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Technical Understanding of the IHI (Why does my Power FALL off?)


New member
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: Tech 103.pdf

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 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.
Thanks for the usefull information :) Just se great


Go Kart Champion
Hey everyone I've got an interesting tidbit to share regarding boosting pressure with APR/Revo/Uni/Giac/etc:

You can actually increase power a little by making an engine slightly bigger or running it a little faster with a bigger cam, but the serious potential for increasing power (say, 50 percent more torque, everywhere!) lies in boosting the average cylinder pressure during the power stroke. Happily, not only is this completely feasible with turbo-supercharging[1] - which can easily generate 3 or 4 atmospheres of boost pressure with a single centrifugal compressor, or more with two-stage turbocharging - but boosting the power this way is easier on the engine than you might think. In fact, peak cylinder pressure and rod stress in a powerplant turbocharged to 2 atmospheres is only moderately higher (perhaps 25 percent) than a normal-charged engine, since less than a quarter of the charge mixture will have burned at the time of peak pressure.[2]
The continuing combustion following peak cylinder pressure is precisely why average cylinder pressure is considerably higher in a forced-induction powerplant. Midway through the power stroke with the piston at 90 degrees after top dead center, supercharged cylinder pressure can easily be three to four times that of a normal-charged engine, with torque over twice as high. True, turbocharging does load the rods and pistons more, and for a longer time, but the loading from higher cylinder pressure is dwarfed by the much higher stress from high engine speeds having nothing to do with the horsepower, the most critical being stretch to the rod and rod bolts when the crankshaft is yanking a piston downward from top dead center at high acceleration during the early part of the intake stroke.
Jeff Hartman. "Turbocharging Performance Handbook" MBI Publishing Company. 2007
1) Turbo-supercharging is just turbocharging. The supercharging suffix refers to the supercharged air going into the cylinder. The compressor is turbine driven. The reason belt-driven superchargers got their more direct name is simply because they were popularized first.
2) This is explained earlier when the author describes top-dead-center pressures and post TDC combustion. Peak pressure occurs "in the 14 to 18 degree range after top dead center"

What he's saying is that peak cylinder pressure is not radically increased with increased boost, due to the relatively slow rate of combustion of the gasses in the cylinder. If one were to draw a graph of cylinder pressure:

Supercharged air would only increase the peak a small amount, but the slope after peak pressure would be considerably less, so average pressure on the piston throughout the compression stroke would be considerably higher.

By the way, I would highly recommend this book so far. Very informative. It's not insulting to my intelligence as an engineer but it is direct enough that anyone could pick it up and expand their understanding of what turbocharging really is.

Feel free to add this anecdote to the OP ;)


I've seen a couple IHI/K03 housings with K04 internals locally. Done to skirt the rules on stock turbo for SCCA class.


Passed Driver's Ed
Came across this awful scan of an IHI RHF5 compressor map:

Crunching the numbers I believe it to be accurate, and it tells the story.

Earlier in the post it was discussed as to the effect of turbine A/R on a compressor map. It should be noted that turbine A/R does not effect the map, the map simply illustrates what the compressor side can do. The turbine A/R can just shift where in the map you are. Anyways....

Decoding the map at stockish boost levels, to make it easy 1 bar of boost, our engine assuming 90% volumetric eff. will flow about 11.5 cubic meters/min into the turbine @ 6500rpm. So draw a horizontal line from 2.0 and intersect it with a vertical line from 11.5 and you can see we are beginning to loose efficiency, but at least still in the map and probably not blowing all that much heat. This is assuming the turbine A/R is adequately sized as to not choke the engine, which we know its not, well at least with the stock DP and its effect on flow through the turbine. So in the real world we are somewhere to the left in the map at a lower pressure.

The map shows us hitting Peak boost 1 bar @ 1800rpm, well it at least states the compressor is capable of such a feat. Look at where the map begins at 2.0PR on the horizontal, draw a vertical line down and we are at approx 3 cubic meters/min. Using a compressor map calculator, our engine outputs 3 cubic meters/min of flow at 1800rpm.

To see the effect of a stage 1 or 2 tune, what are they 18psi? anyways, (18+14.7)/14.7= 2.22 pressure ratio. At 18psi our engine flows 13 cubic meters/min @ 6500rpm . Draw the intersecting lines once again and you will see we are off the map. Regardless of the turbine a/r, this compressor can not support 18psi on our engine, well at 6500 rpms anyways.

Lets look at our midrange though at this boost level, at 5Krpm we are flowing 10 m3/min, and at a 2.22 PR we are in a pretty descent eff. range for this compressor, actually at about all it should be tasked to do(in theory 247chp). Digging deeper we can see that at a 2.2 pressure ratio the compressor is all done at 12 m3/min. That is 6000 rpm with our engine. So if the turbine a/r is not the limiting factor, this compressor can only hold 18psi till 6K rpm with our engine.

In short our turbo is to small LOL, I know captain obvious... If a stage 2 car can hold 18psi till 6K rpm there is nothing even a larger turbine a/r could do to help. But if not, a larger turbine a/r, giving up rpms at spool up would help hold boost on the top end. On a side note the theoretical max output of our compressor is 297 CHP according to the above map(12 m3/min = 29.7 lb/min and rule of thumb lb/min x 10 = crank HP).

Stage 2 proves that adding a down pipe has a similar effect to enlarging the turbine a/r as it extends desired boost into later rpms.

Hope this makes sense, I find it kinda fun to play around with compressor maps, its unfortunate that turbine a/r isn't as easy to figure/plot out.

Compressor flow map calculator:
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