Glenn Butcher
16th April 2003, 08:27 PM
What does the "A/R" mean on a turbo? - taken from another Forum.
The exhaust and compressor housings on turbo chargers use a "scroll" design. For example, the exhaust housing's scroll is where the exhaust gasses enter the housing and are directed at the turbine. It's basically a smooth, tubular chamber that surrounds the turbine with a slot all the way around that acts as a nozzle to direct the exhaust gasses at the turbine. It's called a scroll because it slowly gets smaller in diameter as a goes around the turbine. This pressurizes the gasses, forcing them out of the slot/nozzle at a fast rate. In turbo-terms, the scroll is measured by the cross-sectional area of the scroll's "tube" (A) and the distance from the center of the "tube" to the turbine shaft (R). The values by themselves are not meaningful to the user and for the most part, R does not change much for different housings, but by dividing R into A, you get the A/R ratio. So, the A/R ratio of the exhaust housing refers to the size and shape of the scroll that is cast into the housing. It basically determines how restrictive the housing will be, versus how quickly the turbine will spin up. A lower A/R ratio (smaller scroll area, A) results in a more restrictive housing. This restriction speeds up the exhaust gasses and increases the amount that the gasses will expand. It's the speed and expansion of the gasses that causes the turbine to spin. So with a low A/R ratio, the turbine will spin up quicker, but as engine output and rpms increase, the restriction of the housing begins to build up too much back pressure on the engine, which reduces performance. A good rule of thumb for when there is too much back pressure is when the pressure in the exhaust manifold is more the half of the pressure in the cylinder. So basically, a larger A/R ratio will improve your engine's top end, while losing some mid range power and increasing turbo lag. A smaller A/R ratio will help the bottom and mid-range, but may effect the top end.
On the compressor side, the housing also features a scroll design, but it has the opposite function. The air leaving the compressor turbine has a lot of speed, but not much pressure. The scroll on the compressor housing starts small and gets larger as it approaches the compressor outlet. This collects the air and builds up air pressure. So, the compressor housing is designed to convert the speed-energy of the air coming off of the compressor turbine into pressure-energy, which is much more useful to an engine.
Continued
The T4 is a smaller size then a T04, I'm not sure exactialy how a T4 differs from a T04.
The Txx is refering to the size of the housings. Garret makes several different kinds of compressor and exhaust housings. There are two main measurements for a housing. The first is the overall size of the housing. There are several different sizes. Most of them start with "T", then have a number or two. Generally, the larger the number, the larger the housing. So, for instance, a T2 compressor housing is smaller than a T3. A T04 housing is very large, and the T60 and T70 housings are truly monstorous. (Usually only used on diesel truck engines.) What size housing you want will depend on how much airflow you want to run. A larger housing is capable of passing more air easier than a smaller one. To pick accurately, you have to know A) how much air your engine sucks in per combustion stroke (the displacement of the engine gives you a good estimate of this) and B) how much boost pressure you want to run, because the higher the boost level, the more air you're flowing. When it comes to housings, bigger is usually better - to a point. The price paid for a large compressor housing is very slow spool up. Generally you want to have the smallest possible housing that will still flow enough air at max boost to remain decently efficient.
The exhaust and compressor housings on turbo chargers use a "scroll" design. For example, the exhaust housing's scroll is where the exhaust gasses enter the housing and are directed at the turbine. It's basically a smooth, tubular chamber that surrounds the turbine with a slot all the way around that acts as a nozzle to direct the exhaust gasses at the turbine. It's called a scroll because it slowly gets smaller in diameter as a goes around the turbine. This pressurizes the gasses, forcing them out of the slot/nozzle at a fast rate. In turbo-terms, the scroll is measured by the cross-sectional area of the scroll's "tube" (A) and the distance from the center of the "tube" to the turbine shaft (R). The values by themselves are not meaningful to the user and for the most part, R does not change much for different housings, but by dividing R into A, you get the A/R ratio. So, the A/R ratio of the exhaust housing refers to the size and shape of the scroll that is cast into the housing. It basically determines how restrictive the housing will be, versus how quickly the turbine will spin up. A lower A/R ratio (smaller scroll area, A) results in a more restrictive housing. This restriction speeds up the exhaust gasses and increases the amount that the gasses will expand. It's the speed and expansion of the gasses that causes the turbine to spin. So with a low A/R ratio, the turbine will spin up quicker, but as engine output and rpms increase, the restriction of the housing begins to build up too much back pressure on the engine, which reduces performance. A good rule of thumb for when there is too much back pressure is when the pressure in the exhaust manifold is more the half of the pressure in the cylinder. So basically, a larger A/R ratio will improve your engine's top end, while losing some mid range power and increasing turbo lag. A smaller A/R ratio will help the bottom and mid-range, but may effect the top end.
On the compressor side, the housing also features a scroll design, but it has the opposite function. The air leaving the compressor turbine has a lot of speed, but not much pressure. The scroll on the compressor housing starts small and gets larger as it approaches the compressor outlet. This collects the air and builds up air pressure. So, the compressor housing is designed to convert the speed-energy of the air coming off of the compressor turbine into pressure-energy, which is much more useful to an engine.
Continued
The T4 is a smaller size then a T04, I'm not sure exactialy how a T4 differs from a T04.
The Txx is refering to the size of the housings. Garret makes several different kinds of compressor and exhaust housings. There are two main measurements for a housing. The first is the overall size of the housing. There are several different sizes. Most of them start with "T", then have a number or two. Generally, the larger the number, the larger the housing. So, for instance, a T2 compressor housing is smaller than a T3. A T04 housing is very large, and the T60 and T70 housings are truly monstorous. (Usually only used on diesel truck engines.) What size housing you want will depend on how much airflow you want to run. A larger housing is capable of passing more air easier than a smaller one. To pick accurately, you have to know A) how much air your engine sucks in per combustion stroke (the displacement of the engine gives you a good estimate of this) and B) how much boost pressure you want to run, because the higher the boost level, the more air you're flowing. When it comes to housings, bigger is usually better - to a point. The price paid for a large compressor housing is very slow spool up. Generally you want to have the smallest possible housing that will still flow enough air at max boost to remain decently efficient.