Subaru "boost tube"

Brian...on the subject of BOV's.


I was running an Atmospheric valve previously, and the car ran fine, the issue however was that in the rallying environment, the dust and dirt would sieze the thing up in very short order, and i had to clean it much to regularly.

I went with a re-circ valve to keep the thing clean, and keep junk outta it.



Why we put it so close to the turbo....

1. Packaging
2. we wanted to take the load off the turbo, but also keep the "boost tube" full.



There wasn't really a science to the size of the boost tube, length etc. It was based purely on packaging constraints. I think total length of the 4" section is like 2 feet. As i said, very similar set-up and lengths as the WRC subarus, VTSC cars, etc.

---

Chris

Do It Sidewayz Wrote:
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> I was running an Atmospheric valve previously, and
> the car ran fine, the issue however was that in
> the rallying environment, the dust and dirt would
> sieze the thing up in very short order, and i had
> to clean it much to regularly.
>
> I went with a re-circ valve to keep the thing
> clean, and keep junk outta it.

Good point. I had given some thought to the BOV being located where it is on the Galant - just ahead of the RF tire within the engine bay. It's pretty low in there, so I started thinking about pointing the outlet downward so debris would be less likely to collect, but I'm sure that would expose it to more filth, so recirc seems to be the smart bet.


Do It Sidewayz Wrote:
-------------------------------------------------------
> Why we put it so close to the turbo....
>
> 1. Packaging
> 2. we wanted to take the load off the turbo, but
> also keep the "boost tube" full.

Good points. Unless venting to atmosphere, the BOV doesn't release the excess pressure in the system so much as it re-routes some of it back through the compressor wheel (rather than back against it).


Do It Sidewayz Wrote:
-------------------------------------------------------
> There wasn't really a science to the size of the
> boost tube, length etc. It was based purely on
> packaging constraints. I think total length of
> the 4" section is like 2 feet. As i said, very
> similar set-up and lengths as the WRC subarus,
> VTSC cars, etc.

Appreciate the input. I figure I'll be able run about 2ft of 4" piping between the BOV and TB on the Galant.

Thanks, Chris. Appreciate the feedback.

---

Brian Driggs | KG7KCA | PHX, AZ | 89 Pajero
alterius non sit qui suus esse potest

Pete Wrote:
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> How huge is the spike? (Again, genuine question.
> I can't see it being more than a couple PSI, if
> any)
>

I happen to have some old data from a blow-off valve test SCC did years ago. We only ran at 9 psi in a misguided attempt to eliminate variables introduced by a boost controller. The control run, with no blow-off valve, only spiked to 13 psi. Keep in mind, though, that our data sample rate was pretty low, so there could well have been a very short spike to something higher.

This is part of your answer why bypass valves weren't as critical on old, low-boost turbo systems. The other part of the answer is turbo size. A bigger compressor wheel is more surface area for that spike to act on.

Some radical examples for illustration:

A Turbonetics T72 trim compressor wheel has a 4.03" exducer diameter. That's 12.5 in^2 of surface area. Run 30 psi boost on that, and spike to 40 during surge, and the thrust bearing will see 500 pounds of thrust load (40 * 12.5).

By contrast, a K03 turbo from a 1.8T Volkswagen, which I'll just use as an easily googleable reference for a small turbo, has a 1.81" exducer diameter. So that's 2.6 in^2 of surface area. Run 9 psi, with a 12 psi spike, and you only have 30 pounds of thrust load. BIIIIG difference.

But wait, you say, if the thrust bearing can handle the constant 30 psi, can't it hanlde the spike to 40? The difference there is that at 30 psi, the exhaust is typically running even more than 30 psi of backpressure (or, best case, damn near the same). That puts a thrust load in the other direction. There are a lot of variables, like how much backpressure, and how big the turbine wheel actually is, but the net effect is very little, or even negative thrust load (ie, thrust load on the other thrust bearing) during normal operation, with a sudden series of 500 pound spikes (at 80,000 rpm) when you hit surge.


Even in this worst case situation, thrust bearing failure is a durability concern, not an immediate failure concern, so you will see examples of applications that seem to get away with silliness, but are are actually less durable than more conventional setups.

