Small A/R and peak power

Hi, and Happy New Year to everybody. I am trying to understand something. Small A/R result in faster spool and less peak horsepower, and big A/R slower spool and more peak horsepower. Small A/R produce less peak HP because of backpressure but I Want to understand why even with a big Wastegate we can't reduce this backpressure to achieve the same peak horse power? I think there is a reason why different A/R does exist..

You need a certain amount of energy delivered to the turbine to drive the compressor. At the high end of the flow scale you need exhaust gas at higher pressure to deliver the same power to the turbine with the more restrictive housing. If you open the wastegate more you will reduce boost and usually power. If you have more gate spring than necessary it may be desirable to intentionally open the gate and roll off slightly in the top end to reduce exhaust pumping loss where the compressor is already choked and cannot supply more air, but that depends on the sizing of the turbo relative to the engine.

If the wastegate is open you still end up with a lot of pressure at the exhaust port which affects the performance of the engine.

Thank you both for your answer. So to keep compressor at high pressure we need more energy and the restricted area ask for more pressure to work if I understand well. So open wg will reduce boost pressure. Can we read on a turbine map the max boost an engine can run at rev or it’s better to log map and emap?

The compressor maps and turbine maps don't directly relate to how a real firing engine behaves. They are based on turbocharger test bench data (gas stand data) that basically burns propane to map the turbocharger. The compressor map is based on the pressure ratio across the inlet and outlet of the turbo (NOT the manifold boost pressure); so restriction in the inlet piping (airbox/ducting, filter, etc) affects it, and so does altitude. The turbine map is based on the pressure ratio (expansion ratio) of the turbine and mass flow. That depends on things like exhaust manifold design, restriction of the downpipe/exhaust system, and the temperature of the exhaust gas.

If you want really accurate data you need compressor inlet and outlet temperature and pressure, plus turbine inlet and outlet temperature and pressure, plus intake manifold pressure and temperature. That gets expensive and complicated and usually only done on higher budget builds that are run on an engine dyno before going into the car.

Also, in terms of spool the A/R of the turbine is most important for the "boost threshold" or steady state boost. It affects what RPM where you start making boost. Think of lugging the engine in top gear at WOT - what rpm will it start making boost? Smaller A/R helps with that. The time to spin up the wheel is going to depend on the size & inertia of the wheel. So if I go closed-->open throttle at 4000rpm, I am normally past my boost thresh hold, and the size of the wheels and the bearing design (journal or ball bearing) are going to have a big effect on how quickly it builds boost.

Wow I did read it twice as it does contain so many great infos. Thank you for this (complete ) answer. So the way to know is trial if I did correctly understood you. But if I still understand correctly it means that at High RPM with a low A/R it's better to run low boost to help keep Exhaust MAP low Enough (and probably get more power or less lose).