333pg333

Just wondering what people have seen when measuring their X over b/pressure?

Thom

~1.3 bar for 1.2 bar of boost at peak rpm.

divil

I haven't checked mine, but section 28 of the Turbo factory manual says it should be 0.8 +/- 0.1 bar for 0.7 bar of boost/full load/3500 rpm.

964-C2

It will be a lot higher on 6000+ rpm...
I got 25psi back pressure with 13psi boost pressure on my 26/8-turbo

333pg333

Thanks guys. Any more?

Alan 91 C2

Seems like the biggest part of the back pressure is the turbine in the turbo. The crossover pipe is likely to have .1-.3 bar (maybe 3 PSI) at 6,000. The turbo by design converts the exhaust gas into shaft energy within the turbo. If we look at the compressor side of the turbo and allow efficiency of even 60 percent, then 15 psi at 500 CFM is about the expected value for the 23-25 PSI hot side.

For 15 PSI at 500 CFM needs over 5 brake horse power at the compressor shaft. The turbine side must make the 5 hp and the turbine side has losses.

So the short answer is the pressure drop on the hot side will be around 1.5 times the cold side, if the turbo is in the efficient range. The pipe loss is 5-10 percent of total pressure.

So think about the K26-6, optimized for 10-12 PSI and maybe 400 CFM. The efficiency falls off above 12 PSI, meaning more PSI on the hot side. The K26-8 increases the shaft hp to drive the compressor, more flow and PSI. The net is desired CFM and pressure, and at what efficiency.

The math looks like this:
400 CFM/13.2 Pounds of air per CU Ft= 30 pounds of air per minute
at an AFR of 12.5 then 30/12.5= 2.4 pounds of fuel per minute
pounds per hour= 2.4*60= 145 pounds per hour
divide the pounds per hour by 4 cylinders 145/4=36 pounds per hour per injector
Just as Porsche specified for the 951

TurboTommy

All good thoughts, but I don't think the formula or theory is that consistent.
There isn't really a direct correlation between compressor efficiency and turbine inlet pressure; it effects it some but only a small part.
The turbine side has always interested me and the way I understand it is that the exhaust gases have 3 components that provide the turbo shaft power: pressure, heat and volume; of which only the pressure is the evil component. It obviously takes pressure to create the pressure differential from the inducer to the exducer, but once that's there, volume and heat can take over and provide a great portion of power. So another words, if the hot housing is larger (it will take longer to build pressure = laggier), it will be able to utilize more of the heat and volume to keep up with the demands, instead of having to necessarily increase pressure.
Unless, the compressor is ridiculously over-sized, you would notice on a compressor map that a turbo has to increase its' speed for higher CFM, at a given boost, which will also land it in a probable lower efficiency area; yet high flow turbines will barely have to increase the turbine inlet pressure to keep up with the higher CFM as RPMs increase.
Small hot-housing are the oppsite: the pressure increases rapidly, but as flow increases the restrictive housing can't utilize the rest of the mass flow of the exhaust, and therefore has to increase the pressure component to keep up.

Alan 91 C2

I understand your qualitative response. But turbos are quantitative machines.

And as such the math I presented is a simple overview of how the turbo works.

You are correct the larger hot house and compressors have vastly different boost maps. (and efficiency varies throughout the map from 50-80 percent) I was trying to keep the analysis somewhat simple. I think everyone can understand the pump principle, where the impeller imparts kinetic and potential energy to the mass flowing through the pump. A portion of the brake horse power is converted to heat.

So back to more depth in our challenge of turbo operation.

Assumption1. The mass flow of the intake air plus minimal combustion products=mass flow of the exhaust. Here is where casual logic fails; because the air is hot and should be more volume, but the mass per unit volume is very low.

So a given mass flow (intake air plus combustion products at say 1/12.5 pounds per pound) is flowing over the turbine. So lets say the turbine flow is 1.1 mass of the intake. Assumption 2.

Now the turbo is like a teeter tooter. The energy on the turbine less losses equal the shaft hp. Since the compressor is a pump, even if we are in the sweet spot, efficiency is at best 80% Assumption 3.

So we have the turbine at 1.1 mass flow less efficiency (say .8 in the sweet spot) Input energy =1.1*.8 =.9 Assumption 4.

the .9 energy is driving the compressor, also 80 percent efficient. Net= .9*.8= .72

Since we normalized the mass flow (with the 1.1 multiplier above) the only variable left in the equation mass*PSI in= Mass * PSI out

assumption 5. We embedded the efficiency losses in the mass for simplicity.

