVs onto their cars as a measure to reduce double metering of air and rich fuel spikes, and to quiten down intake noise, both aimed at new emissions control regulations which came into force around the introduction of the R32, S13 etc.

If I was running 30psi of boost in a race situation? Maybe there might be something in it. Then again turbo longevity wasn't a major issue for the 300rwkw RB20s of Gibson Motorsport, or big horsepower FJ20ETs, which didn't run any form of BOV or compressor bypass valve.

In short my feeling is that the only people who will constantly use the turbo lifespan argument are forum wankers who need a noisy BOV to make up for their lack of social skills and intelligence.

And as for the "but the cars are mostly standard that those old turbos are on" argument, that too is bullshit as the cars which most BOVs are on are mostly standard too.

Also, as noted in the text GTST quoted, generally turbo cars with no BOV are quicker around a track, so the throttle response thing (which I have noted personally to be false) mustn't be too well supported.

In short they're a wank, unless standard fitment.

Edited by floody
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