Supra FAQ page 9

Contrary to popular belief, lag is NOT the engine RPM at which the turbo starts producing noticeable boost pressure (see below). Turbo lag is the delay between when you first mash the gas pedal (when above the boost threshold) and the turbo begins to produce noticeable boost pressure. This can also be referred to as boost or turbo response.

Boost threshold is the minimum engine RPM at which the turbo starts to produce noticeable boost pressure. This is incorrectly referred to as lag.

No. Throttle response is the time it takes for significant amounts of air to be drawn into the engine after mashing the throttle (gas pedal). This can occur in the absence of boost.

It is a device used to control the output of the turbocharger. If boost pressure rises above a certain point, the wastegate releases some of the exhaust gas that was destined to the turbine to bring the turbo output under control.

It is used to prevent compressor surge by venting the pressure build-up, between the turbo and throttle body, and the resulting reversion of air into the compressor after the throttle closes abruptly when on boost.

It is an exhaust manifold which connects to the exhaust ports on the cylinder head at one end, and the turbo's turbine inlet at the other. It is both responsible for directing exhaust flow into the turbine, and acts as a mount/support for the turbocharger.

It is the section of the exhaust system that connects the outlet of the turbine, to the rest of the exhaust system.

You have the compressor section on one end, the turbine section on the other. A single shaft connects the two sides, and the center section supports the shaft and housings, and is responsible for lubrication and cooling.

The compressor acts as the air pump. It is what actually forces air into the engine, creating more power.

The turbine acts as the motor that drives the compressor side. It is driven by the energy of hot exhaust gases expelled from the combustion chamber.

When looking at turbos, it's important to keep in mind the relationship between the compressor and turbine. The turbine side is what by far has the largest impact on boost threshold and turbo lag. The compressor side has a minor impact on low-mid range rpm lag, and has almost no impact on boost threshold (within reason). Both the turbine and compressor share a huge roll in determining the turbos efficiency. It is also very important to match the characteristics of the turbine, to that of the compressor. If the compressor is too large for the turbine, surging will occur, and efficiency will plummet. One thing to keep in mind is that all the air that is pumped through the compressor, will also have to flow through the turbine. If the turbine is too large for the compressor, then not only will lag and boost threshold be unnecessarily high, but compressor efficiency and flow potential will be low.

It is the ratio of AREA to RADIUS. This is referring to the design of either the compressor or turbine housing. The AREA is referring to the area of the nozzle at any given point in the housing. The RADIUS refers to the distance between the wheel center, and the center of the nozzle area. A lower A/R number indicates a less efficient housing with faster spooling. A larger number indicates a more efficient housing with slower spooling.

A boost controller regulates the manifold pressure that the wastegate sees. The wastegate relies on boost pressure to know when to open, but the boost controller intercepts this signal to make the wastegate stay closed longer so more boost can be produced.

What are ball bearing turbos? Are they worth the extra money? Ball bearing turbo have the shaft of the turbo mounted on ball bearings as opposed to floating on a film of oil. The advantage is a reduction in drag by eliminating the shearing of the oil film. This is not a huge drag on the turbo, and can only be noticed at lower engine speeds when flow into the turbine is low, resulting in slightly increased lag. Ball bearing turbos do not provide a noticeable benefit at high engine speeds, and should make no difference when it comes to boost threshold. Consider these to be truly beneficial only for low speed driveability, and not ultimate performance.

Pressure is created by restricting flow. In an engine the restriction is caused by everything from the intercooler piping to the cylinder head to the turbine housing to the exhaust tip. But, all things remaining equal, especially charge temperatures, the higher the boost pressure, the more airflow is taking place. But if the increase in volume entering the engine, results in a proportional decrease in air density as a result of drastically increased temperatures, then you are creating no extra power. One thing to keep in mind, if you remove restriction to the airflow post compressor (ported head, high flow intake manifold, higher lift cams) then you can increase airflow and power, while DECREASING boost pressure.

510-525rwhp is a reasonably safe level for long-term reliability. However this is only the case with an efficient turbo, and all the needed supporting modifications. And of-course, tuning is as always, critical. The use of a properly tuned alcohol/water injection system can increase this limit.

What is the limit of the stock fuel system and MAF sensor? The absolute 100% limit is between 525-540rwhp depending on conditions and the particular vehicle from what I have seen. But you should never try to push the stock fuel system this hard as you are asking for trouble. The consensus is 500rwhp is a reasonable safe limit for the stock fuel system with the fuel pulsation damper by-pass performed (if the system is in 100% condition). If you want to get the most out of the stock fuel system, you can also add a second in-tank fuel pump, and an adjustable fuel pressure regulator. The stock MAF has maxed out it's output signal by 540-550rwhp. But it's a significant restriction to flow as well.

Do I have to upgrade the stock intercooler to a front-mount intercooler when going with a single turbo?

Because an upgraded single turbo produces less heat and is much more efficient than the stock twin turbos at the same boost levels, an upgraded intercooler is actually slightly less important for use on a single turbo car than on a stock twin car at moderate boost levels (18psi). However it is wise to upgrade the intercooler to a front-mount whenever your budget permits. The stock intercooler should not be used over 20psi in my opinion.

