Originally Posted by mrdoc442

with the factory U.S boost gauge can answer this thread intelligently unless the needle on the boost gauge is exactly on 10, 15 or 20. Even then you are believing the typically erroneous factory gauge to be accurate. The demarcations between these numbers are not logical and make no sense. I contacted Autometer and spoke with a couple of their tech's who work for the company who made the gauge, and they were not able to read or interpret it. They said they just built the boost gauge the way GM specified, and in the future would endeavor not to put their name on something so shoddy. Scan tool or tuner info would be much more reliable for those fortunate enough to have access to them. The Canadian gauges make a whole lot more sense. Autometer suggested I warranty mine for a Canadian version. The best thing to do would be to get an after-market Autometer liquid filled boost gauge. I had them on other boosted vehicles and they appeared to be reasonably accurate.

I have seen many folks reference 12.5 # boost with the stock 85 MM 3.346" pulley. This is incorrect. According to the Cobalt SS S/C factory shop manual, under engine controls, 2.0L, 6-1395, "Max engine boost is 83 kpa/12 psi."

I stumbled on this chart on another forum and thought I'd share it for anyone interested. I did the math with my car-2006 SS S/C-Stage II, K&N factory filter in opened up filter box-Borla cat-back, extrude honed exhaust manifold-2.8" pulley, and the reference numbers for loss at altitude are very close to what I calculated using the complicated (for me at least) formula given, for each altitude with my car. Remember I am not a mathametician. The numbers to the far right are subtracted from the boost you should have. For example, I should have about 17 psi with my mods at sea level, but at my altitude of 4000' my boost gauge shows 15 psi (-2 on the graph for 4000').

Altitude/ Air press(in. Hg)/ Air press(psi)/ Loss as referenced to sea level (psi)


Sea level/ 29.92/ 14.7/ 0

1000 ft/ 28.86/ 14.18/ -0.52 psi

2000 ft/ 27.82/ 13.67/ -1.03 psi

3000 ft/ 26.81/ 13.17/ -1.53 psi

4000 ft/ 25.84/ 12.7 / -2 psi

5000 ft/ 24.9 / 12.23/ -2.47 psi

6000 ft/ 23.98/ 11.78/ -2.92 psi

7000 ft/ 23.09/ 11.34/ -3.36 psi

8000 ft/ 22.23/ 10.92/ -3.78 psi

9000 ft/ 21.39/ 10.51/ -4.19 psi

10000 ft/ 20.58 / 10.11/ -4.59 psi

I had to do this piecemeal to fit this screen-hope it shows correctly. I can't get them to line up, use the dividers and read corresponding with dividers at the top.

This chart is "ideal" ie., it assumes 100% efficiency and doe not factor losses due to backpressure or heat. S/C boost is somewhat of a black science, with many variables to consider. However in my case, at 15 psi at least, it is right on, considering my mods and altitude. Everyone's situation is different, so just use the table as a ballpark guide, don't consider it to be 100% exact. Remember, noone so far has been able to tell me how to read the factory gauge in between the major numbers. Not even Autometer tech's can tell me how to read their gauge. I E-mailed Chevrolet and they said they would refer me to someone in engineering who could tell me how to read the gauge. Never heard back, so I guess they can't read it either. I have raced cars for many years, and I have never seen a gauge that is any harder to read than the factory (U.S. version) Autometer boost gauge in our S/C cars. It moves up very quickly, and it is hard to watch the road and check the gauge, trying not to run up on the rev limiter-trying to look at the tachometer at the same time. I have noticed that since I installed the Stage II, my boost does not start to drop back at 5600 to 6000 rpm. With the boost bypass mod, I don't know if that has any affect on the boost gauge's calibration/accuracy or not. But I would suggest being careful with that mod. The factory shop manual states that damage to the engine can occur if the boost bypass valve is stuck in the position to allow full boost, unchecked.

Maybe the factory Aurometer gauge isn't off that far after all.

Doc