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Alan L's Avatar
 
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boost bleed off

My boost is pegging at about 0.7 bar at about 3000 rpm. As I push it to above 4000 rpm, the boost drops to between 0.5-0.6. (I suppose it is better this than boost creep?)
Internally the wastegate looks good (now), nice new valve face and seat.
Turbo?
Thanks
Alan
Old 12-16-2007, 10:24 PM
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What kind of exhaust? If wastegate has straight dump pipe and turbocharger restrictive damper, there will be some difference in boost produced.

Other from that, tapering boost is quite common for simple wastegate-regulated circuit w/o boost controller. It's more pronounced on turbochargers with internal wastegate though. Only way to keep boost at constant level is by fitting EBC or Dawe's device.

On the other hand, tapering boost isn't such a bad thing as it prevents turbo from overspining. VE is usually best around mid-range and air mass flow is moderate. As engine climbs up the revs VE starts to decrease and mass flows increase which "chokes" the engine and decreases the boost.
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Last edited by beepbeep; 12-17-2007 at 03:14 AM..
Old 12-17-2007, 12:39 AM
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My motor with stock '78 RoW headers, open dump pipe, open "muffler", factory wastegate and 3DLZ would momentarily swing past 13, 13.5 PSI on the VDO mech gauge and then regulate perfectly at 12.5 PSI exactly.

With GHLs, (another, without melted studs) factory wastegate and a K27/HF and a factory muffler (but still an open dump pipe), it does precisely the same thing.

Transient (1/4 of a second, tops) overswing and slight (1 PSI) overboost, followed by rock-solid boost until I get scared/need to watch the road/redline. It might even simply be gauge overswing in my case.


Alan, glad you're driving the car!

Note that boost level is dependant on load as well.

In other words, you will see the boost drop as load drops off - like if the throttle isn't flat on the floor - e.g. if you kick it to 1/3 or 1/2 throttle, watch the boost build and then don't give it any more throttle, the boost will drop off as the RPM's rise. Completely normal.

I'd suggest that to be sure you might want an accurate gauge and a full-throttle 2nd gear run up to 6,000 or so - ideally up a steep hill, preferably with someone else watching the boost.

These cars do change your idea of what a "long, straight stretch of road" actually is...
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Old 12-17-2007, 03:06 AM
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I agree, that the 1/2 mile dead end straight rural road outside my house has suddenly got a lot shorter. I have chosen to ignore the faint internal engine noise at the moment until something more telling surfaces, or I need another reason to pull the engine. Last night was the first glitch free run in the car - no coughs, farts or burbs, just straight line pull. But this boost behaviour is consistent, repeatable, with the pedal floored. I wonder about a boost leak. It did have an EBC, and when turned off, in the one run prior to working on the car - it seemed to peg at 0.8. But I never pushed it into high revs.
What is a Dawes?
Yes, it does have an unusual throttle response characteristic around 0 boost range. Fun tho.
Pic of W/gate, headers attached.
Alan
Old 12-17-2007, 10:06 AM
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Also, you might have "internal hemmorage" somewhere in the intake tract. If boost is leaking out (or back into intake trough faulty BOV), turbocharger will spin faster to compensate for leak but as revs build it will fall short of oomph and boost will start to decrease.

This is boost graph for 3DLZ vs. K7200 on stock ROW headers (stock is pink):
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Old 12-17-2007, 10:35 AM
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Quote:
Originally Posted by Alan L View Post
I have chosen to ignore the faint internal engine noise at the moment
Hmmm. These engines do sound like a bucket of hammers sometimes with the decklid up... But...

Quote:
But this boost behaviour is consistent, repeatable, with the pedal floored. I wonder about a boost leak.
OK, that doesn't sound like load is a factor then.

Like Goran says, the factory BOV can leak boost if the seals are worn, makes a loud clicking noise, IIRC. There was a good thread on it quite recently.

Quote:
What is a Dawes?
Proprietary type of manual boost controller.

It may be that the PO fitted an EBC to band-aid another problem - like a boost leak somewhere...

You re-worked the wastegate, so it's probably not that.

My take (and Goran surely knows more than me here) is that boost controllers work well - especially at higher boost levels than commonly used on 930's - by completely isolating the wastegate until boost is at the desired level it is to be regulated at. They thus prevent premature opening of the wastegate and the (very) undesirable early boost bleed off.

You can't run a very soft spring in the wastegate (and do it all in the MBC or EBC), because you still need to control the valve itself, mechanically. Think I read somewhere that .5 or .6 bar (or was that 5 or 6 PSI?) is the minimum advisable lower limit.

