there is something about bigger turbo's that has had me confused for a little while.

i understand that a bigger turbo can move more air so with extra fueling can produce more power but what has confused me is that you can have a bigger turbo creating the same pressure yet making more power :confused:

for me i have my boost gauge fittted at the throttle body so where the gauge reads its pressure is a fixed size no matter the size of the turbo, so a fixed size bit of tube at a given pressure surely there is always the same amount of air no matter what turbo is creating the pressure? surely in order for more power you need a greater pressure at the throttle body?

or are people talking about pressure in the turbo itself rather than pressure at the inlet manifold?

It's to do with being inside the turbo's efficiency range so charge air is cooler hence denser at the same pressure. Also the pressure is held much higher in the rev range. So you may get say 1.3bar at 4krpm with both a K03S and big turbo but the k03s will then drop off whereas the big turbo will maintain that pressure as the revs increase. A K03s drops to around 0.6bar by the red line IIRC. Also quite a few big turbo cars are larger capacity so a larger volume or air is being pressurised.

hadn't thought of holding the boost, good point what with bhp being torque x rpm

Hi,

pressure is a result of the restriction to airflow, so you have to look at the whole system.

If the gas can flow through the throttle body, through the head and past the exhaust turbine and down the exhaust easier then there is less backpressure, hence it's possible to have a higher volume of flow at the same pressure, can be achieved with a less restrictive exhaust, downpipe, turbo, ported head, etc.

Turbo sizing is basically a compromise between flow capacity and response, however with roller bearing turbos, more efficient turbine and compressor blades then you can get closer to the best of both worlds.

HTH

Cheers

Neil

Welcome Neil, and a good discription.

Compressing the air means it gets hotter.

The less extra aggitation the turbo does to it (the smoother or more efficient if you like), the less heat the act of compressing it adds.

Larger turbos are likely to be in a better efficiency range at the high end of the rev range.

As previously mentioned, they can hold boost pressure higher into the rev. range before dropping off.

Someone once said "buy the smallest turbo that reasonably fulfils your power goals".

No point buying a huge lag-monster if what you want is 20BHP more a a more responsive drive.

J.