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Is a low compression engine better for forced induction than high compression?
Depends on how much boost you're putting into the engine. The big issue here is managing the amount of internal pressure within the cylinders; by making sure it's not going to damage the engine whilst still making the most power possible. Too much pressure can cause catastrophic failure where you literally blow the head off the engine. Hence why Top Fuel drag cars have big straps to hold the superchargers down in case they get blown off. The higher the compression ratio, the more natural torque an engine produces. Adding forced induction increases the effective compression of an engine, because although you have the same compression ratio, air and fuel are entering the cylinder already at a higher pressure. This increase in pressure translates into a bigger bang at ignition, and a larger pressure from the expanding exhaust gases - resulting in more power. Dropping the compression ratio allows a higher amount of induction pressure to be used, meaning a greater volume of fuel and air can be squeezed into the cylinder. This results in a big increase in torque and power - as long as that volume is being delivered. When the turbocharger or supercharger is not delivering the full volume - when it's 'off boost' then the engine is relying on a lower amount (and pressure) of air coming in, which results in less power. This breathless lack of power is often mistakenly referred to as lag. A low compression engine with big induction pressure will perform very poorly 'off boost' (i.e. when the turbo/supercharger is not delivering), and will very rapidly build power as it comes 'on boost'. In extreme cases this can literally be like flicking a switch from no power to instant full power - and a car that will be quite a handful to drive hard. Depending on the induction device, this 'boost threshold' can be quite high in the engine rev range. A higher compression engine with low induction pressure will perform much better 'off boost' because it still has its own natural compression to generate power; it will generally not have a big jump in power, and as the induction device is generally smaller, its boost threshold will be much lower. A low compression, big boost engine will make an insane amount of top end power, but be very wheezy and powerless down low, whereas the same sized engine with higher compression and lower boost will be very torquey low down, but won't make as much top end power.
Is a low compression engine better for forced induction than high compression?
Depends on how much boost you're putting into the engine. The big issue here is managing the amount of internal pressure within the cylinders; by making sure it's not going to damage the engine whilst still making the most power possible. Too much pressure can cause catastrophic failure where you literally blow the head off the engine. Hence why Top Fuel drag cars have big straps to hold the superchargers down in case they get blown off. The higher the compression ratio, the more natural torque an engine produces. Adding forced induction increases the effective compression of an engine, because although you have the same compression ratio, air and fuel are entering the cylinder already at a higher pressure. This increase in pressure translates into a bigger bang at ignition, and a larger pressure from the expanding exhaust gases - resulting in more power. Dropping the compression ratio allows a higher amount of induction pressure to be used, meaning a greater volume of fuel and air can be squeezed into the cylinder. This results in a big increase in torque and power - as long as that volume is being delivered. When the turbocharger or supercharger is not delivering the full volume - when it's 'off boost' then the engine is relying on a lower amount (and pressure) of air coming in, which results in less power. This breathless lack of power is often mistakenly referred to as lag. A low compression engine with big induction pressure will perform very poorly 'off boost' (i.e. when the turbo/supercharger is not delivering), and will very rapidly build power as it comes 'on boost'. In extreme cases this can literally be like flicking a switch from no power to instant full power - and a car that will be quite a handful to drive hard. Depending on the induction device, this 'boost threshold' can be quite high in the engine rev range. A higher compression engine with low induction pressure will perform much better 'off boost' because it still has its own natural compression to generate power; it will generally not have a big jump in power, and as the induction device is generally smaller, its boost threshold will be much lower. A low compression, big boost engine will make an insane amount of top end power, but be very wheezy and powerless down low, whereas the same sized engine with higher compression and lower boost will be very torquey low down, but won't make as much top end power.
