Ive heard these two terms used before but I dont know what they are referring to? They have been symptoms of a full exhaust system without a big enough wastegate. If someone could explain these to me that would be great.

Thanks.

 

They describe two quite different problems. I assume you know how a wastegate works, but just in case, it opens to bypass some exhaust gases around the turbine instead of through it. By doing that, the turbine speed is kept under control and boost stays at the same level.

Boost creep is a very common problems on FC's, particularly S4's. It's caused by the wastegate being too small. Even when the wastegate flap is fully open it can't pass enough gas to control boost , so it keeps rising uncontrollably. If you don't lift off this will almost certainly cause a lean condition resulting in engine death. Increasing the wastegate's flow capacity is the only way to stop boost creep.

A boost [i]spike[i] is when boost overshoots the set max level and then drops back down again. On a graph of boost vs. time it would look like a spike in the curve, hence the name. It's usually caused by electronic boost control systems that can't react to rapid changes in boost quickly enough. Since boost spikes usually occur in the mid-range, when fuel requirements are far less, they aren't generally dangerous to the engine.