Increase Compression Ratio (Increase Performance)

Courtesy of HowAutoworks.com

There are several ways to increase the compression ratio of an engine, and providing you don't go over the top with a ratio that would be better suited to a dragster, you should see some form of improvement on most engines.


We'll skip the lesson in thermodynamics, but I'll tell you that the compression ratio is directly related to engine efficiency. Engines with higher compression ratios usually produce a bit more power than their low-compression counterparts, but most importantly, they can produce the same amount of power while using less fuel. This is because they can burn the fuel more efficiently.

But don't go running out and rip off your cylinder head yet. This information is not that privelaged - the engine manufacturers with their million dollar research budgets have also stumbled upon it (most of them knew before 1940). The reason why we don't see compression ratios approaching 100:1 is simple. With the quality of pump fuel that we have available, most engines cannot handle more than around 11:1 before detonation sets in; i.e. the temperatures in the combustion chamber gets so high that the mixture starts to burn uncontrollably. Not good for keeping an engine in one piece. So, the manufacturer has to leave a margin of safety in his engine design, and in order to ensure that it will run well on the lowest quality of fuel, he plays it safe with the compression ratio. Don't overdo it. Consult with a good engine-builder before deciding by how much you want to lift the compression ratio. An irresponsible descision might cost you a new cylinder head, or , at worst, ruin your engine.

The correct way to go about the whole procedure, is first to determine the compression ratio of your standard engine. No, looking it up in the specifications that the manufacturer supplies, is not good enough. Individual engine tolerances vary too much, and if you don't know the history of your engine, it might be disastrous to take a gamble and hope for the best. The previous owner might have had the cylinder head skimmed etc. It is best to physically measure it.

This is done by determining the total volume of the cylinder when the piston is at the bottom of it's stroke, including the volume between the piston and cylinder walls down to the top compression ring, the thickness of the cylinder head gasket, the volume of the piston crown if it is not a flat-topped item, and the total volume inside the combustion chamber. This figure is then divided by the total volume of the cylinder when the piston is at the top of it's stroke. This will give the true compression ratio of the engine. A good engine builder will repeat this process for all the cylinders of the engine to ensure that the compression ratios of the individual cylinders are all equal.

The easiest way to increase the compression ratio, is to have the desired amount of material machined off the face of the cylinder head, thereby decreasing the volume of the combustion chamber. Alternatively, you can fit a thinner cylinder head gasket, have the valve seats raised, or fit high compression pistons. This will leave the maximium amount of "meat" on the cylinder head, should it later become necessary to have it machined straight. Most newer heads won't stand two skimmings - should the engine overheat in future after being skimmed once, you will most likely have to throw the cylinder head away.

Like with all modifications, this one will also change the requirements of the engine to give optimal performance. It is likely that the engine will now need less total ignition advance. It might also be able to burn a leaner mixture under part-throttle conditions. The fuel system doesn't know that the compression ratio is higher, and neither does the ignition system, and it usually won't compensate for the new conditions by itself. So, like with almost any other modification, it is vital that the engine is optimised on a loading type dynamometer, so that you can enjoy the full benefit of the changes made to the engine.