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What is Turbo Surge and Volumetric Efficiency?
Contributed By: enginebasics.com
Surge or Surging is a term that is thrown around a lot with little understanding of what it is or what is really occurring in the motor and with the turbo. Hopefully this article will help you understand what is really taking place when a turbo surges.
It is best to think of the motor as an air pump. It is always sucking in and blowing out a certain amount of CFM at any given RPM. Now depending on the motor, because every motor is different, the motor is sometimes sucking air in more efficiently than at other times based on the motors set-up. By set-up I mean the motors cams, cam timing, intake design, stroke, bore, and the list goes on and on. This idea that the motor has “sweet spots” in its breathing we refer to as the motors efficiency. To be more specific it’s the motors Volumetric Efficiency (VE).
Now understanding that the VE of a motor is constantly changing through the RPM band we can say the efficiency of a turbo is changing also through the RPM band based on the size and other components with the turbo set-up. SO WHAT IS SURGE? Well, turbo surge is when the air that the turbo is pushing towards the motor to be ingested is not ingested and comes back at the turbo.
How does this happen?
If the motor is currently at an RPM where it is not very efficient, meaning it is not ingesting a lot of air, yet the turbo is at a RPM where it is efficient and making lots or air, then that air needs some place to go and since the motor can’t take it, it just turns back on itself putting pressure on the compressor wheel. This in reality is surge.
For another example lets use some numbers. Lets assume you have a 2L turbo motor that is running at 2800 RPM where it can ingest 200 CFM of air, but the turbo on your car at 2800 RPM is making 410 CFM of air. That extra 210 CFM of air is going to have no where to go and is going to be “pumping” back on the compressor wheel surging it. The other time a turbo surges is when you get off the throttle rapidly. This could be to shift, or because you are braking, but when you let off the gas and close the throttle plate the air again is not going to have anywhere to go and will turn around to go backward back on the compressor wheel surging it.
The two most common ways to avoid surge is:
1. Installing a blow off valve
2. Properly sizing your turbocharger for the CFM of your motor
Hopefully this article helps you understand surge and why some turbos will surge on certain set-ups and that same turbo won’t surge at all on other set-ups. Like tuners always say: “its all in the set-up”.
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