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Alan L's Avatar
 
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why no boost at clock gauge?

This is weird.
I'm sure I measured boost on my clock gauge when I did this before. But I have had the engine out and all intake system apart since. Now trying to test for leaks and find cause of rough running. When I pressurise the intake to say 14 psi, I get nothing on the clock boost gauge. This gauge hooks into the intake manifold - by the injector blocks. When I hook directly on to that line , it works.
Am I missing something here?
Alan
Old 04-16-2008, 02:47 AM
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Dont have an answer to your Q, but where did you get that set up? how much was it?
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John A.
1979 Porsche 930: 3.4L, SC cams, Twin plug, Leask WUR, Custom SSI turbo exhaust, Tial WG, K27HFS, and we can't forget the Zork (short lived depending on my homeowners assoc.)
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Old 04-16-2008, 08:46 AM
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Is the throttle body butterfly open? And just what *IS* that setup?
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Old 04-16-2008, 09:02 AM
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You can't pressurize the intake manifold from where it appears you're hooked up. It's hard to tell how you plumbed in from that pic, but you're above the intermediate manifold and a good ways above or upstream of the throttle body.
If your blowing air in to the big rubber elbow it will easily escape around the edges of the air flow sensor plate under your air cleaner, and if the throttle butterfly is closed very little air pressure/flow will get by into the intake manifold.
Old 04-16-2008, 09:33 AM
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Yes, perhaps you should be pressurizing via one of the throttle body Ts...which I think are below the butterfly?
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back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
"Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)
Old 04-16-2008, 10:58 AM
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If your clock gauge is the same as the Andial gauge it will "T" into your brake servo in the front of the car, check there as it's most likely a leak there.
Old 04-16-2008, 10:58 AM
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The unit is pressurising thru the elbow rubber that runs down into the turbo intake.
I did use this previously to find a lot of leaks - at the injector blocks, and all over the place. Pulled the engine, new injector blocks, intake seals, replaced hoses etc. Now I try and test again same way, I get no pressure at the clock gauge. The system is able to pressurise with relatively little gas flow - so even if there was a leak at the brake boost T or similar, the pressure should still show on the gauge.
I also tried pressurising off the nipple on the intercooler - than runs to the waste gate - same result. The butterfly is ever slightly open at rest - as it is supposed to be - and without the intercooler, I can detect gas passing it - altho I did try opening it to see if any difference- none.
The unit cost about $5 - from plumber hardware, and 1/2 hr in shed.
Alan
Old 04-16-2008, 12:23 PM
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Quote:
Originally Posted by Alan L View Post
The unit is pressurising thru the elbow rubber that runs down into the turbo intake.
I did use this previously to find a lot of leaks - at the injector blocks, and all over the place. Pulled the engine, new injector blocks, intake seals, replaced hoses etc. Now I try and test again same way, I get no pressure at the clock gauge. The system is able to pressurise with relatively little gas flow - so even if there was a leak at the brake boost T or similar, the pressure should still show on the gauge.
I also tried pressurising off the nipple on the intercooler - than runs to the waste gate - same result. The butterfly is ever slightly open at rest - as it is supposed to be - and without the intercooler, I can detect gas passing it - altho I did try opening it to see if any difference- none.
The unit cost about $5 - from plumber hardware, and 1/2 hr in shed.
Alan
It's not clear from your writings that when you pressurize your engine using the system in the photo that it holds pressure. If not, one of the cylinder's intake/exhaust valves are in overlap timing. if in overlap you should be able to hear airflow in the exhaust while the air supply is on.
Old 04-16-2008, 12:40 PM
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I'm glad you raise the point about valve overlap. This was the problem I had earlier - apart from lots of leaks. I had my compressor hooked into the system via as per pic, to generate enough air to create a pressure. Apart from all the leaks - air pissing out in many places - open hoses, cracked injector blocks etc - air was exiting the exhaust. The only conclusion I could come to was that the cams had some overlap. Don't know what cams it has - but if they have overlap I can't see how it will hold a static pressure. I have removed the exhaust for this test and made a blanking plate to go over turbo outlet. There is still no static pressure, but a modest flow of gas from a bottle is required to very quickly bring pressure up on my test gauge - but not the boost gauge.
