Frequently Asked Questions about Turbocharging

Ok guys. Here's a post about frequently asked question regarding turbocharged cars. These will enhance your knowledge about turbocharging, differences between turbos in general, etc. These was compiled By Trent Kendall.

What is a boost controller? What is boost spike?

External wastegate or internal wastegate actuator has a base spring setting. For the sake of description lets say it has a 5 psi spring. That means that when 5 psi boost hits the diaphragm the wastegate opens. The boost controller is installed inline with the hose that leads to the pressure side of the wastegate and the other side of the boost controller is a vent. When the boost controller is in the closed ( You cannot blow through it ) position, all the boost pressure goes to the wastegate and you have a stock boost level. As you open the boost controller, boost pressure that would have gone to the wastegate now diverts through the boost controller. What this does is fool the wastegate. If you bleed off 5 psi on a 5 psi system it will take 10 psi to open the wastegate resulting in a 5 psi boost increase on your boost gauge and a lot more power from the engine. This is the simple version and results vary with the system, the size of the hose being used, the sensitivity of the wastegate spring. Manual boost controllers are also very prone to boost spikes. This occurs when the boost controller is open all the way and small diameter hose is being used. The boost pressure builds faster than the hose can flow which lets the turbo spike up big boost numbers until the pressure fights its way through the small hose to the diaphragm, then the boost drops back down. This is a dangerous situation since the spike can blow up your engine before you know what happened. The best way to minimize spike is to keep the manual boost controller located under the hood with as short a length of hose as is possible. This is easily demonstrated by you picking up a piece of ¼" hose 4 feet long and trying to breath through it. Go ahead, try it. Pretty tough isn't it. Now cut the hose in half and try it. Much easier. Your turbo and wastegate feels the same way.


What is boost creep?

As greater exhaust output is created from higher levels of engine performance, the stock internal wastegate actuators begin to lose their effectiveness because the amount of exhaust flow to be bypassed is now beyond their intended operating range. As the result of exhaust flow being greater than the valves’ capacity to discharge, more exhaust flow is directed to the turbine and additional boost is created, which is commonly referred to as boost creep.

Which gears should I get?

Turbochargers rely on exhaust gasses to drive the unit. Useable Exhaust gas is created when the vehicle is under a load. This is the reason many people recommend 3.27's or 3.55's for a turbocharged street car. These less aggressive gear ratios place a higher load on the car for a longer amount of time per gear. Of course for a straight drag car you want to pick the set of gears that puts you in your powerband for every gear and so you are passing through the traps at about redline in your final non OD gear.


Which boost controller should I get?

Any manual boost controller valve (air compressor valve) will work fairly well at controlling boost. The $90 Turbo XS manual boost controller probably is a little more reliable and steady. For electronic boost controllers many people go with a Greddy Profec B, Apexi AVC-R, the newest the E-Boost Controller, etc.

What is the difference between a BOV and bypass? How do they work, are they necessary?

A BOV is a Blow off Valve. These horn shaped device is mounted along the intercooler piping somewhere between the turbo and the throttle body. It will open up once the throttle blade is closed in order to bleed off all the air in the system between the turbo and the throttle blade. The blown off air is simply blown into the atmosphere.
A bypass valve looks very similar to a BOV and many BOV's can function as a bypass valve. The difference being is the air that is blown off from the bypass valve is recirculated via a hose back into the air intake system somewhere between the MAF and the turbo. A bypass valve and not a BOV is necessary on draw through MAF cars, otherwise you would be blowing off metered air. A Draw through MAF is where the mass airflow meter is mounted in front of the turbo. There is no definitive answer if a BOV or bypass is necessary. However many people agree that utilizing a BOV or bypass can extend turbo life and increase spool time between shifts.

How does a wastegate work?

A wastegate is mounted in the exhaust plumbing before the turbocharger. A wastegate functions as a boost regulator by diverting exhaust gasses around the turbo once the turbo has reached the desired boost level. They are simply a large valve backed by a spring. Once the exhaust gas has reached the capacity of the spring the valve starts to open, diverting the exhaust gas around the turbo and back into the exhaust and out the tailpipes

What exhaust should I buy?

Turbos HATE backpressure. You want the highest flowing exhaust system you can fit. The main item here is mufflers. You do not want chambered style mufflers like flowmasters. You want a "straight through" type muffler like the dynomax super turbo, ultraflo or any other straight through design muffler. Chambered mufflers will cost you at least 20hp minimum. If you are running a single exhaust 3" is the minimum, for dual exhaust 2.5" will be fine. The overall exhaust system is up to you, like I said the larger the better. If you can fit dual 3" then do it.


What is Turbo Lag??

Turbo Lag is the time it takes from when you smash the "go" pedal to the time the turbos are making an appreciable amount of boost. For some reason people have it stuck in their mind that it takes 10 minutes for the turbos to respond. That is simply not the case. Most of the single turbo kits are making FULL BOOST before 3000 rpm or less (try that with a centrifugal supercharger). Twin turbos have even less lag, the power is there right after you hit the gas basically. You can even launch off the line under boost with a 5 speed if you get a two step..

What are the main differences between a Single and Twin Turbo setup?

First off, you can make as much or as little horsepower as you want with either a single or a twin. The main differences between the two are the turbo sizes and the complexity of the entire package. With a single turbo you only have to deal with one of everything, one turbo, one wastegate, one downpipe, and one set of plumbing. With a twin turbo of course you have two of everything so that makes it a little more complex. With a typical street twin setup you use fairly small turbos therefore there is very little turbo lag.

What are the main tuning problems when dealing with Turbos?

The main thing you need to control with a turbo is the timing. It helps a great deal to have an aftermarket tuning device that will allow you to pull some timing at the point in the power curve where you are making peak torque. An eec-tuner, tweecer or a chip will allow you to do this. This will help ensure that you don't have detonation. You can pull out timing at the torque peak and then add in timing from there to get some more power. Other than that the same tuning issues apply to turbos that apply for all power adders: make sure you have a fuel and ignition system that is up to the task


Which gauges should I have for my turbocharged Mustang?

Fuel Pressure, Exhaust Gas Temperature, Boost. You can use the EGT gauge to help determine if you are running too lean based on the readings. This can help you prevent a blown head gasket or worse... The fuel pressure gauge should be mounted where you can view it easily while blasting down the road. You want to make sure the fuel pressure doesn't start to drop at the higher RPM's, this is indicative of a fuel system that isn't up to par.

If money permits add a wideband O2 to the tuning array. The EGT is more accurate than an O2 sensor gauge, but the wideband in irreplaceable. The EGT is a little slow to respond but it gives you real time cylinder data on 1 cylinder. The wideband responds in milliseconds and is super accurate. There is one kit on the market that is a few hundred bucks.