Hi All

As I know only the bare minimum about how the turbo works, I was curious as to what the negative boost pressure readings I get from my ECU monitor mean. The car seems fine (MY03 WRX), and when under load the boost pressure looks fine (peaked at 1.07bar, sustained around 0.95bar) - but when not under load the pressure reads at negative - -0.75bar (ish) when coasting (no throttle), around 0.65bar on idle. More throttle ups the pressure, but it stays negative until the turbo spools up.

If I had a gauge, I would expect the pressure to read at zero until spool up, and to drop negative when changing gear. So wondered what the readings I was getting meant (i.e. is anything wrong, or are the readings from the ECU correct?).

TIA

Si.

 

Hi Mate,

Interesting and relevant post to me. I've just fitted a data monitor today and mine does the same - I assume this means it's normal but I'm sure someone with knowledge will come along shortly.

 

my defi read negative when off boost, not sure y but it is norm so don't panic

 

When the car is not on boost, ie coasting with no throttle there is no boost pressure being created so the engine is "sucking" air out of the inlet manifold so creating a negative pressure. This is normal and should happen.

 

As he said!

 

I might sound a bit daft here, but if the engine is sucking at negative pressure, what is happening just before you come on to boost when the gauge is reading 0?

 

Thats when the turbine starts to spin and the pressure equalises. Hence turbo lag. The turbo wont start to spin till it has enough exhaust gas through it when it does it will have to overcome the negative pressure first. Depending on the size of the turbo they will provide boost around 3000RPM (roughly). A larger turbo will create more boost but take longer to spin upto speed hence more turbo lag (slower responce). A smaller turbo will have less boost but also have less lag.
Its a crash course in turbo but i hope it explains a bit.
These pressure changes inside the turbo are why dump valves are fitted.

 

i too was confused with it........but they way i look at it was.

N/A cars run at a constant vacuum ie atmospheric. if you put a "boost" gauge on a N/A car it would fluctuate madly from - to + as its sucking in air all the time and firing.

so as an engine sucks in air there is a vacuum left behind - which has to be less than atmospheric because its enclosed. in the case of turbos this happens between turbo and inlet. so depending on the piping and the boost used will determine the magnitude of this "vacuum". also explains the good old turbo kick

hence the old principle of small turbo and large turbo id love to get that set up to work haha