-Dave

Do It Sidewayz Wrote:
-------------------------------------------------------
> Brian...on the subject of BOV's.
>
>
> I was running an Atmospheric valve previously, and
> the car ran fine, the issue however was that in
> the rallying environment, the dust and dirt would
> sieze the thing up in very short order, and i had
> to clean it much to regularly.

If dust and dirt was getting into the valve, it was getting into the engine. Typically there will be brief moments when the engine starts sucking while the valve is still trying to close. Dirt ingestion during this period is a good reason to at least put a filter on the BOV, if not to recirculate.

Recirculating is not actually advantageous on systems without a MAF. That recirculated air is hotter than fresh air, so you're better off dumping it. The boost is already wasted. Its back to atmospheric once its dumped into the intake.

This is not a big enough deal to justify changing your setup at all, though, just something to think about for next time.

> Why we put it so close to the turbo....
>
> 1. Packaging
> 2. we wanted to take the load off the turbo, but
> also keep the "boost tube" full.

Another thing to consider when using a BOV on the hot side of the intercooler is that most BOVs are not designed to be used there, and may not have diaphragms that are up to the increased heat. Some BOVs will have markedly shorter lifespans when used on the hot side.

Again, not a big enough deal, I think, to justify moving it, but definitely big enough to justify putting a spare in the service truck.

-Dave

eyesoreracing Wrote:
> Some radical examples for illustration:
>
> A Turbonetics T72 trim compressor wheel has a
> 4.03" exducer diameter. That's 12.5 in^2 of
> surface area. Run 30 psi boost on that, and spike
> to 40 during surge, and the thrust bearing will
> see 500 pounds of thrust load (40 * 12.5).
>
> By contrast, a K03 turbo from a 1.8T Volkswagen,
> which I'll just use as an easily googleable
> reference for a small turbo, has a 1.81" exducer
> diameter. So that's 2.6 in^2 of surface area. Run
> 9 psi, with a 12 psi spike, and you only have 30
> pounds of thrust load. BIIIIG difference.

Thanks for the numbers... that's just the sort of info I was looking for.

---

Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

That's the first time I had ever looked at the numbers too. I hang out with lots of real turbo engineers, but i'm not one myself. I get lots of knowledge via osmosis, but digging up the actual numbers is pretty interesting.

-Dave

Pete Wrote:
-------------------------------------------------------
> eyesoreracing Wrote:
> >
> > By contrast, a K03 turbo from a 1.8T
> Volkswagen,
> > which I'll just use as an easily googleable
> > reference for a small turbo, has a 1.81"
> exducer
> > diameter. So that's 2.6 in^2 of surface area.
> Run
> > 9 psi, with a 12 psi spike, and you only have
> 30
> > pounds of thrust load. BIIIIG difference.
>
> Thanks for the numbers... that's just the sort of
> info I was looking for.

I call BS. The VW turbo is so small nobody has actually ever seen one so how could they measure anything!!!???
>
>
>
> Pete Remner
> Cleveland, Ohio
>
> 1984 RX-7 (rallycross thing)
> 1978

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

I see them all the time on Passats and A4s. Pull off two or three engine covers, curse at the airbox for ten minutes trying to remove it, and there it is!

I have heard that they were put in Golf/Jetta cars and I've seen "1.8 Turbo" badges on 'em but even after you pull off the Shroud of Wolfsburg you still can't see it.

Funny thing, those are always the ones with the fancy expensive recirc valves that go "PSHOOOO" every time you lift.

---

Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Pete Wrote:
-------------------------------------------------------
> I see them all the time on Passats and A4s. Pull
> off two or three engine covers, curse at the
> airbox for ten minutes trying to remove it, and
> there it is!
>
> I have heard that they were put in Golf/Jetta cars
> and I've seen "1.8 Turbo" badges on 'em but even
> after you pull off the Shroud of Wolfsburg you
> still can't see it.
>
> Funny thing, those are always the ones with the
> fancy expensive recirc valves that go "PSHOOOO"
> every time you lift.


You use those magnifying goggles to see them?
There was one on the table one time but before I could see some
body opened the door and the breeze blew it away!
They told me its smaller than a dime.>
> Pete Remner
> Cleveland, Ohio
>
> 1984 RX-7 (rallycross thing)
> 1978

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.