Does bigger turbo mean more lag? An in-depth look

Hey there! If you‘ve ever wondered how turbo size affects that annoying delay in power called turbo lag, you‘re in the right place.

As an engineering geek with a love of turbos and ECU tuning, I‘ll walk you through everything you need to know about how upgrading to a bigger turbocharger impacts turbo lag. You‘ll learn how lag happens, how much extra power you can get from bigger turbos, and most importantly, tips to minimize lag when you upgrade.

So strap in and let‘s dive into the world of turbochargers!

What exactly is turbo lag?

Before jumping into bigger turbos, first we need to understand what causes turbo lag. Here‘s a quick 101…

Turbo lag refers to the time delay between pressing the accelerator and feeling the surge of torque as the turbo starts to generate boost. It‘s the result of the turbocharger needing to physically spin up to high RPM before it can produce added power.

Turbo lag visualized – the delay from accelerator to boost buildup

At low engine speeds, exhaust flow is low, so there isn‘t enough exhaust gas volume spinning the turbo‘s turbine wheel rapidly. The turbo spins slowly, compressing little extra air into the engine.

Once the engine RPMs climb, exhaust flow increases, accelerating the turbine. At around 2200-3000 RPM the turbo hits its peak speed, pressurizing intake air and delivering the full boost.

So in essence, lag happens because it takes time for the turbo to spool up. Bigger turbos with larger rotating assemblies tend to exacerbate lag, while smaller turbos spool quickly minimizing lag.

Bigger turbos deliver more top end power

The appeal of upgrading to a larger turbocharger is simple – bigger turbos flow more air allowing them to produce more maximum boost pressure and horsepower.

For example, swapping a small K03 turbo on a 2.0L engine for a GT2876R will likely increase peak boost from 10 psi to 20+ psi. This can add 90+ horsepower to the tops of the powerband!

Larger compressors and turbines have higher flow capabilities. Just like how a bigger pipe can flow more water, a larger turbo can flow more air.

More airflow means the engine can burn more fuel resulting in huge power gains. A turbo upgrade is one of the most effective ways to add substantial horsepower.

A larger turbo compressor wheel like this billet 68mm unit flows a lot more air.

The downside – increased turbo lag

So it seems like an easy choice right? Just bolt on the biggest turbo you can find!

Well here‘s the catch – bigger turbos come with a nasty side effect of increased turbo lag. Their larger size directly leads to slower response times.

This happens primarily because of rotational inertia, which you can think of as the "resistance" of the turbo to spinning up quickly.

The larger the compressor and turbine wheel diameters, the higher their inertia. More force is required to rapidly accelerate bigger, heavier wheels.

Additionally, larger housings and castings have more surface area creating increased friction and resistance to quick spool up. These combined effects result in a slower acceleration rate and therefore more lag with a bigger turbo.

So while that huge turbo will ultimately generate enormous top end power, you‘ll be plagued with lag off idle and through the low RPMs. Acceleration will be sluggish and throttle response dull until the big snail starts spooling.

Selecting the right turbocharger

Picking the turbo that‘s right for you involves finding the sweet spot between rapid response and maximum boost.

First, consider your engine specs and power goals. Are you aiming for 600 horsepower or a tractable 400? This helps determine the flow requirements for your turbo.

Next, think about the engine and RPM range where you want power. Turbo response below 2000 RPM? Go smaller. Big power up top over 5000 RPM? Go bigger.

Analyzing the compressor map is critical for matching the turbo to your target powerband. Good map overlap at the engine‘s mass airflow needs is ideal.

Here‘s a comparison of 76mm and 61mm turbine compressor maps showing flow differences:

76mm turbo compressor map (red) vs 61mm (blue)

While there are similarities, you can see the 76mm (GTX3076R) flows more air up top, while the 61mm (GTX2863R) comes on boost sooner at low RPM.

For a responsive daily driver, I might choose the GTX2863R. But for big power in a track car, I‘d spec the GTX3076R despite increased lag.

Twin scroll and dual ball bearing turbos

Modern turbo tech like twin scroll housings and dual ball bearing center cartridges can offset some of the lag from bigger turbos.

Twin scroll splits the exhaust pulses into separate channels feeding each side of the turbine wheel independently. This allows it to spool faster, partially countering the larger turbo size.

Dual ball bearings have incredibly low friction allowing the shaft and wheels to spin faster with less restriction. When paired with low inertia wheels, they can dramatically improve response.

For example, a GTX3076R twin scroll ball bearing turbo will have noticeably better response and less lag than an older GT3076 journal bearing version.

Reducing lag through tuning

Proper ECU tuning is mandatory when upgrading turbos to manage the increased airflow and minimize lag. Here are some effective tuning strategies:

• Optimize VVT/cam timing – advancing intake, retarding exhaust
• Lower boost threshold – build boost sooner
• Raise two-step/launch RPM limit
• Increase injector flow rates
• Enable anti-lag features like rally burble
• Adjust boost by gear

With the right tune, you can achieve excellent drivability and snappy acceleration even with a much bigger turbo. It‘ll really unleash that newfound top end power while keeping lag in check.

Real world results from turbo upgrades

I‘ll share some examples of turbo swaps on common tuner cars and the real world differences they made based on my analysis of dyno data and owner reviews:

2.0T Volkswagen GTI:

Stock K03 turbo: ~220WHP

Upgrade to GT28RS: ~300WHP, 20+ psi boost

Felt slower under 3000 RPM but much faster up top

Ford Mustang GT:

Stock turbo: ~380WHP

Upgrade to 67mm: ~570WHP, 24 psi boost

Major top end power gains above 4500 RPM but increased lag

Honda Civic Si:

Stock turbo: ~200WHP

Upgrade to GTX2863R: ~270WHP, 18 psi boost

Faster spool up and response. More power through entire RPM range.

Subaru WRX STI:

Stock twin turbos: ~310WHP

Upgraded twin 67mm ball bearing: ~410WHP

Huge gains over 4000 RPM, moderate low end lag

So in summary, bigger turbos absolutely bring big power gains up top, but at the cost of worse low end response in most cases. Proper turbo selection and tuning can minimize lag.

I hope this guide gave you a better understanding of how turbo size affects that dreaded lag we all hate! Let me know if you have any other questions.

Your turbocharged bud,

Terry