On a typically standard turbo'd car. I understand there will be a differences from car to car.

A friend's boost guage is showing -18psi on his VX220 Turbo and the only addition is a milltek turbo back system with standard cat.

he thought it was a bit low and he was getting answers of "it should be at 0 on idle" on the VX forums so I thought i'd ask on here

any help will be much appreciated

 

I'm no expert but I'm pretty sure a boost gauge will only read ZERO when the engine is turned off.

at idle my blitz boost gauge reads -0.42 ish depending on the air temp.

 

only way you guna get boost on idle is by having anti-lag, sure you wasnt reading vacuum

 

if it shows -18 psi,,, your friend has just broken some laws of physics

 

will only read zero at idle if he has anti lag

-18 is normal for a turbo car as long as the gauge goes into the plus when he comes on boost. at idle it is measuring vacum

 

No way that -18 psi is normal, you wont ever see that, even when coasting down or whatever the english word is for when you let off the throttle.

 

you wouldnt see -18 if the engine was idling in deap space dude

 

Ok ok, I know enough about engines to know that without antilag, a turbo'd car will never show a positive boost pressure. I guess I just worded the initial post wrongly

So -18psi on idle is typically the norm? and scott, yeah it goes well into the positive at full chat

 

it will have a vacuum yes, but not negative 18 lbs worth !

 

usually around -5psi on vac at idle i think

 

-18, that's fine, it's reading the negative vacuum on idle, then when the turbo cuts in it should go into plus figures, as does the pressure in the plenum or inlet manifold or wherever the takeoff is for the boost gauge!
my frst used to read about -10 - -13 but that was with a cheap boost gauge!

 

Thats what I wrote "you WONT EVER see that, EVEN when coasting down...." Not my first language so perhaps came out wrong?

Shoot down the others in this thread instead that seem to think that -18 is fine.

 

thanks for the input guys

Time to ditch the "auto gauge" gauge and get a proper one I think

 

-15 or so is what i used to see.

 

the amount of vacuum (even though i dont like that saying) it pulls at idle depends on a few thing,,, (cams bla bla) so dont worry abut the number it if pulling,, aslong as there is a vacuum there your happy

the reason why i was laughing at -18psi,, is because the lowest pressure that is possible to create (nothing-ness) is -14.7psi gauge pressure ( or zero absolute) so minus 18psi,,, is more than nothing

everything in our would has an absolute zero,,, pressure being -14.7 psi gauge, and temp being -273 degrees c below this temp, electrons stop moving and the atom does not excist no more

we really shouldnt use guage pressure,, as zero drifts on guage pressure,,, atmopheric pressure changes by the minute with standard atompheric pressure being 1.013bar absolute, or 14.7psi absolute

 

^^^ This guy knows his shindig! ^^^ most of what you said just went over my head!

 

btw the scale of the vacuum side might not be PSI,, it could be inches of mercury thats where your getting the 18 from

 

i see -9 on my turbo at idle.. then boosts to +8

 

well get you car chipped and get that boost up

 

i see -15 on idle and approx -20 when coasting, i'm pretty sure thats not psi though

 

Mine reads -9psi on idle - standard car basically is this good or bad on an S1

 

if you read my post a few up,,, you would my 100% that isnt psi

 

-9/10psi is a reasonable normal idle absolute manifold pressure with "sensible" cams.

 

Why on earth are there negative readings on a boost guage in the first place?

Negative pressure is just impossible

Why don't the guages just read into fractions of 1 bar?

 

A minius reading at idle is correct this is due to there being a vacuum, all the turbo vehicles I've ever had have read minus at idle.

 

A certain well known tuning garage told me
that one of the first checks they do on a car that comes in is check the idle vacuum with an accurate, large liquid filled gauge
on a standard car it should be x psi (can't remember exactly but something like 10 or 11?)
and if it's not that high (i.e. less vacuum)
then it's an indication of incorrect cam timing, inlet leak or some other problem....

 

Boost gauges indicate "gauge" pressure, ie relative to atmosphere, hence a vacuum is indicated as negative gauge pressure.

 

my bad dude i only read the first few posts, good explanation btw

 

wa the hell are you guys on about

 

My boost guage says "in. hg" if that makes any odds to anyone- i'm intrigued as to the units its measured in now

 

in.hg. = "inches of mercury"

nominal atmospheric pressure = 29.92 in.hg.

so nominal idle vacuum of -10psi = 20 in.hg.

 

thanks rich sorry for hijack