The Compression Ratio

Increasing the compression ratio helps generate more horse power by improving the engine's thermal efficiency and by raising the cylinder pressure. However, there are limitations to how high the compression ratio can be raised. Firstly, increasing the cylinder pressure increases the stress placed on the pistons and on the ring lands. Secondly, the more compressed air/fuel mixture would be more prone to pre-ignition and detonation that will wreck an engine sooner rather than later. This problem is exacerbated by the low octane nature of unleaded fuel that we use today. You could mitigate against this problem by retarding the ignition timing, but as Double H mentioned in ignition timing the correct ignition timing is crucial to producing optimal engine power. Thus, retarding the ignition timing would reduce optimal engine power. It would also loose responsiveness and fuel efficiency. Thirdly, the design and size of the combustion chamber as well as the cam timing and overlap period also affects the possible compression ratio. Cylinder heads with pent-roof and semi-hemi combustion chambers and longer duration camshafts allows for higher compression ratios than bath-tub or wedge combustion chambers and short duration cams.

Increasing the Compression Ratio

On modern engines the compression ratio is quite often close to the optimal compression ratio allowed by the octane rating of modern fuels and the valve lift and valve overlap of the stock camshaft. Thus, there is often not much room for increasing the compression ratio. However, installing a performance camshaft with a longer valve overlap or duration reduces the pressure in the combustion chamber as the intake valve opens sooner while the exhaust valve is still open and some of the intake charge escapes through the exhaust valve that is still open. This reduces torque at low rpm. One way of overcoming this drop in cylinder pressure is by increasing the compression ratio.

There are a few ways in which you can increase the compression ratio:

  • You can mill the cylinder head to reduce the combustion chamber. This is the easiest way of increasing the compression ratio.
  • If your engine has deck clearance, in other words, if the piston tops do not reach the top of the engine block, you could mill the top of the engine to reduce the deck clearance. This would be preferred over milling the cylinder head.
  • You could fit pistons with a raised top, or if the stock pistons are dished, you could fit flat-top pistons.
  • If you have an aluminum alloy cylinder head, you could use aluminum welding to add metal to the combustion chamber. This is the best way to increase the compression ratio.

All of these methods, except welding the combustion chamber, reduces valve to piston clearance and might require that you add valve pockets to the piston top. Fitting high-top pistons also has the added potential of shrouding the spark plug, which could inhibit the effective burning of the air/fuel mixture and may leave pockets of air/fuel mixture that are only partially ignited or completely unburned. Finally, use aluminum welding to add metal to the combustion chamber requires a bit of skill to ensure that the heat caused by the welding does not warp the cylinder head. The threat of warping can be reduced by not working on one spot too long. The added metal should be ground smooth to prevent hot spots and each combustion chamber must have the same volume or the cylinder pressures would not be equal across the engine. You can cover the combustion chamber with a flat piece of perspex with a small hole in it and use a burette to measure the volume of the combustion chamber under the perspex. An added advantage of aluminum welding is the building of a squish area. A squish area purges the air/fuel mixture across the combustion chamber, mixing it with any residual exhaust gas that may still be present in the combustion chamber, resulting in a more homogeneous fuel/air mixture that allows for better flame propagation.

Compression Ratio and Forced Induction

Forced induction has different demands on the compression ratio, especially when a forced induction system is added to a normally aspirated engine or when boost is increased. In this instance the compression ratio often needs to be lowered to compensate for the much higher cylinder pressure generated by the boost. This can be accomplished by fitting dished pistons but the outline of the dish should mirror the outline of the combustion chamber to retain the advantages of the squish area.