Bore and Stroke have no effect on Horsepower (Part 1)

KTM RC8 Engine LC8

KTM RC8 Engine [via]

Last week I wrote about what I learnt about bore and stroke, and it seemed that increasing bore made engines more ‘revvy’ and increasing stroke made engines more ‘grunty’.

So I thought if I increased either of these attributes, I would actually increase the horsepower of the engine right?

The answer is no, bore and stroke has no effect on horsepower.  Let me explain how I figured this out.

Let’s assume that there are 2 versions of the KTM Duke 125 engine – one which is over-square (bore : stroke >1) and one which is under-square (bore : stroke < 1).

The bore stroke measurements would look something like this:

KTM Duke Engine Comparion

2 similar capacity engines with different bore and stroke combinations

The objective of this post is to prove that these 2 different bore/stroke combinations have no effect on the horsepower made by the engine.  

Before I do this however, I need to explain 2 concepts:

  1. The relationship between horsepower and torque
  2. How you calculate torque inside a bike engine

Horsepower – Torque relationship

Horsepower v Torque

Horsepower determines top speed while torque determines acceleration [Photo via]

Horsepower is the power made by the engine. For a given gear ratio, it determines how fast a bike can go. The more horsepower a bike has, the better it can punch through the air and reach a higher top speed.

Torque is the amount of turning force exerted at the rear wheel of the bike. It is how much acceleration or ‘shove’ a rider feels when he twists the throttle at any given speed.

Horsepower and torque are related by the following equation

Horsepower = Torque x Engine rpm

(You have to apply different conversion constants depending on whether you use lbft or Nm)

So if if engine rpm were constant,  and we can prove that torque is independent of bore and stroke, then that must mean that horsepower will also be independent.

Next week, I will continue to explain the second concept – How torque is calculated inside a bike engine.

What do you think? If you have any comments or feedback, leave a note below!


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