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More McMath - Just Pressure Ratios
The whole Pressure Ratio discussion, Pr (DL) and r (DW) or PR (SJ), makes me
think some about the accuracy of the stage II mods. I personally have
measured 22psi on my q with my old "stage II" box mod, which at my altitude,
means a Pressure Ratio of:
Po = atmospheric pressure 14.5 @ sea level, 12.6 @ 5000 ft, and 10.7 @ 10,000
Pb = Absolute Boost Pressure = 22psi (k)
PR = Pb + Po/Po
PR = 22 + 14.5/14.5
PR = 2.52
Turbine speed: ~125,000
Now, lets take a drive to Lawsons' house, and see what happens.
PR = 22 + 12.2/12.2
PR = 2.80
Turbine Speed: Supersonic, vortex = heat soak
(Beyond a given speed a turbo will spin so fast, that any given speed only
increases heat, no PR or Flow increase, on a stock k26, that is at 2.7PR and
a ~125,000rpm turbo speed)
Yikes sakes, batman! But that isn't even the worst, let's take a drive up to
the audi viewing area at Pike Peak (assume 10,000ft)
PR = 22 + 10.7/10.7
PR = 3.05
Turbine speed: Supersonic, Boom immenent, (heat soak complete on the
aluminum cold side, causing warpage of components, once they touch... And
Yeeha. Considering the Pressure Transducer is only accurate on the MC motors
to 2.0 bar, looks like we have a problem with heat, even at my Chicago home.
What does this mean? In terms of heat, let's let DL and DW fix their
equations and calculate for us. What concerns me more, is what this all
means in terms of the computer. The absolute best accuracy one could hope
for on the measured Boost Pessure, is the best accuracy of the PT. That is a
given at 2.0 max. Since the PT translates Pb to a voltage, and that voltage
max is 4.9~5.0, and that voltage is achieved before 2.0 (btmt), what is
happening beyond that?
Well specific to the tables, nothing. Once that maximum voltage is achieved,
the table input goes flat to 4.9 volts. Translated to common language, this
means that, by definition, all boost in excess of 2.0bar will be ASSUMED by
the computer to BE, 2.0 bar (or less, depending on how many times you whack
the PT at maximum design parameter). So, in my case (and hardly "special"),
this means specifically, at best, from 14psi to 22psi, there is NO difference
in parameters accepted in the tables by the computer. So, add half again as
much boost, and the computer changes nothing. Hmmmm... Can see the scramble
for 3.0bar PT.
Taking this a step further, since your Pressure Transducer is only measuring
to 2bar, what is letting you have the excess? Baseline spring pressure, in
some cases, yes. Interesting to note, in my case, the WGFV was adding
pressure to the top of the WG after 2.0 indicated, so in reality, it seems
that the whole table of PT readings must be "off". Makes ya think on what is
really in that box, donut?
PR of 2.5 on a stock k26 turbo is the absolute maximum PR to maintain 65%
turbo efficiency. Sounds ok, however, to get 2.5 PR you are exceeding
120,000rpm on a turbo designed to run at 100,000rpm. What Ross learned the
hard way, I argue, may not be the exception. Especially, if you are running
What has saved us all from impending 'boom'? Well, thankfully, your other
sensors are doing their respective jobs, specifically, the charge air temp
sensor, and the engine temp sensors. As you heat soak the turbine, the water
and oil temps start to rise, as does the charge air temp sensor (CATS). The
water is slower than the CATS, the CATS is immediate, the water longer term.
But given the accuracy of the water temp sensors in our cars, I'd be wary of
that 'save'. The CATS is very accurate, but a constant ck of it's
connections (rustling around the CATS tends to make the leads touch, a major
nono), and make sure the boot is protecting it from the elements.
Once Dave & Dave do some McMath, it will be pretty easy to show why more
isn't necessarily better when it comes to boost levels on a stock k26 turbo.
I will argue, that any boost pressures in excess of 2.0 is heading you in
the wrong direction with the wrong turbo. Adding an IC addresses the PR to
Density to an extent, but turbo efficiency is still a hindrance. Remember
increasing turbo speed only increases the PR, NOT the flow. So now, heat
becomes your biggest enemy.
Some thoughts. Easy on that "gas" when you are getting "high" :)