Subaru "boost tube"

I was just standing in the garage staring at my engine and was thinking about this boost tube idea.

My street car background has me apprehensive about disabling the BOV on the car for fear of compressor wheel damage, but would it really be so risky on a rally car? Couldn't this present a risk of damaging the oil seals in the CHRA on the turbo over time? Would the increased volume of the intake plumbing mitigate this risk?

I'm also wondering how one might determine the size (diameter and length) of such an implementation. Within the next few weeks, I'll be having hard piping made for my front mount intercooler installation and was considering trying this out. The Galant isn't likely to spend too much time on the street, so the off-the-line lag isn't really a concern.

I wonder if I might fit a barb to the intake pipe just before the throttle body for a secondary boost gauge to monitor pressure between shifts or something in the absence of a functioning BOV.

Just curious if anyone had any ideas on this.

---

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

You know, if your car was normally aspirated and rear wheel drive, you wouldn't even be worrying about this.

DR1665 Wrote:
-------------------------------------------------------
> I was just standing in the garage staring at my
> engine and was thinking about this boost tube
> idea.
>
> My street car background has me apprehensive about
> disabling the BOV on the car for fear of
> compressor wheel damage, but would it really be so
> risky on a rally car? Couldn't this present a
> risk of damaging the oil seals in the CHRA on the
> turbo over time? Would the increased volume of
> the intake plumbing mitigate this risk?
>
> I'm also wondering how one might determine the
> size (diameter and length) of such an
> implementation. Within the next few weeks, I'll
> be having hard piping made for my front mount
> intercooler installation and was considering
> trying this out. The Galant isn't likely to spend
> too much time on the street, so the off-the-line
> lag isn't really a concern.
>
> I wonder if I might fit a barb to the intake pipe
> just before the throttle body for a secondary
> boost gauge to monitor pressure between shifts or
> something in the absence of a functioning BOV.
>
> Just curious if anyone had any ideas on this.
>
> Brian DR1665 | Phoenix, AZ
> 92 GVR4 - daily | 91 GVR4 - endless rally project

---

"Talk about drugs. Driving a car like that, going that fast, it’s like all the drugs at once." - Tommy Byrne

"Now, Pinky, if by any chance you are captured during this mission, remember you are Gunther Heindriksen from Appenzell. You moved to Grindelwald to drive the cog train to Murren. Can you repeat that?" - The Brain



Edited 1 time(s). Last edit at 02/20/2010 04:24PM by wvonkessler.

True, but if everyone here just left good enough alone, stayed on the pavement, and within the speed limit, we wouldn't have to worry about much of anything, now, would we?



I'm not even slightly worried about this. It just seemed that, if one was in the process of whipping up some interfooler piping, one might mess around with this idea if one was so inclined. It might be beneficial said one to obtain information from others more mathematically inclined to minimize the risk of wasting half an hour and $10 worth of pipe.

wvonkessler Wrote:
-------------------------------------------------------
> You know, if your car was normally aspirated and
> rear wheel drive, you wouldn't even be worrying
> about this.
>
> DR1665 Wrote:
> --------------------------------------------------
> -----
> > I was just standing in the garage staring at
> my
> > engine and was thinking about this boost
> tube
> > idea.

---

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

DR1665 Wrote:

> My street car background has me apprehensive about
> disabling the BOV on the car for fear of
> compressor wheel damage

You mean, running with no blowoff valve or recirculation valve, like every turbo Ford 2.3 or Subaru 1.8 or (gack) Isuzu 2.0?

They only made several bazillion of them.

Oh, I don't think the engine I grabbed from an old Audi 5000 had one, either.

---

Pete Remner
Cleveland, Ohio

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

In the end we did do this.

What i can tell you is it did make a big difference. Throttle response is insane. And spool-up is much better as well.

The car does not run anti-lag and we are running a re-circ BOV.

---

Chris

Chris, you're saying u did run the 'boost' tube, just with no antilag? I was considering the inverse: no boost tube, but no recirc valve either. (like Pete just said....how come no one does that with a wrx or sti?)

I'm getting re-tuned next week and was considering bringing this up with my tuner.


To Wilson: if my car was NA RWD and I didn't have to worry about anything, i'd get bored and go do something else (harder)

---

Jon Burke - KI6LSW
Blog: http://psgrallywrx.blogspot.com/

We ran a boost tube.

I have fun a custom FMIC for some time, which resides behind the grill on my GC.

We fabricated a 4" boost tube which runs in very similar fashion to the WRC car one.

We put the REcirc valve, about 6 inches from the turbo outlet. IN actual fact we used a Mitsu Part, as they hold boost really well.


Remember that it is not all about just stopping compressor stall....welll ok it is. But think about it....you let off the gas, you stall the compressor, you then have to start the thing spinning again. If there is no load on the Turbo, and it can just free wheel....when you get back on the gas it just picks up and goes.

