Often when building custom turbos people will want to make a good deal of power and still have decent spool time. The best way to accomplish great spool up on a higher flowing turbo is to put a smaller turbine housing on it. A good example is taking a holset hx40 and replacing the factory .89 a/r housing and replacing it with a bep .55 a/r bep housing for a dsm. The hx40 with the bep housing will be limited to 550-600 hp range instead of the 700 hp mark that people have made with the .89 a/r, but the spool time will increase which makes the turbo more streetable. Another good example is having us machine a garrett t3 .63 a/r turbine housing to fit an hx35 or hx40. This will limit what this turbo is capable of flowing, but it will bring down the capable flow rates of the turbo down to size with a smaller engine to help spool time and over all efficiency. A good example is if you pair an hx35 with a 16 cm^2 turbine housing with a 2.0 liter 4 cylinder motor. If the car can ever spool that turbo up it will make huge amounts of power, but it wont start seeing boost until around 5,500 rpm in 3rd gear. If you had us machine a .63 a/r turbine housing for the same hx35, then the spool time will be moved to the 3,000-4000 rpm range depending on your boost level and it will still be capable of 450-500 hp. The turbine housing choice that is best for your application depends on the displacement and number of cylinders of the engine that you are turbo charging. Another deciding factor is what kind of power you are looking to make and drivability. When choosing a turbine housing you are sacrificing capable horsepower for spool time.
Often people will ask: should I go t4 or should I go t3 for my exhaust housing? This best way to answer this is by asking your self what kind of HP you want to make, and what kind of spool time you want to see. Also keep in mind the motor that you are using too. If its a 2.0 and your trying to make 450 awhp and you want to have decent spool time, I would recommend that you choose a t3 .63 a/r turbine housing or close. If you are turbocharging a 5.0 v8, the motor is naturally flowing 5 liters of air, so it makes sense that you need to used a larger turbine turbine housing in the t4 format to avoid choking the flow of the engine at higher rpm. If you want decent spool time you want to put a turbo on the car that has flow rates close to the engine. If you use a turbo that flow less than the motor at 6500 rpm, then at 6500 rpm at wide open throttle you will feel the car stop pulling because the motor is having a hard time flowing more air because turbo turbo is limiting the flow. The fix for this would be to upgrade the turbine housing or the turbine wheel and compressor wheel. A good example is an evo 9 20g that we built that made 493 awhp, but the car choked at 6500. We never upgraded the turbine shaft of this turbo so the measurement were 52mm x 68mm for the compressor wheel and 49mm x 55.6mm for the turbine wheel. The reason the car was capable of 493whp, is because the turbine housing is twin scroll and 10.5 cm from the factory. If we wanted to help the car pull harder in the higher rpm range, we upgrade the turbine shaft to a tdo6h4R (58mm x 67mm) or tdo6sl2R turbine (54mm x 61mm). Though both turbines would help the turbo pull to redline, the tdo6sl2 turbine would be the best for spool time and the tdo6h4R turbine would be better for more power. The main reason why we upgrade the turbine wheels in most turbos that we build is because it is very limited for the turbine housing sizes for factory cars. In most cases there are no upgraded turbine housings, so we upgrade the turbine shafts as an alternative, which works better if your trying to retain some of your spool time.
There is not a direct conversion for an A/R to cm^2 estimate but this chart works well for an estimate.
With more and more billet compressor wheels on the turbo market than ever, people raise the question: Is it worth it? The truth is it depends on the billet wheel. The first reason for billet wheels was for making a light weight compressor wheel out of a solid piece of aluminum for spool time and over all flow. PTE has the lightest billet compressor wheels on the turbo market today, because they actually remove the most metal from the wheel than any other companies, but they are commonly known for thrust bearing problems which i will explain in another article. Our extended tip compressor wheels are capable of generating more air flow because of the higher blades, however the extended tip wheels are heavier than the regular cast wheels by about 10 grams, but they are worth it because of the wheel capturing more air. These wheels are machined with a 5 axis endmill which precisely cuts the wheel from a solid piece of aluminum. These wheels range from 120 to 400$. The batmowheel billet wheel has proved to flow well, the wheel was derived from GE jet engines from air planes. This shape of the wheel was created to allow air to easily flow behind each blade in front of it. The tips are also extended to grab extra air, just like GE’s jet engines.
The reason for changing the number compressor blades is to determine at what rpm the turbo will flow the most air. The less number of blades the more air it will flow at higher boost levels compared to a compressor wheel with more blades. A compressor wheel with more blades will flow very well at higher boost levels(~30 psi) but will not flow as well at lower boost levels. The Lower the blade count on the compressor wheel will help the turbo flow more air than the same compressor wheel with more blades. The more blades on a compressor wheel will help the compressor wheel have a peak flow at lower boost levels (20-25 psi). However some companies have started to make compressor wheels taller to allow a compressor wheel with more blades to grab more air and to flow better at higher boost levels as well. You will see taller 11 blade compressor wheels in the GTX series compressor wheels which are created by garret. The higher the blade count also helps with spool time, because it captures more air at lower rpm of the turbo. The choice of compressor wheel depends on the what your goals are as far as spool time and the boost level that you plan to run. google622582fe1d69e3ee
Boostcreep– Boost creep is caused when a free flowing exhaust system is put on a turbo charged car. When revving out to higher rpms your boost pressure(psi) will rise as your rpms of the motor rise. This happens because the waste gate passage can not flow enough air to by pass the turbo to control the boost efficiently. The reason why this happens when you change out the exhaust to a free flowing exhaust is because you the smaller exhaust provided back pressure which caused resistance to flow air passed the turbine wheel.
How to fix this: You can port the waste gate flapper area and add a bigger flapper valve to help prevent boost creep, but the best way is to go external waste gate and select the appropriate waste gate spring for the boost level that you plan to run. We find that it works best that the wastegate spring base pressure should be half or more than the boost pressure that you plan to run. So if you are running 30 psi, then you should have a 15 psi wastegate actuator. The problems with porting the flapper hole and adding a bigger flapper is that it makes it harder to run higher boost levels. Dsm guys will see that there boost level will spike to their set boost level (lets say 20 psi), then when their rpms increase the flapper valve will have a hard time closing, because of the increased surface area of the bigger flapper valve in combination with the waste gate base spring pressure being to low, which makes the flapper struggle to shut to control the massive amount of air pressure that is coming through the turbine housing. When the valve is having a hard time closing it is releasing more pressure out of the turbine housing than it is supposed to their for even though your boost level is set to 20 psi it will fall off to 16 psi and hold a lower boost level to redline. The best way to go is to go external waste gate.