Can we increase the horsepower and not increase the stress on things like engine bearings and the gearbox?
How Are Torque & Power Calculated?
Engine power is a product of torque and RPM.
The formula to calculate engine power is
(RPM * T) / 5252=HP
If we increase torque, we increase power. The torque and engine produces is directly related to how much air and fuel we can get into the engine ie more air in the cylinder, the more power that cylinder makes.
More Torque = More Power.
Engine & Gearbox Stress
The stress on the engine is produced by the twisting force (the torque) of the engine.
If we can keep the torque the same but at a higher RPM we can increase the power of the engine without increasing the stress on the engine.
Worked Example Using A Focus ST
Ford are famous for being conservative with their turbochargers sizes.
Generally Ford do not want customers using their engines at high RPM (presumably to help durability). Generally Ford engines makes their peak torque very low in the RPM range and then falls off towards redline.
Prime Example – 2018 Ford Focus ST
Peak engine stress and gearbox stress (peak engine torque) is 370NM @ 2500RPM.
How much power could this engine make is peak torque was moved to something more, let’s say Japanese (read advanced) and call peak torque at 5500rpm.
What power would the engine produce at 5500rpm with torque of 370NM?
Because we are dealing with BHP we need to convert NM to lbs/ft
Multiply 370NM by 0.737 = 272lbs ft
Bolting the numbers into the calculator below
We get 284bhp, an increase of 32bhp over the stock power number of 252bhp
Increasing the torque number to 6000rpm we get 310bhp, an increase of 58bhp and no increase in stress on the engine and gearbox because of the increased power (higher RPM will lead to faster engine wear)
What can we take away from this example?
If you don’t mind the shorter service life of an engine, there is stress free power to be had by fitting a larger turbocharger to almost any factory stock engine.