Boost Gauge as an exhaust back pressure gauge?

[drawson]

After I have the Mighty Cummins overhauled I'm putting the PDR twin Turbo set up on. this involves turning the ATS exhaust manifold over and redrilling the other side for the Pyro. I also will need a 0-100 boost gauge to replace the 0-60 gauge I have now. I have been thinking of using the old hole which will be in the bottom of the manifold and a couple of feet of copper tubing and hooking it back up to the old Boost gauge.



Anyone else tried this??



Your thoughts??

 

[Wirenut]

Try it! You may be the first... you can never have too many gauges

 

[CPjMech]

I don't how much you will see with a gauge reading PSI. Exhaust back pressure is usually measured in inches of H2O. With the stock exh on my truck I measured 65 "H20 at 3000RPM full load, that is only 2. 3PSI (I now only have 11"H2O or 0. 3PSI). If you have an open exh sys you may never see anything. But anything is worth a try once.

 

[snowracer69]

Originally posted by CPjMech

I don't how much you will see with a gauge reading PSI. Exhaust back pressure is usually measured in inches of H2O. With the stock exh on my truck I measured 65 "H20 at 3000RPM full load, that is only 2. 3PSI (I now only have 11"H2O or 0. 3PSI). If you have an open exh sys you may never see anything. But anything is worth a try once.

Where you taking your readings from? Everything I've seen, for tuning purposes, you want to take your back-pressure reading from your EGT probe location in the exhaust manifold, and it should run close to the same pressure as your boost, but you would like it to be lower, right? I believe you are reading it downstream of the turbo, where, yes, I'd hope it is almost 0 PSI backpressure, and I want it flowing as best as I can down that way.



Josh

 

[Hohn]

Measuring exhaust backpressure is very wise, and you CAN do it with a boost gauge. A lot of people don't realize that boost pressure and back pressure are usually pretty close to each other in number. At 30psi of boost, you probably have close to that in backpressure.



Comparing the back pressure to the boost pressure is probably the single best meausre of turbo efficiency. You really want your boost to be higher than your drive pressure (backpressure) at all times. The bigger the difference, the better. The want to lowest possible drive pressure for a given boost number. This is known as a "positive" pressure ratio.



The opposite is also true-- when boost is lower than drive pressure, it's very bad. The more that DP exceeds boost, the worse it is. This "negative" pressure ratio causes bad things to happen.



When boost exceeds drive pressure (positive ratio), then lots of good things happen. First, the boost will actually blow some air out the exhaust at overlap, which helps to cool EGT. It also evacuates the cylinder, ensuring you have the most oxygen available for combustion.





When you have a negative pressure ratio, then the exhaust will actually push back the intake charge at overlap. Thus, you have a lot less oxygen in the cylinder. Second, you will have VERY high EGTs, because the hot exhaust is remaining in the cylinder instead of getting pushed out. There's a lot more to consider, but this is the basic idea.



So, in short, you want to measure your backpressure in the exhaust manifold and make sure that it is LESS than your boost pressure by as much as possible.



Justin

 

[CPjMech]

My bad. I forgot drawson was checking in the exh man. I checked mine right after the turbo. Still 65"H20 is a very high reading in the exh system, turbo sure spools up faster now

 

[drawson]

Thanks Guys!!! I Let you know how it works out