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Old 05-04-2011, 09:41 AM   #1
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Technical Understanding of the IHI (Why does my Power FALL off?)

Lets have some technical discussion. Sticky if you want, I feel its good to have an understanding of these matters. It helps better diagnose problems, and frankly, its just plain cool. You can never go beyond the realms of phyics, thats why when you explain the theory behind it and get a better understanding of it, you can solve problems easier.

I wont go into exactly HOW to read compressor maps should be read, but this link will do a fantastic job in explaining what you're looking at. It also goes into more about the different type of turbos and is EVERYTHING you need to understand your turbocharged engine better:

Basically its a mathematical means to determine turbo efficiency and how well you can hold boost without:
1. Surging your turbo (Having too much pressure to counter spin the turbine wheel)
2. Running out of breath. (Too much flow and too little pressure)

Here is the compressor map for the KKK K03 turbo which was used in the FSI engine....very very similar to the IHI RHF5 Compressor:

The Y-Axis is pressure ratio relative to atmospheric pressure. The X-axis is volumetric flow rate through the turbo compressor.

As you can see (Well....sort of), when you have low boost (stock 10-12psi), and go further up the RPM range, it'll hit the "Choke Line". The choke line is exactly what it sounds like, the point at which the turbo cannot hold the pressure and enevidtably loses boost with increasing flow rates.

So why does stage 1 do it so drastically?
16psi is roughly a 2.0 pressure ratio. Its requested sort of in a reverse parabolic shape across the RPM range:

Taken from KaJ's logs

Notice the upper left graph. At the end of the RPM the requesting boost is ACTUALLY at 8psi (1.5 ratio). Again, go back to the compressor map. This is the turbocharger choking itself and falling out of its efficiency range. But theres another reason why. What can it be?

I'll tell you what it is! Guess what else starts to choke the turbo? Thats right...the downpipe. Especially the stock downpipe. Its designed to keep emissions low and sacrifices some of that nice free flow to meet the standard. Now you can see why stage 2 with a free flowing downpipe is a nice touch to your tuning needs. I wont neccessarily go into the whole 2.5" vs. 3" debate (maily because different tuners tune for different things) but both are considered "free flowing" exhausts. They allow for better flow and less choking.

With that being known, you can refer back to the compressor map and see that with the right boost and the right flow rate, you can hit the optimal edge of the map and create some nice high RPM power, which is exactly what stage 2 tunes do.

What about bigger turbos?
Well...plain and simple. The only thing that changes for a bigger turbo is where the map is on the scale. It essentially moves up the flow range and can allow for more boost. Hence why you see more lag per more volume with bigger turbo systems. However, most tuners know that people love their low range torque. That is why you tune as close to the surge line as you comfortable with in the lower flow ranges.

Here is an example of what I'm talking about (K04 Content)

That line is the "performance line" plotted across an RPM range. This line is close to the surge line but within the turbo limits. Right around the higer rev range you can see where this turbo shines. But now you have a decision, you can tune for boost, or you can tune for flow. Thats where almost every tuning company is different. Theres a trade off between high boost and surging the turbo and destroying it and running low boost and have less power. Its all about what the end user wants. If you can find your happy medium, then more power to you!

You can see, however, if you tune for boost and basically follow the surge line up, it'll choke out up top and start running around the speed limits of the turbo. Not only will you DROP OFF power drastically, but you can risk grenading your turbo. Linearity is quite common and normal tuners will tune for the wastgate cycle to allow the compressor line to run RIGHT THROUGH the middle of the map (The most efficient point)

Also, notice the difference between the IHI and the K04. Look at the MASSIVE difference in Flow rates the turbos can handle. (X-Axis) What does this all mean? In the most simple terms....

IHI Compressor is too small and inefficient to hold pressure at upper RPMs

I'll update this later. I just had to brain dump most of the content to get an idea of where I was at in the conversation. If you have any questions, comments or concern or even things I should add, I'll be happy to do so.

Keep it technical and on topic.
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Last edited by grambles423; 06-08-2012 at 09:54 AM.
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