Best case the PSI turbine= 1/.72 PSI compressor

So back to our example at 15 PSI boost we have ~21 PSI turbine plus 2-3 PSI pipe.

And maybe the best way to show how the turbo works is to consider the air compressor most of us have say 3-5 hp, and we use the air sander equivalent to 1/4 hp. We can bleed down the compressor with the 1/4 hp load because of the efficiency loss pumping air and the efficiency loss in the air motor (sander). Both the compressor and sander are much lower efficiencies at 30-50 percent with line losses a little more also.

JacRyann

I've tried to combat the diving torque curve by maintaining a flat boost-curve.

1. K26/6 with manual-controller drops 3-4 psi by redline.

2. Upgrade to 38mm Tial, still dropping boost.

3. Went with EBC and was able to program in perfectly flat curve.

4. Programmed rising boost-curve, 18 to 21psi by redline. Got flatter torque-curve with higher HP than torque

5. Tried 21-25psi boost curve, not much improvement due to turbo being way off map


With k26/6 above 20psi @ redline, I was seeing pressures in the crossover of 55-65psi.

TurboTommy

Turbocharger manufacturers have various turbine housing sizes available for a given compressor; and they will have corresponding different turbine inlet pressures for the same engine exhaust gas.
Your math does not reflect this.

333pg333

The reason that I'm asking this is not so much on stock setups but modified. I wasn't there but I believe we reached 40psi on the dyno last night and I wanted to get an idea of what other people had seen. This is on a setup where there is a very small % that is still stock on this motor and ancillaries. Discussing this with a tuner (not from last night) who uses pressure sensor readings as one of his main sources during tuning. He said for track cars you want about 2:1 the boost pressure. For drag 1.5 times. This being a generalisation. So we maxed out at 26psi when we saw 40psi backpressure in the Xover. Seems about right according to him.

Alan 91 C2

Hello Jac
Your comments are correct, you can preserve operation in the sweet spot by insuring all the mass goes through the turbine, not the waste gate.

Tommy,
Your comments do not present any physics. Yes, manufacturers have various hot and compressor packages to optimize the mass flow (Rpm, engine size, where the torque is desired) for each application with peak efficiency. But the turbo acts just like my air compressor example.

It was not my intent to get into aggressive discussion, I like this board and was only trying to answer the original question in a simple way.

Alan 91 C2

Hi Patrick,

The turbo application technology does not change from stock to highly modified. The engine is an air pump, the more air you can put in, the more fuel and power you can make.

The numbers you discuss validate the numbers I presented with the turbo properly selected for the mass flow (horsepower when fuel is added) you have as your target.

The various hot and cold side combos just cloud the issue of mass flow at peak RPM=peak HP.

JacRyann

Patrick, sounds like you've got a well-matched turbo for your boost-levels, between 1.5-2.0:1 ratio.

In my case, I was experimenting to gather useful data. With an undersized turbine, trying to make it generate flow & boost way outside of its efficiency ranges required excessive turbine inlet pressures. The max for K26/6 is about 18psi rising to 20/21psi @ redline (about 300rwhp). I measured about 40-45psi in the crossover, right around your 2:1 ratio. More boost than that saw a much more rapid rise in backpressure and resulted in no HP gains.

Alan 91 C2

Jac,

Your experience verifies the K26-6 was optimized for around 300 ish CFM, and you move so far off efficiency the back pressure consumes any addition mass flow HP you make.

One of the things to know is the all waste gates have a throttling range. The throttling range is the pressure the WG begins to open, and the pressure to full open PSI. The throttling range for the WG was set by Porsche for stock good performance. The EBC modifies the throttling range to give a much faster response and narrow throttling range. So you can get right to the boost you want and pop the waste WG at the correct rate to not over boost, but also not steal critical mass flow from the turbo. It turns out for modified cars, with big pressure going to the turbo, say 25 PSI, just barely opening the WG can steal your power. Where as the stock back pressure of say 15 PSI backpressure would only flow a small mass flow through the WG as it cracks open.

The recurving of the WG opening by the EBC is why you can get higher boost and stay at the edge. A manual boost controller relies on throttling range of the mechanical adjustment and is not as "smart" about high RPM boost bleed.

Hope that helps.