Learn me turbo lag

So something in the back of my mind I've been wondering... I've test drove a bunch of cars with turbos. The Hyundai Genesis 2.0T has the biggest noticeable turbo lag but also the biggest boost from turbo, was like a gust of wind 2 seconds later. I didn't really like that, as I've always drove big V8s with immediate power delivery. But then I drove a few other things with hardly any turbo lag; 2015 mustang ecob000st, fiat abarth, fiat 500 turbo. Is it because they were smaller turbines?

Usually.

But turbo lag can also be caused by the piping size, exhaust flow, boost control, engine management, etc.

But generally speaking, most cars with noticeable lag have big turbos.

Try driving my Powerstroke- the turbo is as big as my head! You can track the lag on a calendar.

It's much less prevalent in modern turbo cars. It's essentially nonexistent in my truck (3.5L Ecoboost), but my '86 951 had huge lag, enough to make it a pretty terrible autocross car because it was so hard to modulate the throttle. It was kinda fun getting on the freeway, though. Anticipation can make lag a lot of fun.

It's about 90% the size of the exhaust turbine housing..wheel plays a role too but a/r size is the biggest contributor. Also most turbos now are dual scroll ie theres two inlets into the exhaust housing both being different sizes and iirc the older genesis is single scroll whereas the later models are dual scroll.

The new stuff gets rid of lag with a variable vane setup.

Lag can also be a product of what the engine is doing right before WOT, in my 06 MazdaSPEED 6 if your sitting at 1500 rpm off throttle coasting down and go WOT = TONS OF LAG, if your at 2.5k rpm holding steady = minimal lag, if your moderately accelerating at 4K even below boost thresholds and mash it = NO LAG

In reply to bigbens6:

You are describing boost threshold and not lag. I doubt your turbo can make any boost at 1500 rpm.

New turbos have low lag because they have smaller turbine and compressor wheels for the same amount of airflow due to better internal aerodynamics. The new Borg-Warner turbos are crazy for this, people are making the same power with a turbo one frame size smaller.

The lag exists because the turbo is driven by exhaust gases, so there must first be exhaust gases to drive the turbo so it can start to compress air going into the engine so there can be more exhaust gases to drive the turbo. There's a positive feedback loop that needs to get bootstrapped by you opening the throttle, so there will be that bit where not much happens.

Another factor is that we're a lot better at controlling heat and detonation than we used to. You still notice it, though, in high specific output turbo engines. In a nutshell, if you make the turbo super responsive, it not only results in a turbo that doesn't work as well at high pressure and flow, but it also results in a lot more heat being trapped in the engine. So the Old Ways were to have a turbo that let the heat out (big turbo, big turbine A/R) and low compression in the engine so the engine could tolerate what was left, and you ended up with something that was soggy off-boost and lag you could measure on a calendar.

ANOTHER thing is much better packaging. Look at the Ecoboost for example, or the 2.0TSi Volkswagen engine. Super tiny turbos, but if you look at the exhaust manifolding and turbo, there is very minimal cross sectional change from ports to turbo. Changing cross section makes the exhaust gases speed up and slow down, this takes energy that should be used to drive the turbine instead. Keeping velocity steady means more of it can be used to spin up the turbo.

ANOTHER thing is modern turbo cars are, generally speaking, high compression/low boost because the turbo is there as a side dish and not the main course. At low boost you don't hardly even notice the turbo spooling up because the difference from atmospheric to 5psi or so is minimal. When I turned my Volvo up to 12psi, I began to notice the lag. It's minimal but there is a definite rubber-bandy effect where the engine torque surges higher after acceleration begins. (Solution: Accelerate earlier but drag the brakes until boost comes up. Rotors are SUPPOSED to be purple, right? )

T.J. wrote: In reply to bigbens6: You are describing boost threshold and not lag. I doubt your turbo can make any boost at 1500 rpm.

Excellent point (and one that I frequently make too) but driver perception is a factor that shouldn't be discounted.

