My 2.8 seems to get to 15 psi alot faster then the stage II did and i hit the smaller line after 15 anyone know what that is for sure.

 

17lbs

 

Without the boost bypass it is. With the boost bypass and a 2.8" you need at least a MAF-T and some 42.5# injectors.

 

and I think it is worthy of sharing a couple of times. So here it is.

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 (which corresponds with -2 psi on the graph across from 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


There are other environmental factors to consider as well-for example increasing ambient temperature decreases ambient density which in turn reduces pressure. What this means is that the outdoor temperature also has some effect on boost.

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.

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 corrseponding with dividers at the top.

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

Doc

 

hhmm something isnt rite there...i have the 2.9 and the 76mm GTP tensioner and i see 16#'s. hhmm

 

given your mods, then your altitude. My numbers varied just a hundredth or two at some altitudes, but was dead on at 4000'. I am not sure what effect the boost bypass might have if any.

Doc

 

Yes altitude and ambient temperatures is wholey to blame here. mrdoc442 beat me to the punch here. I wouldn't think such new cars woudl have problems with leaks unless when you removed the supercharger you mistichtened a bolt or two. (or the person who did it for you)

Remember, superchargers can only spin so fast regaurdless of altitude, unlike turbos that can spin faster untill the desired boost is realised.

 

Stupid double editing threads.

 

Some people with stage 2 with a 2.8 swap arent seeing any boost gains. Thats the problem.

 

Some people with stage 2 with a 2.8 swap arent seeing any boost gains. Thats the problem. Well mine atleast.

 

Some people with stage 2 with a 2.8 swap arent seeing any boost gains. Thats the problem. Well mine atleast.

 

im hitting 15 even with the 3.0, so it doesnt sound right to me either.....

 

to retorque those S/C bolts. I had to make a tool using a cut off 6MM allen wrench. The torque called for is 18 ft. lbs. I gradually bumped the bolts up to 20/22 ft.lbs., just to be sure. I tightened/torqued the top left S/C bolt first, then bottom right, then bottom left and then the top right last, like the sequence used on the base of the holley carburetor on my race car. The manual does not specify any particular tightening sequence for the S/C bolts.


Doc

 

holy X10 of the same post batman!!!!

 

 

just like it did on the other dude. :+) I'm trying to get them off. It is a very slow process.

Doc

 

Alright so after playing around with some crap i figured out that my problem was the Boost Bypass Mod, so i put that back to stock and im hitting 18lbs of boost no problem. i took it all the way up to 140!~ and hit 19.5 lbs of boost! it was SWEEEEEEEEEEEEET

 

Sorry, but there is a blatant flaw to your logic. And you can pass this on to the other forum too if you would like. When your car measures PSI in the manifold, it is measuring RELATIVE to current atmospheric pressure (this is why the boost gauge reads "0" with the key sitting in ON without the engine started, regardless of air pressure -- this is the instrument "zeroing" itself, making PSI measurements accurate RELATIVE to ambient air pressure).

Now this is not to say that altitute does not affect max boost levels, but let's call it for what it is: not air pressure, but AIR DENSITY. The efficiency of the surpercharger (or any compressor for that matter) is reduced when there is low air density. Examples of low air density include: high ambient temperatures or high altitude (think of it this way -- a compressor can't really compress anything in space (~ infinite altitude by your equation).

So, essentially, your boost gauges ARE accurate at ANY altitude. Nevertheless, extremely high altitudes or temperatures are likely to reduce the "available air" for compression by the supercharger, hence lowering max boost levels.

Ciao!

 

What happens if your running the 2.85 pulley ZZP with the Stage 1 and the bypass mod I am getting maybe 15.5psi??? I have the bigger tensioner pulley with the STock belt. Can I hook up the bypass mod and get more boost or not??? Or do I have a boost leak??

 

I see 18 PSI @ red line but I should only be seeing around 17 PSI. DP and tune should take car of that.

 

Thats exactly what im seeing now i took the bypass mod off and it fixed my boost leak whatever problem i used to sit at 15.5

 

some of your points are well taken.

Air density altitude is calculated by measuring atmospheric pressure, temperature and humidity.

I am not sure what point you are trying to make, as you say what most of us have been saying, that altitude effects boost levels. Your comments pretty much paraphrase what I said.

As you intimated, there is actually no true -0- boost. Atmosphereic pressure starts at 14.7 lbs. So 10# on a gauge if you want to be technically correct would be 24.7# without the gauge "zeroing" in. I do not believe that even high end boost gauges measure temperature and humidity, which is needed in order to calculate air density. So to follow your logic, boost gauges have a built in inaccuracy as they cannot measure and accurately calculate air density without measuring temperature and humidity.

I know for a fact that altitude affects boost Pressure measurements. I had a Buick Grand National with a high end Autometer liquid filled boost gauge. When I took the car to a significantly lower altitude, my boost Pressure readings on the boost gauge increased significantly. I do however, generally question how accurate a lot of hard to read factory gauges are.

Doc

 

*taking notes*
boost bypass with stage 2... bad
altitude... bad

 

haha nice

 

so is it decided that the 2.8(2.79) is safe with S2?