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Tuning and understanding Boost By Gear

Contributed By: Enginebasics.com

When rookie tuners or drivers hear the term “ Boost by Gear “ they often think that the vehicle is being tuned to run a certain boost level, when the gear shifter is in a certain gear. Well, this is only sort of right, but hopefully after this article everyone will understand the true meaning behind Boost by Gear.

The Background

Boost by gear is a term that is used by tuners and drivers to infer that there car is tuned to run different amounts of boost pressure, based on the gear they are currently in. The benefits to doing this are that the vehicle gains better drivability, as the motor is restricted in its power by running lower boost levels. The reason one would want to do this based on gear is the fact that gearing gives the motor such a mechanical advantage, that this advantage must be controlled by varying the engine’s horsepower output by varying the amount of boost the motor see’s.

One thing that should be understood is that this type of control can only be done on a turbocharged car. Currently there is not a system on the market that can vary the boost of a supercharger, do to the fact that it is run by the motor itself, and therefore based on the motor, and not a wastegate like a turbocharger is. One could add this argument to the comparison between a turbocharger and supercharger, showing that with a turbocharger the user has more control over the tune and drivability and therefore superior, but the reality is that Boost by Gear is not used enough by turbocharger users to have it be a large weighting factor on the fight between which one is better.

How it really works

As mentioned, the common misconception is that when setting up and tuning boost by gear, you are setting up the wastegate to be controlled by the transmission gear. While this MAY be available on some very rare set-ups, most are instead controlled by the Vehicle Speed Sensor (VSS). This means that the wastegate solenoid is controlled by how fast the vehicle is going not by the gear engaged by the transmission. But lets be clear on something, the vehicles speed in any given gear is limited by that gear, so they are proportional. This is why we get the name Boost by Gear. There are only a few examples were the name doesn’t stick.

For example lets say on a particular car first gear goes to 38 MPH, 2nd gear 62 MPH, and 3rd gear 94 MPH. Knowing this we would set the wastegate solenoid to run 5 PSI to 38 MPH, 10 PSI to 62 MPH, and finally 15 PSI (lets say our max boost) all the way to 200 MPH. With the vehicle being set-up this way, the user would see 5 psi in first gear, 10 psi in 2nd gear, and 15 psi in all other gears. Therefore the user might think that the gear controls the boost, and to be fair his thinking is not to far off. The only time it would not appear this way is if the driver got on the gas pedal to accelerate, but chose not to downshift. For instance with our example above, the driver might be cruising along at 41 mph in 3rd gear and jump on the gas to pass a slower vehicle. Because he is only doing 41 mph, the VSS will tell the wastegate solenoid to run 10 PSI of boost, even though the driver is in 3rd gear.

How to set up boost by gear

To set-up and tune a boost by gear set-up, the vehicle must be equipped with a few key components:

1. Must be turbocharged

2. Wastegate Variable Solenoid

3. Tunable ECU with both a VSS input and a wastegate duty cycle output

First make sure that the ECU has an output that can be variable based on one or two inputs. Next, set up a table so that the wastegate solenoid output voltage cycle is based on the vehicles VSS. Then, drive the vehicle and note the max RPM in each gear. Usually the first 3 gears are the most important as max boost may be run after 3rd gear. Last, apply a certain duty cycle to the wastegate at each interval of speed until the proper boost pressure is achieved at each of these speeds (or gear).

I have to apologize that the exact description has to be so bland on how to set up a boost by gear setup, but the reality is that each ECU is different and with so many ECU’s on the market each one will be set-up slightly different. The basics are found above, and should help you in showing you what to look for, and make sure your ECU has these options before you attempt to do this type of boost control. For me personally, I found that once I knew that boost by gear was just a wastegate duty controlled output off of the VSS sensor, it was very easy to set up my first boost by gear set-up. Having now tuned over 200 cars with this type of boost control, I am sold that for any high HP, street driven turbo set-up, boost by gear is a MUST for great drivability and safety. Also there is no reason to strain and push the motor with tons of boost, if the wheels are just spinning anyway.

Some known boost by gear/speed controllers are:

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