Random blow-off valve question (MCM)

"Jake - Has Bad Luck So You Don't Have To" (murdersofa) 03/23/2016 at 12:24 • Filed to: None

0

13

On the latest episode of MCM, which is amazing by the way, they put a blow-off valve on the Saab so it makes more choo choo noises. Did it not have one before? How does a turbo car get away without having a BOV to purge excess boost after the throttle snaps shut?

Your boy, BJR > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 12:26

0

Something something Sweden, but no, no BOVs on Saabs. Waste gate maybe? Idk how turbos turbo.

bryan40oop > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 12:31

0

Vents intake pressure to atmosphere. Forced induction forces air. N/A engines use vacuum to suck in air.

OPPOsaurus WRX > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 12:31

2

i think they do but it goes back into the intake or something so you dont really hear it the same. I think it lowers the emissions by going back into the system

Jake - Has Bad Luck So You Don't Have To > OPPOsaurus WRX 03/23/2016 at 12:34

0

A compressor bypass valve? Possibly, but then I would think we’d have seen Moog removing it or putting the POV where the CBV was.

Rust and Dust - Oppositelock Forever > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 12:36

1

Open vs recirculating BOV. Still have fond memories of teenage me hose clamping a 2 litre bottle cap into my Eclipse GS-T’s intake tubing to turn mine into open air. So lucky I didn’t suck a bottlecap into my cylinder head...

Aaron M - MasoFiST > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 12:36

2

Most turbo cars have a bypass valve that reroutes into an intake, cars with closed-loop air metering (i.e. a MAF) require it because otherwise the loss of air after the meter messes up the fueling. A BOV is cheaper in speed-density systems but otherwise doesn’t do the job any better than a stock BPV.

It’s also worth noting that Corky Bell (one of the gurus of modern turbo tuning) says that while compressor surge sounds really gnarly, you generally don’t really need a BOV for reliability reasons.

Danger > OPPOsaurus WRX 03/23/2016 at 12:41

0

Agreed. Modern turbo cars have a recirc type system.

vicali > Your boy, BJR 03/23/2016 at 12:41

0

Niether did 2sxy’s previous owner..

Jake - Has Bad Luck So You Don't Have To > Aaron M - MasoFiST 03/23/2016 at 12:50

0

So a low pressure 5-10psi system would be fine without any kind of air-recirc valve or vent-to-atmosphere valve?

Aaron M - MasoFiST > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 13:03

1

The early Bell Engineering Miata kits and a lot of Crawford Performance builds run without one. The early Celica Alltracs also didn’t have one.

There’s a lot of debate, both about reliability and performance. That said, the best summary I have found is that a bypass valve increases turbo life at the expense of transient response (every shift boost goes to zero and you need to spool again). Off-throttle compressor surge won’t kill your turbo but it’s pretty sensible to assume that it’s harder on the turbo than normal driving. Beyond the throttle response, another reason that builders like Crawford don’t run them is because the vast majority of aftermarket BOV/BPVs leak, which can be worse than off-throttle surge if you’re building up an engine designed to run 25-30 psi of boost.

Your boy, BJR > vicali 03/23/2016 at 13:12

0

Huh?

Birddog > Jake - Has Bad Luck So You Don't Have To 03/23/2016 at 13:25

0

Turbo Fords from the 80s didn’t use a BOV or Bypass. They would just let the Turbo surge. It makes an interesting sound. I think it depends on how the incoming air is metered.

vicali > Your boy, BJR 03/23/2016 at 14:09

0

Marty’s Evo has all the turbo parts but none of it was plugged in or set up. No engine management for FI at all..

That’s the joke.