Do stock 930 cams have overlap?
Alan
Old 04-16-2008, 01:00 PM
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Quote:
Originally Posted by Alan L View Post
Do stock 930 cams have overlap?
Alan
Yes, stock or modified 930 cams will have overlap.
I do not know if there is a point in the engine's rotating mass cycle that all six exhaust valves are closed but that's the point you want your crank to rest when checking for leaks. It would be ok if a cylinder or two's valves were in overlap provided it's the intake valve that's open.
Old 04-16-2008, 01:16 PM
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Quote:
Originally Posted by Alan L View Post
I have removed the exhaust for this test and made a blanking plate to go over turbo outlet.
Alan
Sorry Alan, skipped over this important sentence. Not to worry about overlap with exhaust closed. I'm presuming you have sealed that blanking plate and it's not one of your leaks. Is wastegate in the system? Sealed?
If clock boost gage is not registering, system must have a gross leak somewhere.
Old 04-16-2008, 01:28 PM
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Waste gate is in system - except when I plumbed in thru the intercooler nipple - which removed the w/gate from the system. No change. W/gate in reasonable nick - altho can detect very small bleed from there. Have stripped and refaced valve and checked diaphragm.
If I have pressure on my test gauge, I can't see how I could have a leak big enough , for the small amount of gas I am putting in, for it not to register on clock gauge?
Have hunted for leak, but can't find any - at least not significant. Have not applied any sealer to exhaust blank plate - but figure with gasket it has to better than straight overlap. Can't hear any gas or feel any at that point. There will be a small bleed at that point - perhaps like waste gate.
So, is there a particular point in engine rotation that this test can be done without exh overlap? I would suspect not, if the cams have overlap.
Alan
Old 04-16-2008, 01:46 PM
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Quote:
Originally Posted by Alan L View Post
If I have pressure on my test gauge, I can't see how I could have a leak big enough , for the small amount of gas I am putting in, for it not to register on clock gauge?

So, is there a particular point in engine rotation that this test can be done without exh overlap? I would suspect not, if the cams have overlap.
Alan
Alan:
According to basic physics and Pascal's principal, a closed pressure system will have equal pressure throughout. If you have a substantial leak and the orifice to the clock gage is restricted, it's possible the orifice is acting as a big resistor, not letting the gage access the pressure where the majority of the system's volume exists.
If you shut off the air supply, how long does the engine system hold pressure?
If the gage isn't even moving, I'm as baffled as you for your 930 has defeated Pascal's law. (A 930 may be able to defeat the law of the street but not physics).
I don't know if there is a point without overlap either.
Chris
Old 04-16-2008, 02:15 PM
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Chris, I suspect there is no point without overlap on at least one cylinder. I have yet to presssurise it without something flowing out the exhaust. (my leakdown numbers are good -2-3% on all). The pressure drops as soon as I cut the gas flow. Does anyone manage to pressurise the system and hold it statically?.
The boost gauge does not move a jot, even when I pressurise to 14 psi.
It is holding a lot more pressure than before (but I have also blocked the exhaust off). My compressor could only produce about 5 psi running flat out before I pulled the motor. But I am sure the boost gauge was responding at that time. Now when I hooked the compressor on I had to shut it down almost instantly - even tho it was dialled right back - it wanted to over pressurise. So, I run a gas bottle to pressurise with a fairly low flow - which suggests minimal leaks - but the boost gauge does not move. Is this weird?
How do others do this test?
