StephenDone

Fuel cut is dependant on atmospheric pressure. This could happen if you are at altitude. But much more likely to be your gauge.

Steve

[Edited by StephenDone - 8/7/2002 12:41:25 PM]

jamesjones

J

mega_stream

Don't know, but MY99 AE801 peaked at 19.5 psi last night (the dawes needed turning down a bit ) Didn't fuel cut though

Mines now running 17.5psi

john banks

Fuel cut is 17.6 PSI held for about 4 seconds over this value.

jamesjones

Thanks

I did think that it was about that but I was getting fuel cut at around 15.5psi. Maybe my boost gauge is wrong.

J

scobymars

mine was cutting at 1,1 bar
how much is it in PSI?

john banks

14.5 PSI is 1 bar.

Do gauges read absolute pressure or relative pressure?

nom

Absolute, don't they? Or relative, depending on what it's relative to
I think if you were up a very big hill, it would read under 0.
So, if the car read +15psi over the local atmosphere (as it does measure locally), and the local atmosphere was 1psi under 'normal', then the gauge would read +14psi (with the car reading +15psi) as the gauge doesn't adjust. If that makes sense...

I think that's how it would work anyway

StephenDone

Boost gauges read relative pressure - if your engine is off, your gauge reads 0 doesn't it. That means 0 psi/bar above atmospheric pressure.

Manifold pressure sensors read absolute pressure, so with the engine off, they'd read around 14.5 PSI / 1 Bar.

Steve

john banks

Well I was thinking relative so fuel cut should always be at the same point on the gauge since it is compensated absolute = relative. However, I wondered since the gauges do not always show exactly zero - could be crap gauge or varying atmospheric pressure? Thinking about the construction I would think they are relative though?

jamesjones

My boost gauge reads -12psi when the engine is off. It is a cheap one made by Mocal, should I take it back. Could it be why I get fuel cut at 15psi with my AE800 ecu? Perhaps it is mis-reading.

j

DBDOM

Heh, Yeah i would take it back

nom

Well, that sorts the original problem out... but I still think that the gauge is absolute but the numbers are offset to read 0psi at 1 standard atmosphere.
Trying to explain more clearly, (assuming the gauge is accurate) the gauge will give a correct reading of conditions in the manifold if the local atmosphere is at 1 atmosphere pressure. However, if you were to take the car underwater into a big cave (!) and therefore increase the air pressure to say, 2 bar, the guage would read 1 bar at rest, not 0 (1 bar above what it's expecting local pressure to be). Driving about the cave...(!) if the car boosted to 1 bar - which it would read as 1 bar as it would be 1 bar above local atmosphere - the gauge would read 3 bar.
If nothing else, I think I can spell gauge now!

scobymars

I'm may be reading less than you guy because the gauge are set up for 1 atmosphere (1015 mBar) and that here south of france we mostly have high pressure and you guy's in england have low pressure

jamesjones

Well i've swapped the gauge and although it sits fractionally below zero with the engine off I am holding just over 16 psi with no cut-off. So that'll be 16.5psi in real terms.

I guess you get what you pay for.

J