What's the +40 on an FC boost gauge mean?
 

Yeah, read the title. I've never seen boost measured in this way. What's it stand for, and what's +40 in psi/bar? Thanks.

It does say pretty clearly on the gauge that the scale is x10mmHg (times ten millimeters of mercury), so it reads from -200mmHg to +400mmHg. 1psi = 51.7mmHg so full scale is ~7.7psi.

Hmm... mine never goes up that high... at WOT it only hangs around the middle of the gauge... is something messed up?

^ same but on my defi boost gauge i get 0.6-0.7 bar

its not suppose to max out...
you have a speedo that goes to 160, is it suppose to be maxxed out when you drive?


millimeters of mercury....

Do you have a proper boost gauge to compare it to? Since you're obviously expecting more than stock boost you definitely should have one.

In a stock car maybe, but if you boost high enough (i.e. over 7.7psi) it certainly will...

My car is stock, but it's only started hitting the + on the 40 since i got a straightpipe. Boosts a lot more. I'm not complaining. If it's only at 7.7, or so, I'm not gonna hit fuel cut any time soon, so I'm safe. Thanks.

true enough! i was assuming he has a stock car. :)

means get bigger turbo and boost controller and boost gauge
simply complex. you need to memorize my car more. jk. terry has your email at work so
I can't send anything till monday. how did you like the hood on saturday? pm me to get info

lol mine reads a few pounds of vacuum when the engine is off (guess what, they arent too accurate)

not to thread jack, but what is stock boost?

for clarification, it reads a few pounds vacuum when they key is in the on position, but the engine is not running

and if you have a FCD that gauge will be almost completely useless since the signal from the pressure sensor will be altered.

the only thing the stock boost gauge is good for is seeing if you actually are producing boost, aside from that the readings are hardly accurate even when it is working right.

For clarification vacuum is measured in inches of water column.

if you dont have an FCD, you will hit fuel cut with a straight pipe. be careful!!

I think it shows the battery strength then... it's in the same location on the n/a clusters, and i noticed it was acting strange after i was trying to start my car after the battery died.

the needle will sit on the peg so it will read a vacuum with the engine off, that does not mean there is an issue with the gauge since it is electrical not mechanical..

Stock max boost is 6.5psi (340mmHg) for S4 and 8.3psi (430mmHg) for S5.

That depends entirely on the type of FCD you have. Good ones are clamping circuits that have no effect on the MAP signal until just before the fuel cut point, which is off the top of the scale anyway. Crap ones just scale the whole signal by a fixed percentage, which not only upsets the MAP signal sent to the gauge and ECU all the time, but it doesn't actually remove the fuel cut, it just lifts it higher.

In the Paul Stokes FCD article, the unnamed aftermarket FCD reduced the MAP signal by ~8%, which would move fuel cut up to ~10.5psi.

For clarification, vacuum can be measured in any pressure units. Technically there's no such thing as a specific unit for vacuum, but some units are used for it more often than others.

I thought the spring on the Zenki turbo was at 5.5psi?

Personally I think the whole idea of rating wastegate springs at a certain boost pressure is silly. There are many, many other variables besides the wastegate spring that determine how much boost is made.

Anyway, the whole 5.5psi thing is a myth. The S4's stock max boost is 6.6psi as per the FSM, which occured at ~4000rpm IIRC at then started dropping.