Boost Spike vs. Boost Creep


Since these two terms are very commonly used in describing turbo/engine behavior, yet they are not always understood correctly, I wanted to attempt to clear them up.

People often mix the two up, or even interchange them, when they are in fact two VERY different things.

Boost Spike: Boost spike is when the boost level initially "spikes" up to higher than the preset boost setting, and then quickly settles back down to where it should be.  As most people with turbos know, once the boost pressure in the intake starts to rise, the rate at which it rises quickly increases until the pressure is increasing at a phenomenal rate.  This means that, if your boost is set at 12 psi, when it reaches that point it will be increasing so quickly that it will go higher than 12 psi and then drop back down once the boost control system can correct it, which is within a half second or so.

Some causes of spike are bad boost controllers (only ball-and spring type MBC's should be used, and only proven electronic boost controllers should be used), long boost source or wastegate activation hoses, and the lack of any boost controller at all.  It's basicially an effect a t slow response time of the boost control system.


Boost Creep: While boost creep also refers to an unwanted rise in manifold pressure, its cause and effect are totally different from those of spike, as is the way it manifests itself.

As you know, boost pressure is controlled by the wastegate, which allows exhaust gasses to bypass the turbine wheel.  In effect, it creates an alternate route for the hot exhaust coming out of the motor to take, which means that any gas passing through it will not spin the turbine wheel.

Now, if this wastegate cannot flow enough to bypass the required amount of exhaust, then that means that too much of the gasses are going to go through the turbine wheel, meaning that it will have too much energy imparted on it (it will be spinning too fast).  As the excess exhaust gas amount gets greater and greater, the turbine wheel spins faster and faster, and the boost level rises.

Creep happens ANY time when the wastegate cannot bypass enough exhaust gas to keep the boost under control.  This can happen when the wastegate is too small in diameter, or when the design of the wastegate doesn't allow it to open enough, or when the wastegate simply doesn't have a good enough flow path to divert a lot of exhaust.  It can also happen when you increase the amount of exhaust coming out of a motor (running more boost/airflow, making more power).

Since this tends to get worse and worse as the engine speed rises (more cycles per second is more conducive to more exhaust gas, to a point), that means that one will see the boost climb to the preset level on the boost controller, and then it will gradually creep up past that line to a "minimum" given the circumstances.

That minimum can be 2 psi above the set boost level, or it can be over 30 psi, depending on how the wastegate is designed, how big it is, the car's setup, and more.

It is also important to not that you certainly can have spike AND creep at the same time, which would result in the boost level jumping up, settling to the preset level, and then slowly climbing back up again as you approach redline.



Kyle Tarry - 2003