Can a Turbo Be To Big? [How to choose the correct size]


Last week a friend of mine arrived in my shop with a Garrett 3076R turbocharger and asked me if I could install it in a stock Miata MX5 because he wanted to more power out of corners on the track.

Unfortunately, I then had to break it to him that the turbocharger he bought was far too big for his car and his needs. If he had asked me can a turbo be too big before purchasing the Garrett, he could have saved time and money.

Every turbo is designed to perform optimally for a specific range of horsepower and engine displacement (size). If you select a turbo that is too big for the engine you will experience a lot of turbo lag resulting in lost power, inversely if you select a turbo that is too small you will not achieve your target horsepower.

Can a Turbo Be To Big?

Despite what you might have heard, bigger is not always better, especially when it comes to turbochargers. Turbos are designed differently for different applications and come in a huge range of sizes with turbochargers to suit everything from 1L engines all the way up to 12L engines and beyond.

Now imagine the physical size difference of a turbo from a 1L engine compared to a 12L. The 12L will obviously need a much larger turbocharger to fill the bigger engine with the air it needs, whereas a 1L engine wouldn’t need a very big turbo at all to achieve a safe maximum performance.

a big turbo

Turbochargers create power by converting exhaust gas energy into mechanical energy by means of a turbine. Once the turbine spins (spools) fast enough the turbo will suck atmospheric air in through the compressor housing and force it into the engine creating more power.

To find out exactly how a turbo works in simple terms you should check out this article

This means that a bigger turbo needs a lot more exhaust gasses to spool the turbine and create sufficient boost pressure inside the compressor housing, so if you put a big turbo on a small engine the engine will suffer significant turbo lag and you will experience power loss and slow acceleration at low RPM while the turbo is trying to fill with enough exhaust gasses to spool the turbine.

Once the turbocharger reaches full boost you will notice a huge increase in power and possibly blow your engine apart if it’s not correctly built and tuned for your setup.

Inversely if you put a small turbo on a big engine you will have excellent acceleration due to the smaller turbo filling up and spooling much faster because of the large volume of exhaust gasses coming from a big engine.

The downside to using a turbo that is too small is that you likely will not achieve your performance targets because the turbo is being pushed to its limit and there is no room for tuning or improvement.

You also run the risk of over-speeding the turbo due to being at maximum velocity all the time, eventually, this would lead to premature turbo failure caused by the extra pressure put on the internal bearings.

Before choosing a turbo its important to fully understand how they work to ensure you make the correct choice when purchasing one, to find out exactly how a turbo works I think you should check out this article to give you a solid foundation on the subject and avoid any unwanted errors.

How To Choose The Correct Size Turbo For Your Engine

When selecting the correct size turbo for your application you first need to decide what exactly your performance needs are.

If you plan on doing track days and require responsive low-end acceleration out of bends along with mid-range power you’re going to need a smaller turbo to achieve this, due to the A/R ratio being lower, meaning the turbo housing can hold less air.

If it’s drag racing your into, your going to want a big turbo that’s going to give you maximum airflow into the cylinders of the engine to create maximum peak power.

But it’s not just a matter of picking the biggest turbo you can find. The turbo must be matched or mapped specifically to your engine, goals, and upgrade plans. This will ensure you get exactly what you need without the guesswork.

The next thing you need to determine is what your target horsepower is. Turbos are sized by the volume of air that can flow through them. The formula for determining the horsepower equates to 1 lb / minute = 10 HP at the crankshaft, so if you hope to achieve 400 horsepower at the crank you will need a turbocharger with an airflow of 40 lbs / minute.

The size of any turbo can be identified by reading the specifications on the plate as shown in the picture below.

turbo size plate

Sizing a turbo this way is called compressor mapping and it is a must-do if you want to achieve the most power from your turbo engine as efficiently as possible. I have included a link to Balancemotorsport.co.uk which has a compressor mapping calculator that you simply input your engine size and target horsepower into and it will tell you the exact size turbocharger you need to achieve your goal and is 100% free with no sign up of any kind.

Conclusion

So in short, yes a turbo can be too big and it can also be too small depending on your needs, so the safest, quickest, and easiest way is to choose the correct turbo from the start by using a compressor map calculator and installing the turbo with confidence knowing you made the correct choice.

Gavin

With over twenty years worth of experience in the motorsport industry and many happy customers you can rest assured that the information you get here on BackyardBuildz.com is 100% free, reliable and without the fluff.

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