Here is a big one, I am going to let a big secret (for some) out of the bag here for you guys so bear with me through this.
First of all, know that there are compressor housing A/R’s and turbine housing A/R’s. Since turbine housing A/R is likely the context of your question I will address that here.
A/R ratio is a term we use in the industry to differentiate flow capacities between like housings with similar exterior dimensions with different size volutes. What is a volute? A volute is the spiral shaped cone on the inside diameter of the turbine housing that begins right about the point in the housing where you can no longer see into it, after the inside diameter changes from the shape of the flange opening to the shape of the volute (usually a teardrop shape). The volutes job is to harness the energy (heat, pressure, velocity and sound) from your motor and concentrate it onto the turbine wheel to generate power to turn the compressor wheel. The number designation given is a ratio derived by taking the cross-section area of the beginning of the volute over the distance from the middle of the cross-section to the center of the turbine wheel (axis).
So what does this number tell you about flow? By itself, …nothing. This number is used to compare housings of similar castings with the only difference being the volute. Here is where I am going with this. Say you have a housing whose area is 1.45 in2 over a radius of 1.69 inches, that makes the A/R= .85. Well, say you have another housing whose area is 1.8 in2 over a 2.1 inches radius. Well, the A/R is still .85 so that means they flow the same …right? WRONG! So what does that tell us? You cannot compare turbine housings between families just by the A/R. Reason I go into this is that I have people ask me all the time what the A/R of a FP30 housing is. Why? There is only one FP30 volute, so how is knowing what it’s A/R is relevant? You can however compare the A/R between all T31 housings, or all T4 housing, etc.
The shape of the volute can affect the way the potential energy is harnessed. The closer to the shape of a teardrop the volute takes, the easier energy is transferred around the housing and into the turbine wheel. Most OEM castings like the 7cm Mitsu housings have a compromised volute (teardrop cut in half) for water line clearance to the bearing housing and ease of casting. On the other hand, most aftermarket housings like the FP30 and Garrett T31 housings have a better shaped volute patterned after Garrett Motorsports housings for maximum efficiency.
Here is where A/R comes in to play. Say you are buying a straight Garrett 50 trim with a Stage III turbine wheel. When it comes to the turbine housing you have 3 popular choices. You have a .49, .63, and .82. They all have very similar exterior dimensions and look exactly alike but on the inside they are very different.
NOTE: The following scenarios include hypothetical spool times and performance numbers used to illustrate the difference between turbine housing sizes and may not necessarily reflect the performance characteristics of this specific combination on your car exactly.
The .49 has the smallest volute so it will make our turbo respond fast, with the least amount of lag of the 3 choices. This small volute makes it spool fast but once the turbo is spooled and the wastegate is open, this small volute now becomes a restriction to the flow of exhaust gas speeding out of the manifold looking for freedom to the atmosphere. Backpressure (pressure that builds in the manifold before the turbos turbine) builds to 63psi before the turbine and less and less exhausted air/fuel mixture is allowed to exit the combustion chamber which limits your horsepower. You reach 20psi of boost by 3400rpm but because of your diluted A/F mixture in the cylinders you reach 345 hp by 5700rpm at 20psi. This makes a great autocross turbo because you make plenty of power down low with enough torque at the bottom of third gear in turn 4 to pick up half a second lap time.
Say you decide to go with the .63 turbine housing. Instead of 63psi of backpressure you only get 33psi. This allows a cleaner charge in the combustion chamber. A lot less backpressure builds in the manifold and as a result you make much more power on top, as much as 360hp by 5700rpm. The penalty is that instead of reaching 20psi of boost by 3400rpm, you don’t see 20psi till 3700. This can be a much better compromise for a car driven on the street that sees occasional duty at the local drag strip.
All out setups that demand every ounce of performance from their turbocharger require the largest A/R they can tolerate within reason. The .82 housing offers the least restriction to flow of the three. You may not see 20psi till 4000, but you’ll be able to hit 380hp on pump at that boost. Backpressure is kept in the high 20’s.
Restriction varies with mod level. Guys with fewer mods that affect VE are less affected by the restriction a smaller housing. So, for a guy with a stock motor with basic upgrades a .49 housing might be nice. By the time you add heavy cams, port the head, install a sheet metal intake many, etc. even a .63 A/R housing might be out of the question. Conversely, a guy with a stock motor might not see any better backpressure readings with a .82 than with a .63, now all he has is a lazy car. For help on where you should start, share your data with friends that have similar setup’s that have backpressure data, or get the advice of an experienced turbo professional.
Most people never know what their back pressure is. I mean how many of us really have a pressure transducer tapped in their manifold? I’ll tell you, I see it without fail. All these guys have that ridiculous EGT gauge tapped religiously in their #1 runner. I honestly think only the Autometer blinky light A/F meter for a narrow band OEM O2 sensor a more worthless piece of shit gauge to occupy gauge space, but I digress. You don’t have to have a gauge permanently mounted in sight for backpressure. Only need to make this measurement after you make a change that affects your engine VE. VE is volumetric efficiency. VE is a measure of your engines ability to move air through it. Head porting, camshafts, intake manifolds, air filters, intercoolers, turbos and exhaust systems all affect your engines VE. Examples of modifications that don’t affect your VE are fuel injectors, piggyback ECU controllers, blow-off valves, fuel pressure regulators, fuel pumps, and MSD ignitions. You also don’t have to have a fancy stand-alone ECU with an expensive 5 bar MAP to take backpressure measurements. Next time you are in AutoZone, take a look in the hillbilly section. Next to the chrome naked girl silhouette mud flaps you’ll find cheap oil pressure gauges. Pull that stupid EGT probe out temporarily and run some copper tubing to the hole with a 1/8 NPT compression fitting. Run enough tubing to protect the gauge from the heat of the exhaust manifold. Then run rubber tubing to a generic fuel filter assembly, then run more tubing to the cockpit to that bitchin oil pressure gauge. The fuel filter acts as a dampener to reduce the pressure oscillations from the exhaust “putts” and allow you to make a cleaner measurement. Get a friend to ride with you to watch the gauge. The gauge may not even flinch till you hit boost, but then should rise quickly with boost and continue climbing with RPM till you reach your engines max VE point then taper off (usually around 5700 with a stock intake manifold).
So now that you have your backpressure reading how much is too much? Generally speaking, you don’t want more than a 1:1.5 ratio of boost to backpressure. So if you’re at 20psi of boost you should not see more than 30-35psi of backpressure. If you do then you should upgrade the turbine side, you’ll make more horsepower for every pound of boost you run.
Turbine wheels and exhaust system diameters can also affect your backpressure reading, but to stay on topic I will address turbine wheels and pipes later on. For the most part, turbine wheels being part of the CHRA, are not easily changed by the average do-it-yourself tuner. For now, just know that if you upgrade your housing and your backpressure reading doesn’t come down where it should, your problem might be your turbine wheel. If this is the case or if larger turbine housings are not available you might consider a complete turbo change.
Stay tuned…
