Boost effects on low compression engines vs high compression engines

[RealMcCoy]

Let me see if I understand, by lowering compression you are able to run more boost effectively by adding more air to the cylinder thereby generating more power. In higher compression engines power is derived not so much by adding air but more along the lines of compressing the air tighter.

Is this why big supercharged dragsters run compression ratios as low as 5 to1?

You're starting to catch on....

There are two completely different factors at play. A motor can be envisioned as simply an air pump... The more air you can pump, the more power you can make. The secondary factor is how much energy(power) you can extract from the mixture you have. The more you compress it, the more energy will be released during combustion. This is not only mechanical compression ratio in play, but also camshaft profile and cylinder head design. There is always a limit to the amount of cylinder pressure any given motor can withstand on the fuel used.... The higher you go with the octane rating of the fuel, (burns slower with higher flash point) the more pressure you can get away with...

Boyle's law states that gas volume and pressure are inverse at the same temperature. As one doubles the ether halves for a fixed amount of gas... That can also be interpreted by if you double the amount of gas in a fixed volume, you double the pressure. Let's imagine you have a perfect intercooler, and you get no temperature or pressure drop as you compress the air: If you increase the the amount of air by one atmosphere(14.7 lbs/sq.in) you have doubled the air charge in the cylinder, and doubled the pressure.

At 10:1 ratio, you decrease the volume by 90%, so your pressure rise on a 100% efficient N/A, with no thermal expansion calculated, would net 147 psi.

At 8:1 compression you decrease the volume by 78%, so your two atmospheres of boost would net you a pressure rise to 134 psi.

At the same boost level (two atmospheres) a 10:1 motor would net 291 psi... Seeing a problem with this picture?

Of course those are all theoretical numbers that would be complete bullshit in the real world when you factor in thermal expansion, pressure drop, and fuel mass... But I think it makes the point.