You'd be surprised how low some of the modern turbos will make boost if you wait long enough. Peak torque on a lot of the modern turbocars is at 1800 or so, or is rated as flat from 1500-4000 via throttle body and boost control. Although I AM told by many people that you do not want to go WOT in a DISI below 3k because the fuel pump can't keep up and that is why they got a reuptation for folding rods. But the little turbo they used will build 21psi at something crazy low like 2500rpm. Remember that engine makes Evo VIII power but peak power is at only 5500rpm.

And this is why turbos and automatics rule. You're never under the boost threshold if you get the stall speed right. Insta-BOOOOOOOOST

Part of the reason why factory turbo engines make E36 M3 power for the boost level they are at, is because the turbine section is sized for a low boost threshhold and quick transient response at the cost of much higher exhaust pumping losses at peak power rpm. This is why you can put larger turbos on, run them at the same boost level, and still gain power.

As far as variable turbines go, i'm pretty sure the 911 turbo has had them for a good while now. They are expensive and only useful if you are trying to make so much power that the appropriate turbo would become annoyingly 'laggy' without it. So the real reason why they are rare is because most factory turbo engines are just trying to duplicate the power of a somewhat larger stock family car engine, and since that goalpost is pretty low, they are able to hit those #s with a tiny turbo that doesn't lag much anyway, thus no need for a variable turbine and all its associated complexity and cost. But there is no reason why they cannot be reliable in 2015.

Variable vane turbos are also failure prone for not all that much benefit. The benefit on a gasoline engine is small and all those tiny moving parts in the hottest part of the exhaust stream tend to sieze or get clogged with carbon. Have done quite a few VW TDI turbos when the VNT bits get sticky or sieze outright. Turbo replacement is acceptable on a Porsche, completely unacceptable on, say, a Fusion.

Ford actually makes it part of the control algorithm on the 6.7 to keep the actuator moving around so it doesn't stick. I'm pretty sure that it also kicks it around after shutoff, like GM used to do with the throttle bodies on Caddy V8s to prevent coking.

I kinda like turbo lag. Maybe that's because my first built turbo car was a GLH. With the bigger turbo and piping the lag was measured in counties, but when it came on, it was like asking for warp speed. Holy cow man!!

ord actually makes it part of the control algorithm on the 6.7 to keep the actuator moving around so it doesn't stick.

That's what was missing with the earliest setups. 'Not that much benefit' depends on how big the turbo is. Dropping boost threshhold by 500-1000 rpm can be a huge deal when it would otherwise be 3500-4000 rpm. But very very few oem cars come with turbos that 'big' relative to the engine to begin with.

In reply to Vigo:

Exactly. When they can use non-VNT turbos to get decent response in a powerband from just off idle (just above torque converter stall speed ) to 5000-6000rpm, then there is no need for the additional expense and complexity. Modern turbocars are mostly engineered so 1.4-1.6l fours feel like big fours, 2-liter fours feel like moderately sized sixes. It's not about max power as it is making a small engine feel bigger on-demand.

As opposed to, say, a Genesis 2-liter, which is about BOOOOOOST in a sporty car. I will admit to being dumbfounded when I discovered that while the 2.0 makes ~270hp, the 3.8 V6 makes ~350

And of course the Diesels need all the help they can get with balancing turbo response with flow efficiency, so that is where we see most VNTs today. Being unthrottled may actually benefit them with respect to working with VNT since even at no load they are still moving a lot of air, and the cams tend to be small/compression rather high so exhaust backpressure isn't as big a deal.

DrBoost wrote: I kinda like turbo lag. Maybe that's because my first built turbo car was a GLH. With the bigger turbo and piping the lag was measured in counties, but when it came on, it was like asking for warp speed. Holy cow man!!

I've heard several cars are actually tuned for that. It is fun.

And all my turbo info is dated...