Our theory is that the "boost tube" also remains a reservior of air.

---

Chris

Now we're talkin.

Chris, any reason why you put the 1G BOV right next to the turbo? Just to keep the recirc route back to the impeller short? You've mentioned a 4" diameter to yer boost tube, but what length? How did you decide on the dimensions you used? I'm curious.

I'm ditching the MAS for a speed/density setup on the Galant, but I was still planning on recirculating the BOV. I *could* vent to atmosphere like all the coolest guys in the shiny wheels, flatbill crowd, but why waste boost?

Curious to get a little theory in my mind before I whip this out.

---

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

A question I have...

How much energy does it take to spin a turbo with the outlet blocked, vs. how much energy does it take to spin a turbo when it can flow a small amount of air?

Bear in mind that a turbo is not a positive displacement device, blocking the outlet doesn't make it stop. (If it did, how come the turbo chatters and giggles for seconds after you snap the throttle shut? Must still be spinning at a high rate)

It's a genuine question, I've never heard of the testing being done.

I personally don't see how it could damage the turbo, though, it's not like an intake backfire where the airflow IS trying to move backwards.

---

Pete Remner
Cleveland, Ohio

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

Pete, I always took the chatter of the turbo in those circumstances to be a rapid increase in pressure. When the TB snaps shut, the volume of the system being pressurized radically shrinks. Now the compressor isn't working to maintain pressure in a system which is also consuming it.

My math is probably ridiculously off, here, but suppose a turbo is pumping 700cfm of air into the intake pipe, intercooler, and intake manifold. That's something like 12cfs(econd). I don't know how much volume there is in the average charge system, but from WOT to NADA that quickly probably means a boost spike or something.

This is part of my previous, pre-NA/RWD post question.

1. Maybe defeat the OEM BOV (lose the vac signal) on a stock system?
2. Might a reducer on the BOV outlet slow the bypass to preserve charge volume?
3. Does the increased volume of the boost tube mitigate the risks of turbo damage because the turbo would still "see" a larger volume to pressurize?

It's just something I'd like to play around with while I'm having piping done, personally.

---

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

I don't understand how a turbo that was generating, let's say 20psi of pressure, and was getting all of the torque to do this from a shaft driven by the exhaust wheel on the other side, is going to suffer damage when the throttle plate shuts, cutting off the drive to the exhaust wheel, so there's nothing trying to spin the turbo but inertia.

Intake backfires DO kill turbos because the exhaust is still driving the turbo. Same for flutter under load because the turbo is being actively driven. But off throttle?

---

Pete Remner
Cleveland, Ohio

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

Pete,

The damage caused by compressor surge is typically to the thrust bearing. When that huge spike in charge pressure pushes back on the compressor wheel, its acting on the full surface area of the wheel. Conventional sleeve-bearing thrust bearings are pretty weak and will eventually fail if repeatedly subjected to this kind of abuse. Ball bearing turbos, though, have virtually indestructible thrust bearings (ie. they are no longer the weakest link), so you can get away with shenanigans like anti-lag, boost tubes, and such without guaranteeing turbo failure.

-Dave

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

Assuming that the main thrust loads are turbine -> compressor, then I can see how a rapid reversal could affect things. But, again, how much loading IS on the compressor, when it's not flowing any air?

OTOH, it seems like the main cause of turbo failure on the early non-blowoff vehicles was due to coking because they didn't have water cooling.

---

Pete Remner
Cleveland, Ohio

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

The spike is going to be dependant on a number of things.

The volume of air post turbo (how big is your i/c piping, how big is the i/c core etc)

and the amount of boost.



Pete...remember that most of the cars you listed ran pretty mundane amounts of boost, and had no I/C.

In the case of the early Subaru's, they ran 7 PSI stock, and had all of about 8 inches of piping from the turbo outlet to the throttle body...so not alot of volume there.

I think you will find that when manufacturers started putting in I/C's and more piping then Re-circ valves were pretty standard.

---

Chris

Do It Sidewayz Wrote:
-------------------------------------------------------
> The spike is going to be dependant on a number of
> things.
>
> The volume of air post turbo (how big is your i/c
> piping, how big is the i/c core etc)
>
> and the amount of boost.
>
>
>
> Pete...remember that most of the cars you listed
> ran pretty mundane amounts of boost, and had no
> I/C.
>
> In the case of the early Subaru's, they ran 7 PSI
> stock, and had all of about 8 inches of piping
> from the turbo outlet to the throttle body...so
> not alot of volume there.
>
> I think you will find that when manufacturers
> started putting in I/C's and more piping then
> Re-circ valves were pretty standard.
>

And if we all remember, that much MUCH larger volume of air inside all that piping and the intercooler is still compressible, its not like the air is concrete or something like that....
>
>
> Chris

---

John Vanlandingham
Sleezattle, WA, USA

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