Understanding turbo choke

I have always wanted a turbo engine. Now that my engine is at the end of its life I now have the opportunity to build one from the ground up. This gets into a whole mess of other issues that connect back to my earlier post about the limitations of the Gen 1 Chevy Small block, but forget about that for now, instead, lets explore the nature of turbos and why some turbos might not work for a given engine.

I have been trying to learn as much as I can about turbos lately. One of the questions I had early on was how a turbo can support 1000hp, but not work on a large displacement engine with a target HP goal of, say, 600hp because the turbo is “too small”. It would seem that this would contradict itself: how could a turbo flow enough air to support 1000hp, but not work at 600hp? The answer to that question is that turbo compressors work on pressure differentials. Lets take a look at a compressor map to see what I’m talking about:

If you look at this map, the X-axis is the amount of airflow and the Y-axis is the pressure ratio between the amount of boost and atmospheric pressure. Each circle is called an “island” and the number near each island is the efficiency of the compressor where the pressure ratio and flow intersect. Notice how on the right side of the map that the islands tend to sweep upward.

The right edge of this island is the choke line. Any point to the right of this line is operating the turbo in a very inefficient range and can actually damage the turbo.

So where would this turbo operate on a 6.3 liter V8? It would operate where these red dots are if the intended output was around 600hp at sea level. From what I have seen (depending on a number of factors), it usually takes about 8 psi to get to this power level with size of an engine. (I used BorgWarners MatchBot to get this plot)

Each dot represents a 1000rpm increase starting at 1000rpm and ending at 6000rpm, a typical operating range for a pushrod v8. You can see at 6000rpm, the turbo will not provide enough airflow. Even though this turbo was designed for a lot of power, it was clearly intended to be used on small displacement engines which would require more pressure for a given flow.

So what is actually happening inside the turbo when it is run beyond the choke line? I don’t actually know whats going on at the micro level, but I do know that the velocity of the air inside the compressor reaches sonic speed (mach), and the compressor blades can only move air when its below sonic. Most likely, when the air becomes sonic, the it extremely turbulent which creates vibrations and heat, but the compressor blades are not actually moving any more air. This is the limit of flow.

But this is what happens when a small compressor is used on a large displacement engine. The compressor, even though its capable of flowing enough air to support a large amount of power, can only do so at a high boost pressure. You might think that cranking up the boost pressure with this small turbo would push it into an efficient range, but that’s not the case since the airflow requirement also increases as the pressure climbs, and the intersection point of the pressure ratio and airflow keeps moving farther right into the choke area.

At 15 psi, the 6000rpm mark moved deep into choke. But, at low rpm, the turbo is efficient.

Furthermore, since boost is so efficient at making power, increasing the boost on a large displacement engine is going to make so much power where its going to become impractical. The spool up on the turbo is also going to be so fast that the amount of torque created might also become damaging. The engine internals can only take so much!

A larger turbo or twin turbos is required for a large displacement engine. A large compressor simply flows more air at a lower pressure which allows it to work in an application like this. You will find that large turbos designed for large displacement applications might advertise the same horsepower range of a smaller turbo, but the tuning of the compressor is what makes it work for these bigger engines. This tuning is called the Trim. In general, large engines need high flow at low pressures and small engines need low flow at high pressures.

Trim

Interestingly, if you search for turbo trim on google, you find very little describing what the effect of trim sizes actually do. Instead, you find the literal definition of it, which is simply the size of the inducer expressed as a percentage of the exducer. The equation for it is simple, (inducerDia/exducerDia)^2*100. If the trim of a compressor was 1, that would mean the inducer and exducer are the same diameter. Trim has nothing to do with the overall diameter of the turbine or compressor. You can have a 80mm OD compressor with many different trim sizes and the OD will remain the same.

The effect of trim is this: the larger the trim number, the more flow that compressor can produce. The smaller the trim number, the peak flow is less but its more efficient at a higher pressure. So large trim: More flow, less pressure. Small trim: Less flow, more pressure.

I you think about trims in the extreme sense it might help understand how it effects flow and pressure. For example, think of a compressor with an OD of 80mm and an unrealistic and extreme trim of .05. This would mean that the inducer is going to have the diameter of a coffee stir. You can see how such a small inlet is going to limit flow. But the OD of the compressor is very large in comparison and have a ton of leverage creating a lot of suction at the inlet and having virtually no chance of flow reversion. Think of it like a gear ratio, like a very small gear driving a very large gear – the torque is high but the speed is low… As the trim gets bigger, the inducer gets larger and allows more flow, but the ability to build pressure is reduced!

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