#6 vs #8 hotside anyone measured the backpressure in the crossover pipe
09-18-2006, 11:33 AM
#16
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Originally Posted by ross255
GPF, no 2:1 is not OK, it is the max you wan to see, and as fast951 said, even that is not good at all!
It will probably explain a few blown headgaskets using a 26/6 to try and get 18psi.
Your data does show that even with a bigger coldside, so you would be on boost at lower turbo rpms, even with the wastegate slightly open to control boost, you still get to high a backpressure figure of 30 psi with a #6 hotside.
Now if only someone has some data on a #8 hotside for us to compare this to.
It will probably explain a few blown headgaskets using a 26/6 to try and get 18psi.
Your data does show that even with a bigger coldside, so you would be on boost at lower turbo rpms, even with the wastegate slightly open to control boost, you still get to high a backpressure figure of 30 psi with a #6 hotside.
Now if only someone has some data on a #8 hotside for us to compare this to.
I checked the backpressure on my 26/8 today. Boostpressure 12,5 psi at 6200 rpm, backpressure 25 psi. No cat.
09-19-2006, 12:44 AM
#18
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"I can't believe no one has even mentioned turbine efficiency in this thread, it is the sole reason for the K27 compressor affecting backpressure using the same K26 model turbine. The turbine has an efficiency curve, and generally is designed to work in correlation to the same shaft RPM as the compressor wheel. When you put on a bigger compressor wheel you can run the turbine efficiency off it's chart creating excessive lag, and excessive backpressure, ala GT2540R."
Yeah,
turbine efficiency is a difficult concept to understand and rarely discussed. In laymens terms I think it's a little like this:
A larger compressor wheel needs more exhaust space and torque to turn the larger capacity compressor. So, if you don't go up in size on the exhaust side, you'll need more and more exhaust flow (backpressure) to make up for the need for more torque on the rotating assembly. One could argue that the larger compressor, now, doesn't have to turn as fast. True, but it's still a net loss, because the need for more exhaust energy is worse than the compressor's ability to produce boost.
Sort of the same thing:
imagine putting a larger prop ("trying" to push more water) on a boat with the same engine. It will be laboring harder (backpressure), and yet the boat, still, won't be going any faster. Kinda the same principal.
Yeah,
turbine efficiency is a difficult concept to understand and rarely discussed. In laymens terms I think it's a little like this:
A larger compressor wheel needs more exhaust space and torque to turn the larger capacity compressor. So, if you don't go up in size on the exhaust side, you'll need more and more exhaust flow (backpressure) to make up for the need for more torque on the rotating assembly. One could argue that the larger compressor, now, doesn't have to turn as fast. True, but it's still a net loss, because the need for more exhaust energy is worse than the compressor's ability to produce boost.
Sort of the same thing:
imagine putting a larger prop ("trying" to push more water) on a boat with the same engine. It will be laboring harder (backpressure), and yet the boat, still, won't be going any faster. Kinda the same principal.
