When somebody says “turbo lag,” they’re talking about the delayed response a turbocharged engine can have when you put your right foot down on the gas pedal. A turbocharger uses the flow of exhaust gasses to spin a wheel, spool up, and feed extra fresh air into the engine intake to ultimately make more power. But that spooling can’t always happen instantly, especially with bigger turbos and older cars.
That turbo lag you can be left with creates a rubber band-like effect, making a car feel lethargic until it suddenly gets a rush of power. Today’s turbocharging technology has come a long way from its origins, and even the big boosted tuner cars of the recent past. In fact, advancements in technology have rendered it mostly a thing of the past, at least as far as unmodified cars are concerned. Let's go more in depth about what turbo lag is, as well as what kind of technology is implemented to minimize, or completely extinguish, its laggy ways.
What's Happening During Turbocharging?
First, let's quickly discuss how a turbo works. Exhaust gasses spin the turbo's compressor wheel, or turbine, fast enough to compress intake air, force it into the engine, and when mixed with the appropriate amount of fuel, it creates more power. Normal intake air is at what's called atmospheric pressure, whereas boosted air is above atmospheric pressure. When compressed air is being created, it's called "being in boost."
A healthy naturally aspirated engine operates at 100 percent volumetric efficiency, meaning it's doing the best it can at producing power with atmospheric pressure. A turbocharged engine operates above 100 percent volumetric efficiency, meaning it's producing more than 100 percent of what the engine could produce while naturally aspirated.
For more info on how a turbocharger works, as well as what are the different kinds of turbocharging, check this out. For even more info, including what happens to unused boost pressure, check out this thorough explainer.
What Exactly Is Turbo Lag?
As boost builds, the delayed response you feel is getting more air to the intake valves to then create more boost. Because remember, exhaust pressure is what spins the turbo. The key aspect here is "as boost builds," meaning it's already being produced, but not enough to warrant a substantial, noticeable increase in power.
Many people confuse being below boost threshold and turbo lag. Boost threshold is the point in the rev band when the engine's exhaust gasses spin the turbo enough to start creating boost pressure (keyword: start), meaning once the threshold is passed, the engine's volumetric efficiency is increasing over 100 percent. Turbo lag is the time it takes to then feel the power come on strong when it hits peak boost, or the maximum amount that the turbo's been tuned to deliver.
From personal experience, I once drove a manual first-gen Audi TT with a turbocharged 1.8-liter engine and some very pronounced turbo lag. It was completely stock, though in good mechanical shape. When I put my foot down in a lower RPM, I could clearly feel when the engine hit boost threshold, and then began to build up to peak boost at a much higher RPM. That latter bit about slowly building up to peak boost was turbo lag. Even when I was in boost, putting my foot down still created a rubber-band-like effect.
What Could Be the Reason For Turbo Lag?
There are several reasons why turbo lag might happen. The turbo's design might not be optimized for fast, efficient spooling, which is when the turbo is spinning at the right speed to compress air up to maximum boost pressure. The series of tubes that the compressed air travels through before reaching the intake valves might be too long or inefficient. The engine's throttle body might be a little on the small side, thus not allowing enough air in to produce more exhaust gas and keep the boost pressure compounding along. The design of the turbo might not be ideal, either, especially if it's older and therefore possesses old, less efficient technology. There are other reasons, but these are some of the most common.
For more information on different kinds of turbocharging, check this out. Twin-scroll turbochargers are a relatively newer form of turbocharging in modern cars, most notably found under the hood of the Hyundai Veloster N and Elantra N.
How to Minimize or Get Rid of Turbo Lag
Minimizing turbo lag is fairly simple, just remedy all of the above. However, it's not always that straightforward due to the time it takes to engineer an ideal, lag-free system, the materials cost of designing and building the right components to do it, coming up with the proper ECU tuning, as well as designing and building the ideal characteristics of the engine itself to optimize intake and exhaust flow, such as having a smaller exhaust housing that will cause gasses to flow faster.
Regardless, automakers have come up with some clever ways to remedy turbo lag. One is designing turbochargers that spin more efficiently and therefore push more boost in, faster. These might feature lightweight components, like thin aluminum compressor wheels or lighter and more efficient ceramic bearings. Another is to simply install a smaller turbocharger that will reach boost threshold sooner, extinguishing any discernible feeling of lag, and thus reach peak boost faster. Then, increasing the engine's compression ratio is another method, as higher compression engines make more power down low in the rev range, bringing the boost threshold down with them. Higher compression ratios used to spell disaster for turbocharged engines, but advancements in technology have allowed manufacturers to bring them up and still have the vehicle operating safely.
In practice, keeping the RPMs up is the simplest way to avoid or minimize turbo lag, at least well past boost threshold. But with modern cars, turbo lag is by far less of a thing than it was even just 15 years ago.
Trickle Down Engineering is Good
Though, the cool thing is that a rising tide raises all boats. What I mean by this, is as high-end technology is developed and utilized, it eventually becomes more and more commonplace, trickles down into other areas, and kicks out old technology. Aftermarket tuners in the late '90s could only dream of the turbos that manufacturers throw on brand-new cars these days, or how they assemble their cylinder heads.
For instance, Honda's turbocharged L15 engine That's found in the 2017-current Honda Civic Si makes a lot of power for its size, and features some really neat tech to build boost as efficiently as possible. One of these is sodium-filled exhaust valves, which help pull heat out of the cylinder head, which not only cuts down on heat soak, but also ensures a more ideal exhaust gas charge.
Then, the turbos that aftermarket tuners are buying and strapping to the side of their engines these days are even more fascinating. Some might look too big for the engine they're bolted onto, but because they're operating so efficiently, they not only reach boost threshold faster, they also have minimal lag when matched with an equally efficient intake system and cylinder head.
We're truly living in some epic, technologically advanced times, where a good mix of tuning, improving flow, and turbocharging can make a lot of power in an otherwise small displacement power plant. The same goes for efficiency, as using these in conjunction with tech like mild hybrid assistance allows for astonishing fuel economy. It's sort of a bummer that eventually this will all cease to make way for electric motor tech, exclusively. But then, that will be a whole new area of technology to tinker and perfect as well.