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Like wtf bro, if you have a piston that's doing 12:1 compression ratio, that's a calculation based on volume. If you compress the air with a compressor, that pressure ratio is factored into the cylinder with the piston at BDC. Where is the calculation for the effective compression ratio where boost is added to the compression ratio at TDC? It doesn't exist, it's a conspiracy man. I mean what is the functional relationship? Is it additive? 14 psi of boost = 2:1 compression ratio, and an engine with a 10:1 geometric compression ratio now has an effective 12:1 geometric compression ratio? That can't be right, because then it would just be easier to use an 12:1 piston and save all the turbo plumbing. Is it multiplicative? 10x2=20? So a 10:1 CR piston with a 2:1 compression ratio from a compressor would effectively have 20:1 compression ratio? Or is it some other non-linear function that can only be gleamed through experimentation?

Like wtf bro, if you have a piston that's doing 12:1 compression ratio, that's a calculation based on volume. If you compress the air with a compressor, that pressure ratio is factored into the cylinder with the piston at BDC. Where is the calculation for the effective compression ratio where boost is added to the compression ratio at TDC? It doesn't exist, it's a conspiracy man. I mean what is the functional relationship? Is it additive? 14 psi of boost = 2:1 compression ratio, and an engine with a 10:1 geometric compression ratio now has an effective 12:1 geometric compression ratio? That can't be right, because then it would just be easier to use an 12:1 piston and save all the turbo plumbing. Is it multiplicative? 10x2=20? So a 10:1 CR piston with a 2:1 compression ratio from a compressor would effectively have 20:1 compression ratio? Or is it some other non-linear function that can only be gleamed through experimentation?

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[–] • 1 pt

Because volumetric efficiency that's why.

[–] • 0 pt

You can assume things and still be in the ballpark, you can assume compression by the piston is adiabatic, when it's very far from it, yet calculation assuming adiabatic conditions gets you in the ballpark. Adding more parameters makes the calculation take longer but you start running into diminishing returns for the parameters you're considering.

[–] • 1 pt

But it's not just compression it's things like cam overlap, manifolds, heads, valves, etc.

This is why the easiest and most accurate way is still a dyno. Like you said you can get close but to perform at the top tier you need real data.

[–] • 0 pt

Volumetric efficiency itself is a measure of how well the geometric compression ratio translates to actual compression of the air. Boost doesn't affect volumetric efficiency, only the density of the fluid. However at certain pressure ratios, the laws of compressibility alter the volumetric efficiency of the engine, because compressibility changes the rules of airflow. When the air approaches sonic velocities(achieved when pressure ratios approach 2:1 between a reservoir and a throat) then a constriction increases pressure, which is the opposite in a subsonic flow.

Think about this when designing intake manifolds, thin long runners are great for low speed operation, because it increases velocity of the air, at higher mass flow, the pressure ratio between the manifold and runner approaches 2:1 and you get sonic choking at the throat. So you design big fat short runners for high mass flow, but then the air doesn't accelerate enough at low speed. So then you try to get the best of both worlds, you use really smooth bell mouth runners to get more flow at low speeds, while still flowing well at higher mass rates.