Turbochargers come in many sizes and physical orientations for all kinds of applications. There are literally hundreds to choose from. Here we’ll explain the basics to understand the technology a little better when selecting your next turbo for your sandcar.
Turbos are rated based on how much air they can deliver. Each model usually has specs that explain the capabilities and limitations. The bigger the turbine, typically the more air it can deliver at full boost. But the bigger the exhaust turbine is, the more exhaust can pass through easier, reducing back pressure. Larger exhaust turbines are also heavier, having more mass, hence it will take slightly longer spin up to produce build full boost. The length of this spin up process is called “spool time”.
According to standard turbo theory, your engine is happiest when the exhaust pressure and intake manifold pressures are identical throughout the RPM range. This matching will produce the best combination of max horsepower and quick spool time. So how do you measure this? For the shade tree mechanic this is difficult. That's where you depend on Outfront to recommend the best turbo setup to achieve your target horsepower, based on your engine setup and displacement.
Everybody wants a fast spool time to get the horsepower to build at the lowest RPM as possible as you accelerate. So the thing to keep in mind is to select a turbo that has a small enough turbine to spool up quickly, but big enough to pump enough air to support the maximum horsepower you’re targeting to achieve. That’s the tradeoff you need to consider, horsepower versus spool time.
Turbos are often referred to by "T" numbers that are really a "class type" . These numbers were originally created by Garrett, the largest turbo manufacturer in the world. You have probably heard people talking about T3 and T4 turbos in their car. The “T3” number is a general CFM classification. The T number also specifies a compatible exhaust flange type which the turbo will bolt onto. It’s important to understand there are many different types of T3 turbos with different air outputs which are rated for different horsepower applications. So when somebody says they have a T3, they are actually saying what class of turbo they have, and the flange type. Generally speaking T3 class turbos can deliver enough air volume for applications in the 200 to 300HP range.
The bigger T4 turbo has a larger turbine on the exhaust side and larger turbine fins in intake compressor side. This allows the T4 to pump out more air at full boost to achieve more horsepower than a T3, assuming the engine displacement is big enough to provide enough exhaust. The penalty you pay is a slightly slower spool time, when compared to a T3 on the same engine. T4 turbos are usually for larger motors and use a different exhaust flange. T4’s can deliver enough air for applications in the 400 to 800HP range. T4’s have been very popular in full tilt 2.5+L Subaru setups to make loads of horsepower.
T3 and T4 turbos have been around for a long time and offer great value, ranging from $750 to $1000 depending on your target horsepower goals. These turbos use oil-lite bearings to support the turbine shaft, which eventually wears out after years of usage. The length of usage depends on how often the oil is changed, and what type of oil filter used in the engine setup. The life of the turbo will also be longer if you incorporate a blow-off valve setup somewhere between the turbo and the throttle body valve. But when the when the time comes most turbos can be rebuilt for $400-500 by Turbonetics, a local Southern California rebuilder that specializes in aftermarket turbo servicing.
There is another popular turbo class called T3/T4. This was designed to fill the gap between 300 and 550HP configuration. This class of turbo is ideal if you want to start out with a stock Subaru motor to produce about 300HP, then later upgrade the internals (rods-pistons-block-injectors) of the motor to produce 400+HP. This turbo is assembled with a combination of the exhaust side turbine and housing of a T3 and the air compressor turbine and housing side of a T4. This hybrid combination fits many applications for Subaru sandrail setups.
In more recent years Garrett has introduced turbos with ball bearings to support the turbine shaft. These have been named GT25, GT30 and GT35 class turbos. As you can imagine the reduced drag of ball bearings in the turbo enhances several aspects. First, the spool time is quicker. Second, the maximum RPM of the turbine is increased at full boost. Third, the cost of the turbo is enhanced too, about double that of the older style T3 and T4 type turbos with oil-lite bearings.
Ball bearing turbo setups will spin about 30% faster, which pump out about 30% more air. This lets you use a smaller turbine for the target horsepower you want to achieve. The smaller turbine means less mass, which results in a quicker spool time. So a T3 sized turbine with ball bearings (GT35) can pump more air volume, similar in volume to a larger T4 (which has a slower spool time), because the ball bearings allow the turbine to spin so much faster. So the addition of ball bearings make the trade-off of spool time versus max horsepower less of an issue.
Garrett has now introduced the new GTX turbos with billet compressor wheels boasting more HP and less lag than its GT counterpart .
You might be thinking….. who would ever buy a turbo without ball bearings? The answer to that is...... a lot of people. Most people buy the conventional T3/T4 without ball bearings because they cost less, about half what ball bearing turbos cost. Another factor is that some ball bearing turbos are not rebuildable, so be careful. But if money is no option, opt for the newer ball bearing setup, they absolutely rip!
So as you can tell there are a lot of considerations to select the turbo to match your long term needs. If all these issues have your head spinning, just ask us at Outfront and we’ll help guide you to the best value turbo for your target horsepower and long range plan.