Turbo manifold pressure.

[LankyTim]

Hi,

Ive fitted my boost gauge in my 1997 944 turbo but im getting confused at the readings im getting and how to interpret them. Under normal driving conditions, just tootling about at 30mph it shows -0.7 bar and under hard accelleration it just about goes over the 0 mark into the + side of the gauge. Is this right? Im about to fit a manual boost controller and im told I can safely boost to about 12PSI (or whatever that is in BAR) Is this 12 PSI from normal, i.e when its running and showing a vacuum in the inlet manifold or when the boost gauge is on zero, i.e no pressure or vacuum at all, then add 12psi of pressure.

Im probably confusing myself here a little, but any advice is greatly recieved!

[Les, slight limp]

+12psi, when the gauge is showing vacuum it means the engine is in effect using more air than supplied so that VE goes down and economy goes up, ie the engine is trying to suck in x cfm of air for a given rpm, the throttle plate is only allowing y cfm to pass and if y is less than x it shows a vacuum on the gauge.

If you have an lpt you've only got 4 psi at the turbo and by the time it's gone through the piping and intercooler you see about 3psi at the inlet manifold, less than that and you've probably got a gunged up intercooler or a boost leak, the "normal" turbo runs about 7 psi which ends up about 6 psi.

Intercoolers are easy to de gunge, a strong washing up water mix poured into the intercooler to about half full, block the inlet and outlet with a couple of rags and shake, repeat until the water coming out looks fairly clean and then rinse with cold water to remove the kind to hands suds.

[LankyTim]

Foot to the floor, at 40mph going up a mild hill the boost gauge just about goes over 0 into the + side. I dont think theres a boost leak as I cant hear any untoward air like sounds. There is a crack in the back of the exhaust manifold though. Its audable when the engine is cold but as it heats up the manifold expands and seals the crack to an extent where it cant be heard. Could boost pressure be lost through the crack? Ive got a spare manifold which needs welding to repair a similar crack. I going to swap them round, something which ill get round to one dy...

Ill have to remove the intercooler and give it a good wash as it could well be clogged. The thin rubber pipe from the inlet manifold to the the turbo (actuator?) looks to be perished a bit so that could also affect things. Its being replaced with a silicone pipe as soon as my suppliers send me the correct colour and quantity of tube!

[ivor940]

Hi re the exhaust leak, check the 3 nuts where the exhaust attaches to the manifold. My 945 2.3 LPT had a loose lower nut. It is awkward to tighten. Suggest a 1/4" drive breaker bar or short spanner.

ivor940

[LankyTim]

Originally Posted by ivor940

Hi re the exhaust leak, check the 3 nuts where the exhaust attaches to the manifold. My 945 2.3 LPT had a loose lower nut. It is awkward to tighten. Suggest a 1/4" drive breaker bar or short spanner.

ivor940

It had the headgasket replaced 6 months ago, it was making the same noise before and after the manifold was removed so I dont think it coud be a loose nut. Ive felt the crack in the manifold with the tips of my fingers, without the engine running of course!

[Les, slight limp]

The cbv could be stuck open venting boost pressure or the wastegate could be stuck open stopping any boost from building, the manifold is a pretty common problem.

[LankyTim]

Originally Posted by Les, slight limp

The cbv could be stuck open venting boost pressure or the wastegate could be stuck open stopping any boost from building,

What can I do about it?

[ivor940]

Hi Tim, First connect a 0-10 psi gage to the manifold nipple. This will ensure that you are seeing the actual boost pressure on acceleration. I will measure this on my 945 LPT as soon as I can borrow a guage from work so you have a figure to compare with.

ivor940

[Les, slight limp]

Erm what boost gauge have you fitted, if it's one of those 30psi ones 4psi will hardly make it flicker, what make is it coz the no name ones are really unreliable unless your shoving lots of boost through them, reconnect your stock gauge and if it shows boost it's the gauge and if not the problem is elsewhere, as soon as i've rebuilt my t3 the mitsu on mine is free to a good home, the turbine housing is badly cracked but the wastegate actuator and cbv are known good, of course your lpt would then be a hpt tee hee, it's been done before with no probs.

[LankyTim]

It goes from -1 BAR to +1.5 BAR. I wanted to get one that read in PSI but this one is made by Smiths (a good make I think) and has the right font on the numbers and an orange needle to match the cars original instruments

I dont have another pressure indicator I could use to test the turbo with, unless I could adapt a tyre pressure gauge.

My 940 was one of the few that was never fitted with a boost gauge from the factory. Better this way as it would have to be replaced with a more accurate one and id have a redundant one to remove on the dashboard.

How easy is it to mess about with the waste gate? Do I have to strip everything down?