And changing springs is a pain, so having the capability to control the gate - even for an extra .1 or .2 bar of boost - without stripping the wastegate out could be a win. You could drop boost if you bought some lousy gas, for example.

I thought about trying an MBC and decided not to. With my C/R, I can't run significant extra boost on pump gas, and I'm really not convinced my wastegate has any leaking issues, so the only possible advantage (that of preventing the leaking of boost prematurely below the cut-off threshold) probably wouldn't be noticable/measurable.

For the sake of $60, I might try one anyway (set to the stock boost level) at some point to see if it improves the area just before boost regulation starts, but I'd also prefer as little stuff on the car to go wrong as possible.

Quote:
Yes, it does have an unusual throttle response characteristic around 0 boost range. Fun tho.
Ahh yes, the old rubber-band-to-the-horizon-with-1mm-of-throttle feeling when the 3DLZ finally gets it together...

I do miss that a little sometimes.

Not so much now that it's wet, slimy and icy here, mind you. I'm tip-toeing about with the throttle welded shut, and if I'm getting RPMs there's very limited boost involved. Every dollar I spent on that asymmetric-lockup LSD was worth it three times over.
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Old 12-17-2007, 11:41 AM
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Quote:
Originally Posted by spuggy View Post

Like Goran says, the factory BOV can leak boost if the seals are worn, makes a loud clicking noise, IIRC. There was a good thread on it quite recently.
.
Mine still makes a clicking sound. I've tried , but can't seem to buy gaskets - made some.
How would I check if it is leaking?
Alan
Old 12-17-2007, 11:56 AM
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THE best way to hunt for leaks on turbocharged cars, done daily by wrenches at dealership I work for:

Yank the rubber off the turbo intake, insert a metal disc of appropriate diameter (with nipple welded on it) into the tube. Attach shop air supply to 0-2 bar regulator, attach regulator to T-orifice. Connect accurate mechanical boost gauge to one nipple of T-thingie and connect another to the disc.

Start adjusting the pressure on regulator from zero. SLOWLY. Monitor the gauge. Turn it up to 0.8 bar or so. Chances are you will hear hissing sound from couple of places. Take a piece of rubber hose, put one side of it in your ear and hunt around with other side for places where you hear hissing.

You can also spray soap lotion on hoses and joints. It will start bubling where leak is.

Actually, we have this "leak hunting kit" in a shop with nippled disc of different diameter so they can be connected to different cars. It's maybe little finicky to put together but it never fails.
We have
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Old 12-17-2007, 12:13 PM
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Beepbeep,
Can you post pictures of this test rig?
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 12-17-2007, 01:12 PM
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I did try this method early on - could not pressurise the system at all. But then subsequently, it seems my wastegate can was blowing straight to atmos - so may work now. I plumbed into the waste gate take off from the inter cooler - about a 3/8 " nipple.
Given the BOV is an integral in- line piece in the intake (pressurised ) system, will this still pick up any BOV internal leak - or are we talking external leaks. (I feel like a turbo head already - waste gates, BOV's , boost bleed...)
Will try again.
Alan
Old 12-17-2007, 01:27 PM
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Quote:
Originally Posted by Alan L View Post
I did try this method early on - could not pressurise the system at all. But then subsequently, it seems my wastegate can was blowing straight to atmos - so may work now. I plumbed into the waste gate take off from the inter cooler - about a 3/8 " nipple.
Given the BOV is an integral in- line piece in the intake (pressurised ) system, will this still pick up any BOV internal leak - or are we talking external leaks. (I feel like a turbo head already - waste gates, BOV's , boost bleed...)
Will try again.
Alan
Hmm...as far as I remember, BOV dumps the pressure to hose that goes from CIS into turbos mouth. If you yank that hose off the turbo intake and pressurize the system from intake and upstream, presumed BOV leak will leak into CIS and hose leading into turbo (the one that you removed) so you should be able to hear it.

BOV dumps the pressurized air from intrmediate plenum into non-pressurized hose that turbo sucks air from, so it should work. You see the metal pipe in lower left corner going downwards to the turbo( It's connected to BOVexhaust with a short piece of rubber hose.)? That's pipe should be disconnected from CIS/BOV and pressurized. The open one is hose feeding pressurized ar back from the turbo (intercooler is removed on the pic).



With other words, you should disconnect the pipe from the rubber hose going into turbo intake (and we are talking compressor intake, the "center" of the turbo), jam a nippled disc into the hose whose other end is still connected to the turbo, tighten the clamp around the disc and pressurize everything from turbo intake upwards. Remember, this means that CIS, BOV "exhaust" and filter box are unpressurized.


