Turbocharger Lag

By Joseph Obernberger

SUMMARY: This text hopes to describe what exactly turbocharger lag is, why it is getting such a bad reputation, and why that bad reputation is usually unfounded.

Turbocharger lag is, for practical purposes, the time it takes the turbocharger to spin up and make usable boost pressure after you plant your right foot. The turbocharger is driven by hot exhaust gases passing through the turbine side of the turbocharger assembly. Before the turbo can make positive boost pressure, that is pressure above atmospheric pressure, there must be enough exhaust energy to spin the turbine. The only way there can be a substantial amount of hot, high velocity exhaust gases passing through the turbine, is if the engine is under a significant load. Once that occurs boost pressure is created, more fuel can be injected, and hence more hot exhaust gases produced to spin the turbo even faster. What a wonderful cycle!

So why am I saying that this nonsense about turbocharger lag is unfounded? Well, in the old days of turbochargers, such as some of the first Porsche 911 turbos, large turbochargers (by today's standards) were used. These turbos contained heavier metals and hence took more energy to spin up. So, when test drivers got on the gas there was a significant delay before (BAM!) loads of power was produced. This was deemed undesirable and given the name turbo lag. These engines also had poor low end torque, because the turbo would not spool up to create usable boost until higher RPMs were reached. When magazine articles were written about cars using turbochargers, lag was 'driven home' to the engine designers as a very bad thing. So, back to the drawing board they went, and they came up with the idea of using much smaller turbochargers and eventually using much lighter materials to help eliminate lag. In my opinion they succeeded greatly. For whatever reason magazine editors, when they see that an engine is turbocharged, have to bring up lag as a negative issue, even if, in reality, it isn't at all.

Here comes my rant. A while back I read an article about the 2002 Audi S4. This car comes with a wonderful 5 valve per cylinder twin turbocharged, intercooled, 2.7 liter V6 engine. It produces 250HP at 5800RPM, and 256lb-ft at 1850RPM. Now that's what I call low end torque! That's just off idle! Audi accomplished this by using two small sized quick spooling turbochargers. The down side in doing this is that top end power can suffer because of the smaller turbo placing a restriction on the exhaust. In any case, the magazine article complained about turbo lag with this engine! What turbo lag! It produces peak torque at 1850RPM! So, for the next year (2003), Audi ditched the wonderful twin turbo V6 and used a 4.2 liter V8 engine. This engine produces 340HP at 7200RPM, and 302lb-ft at 3500RPM. The same magazine praised this engine for its low end torque. While this engine is clearly more powerful, it cannot match the turbo engine for low end torque. They're just giving turbo engines a bad name! Shame on them!

Unfortunately most car manufacturers that are using turbo engines are using very small turbochargers to get away from the 'dreaded lag'. As mentioned earlier, this leads to excellent low end torque, but limits top end horsepower. Thankfully some are not giving in. Mitsubishi's Evo VIII MR uses a 2.0 liter engine with 280HP at 6500RPM and 295lb-ft at 3500RPM. This is almost the same amount of torque as Audi's 4.2 liter V8 and at the same RPM! Hurray for turbos! This engine is, however, highly criticized for its turbo lag. While it does have some lag, it is very marginal. In driving the car, boost pressure basically follows what your right foot does. I would say that this engine has minimal lag, but noticeable at very low RPMs. And there you have it - driving around in top gear at 30MPH wondering where the engine power is. For goodness sakes downshift! It is true that a turbo car must be driven differently than a normally aspirated car, but for the same size engine, you'll never get the power out that you can with a forced induction engine - particularly turbo engines. So does all this talk of turbo lag make any sense? You could have a 2.0 liter engine with 150HP and 130lb-ft or you could have a turbo 2.0 liter with 280HP, and 295lb-ft, but then you must put up with turbo lag, and gosh, who wants that?

And then there is Honda. Wow - what to say here! They take a 2.0 liter engine, spin it to high heaven and get excellent power out of it. Magazines, then criticize it because you have to rev it so high, but thank goodness it doesn't have turbo lag! So here are your choices:

• Low power, low torque normally aspirated 2.0 liter engine.
• High power, high revving, low torque 2.0 liter engine.
• High power, high torque (at a relatively low RPM), 2.0 liter turbo engine.

I know where my vote is going!

In my opinion peak torque should be achieved at roughly half the maximum engine RPM. So, for example, if redline is 6000RPM, peak torque should be around 3000RPM. I feel this leads to a well performing fun to drive car. Audi took it a little too far, in my opinion, with a peak torque at 1850RPM - and they still got hammered for turbo lag. You'll never see a normally aspirated gasoline engine (within reason) with peak torque so low.