Engine Design Guide: Part 3 - Bottom End
The bottom end (also known as internals) is the core of your engine. The components you pick here can change significantly how fuel efficient, environmentally friendly, or reliable your engine is. There are 3 components to pick from here, and your first quality slider.
The first component is a crankshaft. This is what transfers power to the gearbox from the engine. It can usually take more slightly more RPM than the other two components.
Cast Iron - This is cheap and simple. Nothing more.
Forged Steel - This is stronger, can take more torque and RPM and is lighter, but requires a factory with a forge works, which is expensive, and has a higher ET and PU. Cheaper in material costs than cast iron. Available from 1956.
Billet Steel - The strongest crankshaft available. Can take a lot of torque and RPM, and is light, with a bit more ET and PU, but requires your factory to have a CNC Shop. Even cheaper than forged steel in material costs. This unlocks in 1986.
Flat Plane vs Cross Plane Cranks
If you select a 90 degree V8, you’ll be able to choose between “regular” cranks (cross plane) and flat plane cranks. From the little I actually understand about these myself, a flat plane is similar to a crank you’ll find in an inline 4 engine, and has a different firing order to cross plane. This gives it a different sound, so if you care about that, awesome for you.
Cross plane V8s are smoother, a bit more reliable, and have very slightly lower production units.
Flat planes are slightly lighter, and very slightly cheaper. They also give a very minor boost to midrange power. From my testing you’ll usually gain about 1lb-ft of peak torque, at least on NA engines.
The next component is your conrods. This connects your pistons to the crankshaft. It usually has the same limits as the pistons in RPM and torque.
Cast Iron - Cheap and simple.
Heavy Duty Cast - Useful in the early years for powerful and low revving engines as it can take a lot of torque, but has a low RPM threshold and reduces smoothness. They are also slightly heavier. Available from the start.
Heavy Duty Forged - Like the cast version, but lighter, smoother, and can take RPMs equal to a cast iron crankshaft. Requires a forge works, and is available from 1956.
Lightweight Forged - This can take more torque than cast iron, and higher RPM than anything before it. Unlocks in 1967.
Lightweight Titanium - This can take as much torque as heavy duty forged, and more RPM than any other conrod, while also being smoother than any other. Requires a CNC shop and unlocks in 1997.
The final component is pistons. Changing these can drastically change your engine’s emissions and fuel economy.
Cast Iron - Cheap and simple.
Heavy Duty Cast - Useful in the early years for powerful and low revving engines as it can take a lot of torque, but has a low RPM threshold. Also slightly heavier and less smooth. Available from the start.
Forged - This can take as much torque has heavy duty forged conrods with a bit higher RPM, but can’t rev as high as lightweight forged conrods. Forged pistons also lower octane by one RON, but increase emissions and loudness… Requires a forgeworks and is available from 1956.
Hypereutectic Cast - These pistons are like cast, but lower emissions a lot. They are also the quietest pistons. Unlock in 1970.
Low Friction Cast - These are like cast, but improve fuel economy drastically and lower emissions by a lot, while also increasing smoothn. They are also slightly more limited in RPM. Unlock in 1992.
Lightweight Forged - These can take the most RPM and torque, and improve smoothness the most, but also increase emissions a lot. Forged pistons also lower octane by one RON. Unlocks in 1992.
Next you have variant capacity. This is where you can adjust the bore and stroke of this version of the engine.
Some reasons you may reduce the size of the engine:
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To make your car get better fuel economy
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On a performance variant you may decrease the stroke to allow it rev higher reliably.
Reducing bore and stroke here will also reduce weight slightly, but not as much as on the family bore and stroke selector.
Next we have your first quality slider.
In simple terms, quality sliders change the year of a component, and also reduce or largely increase ET and PU depending on the direction you move the slider. On bottom end, increasing quality reduces weight, very slightly reduces cost, slightly increases reliability and largely improves smoothness. Applying lots of quality will quickly make your car very expensive, so carefully watch ET and PU when applying it.
Having tech pool in bottom end will also let you unlock engine family things such as block material earlier.