Fredmeister: I'll take picture of the rig tommorow and post it in this thread.
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Last edited by beepbeep; 12-17-2007 at 02:03 PM..
Old 12-17-2007, 01:59 PM
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Give me a few minutes, I'll draw it for you.
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Old 12-17-2007, 02:04 PM
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See the hose 17 going into turbos intake? That's the hose you should jam the cylinder into and pressurize. You loosen the clamp 17 that sit's on the other side of the hose (hose should still be attached to turbo on the other side), remove pipe 16, jam the cylinder into hose and tighten it again with the clamp. Then you pump up wholw sheebang to 0.8 bar and listen for hissing noises. That way you can pressurize the system from turbo upwards.

It will pressurize the turbo, intercooler, intermediate plenum and plenum going into the heads. Only way for air to escape is trough heads and exhaust valves. Your cam aren't radical enough to have overlap (I hope). There will be some air leaking past the valve seats and their stems but you should be able to pinpoint any external leaks. If your leak is bad enough to make boost taper a lot, you will be able to hear it. I never did this to 911 but I did it on couple of other turbocharged cars and principle is equal:

You pressurize the system (just like turbo would when on boost) but with engine not moving. Then you listen for leaks. Just be careful with pressure and don't go past what typical turbo would do (0.8 bar is safe). We get heaps of SAAB's that we troubleshoot this way. Two months ago I had a very annoying whooshing sound coming from my Audi S4 and just couldn't find the damn leak. Pressurized everything to 0.8 bar and still couldn't find it albeit it was obvious that air was leaking somewhere. Went with my hands across all plumbing and finally found air escaping trough two cracks on the armored rubber hose on intercooler intake.

I would bet 50 öre that your intercooler O-rings aren't sealing. Check those first.

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Last edited by beepbeep; 12-17-2007 at 02:26 PM..
Old 12-17-2007, 02:15 PM
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Here is DIY version that some gent used on his Mitsubishi...err... Dodge...

http://www.stealth316.com/2-pressuretester.htm

All in all...this is the mother of leak testing. It will reveal all leaks in the system. Cracked intercooler, ruptured hoses, leaking intake gaskets, leaking injector gaskets...if it leaks you'll find it.
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Last edited by beepbeep; 12-17-2007 at 02:33 PM..
Old 12-17-2007, 02:29 PM
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Thanks guys - I'm on to it - Gorans link has a simple system to make up - mostly out of CIS/leakdown test components anyway.
To get this right; the little nipple on the plenum chamber by one end (fan) of the BOV head dumps into the inlet tract for the turbo? I have never found where mine goes - it disappears somewhere - but not there.
It seems that Gorans boost bleed chart shows about 1 psi or 0.07 bar bleed. At 0.15 I have a bit much - and should be looking for a leak.
Alan
Old 12-17-2007, 03:12 PM
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Hmm..Dunno which nipple you are talking about. The BOV works this way: When you shut the throttle the vacuum between the throttle and the heads pulls the piston inside the BOV which uncovers the port in the chamber. When it happends, trapped air between the intercooler and closed throttle is bled trough that port back into pipe that feeds the turbo with air.

The "dump" of air is primarily done trough that port. There is also a small disc with three small hoses, whose funcion I never cared to find out. It's the one resting on the top of the BOV on my picture. As I said, I don't quite know what it does but I bet it's crucial for BOV to work
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Old 12-17-2007, 03:50 PM
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Hope this pic helps:
I wonder what the disc does??

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Old 12-17-2007, 03:53 PM
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I think I can see a banjo bolt on your plenum chamber leading to your 3 way nipple thing over your inlet tract. I don't have that thing, nor any way into my inlet tract via a plenum hose. All I have is a nipple where I think your banjo bolt is, and a hose going somewhere - but away from the system. I think I followed it into the rear wall of the engine bay somewhere, and lost it. Hmmm. (Car not here at moment - finally got it going well enough to get into town for import certification etc)
More work - but I am quite sure my BOV is not dumping into my turbo tract. In fact I have no idea what it is doing.
I'm not sure about you guys but my Hose #17 in nathans pic is simply a sleeve over a butt fit between the the plenum and turbo tract. There is no room to insert a plug there. I will need to find somewhere else - I'm thinking the elbow rubber?
Alan
Old 12-17-2007, 04:19 PM
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Goran, Where does the hose on the LH side of your disc go - or is it mounted on the disc?
Maybe it is a closed chamber with some sort of ballast/damping control for the BOV, or is it feeding into your inlet tract?
Alan
Old 12-17-2007, 08:32 PM
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Quote:
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Beepbeep,
Can you post pictures of this test rig?
Here you go. The magic bulet for solving leaks in turbocharged cars. SAAB special tools.


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Old 12-18-2007, 11:02 AM
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