Fellas, I'm looking for a little help here. I've got a digital boost gauge from SPA and a GReddy Profec B Spec II boost controller. Both of which give me vacuum readings in some sort of negative PSI numeric value, which I don't truly understand and can't convert to inches of vacuum like we do here.

Does anyone know the conversion?

My current vacuum is -60 on the boost controller and the boost gauge. My controller is set to read in PSI, so this would equate to -6.0 PSI of vacuum. I'm just wondering if anyone can give me an idea of what that would equate to?

Thanks guys!

Nick

 

6psi in InHG (what most gauges read vacuum in) is 12.2161.

Sooooo it sounds like you're pulling 12InHG of vacuum...?

Someone please correct me if I'm wrong.

 

Thanks Matt. Anyone else have an idea?

If that's true, before I am told I will need a rebuild, I have a large streetport that is idling at about 1,250 rpms. I think those numbers are roughly in line with a healthy engine? Maybe down a touch...?

 

I have the same boost controller and Matt's number is correct, I don't know what a LSP idle vacuum should be, mine usually sits around -75 to -80.

 

That's not even an idle dude. You should be able to get that lower if you really want to. Trying to look at vacuum numbers as a gauge of engine health on such a setup as that isn't going to tell you anything.

 

It's the way it's set, I don't know why the tuner did it that way but it's idled that high since I got it. After I install my new injectors and rails, I would get it retuned and try to bring it down some.

I've done a compression test before and know it's good but want to be able to monitor it's health without having to do another compression test.

Nick

EDIT: Now that I think of it, the idle is set that high because my Microtech doesn't have closed loop idle control and as such, it can't fast idle on startup. That's why it is that way, according to the tuner.

 

Microtech is the only computer I won't touch (because you can't share maps), so I can't help you there.

 

No worries bud. Thanks for the offer anyway!

Anyone else know what the deal is?

Nick

 

So did some Googling around.

First off, there are WAY more pressure and vacuum units out there than you would think. In the US vacuum is typically measured in inches of mercury (in Hg) - it's literally how many inches the vacuum would pull mercury up a glass tube.

It looks like the Profec doesn't have enough display digits to properly display vacuum, so there's a missing 1 at the front. So -60 is actually 16.0 inches of mercury.

The manual is vague about this. They really only talk about the pressure reading, not the vacuum reading.

When you are in boost, I'm assuming it is displaying in PSI and the display should NOT show the "x100 %kPa" in the corner.

More in the manual -

http://www.greddy.com/upload/file/PRofec_Bspec2.pdf

The fact that the Profec B doesn't have enough display digits and doesn't have a period in the display doesn't make it the best way to show vacuum.

Dale

 

Hey Dale thanks for doing that research buddy! I am not 100% sure it's correct though simply because if you're in partial throttle conditions and vacuum, you watch the gauge for example go from -60 at idle to -50 and so on but if a 1 is all that's missing than when it says -10, that's really 11 inches? That doesn't seem right.

Think if it like this: if to go from -60 to -50 is ten places and that takes you from 16 inches to 15 inches. Than how could going from -10 to 0, which is also ten places take you from 11 inches to zero pressure. You get where I'm coming from?

Nick

 

PSI is just being display x10, so 14.5PSI show up as 145 and -8.0PSI of vacuum is display as -80, which is -16.2882inHg. Dale is right that it is short of display unit, it is missing everything after a tenth of a decimal. So it can't display -18inHg of vacuum which should be -8.84077PSI, instead you'll see either -8.8PSI/-17.917inHg or -8.9PSI/-18.1206inHg, but I don't think that's a big deal, as long as your positive pressure is being display properly

 

There's no reason to be idling at 1250 with a large street port----- You should be able to chill at 950-1000 all day long. Fast idle at cold start isn't needed, my FD sits at 1000ish regardless of water temp, electric load etc. What apex seals are you rocking Niko?

 

OK this seems to make the most sense. I'll look up conversions and figure that out. Thanks!

Nick

 

Hey Rich, it's a little bit of an unknown. I was told they were ALS seals but in truth I can't be sure. I also think my RPM's are reading a little high on my tach cause when ignition is on it doesn't zero, it rests a touch above zero. I'll take a video and send it to you next week. Point being, it may be around 1,000 I don't know for a fact...?

Nick

 

Really, I just wouldn't rely on the Profec for a vacuum reading. With the display not having enough room to properly display it and chopping off numbers it's just eye candy as far as vacuum is concerned.

Dale

 

It's kinda like any boost gauge, you can't tell the exact reading in between the line anyways. The more detail display would be the commander or some kinda display fed from the ECU.

 

Maybe I should hook up my laptop to the Microtech...

Nick

 

from my understanding the greddy profec reads in kpa? if that's the case -60 kpa is -17.718 in/Hg

so every -10 is approximately -3 in/Hg

 

it reads in Kpa but can also be put in a PSI mode

 

He stated it is set to read in PSI.