Over Boost Troubleshooting

[cl8ton]

Spring length is 6.25in and wire size is 5mm.
Thanks Fred/All

[cl8ton]

bump

[jwasbury]

I had the same issue with my '79 with the boost guage pegging. Have you tried bench testing the wastegate to see if you can get the valve to open? In my case, the valve would not open, but air was leaking past the valve stem. When I opened it up, I found two springs in there. When I ran it with only one spring, it wasn't pegging the boost guage, but was not getting full boost due to the leakage at the valve stem. I sent it off to a machine shop and after looking at it, they said to toss it in the trash and get a new one. I put on a TIAL with a .8 bar spring and its regulating boost correctly and the factory guage seems to be reasonably accurate as well.

jacob

[cl8ton]

Jacob, thanks for posting and welcome aboard!

Spring in, I can barely get the valve to move. I’m not a girly man, but dam I’m sure I can bench press 1bar=14.5psi (unless my math is whacked)
Spring out, everything moves silky smooth.

Enough already, I am buying a stock spring but PP doesn’t sell them, anyone know a source, PM me, else PM me in a few days for a source.

[JFairman]

The factory boost gauge in the tach measures air pressure above the throttle butterfly in the intercooler or the intermediate blow off manifold.
There is always pressure loss through the restriction of the throttle body, especially at part throttle and the boost pressure the intake manifold sees will always be less.
This is one of the reasons a mechanical boost gauge tee'd into the power brake booster hose, which is plumbed to the intake manifold is much more accurate as far as what the motor is actually seeing.

[jwasbury]

Cl8ton,

Even with the .8bar spring, I wasn't able to push open the valve by hand. I rigged up a hose to a bicycle pump (with pressure gauge) and used it to pressurize the wastegate. I used the same set-up to test the over-boost cutoff switch as well.

[fredmeister]

You guys have to remember that the area under the rubber diaphragm in the WG housing multiplies the pressure of 1 bar to get force to open the valve. If you look at the cross sectional area of the the WG housing, multiply that by 1 bar and that gives you a ruff idea of the actual spring preload you need to overcome to open the valve. Alot more than 14.5 lbs.
The best solution is to order a new 0.7 or 0.8 bar spring to be sure.
Make sure you run a boost gage in the clock hole tee'd into the brake booster for accuracy. Tach gage is useless.

[fredmeister]

hey cl8ton,
PM sent.

Fred

[wcc]

I would test the boost gauge sending unit. That may be why you have an inaccurate reading.

Turbo Boosting Gauge/Vac Leak

[icemann427]

I'm getting WG woosh starting at about .6 bar but I still get to the stock spring's .8 max. What is the woosh telling me? Thanks

[GaryR]

I would go with an Autometer 0-35 PSI guage and see what is really going on. After that I would get a nice Tial WG with a .8 Bar spring so you have something that is easily adjustable (via cheap spring swap-out). If your OB switch isn't installed get a new one in there now..

[Alan L]

Originally Posted by cl8ton

Jacob, thanks for posting and welcome aboard!

Spring in, I can barely get the valve to move. I’m not a girly man, but dam I’m sure I can bench press 1bar=14.5psi (unless my math is whacked)
Spring out, everything moves silky smooth.

.

If you open the spring can you will see the spring fly out with considerably more than 14lb compression force, The spring is holding about 120lb force from memory. Which is why you can't push it open. I have measured it by compressing in my press to normal can length, on bathroom scales. Depending on your spring - you are in the 120 lb region of compression pressure. A 1 bar spring would be near 150 lb, ie about the weight of a slim adult. You won't be pushing that open with your thumb.
Alan

[les_garten]

Originally Posted by Alan L

If you open the spring can you will see the spring fly out with considerably more than 14lb compression force, The spring is holding about 120lb force from memory. Which is why you can't push it open. I have measured it by compressing in my press to normal can length, on bathroom scales. Depending on your spring - you are in the 120 lb region of compression pressure. A 1 bar spring would be near 150 lb, ie about the weight of a slim adult. You won't be pushing that open with your thumb.
Alan

Heh,
When I was younger and dealing with decompression sickness patients. A common test is grip strength. I usually could squeeze about 175# on the grip strength test that maxed out at 200#. Most strong folks were well under 100#. I used to squeeze those grip exercisers and break one every 2 weeks or so. Of course that was a long time ago!! I noticed on my TIAL with a 1 Bar spring that I didn't have a hope in hell of moving that valve with my thumb! I kinda did the 1 Bar/14.7# math in my head and figured there was some kinda fulcrum multiplier inside that wastegate.

Les

[cl8ton]

Hehhhh, I put a .8 spring in and even with my (old man) Kung-Fu grip, that bugger wouldn't move!

Going on faith brothers!

[fredmeister]

Lets see, the wastegate can or diaphragm diameter is roughly 3 inches in diameter. Cross sectional area is then Pi *d^2/4 or 7 square inches.
Multiply that by 14.7 psi and you get about 104 lbs of force needed to crack open the wastegate valve. Seems reasonable you aint gonna do that with your thumb.
Alan L measurements seem in the ballpark then. My dimensions are just a guess from memory.
Pretty cool stuff.

[voitureltd]

Originally Posted by MT930

Here is what I have. Based other comments this may not be stock. The service records show that the PO sent the tach out for repair could they they have put a new face on it that goes up to 1.5 bar (no description on the exact work performed). and can it be calibrated? I plan on getting a clock mount anyway. Can anyone tell me if this is a stock tach face?
I appreciate your help.

That Tach face is a stock 1977 Turbo gage . Why the factory used a 1.5 boost calibration with no inter cooler pre 1978 turbos is odd unless it was a carryover from the 934-5? The question for you to figure out is is the innards calibrated to one bar needle movement and showing 1.5 on a wrong face plate or do you have a true early 1.5 unit and you are indeed over boosting. Sometimes there is a simple solution to a seemingly complex scenario