Of the umpteenth turbo cars I've had my current '12 TDI Jetta has the worst lag of all of them, I go through 5 round-abouts on the way to work (its glorious) and have to run the car in the 'sport' mode to keep the revs a little higher or else its scary merging out into them when you need to get going like, right now.

like a couple people said previously it could be in the engine managment. I know on my volvo 850 it only pushes 5 psi until 3000 rpm then lets it jump to 10 psi, after some research its because if you push over 5 psi under 3000 rpm then rods start bending. the Genesis may have something similar, especially since its the stock turbo.

I see these conversations about turbo lag, but In my mind I frame it a little bit different...

Any engine is going to have a power band where it produces the most power. On turbo cars the turbo determines at what RPM the engine produces the most power. Large turbo = high rpm power band. Small Turbo = low/mid rpm power band.

In my car I am pulling almost 30# of vacuum at idle. The car does not reach atmosphere pressure (0 PSI) until 3500 rpm. after that , watch out because it's time for baby to eat...

If I hit the throttle off idle, I will have "Lag" but at the same time... I'm just not in the car's power band yet.

My old '88 Thunderbird Turbocoupe was fun. 1st and 2nd was limited to 12 lbs of boost with 3rd and up getting the full 20+ psi. Lag was big but it wasn't an autocross car so who really cares.

The downside to smaller turbos and instant boost is that they are at or above their threshold at redline. Its a sizing/tuning choice; sizing it small makes boost come on quicker which sells cars during test drives.

Ever drive an air cooled 911 turbo? They were sized on the big side. One of the reasons was to keep the heat down during normal driving since there wasn't a ton of cooling available. But man, when it came on

No one's mentioned the huge advantage of direct-injection for turbo engines. Instead of risking detonation by letting the fuel cloud linger in the combustion chamber as the piston comes up, wait until right before you want to light it off, then inject it. It has the double benefit of helping to cool the hot air, but mostly allows buiding turbo engines with 11-13:1 compression, which greatly helps the bottom end of the rpm range before the boost kicks in.

In my Ford 3.5 Ecoboost, it always amazes me to see, under the right conditions, that I'll be traveling up a hill in 6th gear - at 1500 rpm. That takes some serious engine management to pull that off with a turbo engine.

kb58 wrote: No one's mentioned the huge advantage of direct-injection for turbo engines. Instead of risking detonation by letting the fuel cloud linger in the combustion chamber as the piston comes up, wait until right before you want to light it off, then inject it. It has the double benefit of helping to cool the hot air, but mostly allows buiding turbo engines with 11-13:1 compression, which greatly helps the bottom end of the rpm range before the boost kicks in. In my Ford 3.5 Ecoboost, it always amazes me to see, under the right conditions, that I'll be traveling up a hill in 6th gear - at 1500 rpm. That takes some serious engine management to pull that off with a turbo engine.

Well, except that's not how anyone runs a DI engine. I've yet to see one that consistently runs compression stroke injection. All the ones I've seen inject fuel as early as possible- some near TDC before the intake stroke even gets going. Most around 270 deg before TDC of ignition. (and for sure on your truck engine)

Compression injection leads to a lot of particulates, which is bad.

It works for the cold start, for different reasons, though.

I'm surprised at you people. The true response would have been to keep him waiting for a week or two. Don't you know when to 'lag' on your responses. Good grief!

My Esprit (RIP) had a T3. Boost started to come on about 3600, mostly there about 3800-ish, with the rebuilt balanced lightened turbo. Lag off the line was significant. I recall, umm accelerating from a stoplight next to a Mustang once, door to door until I hit about 3600 RPM, after which I jumped ahead 2 car lengths. On a race track, the lag was non-existent. On a race track, every little quirk with that car suddenly made sense and helped you out.

NordicSaab wrote: In my car I am pulling almost 30# of vacuum at idle. The car does not reach atmosphere pressure (0 PSI) until 3500 rpm. after that , watch out because it's time for baby to eat...

Say what