Turbo Lag

[Morrac32]

I haven't found a thread explaining this yet, and I might not have dug far enough either. Anyway, the question I have is what exactly is turbo lag? From what I have gleamed it seems to be the time it takes for the turbo to build enough pressure up to kick in and start adding horsepower. I have no idea if I am right or not. Also, is there a way to keep it from happening and is there such thing as a turbo or a car with no turbo lag or is this something that only happens with large turbos?

 

[deadcell96]

Yeah, you are right about the turbo lag. As far as I know, it is unavoidable.

 

[Leonidae@MFT]

Not unavoidable. Difficult to get rid of? Yes. VGT and twin ball bearing turbos have low lag factor. Also, amount of exhaust gas to spin the turbo, the compressor and turbine materials and specifications play a major role in this. Example: you will get lag if you put a turbo meant for 2l and up into a 1.6l engine.

 

[Jim Prower]

Exotic systems like anti-lag kits can eliminate lag...at the cost of reduced turbo life, as it changes timing off-throttle, dumps fuel into the turbo, and introduces fresh air to burn the fuel in the turbo. This what Rally Cars use.

 

[Time_Attack]

Turbos spin at an incredible RPM. When you step on the gas it will take a few seconds for enough exhaust gas to flow through the turbo and thus spooling it up to operating speeds.

There are various ways to lessen turbo lag such as using a smaller turbo with a dual entry design.

 

[motortrend]

Ive never experienced a single turbo spooling up, must be cool to have lag at first and then a surge of power. I think the only turbo car I've been in is a 335i and that was a twin turbo which had pretty much no lag that I remember

 

[gary31]

Yeah the torque curve for the 335i engine is flat from about 1000rpm to close to the redline.

Amazing engine...

 

[Crash]

Anyway, the question I have is what exactly is turbo lag? From what I have gleamed it seems to be the time it takes for the turbo to build enough pressure up to kick in and start adding horsepower. I have no idea if I am right or not.

You got it pretty much right. So inside a turbo is a turbine. In order for boost to be generated, the turbine has to spin up and increase in RPMs. The acceleration of the turbine is what causes turbo lag, since as you know, acceleration of things are never really instantaneous. This is also the reason why large turbos have a large amount of lag, since a larger turbo often has a larger mass, and also if the diameter of the turbo is larger, there blades of the turbine will have a larger moment of inertia.

I'm not sure if there really are ways to totally eliminate turbo lag. Maybe someone somewhere can invent a device that can provide secondary pressure equal to the exhaust pressure to keep the turbo spooled coupled with a blowoff valve. As others have alluded to though, a much simpler way of minimizing the lag until it is essentially imperceptible would be just make the turbos smaller, and add more of them.

 

[CodeRedR51]

I had a small (Garrett 2554) turbo on my 1.6 Miata. Was a hoot to drive. Hardly any lag at all. Small turbo's are great for that, but unfortunately the larger the engine the larger turbo you have to use to make any kind of power.

 

[Time_Attack]

The trade off of a smaller turbo is less top end power because they max out sooner.

 

[Punknoodle]

As well as what others have said, an appropriately placed blow off valve helps to reduce lag, I'll explain for those who don't know, imagine a turbo spinning away pushing air into the engine, suddenly the throttle body shuts and the air has nowhere to go, so the pressure builds up, slowing down and stalling the turbine. Now if you put the blow off valve on the intake, the air has somewhere to go when the throttle body closes, thus letting the turbine keep spinning so it doesn't have to speed up again (as much).
Put that BOV close to the turbo and it will be louder but can increase lag, as all the air already pushed into the intercooler will be flushed as well and need to be pressurized again, however if you put the valve closer to the throttle body the air in the intake is flowing as if it was going into the throttle body and will flow better once the throttle body opens again the the valve shuts

 

[Time_Attack]

A lot of older cars placed it on the hot pipe going to the intercooler. Thats how it was on my MR2 turbo. I see a lot of custom setups place it like you said which does make more since.

 

[Punknoodle]

Yeah when I bought my car a cheap one had been installed just after the turbo, but the way the intercooler is set up the outlet of the intercooler goes straight into the throttle body so there is no other place for it. I've since put a good one on but had to leave it in the same place until I end up installing a front mount.

 

[motortrend]

Yeah the torque curve for the 335i engine is flat from about 1000rpm to close to the redline.

Amazing engine...

Definitely a great engine, but the HPFP problems are a bit of a turn off. Still, great engine which with some not so big mods can get big power

 

[YSSMAN]

I'm not sure if there really are ways to totally eliminate turbo lag. Maybe someone somewhere can invent a device that can provide secondary pressure equal to the exhaust pressure to keep the turbo spooled coupled with a blowoff valve. As others have alluded to though, a much simpler way of minimizing the lag until it is essentially imperceptible would be just make the turbos smaller, and add more of them.

Porsche has their variable geometry turbo setup that actually "adjusts" the size of the turbo according to the power demands. That's done by adjusting the angle of the blades from within the turbo, allowing for pressure to be built up more quickly (or slowly). Generally speaking, Porsche's system allows for its performance to be nearly seamless, and really crank up the power figures of the 911 as of late.

 

[Keef]

Not unavoidable. Difficult to get rid of? Yes. VGT and twin ball bearing turbos have low lag factor. Also, amount of exhaust gas to spin the turbo, the compressor and turbine materials and specifications play a major role in this. Example: you will get lag if you put a turbo meant for 2l and up into a 1.6l engine.

Exotic systems like anti-lag kits can eliminate lag...

Eliminate? Minimize at most. Turbo lag is impossible to get rid of because of air's ability to compress. Turbos will never be as instant as a mechanical system unless they've got a separate proactive system to support them, like nitrous or air injection, but those have other side effects in the exhaust and converters that aren't necessarily good. Beyond that, the backlash from a mechanical system will never be removed so even that isn't perfect.

 

[VolvoV50Driver]

Porsche has their variable geometry turbo setup that actually "adjusts" the size of the turbo according to the power demands. That's done by adjusting the angle of the blades from within the turbo, allowing for pressure to be built up more quickly (or slowly). Generally speaking, Porsche's system allows for its performance to be nearly seamless, and really crank up the power figures of the 911 as of late.

Not strictly true... The air guided TO the turbo is changed. It's called the venturi effect, when air passes through a narrowing space it speeds up. Couple this with the angle it hits the turbine (90*) it helps the turbo get upto speed quicker. (PUB FACT: 150,000rpm!)
When the throttle is released the airway opens slowing the air and slowing the turbo so no wastegate or dump valve is utilised.

It fact almost all modern diesel engines use this type of turbo. It's just very expensive to use this on petrols because of the higher temps involved, hense why only high end cars use them.