Turbo Lag

In any discussion about turbocharging, the issue of turbo lag is sure to rear its ugly head. For many, turbo lag is a major issue but the issue of turbo lag is greatly misunderstood. No, turbo lag is not a myth, and yes, it does exist - in technical terms, turbo lag can be defined as the time it takes the turbo to spin fast enough to produce boost pressure.
However, it is not as bad as most people believe.

The problem originates in the 1980s when turbos and superchargers became so popular that several marques started producing OEM turbo vehicles like the 1983 Nissan EXA Turbo and the Mitsubishi Colt 1600 Turbo. With these vehicles, the issue of cost and reliability was more important than performance. In a sense the turbo was more of a marketing gimmick as a 78 kW turbocharged 1500cc motor cannot be described as a performance vehicle.

There are two things these manufacturers did to give turbo lag a bad name: to minimize costs they produced turbocharged vehicles without intercoolers and then, to deal with the increased temperatures of the compressed air that was pumped into the combustion chamber, and to minimize detonation, they lowered the engine's compression ratio. The Nissan EXA Turbo, for example has a compression ratio of 7.4:1 compare to the normally aspirated engine's compression ratio of 9.0:1! This robbed the engine of power and performance that was not compensated for by the turbo until the turbo spooled up and started producing boost pressure!

Fortunately, these vehicles were also fitted with small turbos so they spooled up quickly enough, but the small turbo also contributed to backpressure in the exhaust system. This affected the exhaust's scavenging ability and increased the possibility of reversion. In essence, it reduced the engine's volumetric efficiency, further robbing the engine of power and performance. Again, these deficiencies in performance were only overcome when the turbo started producing post pressure.

With a much lower compression ratio, increased back pressure and reduced volumetric efficiency, an OEM turbocharged vehicle was often out performed by its naturally aspirated brethren in those precious few seconds while the turbo spooled up to spin fast enough to produce boost pressure. Once the turbo spooled up and started creating boost pressure, it more than compensated for the deficiencies caused by the lowered compression ratio and lowered volumetric efficiency.

But today, with the advances in turbo technology and the use of an intercooler to remove some of the heat that is a natural side effect of compressing air, it is not necessary to lower the compression ratio. Indeed, with the proper use of an intercooler turbocharging has become a bolt-on accessory with no need to open the engine. Also, because you don't need to get boost pressure so quickly, you can run a slightly larger turbo that is better suited to the exhaust system and produces far less back pressure. This effectively takes turbo lag out of the equation. Even though the turbo will still have turbo lag, the engine's power and performance is not compromised by lowered compression ratios and increased backpressure.

So the use of a properly sized and efficient intercooler should be a compulsory part of any turbocharger system if you do not want to suffer the consequences of turbo lag.

Brent Harris served as an apprentice motor mechanic at Reitz Nissan in the late 1980s before going on to the University of the Western Cape where he obtained a MA. His passion has always been cars and he has extensive experience in modifying Nissan engines. He is currently self-employed and serves as the editor for http://www.custom-car.us

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