bigdog

Just fitted a boost gauge.
Problem is:
on idle it show -20psi.
on light acceleration to 4000rpm shows +5psi for about a second then drops to -15psi.
on cruise at (say) 85mph shows -5 needle.
on hard acceleration shows +15psi for about 2secs then drops to 0.

confused? I am...

Help!! do you know why.

It's an Autometer gauge connected to one of the two hoses that go off from the manifold to somewhere on the right(when looking from the front) of the car. ie not connected to the hose on its own that goes off to the left

[This message has been edited by bigdog (edited 29 January 2001).]

Andy Tang

iTrader: (3)

Bear in mind that the gauge is reading pressure. If I remember correctly, the engine pressure is lower than atmosphere, and that's why it read a negative figure.

Each time you lift off the accelerator, the pressure will drop. You'll find it peaks at about 15psi (14.75 or there abouts)

just want to reassure you that the readings you give are prefectly fine!

Andy

bigdog

Thanks Andy.
I should have made it clear that I get the -Ve readings or brief positives without me backing of the gas pedal.

car is an STI..could I have tapped into the wrong hose?

Ian Cook

While accelerating the gauge should read positive, at idle or when you lift it should read negative, the gauge will drop off at times, it shouldnt go negative unless you lift the throttle though !

Blimey only not had my car for a month and still wondering if i sadi that right !!!!

[This message has been edited by Ian Cook (edited 29 January 2001).]

Andy Tang

iTrader: (3)

Which year of car is it??

bigdog

Thanks Ian,
on cruise ie steady throttle it drops off, very quickly and reads negative. sound like this in normal. I'll relax and monitor it (no pun intended) for a few days.
BD

bigdog

Andy,
its a year 2000 model (STI V6)

Don Palmer

Your boost gauge is actually reading the inlet manifold pressure.
Manifold pressure in a nastily aspirated engine gives a good impression of performance of the whole system for a given output or a given engine speed.

In turbo engines it registers the same thing but also indicates turbocharger performance.

You get low pressures on low load high engine speed because the throttle is closed. High loads at medium to high engine speed with wide open throttle gives high boost pressure. (The waste gate controls the maximum pressure in the system - without which the boost would run away, the turbocharger would become unstable and the engine would almost certainly destroy itself through detonation.)

If you study the manifold pressure as you run from a standing start in a low gear (first) with the engine ticking over and the car trickling along. Open the throttle quickly and you will see the gauge go from high vacuum to zero as the throttle opens then as the turbo warms up, the pressure increases dramatically as the engine comes on boost, and registering full boost after a second or two. ( lag ).
Do the same trick at tick over in top gear and you can wait half an hour of so before you get boost. (I have been known to exaggerate a bit too!).

The point I am making is that the manifold pressure is a function of engine load and speed. What you really want from an engine is loads of boost over as wide an engine speed as possible. Engine tuners make a real meal out of matching the turbine characteristics to those of the compressor and the engine they are fitted to.
Prodrive tuning In particular is designed to produce boost over a wider range than standard, rather than at very high engine speeds only.
Because….. it's much more drivable.

Hope this is at least plausible and perhaps even understandable.

In short the wobbles are more a function of your right foot wobbling than the gauge telling porkies!

Best fishes
Don

bigdog

Don Palmer you're a star!!
sounds not only plausible but I recognise some of the boost activity you mention (I've been playing with my right foot most of the afternoon).
Thanks again, that's helped a lot i can sleep now.
BD