Boost pressures at altitude?

[JimR]

My boost gauge is reading lower than I believe it should, even though the car has plenty of power. So I did some calculations but the numbers still don't match. Here's what I'm thinking:

- Stock boost (manual) = 6.8 psi

First question. Does this 6.8 number mean boost pressure above ambient air pressure at sea level? I'm assuming yes...

- My gauge is reporting 4.4 psi (actually 220 torr, metric gauge)

- I'm at 1140 meters above sea level, so the air pressure here is ~12.8 psi as opposed to 14.7 psi at sea level.

Second question. Does this math make sense?: 6.8 psi / 14.7 psi * 12.8 psi = 5.92 psi
If so, then my turbo should be making 5.92 psi above ambient, and that pressure should be reflected on the gauge.

Why is it reading 4.4 as opposed to 5.92? What am I missing here? I am fairly sure I am making the boost I should be... it's just not reflected by the gauge.

Thanks,
Jim

 

[Nocheez]

Are you using your stock gauge?

 

[JimR]

Are you using your stock gauge?

Yes. I know it's very slow to respond, but it should still be quite accurate. I took those readings in 4th gear on a long pull, so it had plenty of time to adjust.

 

[Bishop92t]

Stock gauge is both slow and inaccurate. I know you will lose some boost at higher altitudes however I don't know the formula.

 

[JimR]

Ok, how does this math sound?

From what I've been reading, the turbos work on a pressure ratio. So if the turbo had a ratio of 2:1 and it was 100% efficient, then the output pressure at sealevel would be 2 * 14.7, or 29.4 psi.

Obviously our turbos can never be this efficient. However I can get the relative efficiency by dividing 21.5 (14.7 plus 6.8) by 14.7. This gives me a ratio of 1.4626:1 for our turbos at sealevel.

12.8 psi (approx pressure at this altitude) multiplied by 1.4626 gives me 18.72 psi.

18.72 minus 12.8 is 5.92, exactly the same number I arrived at earlier. This is (at least) the amount of boost I should be making at this elevation.

I'm looking forward to getting an aftermarket gauge so I can settle this. ;-)

 

[luvs2slide]

You got the math part down, and it makes complete sense. An aftermarket boost guage would definately give you what you're looking for. Now it's just comes down to taste. :beer: Which brand?

 

[JimR]

Well... I've solved this whole issue for myself by burying the stock boost gauge... so I don't really care what it's reading anymore! lol

Just got the 3" turbo back on today, already had an intake, and I put 3 washers on the wastegate. It's going to approximately 9 psi.