Re: [DML] Re: Turbo PRV.

["Joseph Kuchan" <josephkuchan@xxxxxxxxxxx>]

Martin has the most correct explanation of the function and purpose of a 
BOV. It *is* a safety device, but it provides safety primarily for the turbo 
components in a blow-through installation where the throttle plates can snap 
shut and present a sudden and formidable barrier to the air flow from the 
turbo compressor. That air wants to go somewhere, and assuming there is no 
BOV when it can't get past the throttle it circulates around in the 
compressor housing, and can even back up through the air inlet tract. That 
actually imparts a shock to the turbo components and shortens their life. It 
also slows down the compressor considerably which leads to the performance 
reason for installing a BOV: to prevent the turbo from slowing considerably 
during shifts, which in turn requires  more of a spool-up after the shift. 
(Turbo lag on every shift.) Martin is also correct that for performance 
reasons it is better to recirculate that air through the compressor than to 
woosh it out to atmosphere.

Martin is correct that in a suck through design such as the BAE, no BOV is 
necessary. (Or desireable)

David is correct that a BOV is desireable in a blow-through design.  It is 
not a motor safety device per se. The reason it is normally adjusted to blow 
off at higher than the boost setting of the wastegate is because you 
normally don't want that thing venting your precious boost to atmosphere, so 
you set it where the only time it does so is when pressures rise suddenly - 
and potentially harmfully - as happens when those throttles snap shut.

Overboost is most often due to a wastegate that is sticky, defective, or has 
too small a vent for the exhaust gases, that is, was improperly sized for 
the turbo.

A well-designed turbo installation should probably have some additional 
safety device other than the wastegate. This might include fuel and/or 
ignition shutdown devices should boost rise too high.
(Maybe there's another "Gullwing" article brewing here!)

-Joe Kuchan


>Hi David
>
>I'm afraid you're barking up the wrong tree here. Either that, or I've
>misunderstood how my engine works :-)

>Martin





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