WOAH! It's easy to loose perspective on power when you see so many people throwing around big numbers. Jumping into a 700rwhp street driven Supra in frankly just plain stupid. That level of power makes more for bragging rights than a fun drive. You will be required to run on race gas, low/mid rpm range lag will be significant, and the power wont even be usable on the street. Roll on throttle in 3rd gear at 80mph would have the rear tires spinning like crazy, and the car fish-tailing. Quick bursts of acceleration will have you into 160mph plus speeds before you have a chance to realize it. Baby steps are crucial, try 400rwhp first. That's enough power to beat almost anything on the street, and is enough that any burst of acceleration more than a few seconds has you breaking every speed limit in the US. 500rwhp will beat almost any production exotic car there is, and can kill you or land your ass in jail in short order, if you do not practice significant restraint. So start out simple, and work up from there.

Twin turbos are most advantageous over single turbos on larger displacement and split bank engine ('V' and Horizontally Opposed). On a small-moderate displacement single bank engine like the 2JZ, the merits of a twin turbo set up do not significantly out-weight the disadvantages (added cost, complexity, weight, packaging, etc).

I am going to be keeping the stock fuel system and maybe the stock MAF. What would be a good turbo to use, and how much power will I get?

A turbo with a 59-61mm inducer, and P-trim turbine (or equivalent) with ~.58 a/r should work nicely. If you want super quick spooling, try the SP57 which has a non P-trim wheel. 500rwhp is a reasonable goal with the stock fuel system (in 100% condition) and MAF sensor while running pump gas. You should be able to make a little more than that with good tuning and high octane fuel. Also, consider a fuel pulsation damper by-pass a must.

I autocross my Supra a lot. What would be a good turbo for me? A Sound Performance SP57 or the newer SP58 would be perfect! Excellent spooling characteristics, and very usable power output.

I road-race my Supra a lot. What would be a good turbo for me?** The most important thing to keep into consideration for a road race car, is that charge temperatures need to be kept at a minimum. So it would be wise to go with a slightly larger more laggy turbo with better efficiency. I would recommend a turbo with a 61-64mm inducer diameter, and a P-Trim turbine wheel (or equivalent) with a ~.70 a/r housing.

I want to go with a single turbo, but won't have the money to upgrade the fuel system for a while. What's a turbo that will work well on stock fuel, but make even more power when I upgrade the fuel system and am able to turn up the boost more?** I would recommend using a turbo with a 61-63mm inducer, and a P-Trim turbine or equivalent. You could expect 500rwhp on pump gas and stock fuel (in 100% condition), 520rwhp on pump gas with upgraded fuel, and around 600-650rwhp with proper upgraded fuel system and race gas.

I am upgrading the fuel system, and have ditched the stock MAF sensor. Most of the time I am going to be running pump gas as this is my daily driver, but occasionally I will put in race gas when I go to the drag strip and want to crank up the boost. I want something that doesn't have too much lag. What would be a good turbo for me?** I would recommend a turbo with a 61-67mm inducer. On 92-93 octane pump gas, you can keep the boost down and make 500-525rwhp safely. When you have race gas in the tank you can crank the boost to as high as 27-29psi and be making 600-700rwhp depending on the turbo.

I want a single turbo that will make tons of power on the street, and I don't mind running race gas all the time. I also might be drag racing it a fair amount and I want enough power to run deep into the 10s. What would be a good turbo for me?** Focus on turbos with 66-70mm inducers and Q-trim or equivalent turbine wheels. A ~.70a/r should work well on most of these turbos, but not all. These turbos should be capable of 650-700rwhp on race gas depending on the turbo.

I want a TWIN turbo kit that will make tons of power on the street, and I don't mind running race gas all the time. I also might be drag racing it a fair amount and I want enough power to run deep into the 10s. What would be a good turbo for me?** You should be able to make 750+rwhp with this kit with the proper trim on race gas.

I want a single or twin turbo setup that will make enough power to run deep into the 9s in the quarter-mile. What should I get?** These turbos are capable of over 800rwhp on race gas.

What are the largest turbos that can be installed on a MKIV Supra?** These turbos are capable of over 1000rwhp on race gas.

Which turbo kit should I use?** The RPS, Sound-Performance, PHR, GReddy and HKS kits are all proven performers, and very well built, and all of them can create a lot of power. The GReddy turbo kits (T78, T88 33D and T88H 34D) are without a doubt the biggest bangs for the buck, with all of those kits capable of huge horsepower, and available for around $4,000. The HKS, PHR and Sound-Performance Kits are some of the more expensive kits out there. But the build quality of these kits is stunning, and the SP and PHR kits offer many more choices and options than the Japanese Kits. The SP kits are based on their own manifold, and PHR uses the HKS manifold on their Stage 1 and up kits. RPS falls into the mid-range price-wise, but are also very well built, and offer plenty of choices for turbos, and are based on their own manifold design.

Some lower cost turbo kits are finally showing up on the market. Directed Engineering was one of the first to offer a very low cost option to the $4,000-$5,500 kits, offering a good street kit for $3,200, and based around the Turbonetics cast manifold. But that kit is out of production for the time being. Sound-Performance has recently released a "budget" kit that sells for well under 3k. It uses a manifold based on a modified stock unit. RPS also now offers a less expensive kit (~$3,600), and PHR has been offering a lower-priced "street" kit for awhile, which is based around the Turbonetics cast manifold (~$3,800).