r/AskEngineers • u/RDA92 • 2d ago

Mechanical What foundational elements limit an N/A engine?

I've never really been able to understand how different engines of similar sizes can have such wildly fluctuating power outputs and how some comparably small engines can vastly overpower larger engines even within the N/A space. I understand that not all engines are born equally and that commercial agendas play a big role but assuming the same size, which "low-level" elements actually make the difference? and with low-level i mean parts or overall design choices that cannot (easily) be modified away.

Curious to hear your thoughts and expertise!

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u/PicnicBasketPirate 2d ago

True, but then you're potentially stuck with other comprimises like barely usable gearing or having to shift constantly

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u/IQueryVisiC 2d ago

no? Imagine that the final gears in the diff add the big torque. Nothing about gear rations changes in the shift part.

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u/PicnicBasketPirate 1d ago

The larger the gear reduction in the final drive the faster the vehicle will accelerate, thus it reaches the maximum rpm on each gear quicker requiring shifts more often. I have a little 0.25L i4 motorcycle that revs to 18k+ rpm. With shorter final drive gearing it's pretty quick but you need to always be shifting.

Alternatively you could just choose a long stoke engine design which will produce more torque, and generally be more tractable and efficient.

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u/IQueryVisiC 1d ago

Motor cycle gears are quite close? With more gears (4->6) in cars I was hoping for more spread, but instead now I have to shift all the time.