Alan
Old 04-16-2008, 05:51 PM
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Alan:
Theoretically, with your exhaust outlet capped the only leak path for air to escape a pressurized intake system would be past the valves and rings and that path is miniscule in a good engine. Thus, given the volume of the intake system, i.e. turbo feed pipe, turbo, turbo outlet pipe, intercooler, intake manifold, power brake boost line, distributor vac/retard and pressure gage lines etc., I’d guestimate a 930 engine charged to 15-psi should hold for hours before it drops even 1-psi.
You note, “My compressor could only produce about 5 psi running flat out before I pulled the motor.” Are you saying your turbo (compressor) boosted to a max 5 psi? If so, less than half what it should be capable of with a stock wastegate. A turbo can flow a whoollee bunch more air than the ¼” airline you have on your test setup. It was doing a good job keeping up with a huge leak.
Unless I’m missing something here, you’ve still got a huge leak.
It’s going to be a process of elimination. Pull the intercooler and plug all the accessory lines/ports coming off the intake manifold. (Some of the 930 vac control valves are designed to vent the intake to atmosphere is certain cases. Perhaps one is open all the time?) Pressurize the throttle body– it and the intake manifold and any open cylinders should hold 15 psi for a long time. If ok, start adding back the accessory lines and accessories, test the intercooler, hoses etc.
I recommend you test the integrity of the intercooler by itself. It’s basically a radiator and many a radiator leaks only under pressure and the higher the pressure, the greater the leak. It may have no appreciable leak under vacuum so you wouldn’t notice it at idle or no/low engine load.
Old 04-16-2008, 06:45 PM
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Sorry, I meant my w/shop compressor - it has a capacity of about 2 cuft/min. I guess a turbo compressor can do better than that.
Will attempt what you say. I wondered if I couldn't detect a faint exit of gas over the intercooler.
Will try and check.
Alan
Old 04-16-2008, 07:39 PM
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I can confirm there are overlap and non- overlap phases. The initial problem I had was I was using my leak down regulator system - this creates a strong pressure differential at the gauge - hence the gauge read high pressure but the gas flow on the other side was quite low. Fixing that, I can pressurise to about 0.5 bar now, but have large gas loss somewhere.
When I rotate the engine under these conditions I get peaks and troughs of pressure - which would relate to the overlap phases. Makes sense if say 10 deg overlap, then = 60 deg of total overlap = 300 deg with no overlap. Seems about like that. My clock gauge now registers, but I have to find where my 2 cu ft/min is disappearing to.
Alan
Old 04-16-2008, 11:00 PM
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I have found the source of the problem - I did not have the intake manifold tightened down enough on its new injector blocks and gaskets. I finally found a torque setting which suggested 18 ft/lb. I was using about 5. This has improved things a lot pressure wise. But I still am not convinced you can hold a static pressure in the system as a means of testing it. I get significant loss via the idle bleed screw - which would seem unavoidable - given how it functions. Unless it shuts completely off if you screw it hard in.
Alan
Old 04-17-2008, 12:39 PM
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Quote:
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I still am not convinced you can hold a static pressure in the system as a means of testing it. I get significant loss via the idle bleed screw - which would seem unavoidable - given how it functions. Unless it shuts completely off if you screw it hard in.
Alan
The idle bleed screw gets its air from just above the throttle body, an area within your "pressure vessel.” Other than leakage past the rings, which at 15-psi will be nil, the only added leak source should be the throttle plate shaft and that should also be nil.
Have you removed all hoses from the intake and intercooler, plugged the orifices and pressurized? Once that part of the intake system is found leak free or damn close to it, start adding back the external goodies one at a time – you’ll find what your looking for.
Old 04-17-2008, 03:33 PM
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I found it difficult to plug some of the gaps - where the intercooler fitted in to - so have chased whatever I could find as a possible leak source. I definitely have a leak then near the idle screw if it takes its air internally. Thanks. Will have to investigate further. It is h(p)issing out past the screw at 14 psi.
Alan
Old 04-17-2008, 03:59 PM
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