Compression ratio Basic considerations

14 October 2002

I recently had one of my best mates to stay for a week. Don't get to see him very often, so when we're together we spend most of the time pontificating on all things A-series since he's a long-time Mini enthusiast too. Well, at least from a racing point of view. Anyway, of the various subjects covered one that prompted me into doing this little piece of verbiage was that of compression ratios (CR).

It's another for those subjects where folk can be heard waxing lyrical about their personal recommendations for CR levels, or stipulating hard and fast 'facts' about CR required for certain camshafts. And again, it's one of those generally misunderstood areas of engine tuning. So I'm going to try and cut through the haze a bit.

CRs are generally spoken about and quoted in reference to the 'static' ratio. That is a bunch of calculated or measured volumes calculated by means of an equation. The major stumbling block here is that this equation really assumes another all-influencing factor is performing at its best - volumetric efficiency (VE). VE is how capable any engine is at filling its cylinders - the optimal 'best' on a normally aspirated engine being 100%. So if a particular cylinder is measured/calculated to contain 250cc, it would need to actually breathe in 250cc of fuel/air mixture in one intake stroke. This rarely happens on standard engines. In fact many engines are hard-pushed to achieve this in full race trim. The main components affecting VE are the cylinder head and camshaft (providing other items like induction and exhaust systems are not causing further restriction). Cylinder head because it depends how well it flows air in the required areas for any application (road, rally, race, etc.) and cams because this directly affects how fast, far, and long the valves are open in the head to allow the all-important fuel/air mixture to get into the cylinder.

Standard road cams with very little overlap generally require less CR to make reasonable power than 'sportier' cams. These sportier cams generally have more valve over-lap - a condition where the valves are open at the same time - which directly affects VE at low rpm. Poor flowing cylinder heads reduce VE on any engine, so will require a higher CR to generate a higher dynamic CR.

Having read this far and concluded that this CR business is far from straightforward, then I've achieved what I set out to do. The point being that just because someone says 'oh, I'm running 11.5-1CR in my road going engine (where you are running 10-1CR) with the same engine size/cam (and general bits and pieces) you are without any detonation problems' doesn't mean you can rush out and have a heap more metal chopped of your cylinder head to get more power. It may be this 'someone' has a very naff cylinder head that is causing a dramatic VE deficiency. I am saying this as a few folk have done this and destroyed pistons - an expensive exercise. Too high a CR for a given fuel octane will cause detonation which, in turn, usually destroys pistons. Always seek guidance on this from a more experienced person - like one of the many engine-building specialists. One phone call could save you a fortune.