Choosing the Right size Turbo:
Choosing the right size turbo for your vehicle can be a daunting task. With what seems like an infinite amount of variables that can affect your choice; A/r, trim, compressor size, high hp, high torque, lag, etc. This process can be a challenge, but with the general information you will find below you can have a better understanding of how a turbo works and how to figure out the correct turbo for your application.
1: How a turbo works.
A turbo works by spinning the turbine side of the turbo with hot exhaust gasses. The turbine wheel is connected to a common shaft which spins the compressor wheel of the turbo sucking in air from the atmosphere, compressing it and ultimately forcing that now denser air into your engine.
Trim describes the ratio of the inducer to the exducer of either wheel of your turbo.
To understand trim we first need to understand how compressor and turbine wheels work. Each wheel has an inducer and an exducer.
On the compressor wheel the inducer is working to suck in atmospheric air while at the same time the exducer is pushing that air through the compressor housing.
On the turbine wheels the inducer and exducer are working opposite the compressor wheel. The hot gasses from your exhaust hit the larger area of the inducer and expel the hot gasses out the exhaust via the exducer.
So taking the measurements of the inducers and exducers we can find the trim of the turbo by using the following formula: Trim= ( Inducer² / Exducer²) x 100.
So for example let's use the Garrett direct replacement for the RB26DETT, the GT2860r.
Compressor Trim: ( 47² / 60² ) x100= 62
Turbine Trim: ( 54² / 47² ) x 100= 76.
Generally speaking the higher the Trim, the more air a specific turbine or compressor wheel will flow.
3: A/R (Area Over Radius)
The A/R of a turbine or compressor housing directly relates to that specific housing’s ability to flow. The larger the A/R, the more that housing can flow.
The turbine side of the turbo will directly relate to how you want your car to make power.
A smaller turbine housing will have a smaller peak power but will generate boost pressure quicker resulting in more torque and a more responsive powerband.
A large turbine housing will have higher peak power but will take longer to spool (lag).
The best tool you can use to pair a turbo to your specific engine. A compressor map is used to show how much air a turbo can flow.
Before looking at a compressor map you should double check that the displacement of your engine and the HP range for that specific turbo match the manufacturer recommended ratings for your given power goals.
Corrected Air Flow (Lbs/ Min): Roughly 1 Lbs/Min= 10 Hp
Example: If you want to make 600hp (at the crank) you would need to find a turbo that can handle 60lbs/min.
***Drivetrain loss is typically about 20-30% from crank to wheel. SO if your power goal is 600hp at the wheels, you would want to look for a turbo that can flow at least 65lbs/min+
The ratio between the atmospheric pressure in front of the turbo (14.7psi) to the pressure being forced into your engine from the compressor housing.
Pressure ratio is found using this formula: Pressure Ratio= ( 14.7 + Desired Boost PSI) / 14.7
So for example let's say I Want to run 19lbs of boost on my RB26 the equation would read: (14.7 + 19 ) / 14.7= 2.29
So we can now use the compressor map, using your desired HP (corrected air flow) and desired Boost level (pressure rating) to chart out where a given turbos efficiency will land.
Each oval on the Compressor Map is called an efficiency island and directly relates to where the turbo will be most efficient. You can use your values (Pressure Ratio and Corrected air flow) to find a turbo that is most efficient for your application.
Simply put the lower the efficiency, the hotter it makes the air for a given boost pressure.
Selecting a turbo:
First you need to ask yourself some questions:
What HP do I want to make?
How do I want to make that power? (Responsive? Peak Higher HP? Low end? High RPM?)
What is my desired Boost Pressure?
What fuel am I going to run?
I want to upgrade my RB26 to a single turbo application making a responsive 650WHP running around 18PSI of boost on pump gas.
First i'm going to narrow down my search to turbos that are within my 3.0L displacement and at least 900HP rating (accounting for drivetrain loss).
Then I am going to Find my Pressure Ratio: (14.7 + 18 ) / 14.7= 2.22
Then I am going to find my Corrected Air Flow: (1lbs/minute= ~10hp) = 70Lbs/min (accounting for drivetrain loss)
So now I can take my values of PR 2.2 and 70Lbs/Min and correspond that with a turbo that will fit my desired application.
If a turbo says it is rated at 1000hp, that number is often found using the highest engine displacement, with around 50psi of boost.
Bigger turbo means more power. You can have too big of a turbo, the turbo can actually produce more boost than your motor can breathe, effectively choking your motor.
Bigger is not always better. Depending on your application a smaller frame turbo can actually net more power in a desired RPM range.
General Rules Of Thumb:
If you want to make 600hp then generally you should buy a turbo that is rated 35% higher than your desired HP rating.
If you are on the edge of being out of your turbos efficiency range, don't push it. Find another turbo that fits better within your operating range.