Compression

[lucaneves42]

What is the relation between compression, bore and stroke, engine material etc? I do want to build a high compression engine, but I can't find the sweet spot.
Basically, what I want to know is: how one manage to build HC engines?

[Dragawn]

Compression is not really influenced much by these, but from my experience an as oversquare (higher bore, lower stroke) as possible engine gives the least octane rating (how prone the engine is to preignite, so less knock means higher compression possible). A nice bonus with this is that you'll get a high revving engine, which can use a more aggressive cam profile, which further reduces knock. I'm not sure if material influences knock, iron gives a tiny bit more hp, but more emissions too, but I forgot about knock. If there's a difference in materials it's like 0.2 octane ron.

Recently I made an v8 revving to 11k rpm which ran at 13.6:1 compression, so yeah.

EDIT: timing also heavily influences your octane/knock, but I really wouldn't recommend to retart it since that'll give you poor engine stats.

[Jakgoe]

Another two factors that influence the octane requirements for the engine are the fuel mixture, and the choice of fuel delivery. Choosing a direct injection engine with a throttle body per cylinder will result in a lower octane rating than a single point EFI engine.

[Daffyflyer]

Main ones that come to mind (IIRC there are more than this though)

Bore - Smaller = a little lower octane requirement
Cam Profile - More Agressive cam profile = more overlap = less dynamic compression = more static compression possible
Fuel System - More Sophisticated = lower octane. (EFI is good, DI is better)
Fuel Mixture - Richer = Lower octane requirements (thus higher compression possible)
Ignition Timing - retart = Lower Octane

[utopian201]

...
Cam Profile - More Agressive cam profile = more overlap = less dynamic compression = more static compression possible
...

Whats the difference between dynamic and static compression

[gt1cooper]

Static is the measured compression (i.e. 10.5:1), whereas dynamic compression is the actual change in cylinder pressure. For instance, an engine with 10.5:1 compression can have 250 lbs of cylinder pressure, and an engine with 14.5:1 can have 150 lbs. The reason for this is that the engine (in anything but a perfect case, or a boosted case) will not have 100% volumetric efficiency in normal operation, since the intake valve closes after BDC (bottom dead center crankshaft lobe position for the cylinder) and a noticeable amount of the fuel mixture can escape. Now, there are exceptions. As stated before, if the engine is boosted, some more mixture can be forced into the cylinder even after BDC, though usually not much, and sometimes, not any.

Most engines have a rpm at which you will find closer to 100% volumetric efficiency, and this is selected by your (in game) cam profile. Changing that value up in game raises that point higher. One has less than 100% volumetric efficiency at low RPM, before you reach that point, and high RPM, after you reach that point, as you are either not pulling in as much mixture as you could've, or you are actually forcing some out before closing the valve. Either case is not optimal.

This also ties into octane, as at 100% volumetric efficiency, you are getting the most fuel mixture into the cylinder, leading to the most cooling between compression strokes and also more mixture to have to heat under compression. This also shows why going to higher RPM (past the optimal volumetric efficiency) hurts your octane rating. Of course, there are many other factors listed before.

[Dragawn]

Static is the measured compression (i.e. 10.5:1), whereas dynamic compression is the actual change in cylinder pressure. For instance, an engine with 10.5:1 compression can have 250 lbs of cylinder pressure, and an engine with 14.5:1 can have 150 lbs. The reason for this is that the engine (in anything but a perfect case, or a boosted case) will not have 100% volumetric efficiency in normal operation, since the intake valve closes after BDC (bottom dead center crankshaft lobe position for the cylinder) and a noticeable amount of the fuel mixture can escape. Now, there are exceptions. As stated before, if the engine is boosted, some more mixture can be forced into the cylinder even after BDC, though usually not much, and sometimes, not any.

Most engines have a rpm at which you will find closer to 100% volumetric efficiency, and this is selected by your (in game) cam profile. Changing that value up in game raises that point higher. One has less than 100% volumetric efficiency at low RPM, before you reach that point, and high RPM, after you reach that point, as you are either not pulling in as much mixture as you could've, or you are actually forcing some out before closing the valve. Either case is not optimal.

This also ties into octane, as at 100% volumetric efficiency, you are getting the most fuel mixture into the cylinder, leading to the most cooling between compression strokes and also more mixture to have to heat under compression. This also shows why going to higher RPM (past the optimal volumetric efficiency) hurts your octane rating. Of course, there are many other factors listed before.

Holy- That was an interesting read, Cooper, thanks!

[gt1cooper]

Thanks might want to get someone who actually took a class on this type of thing though, this is my basic understanding (very basic)