Combustion chamber shape
Since the reasonably recent publishing of a certain Mini tuning, performance and maintenance guide that has strangely coincided with a batch of new readers/devotees of a certain prolifically quoted 'bible' on A-series engine modifications - the subject of all things cylinder head have bounced into the fore once more. Something I was heavily canvassed over at this years (2003) Mini in The Park event whilst doing duty as Mini 'agony aunt' and 'doctor'… In particular the subject of the combustion chamber was foremost in discussion since the aforementioned 'guide' exhibited a number of wildly different chamber profiles with very little in the way of supportive technical information. Now, I have considered the pros and cons of doing this missive. On one hand, I am not out to increase sales of said guide since I'm not one of its supporters in any way. On the other I am certainly not attempting to belittle the efforts of the featured company as they have long been at the forefront of performance A-series engines and components, nor do I wish to be-little the grandiose efforts of an eminently brilliant engineer who produced a valuable plethora of information to the A-series masses. I do, however, feel a little more information than has been presented is needed to provide those general readers with some much-needed relevant and tangible information. I am not about to start conjuring up a vast array of test data either, since this is all relative (more later) - simply some basic information to consider.
There is absolutely no denying the fact that a certain number of folk will always be drawn by something that is 'different'. It's generally what helps vendors keep selling wares over a protracted period. Reincarnations of innumerable goods will attest to this. Cylinder head chamber shape is no exception - more than any other part of the cylinder head because it is something that is easily seen with the naked eye. But all that glitters isn't necessarily gold…
The chamber is responsible for four basic events. The first is least restriction to air flow, the second to promote the best fuel/air mixture, the third a good flame front to promote an effective burn of the trapped fuel/air mixture, and lastly to contain as much as possible the heat generated by that burn.
Airflow. On the face of it, the simplest answer is to move anything that could be viewed as an obstruction to this parameter as far away from the valves as possible. And would be a reasonable approach if it weren't for the fact the chamber has to be contained with the perimeter of the gasket fire ring. Still, that gives fair latitude…
Fuel/air mixture. That does not encompass the correct air/fuel ratio required for best power/torque/fuel consumption since that is mainly governed/controlled by the carburetter/induction set-up. It does encompass how the air/fuel is mixed together within the combustion chamber and where the majority of that air/fuel mix ends up prior to being lit by the spark plug. Some will be familiar with the term 'swirl' when referring to distribution of the incoming fuel/air mixture into the chamber/cylinder. Creating maximum swirl is most definitely NOT the ultimate goal, since too much swirl will centrifuge the heavy fuel out of the lighter air, creating an inefficient mixture within the confines of the chamber.
Flame front. The compressed air/fuel mixture in the combustion chamber is lit by the spark plug. The flame front is the way in which the burn spreads throughout the mixture contained in the combustion chamber. The more effective the flame front, the better the burn, the more power and more fuel efficient the engine will be.
Heat containment. What really provides drive power from an engine is heat. The more heat generated within the combustion chamber, the more power is produced (calorific value). The more heat escapes, the lower the power potential from each burn in each cylinder.
Unfortunately - all these are inextricably entwined, so it's necessary to view the combustion chamber as a whole then consider its effects on each of the aforementioned. To this must be added a fifth component - the piston. Or more exactly - the crown shape since this has an immediate effect on swirl and burn and a small effect on heat retention.
This addendum to the Cylinder Head section was fuelled mainly by pictures of combustion chamber shapes and folk's excitement at what they could possibly do for them. It is also prompted by the 'new isn't always great' teaching. So what's in these varying and dramatically different chamber shapes? The answer to that lies in just why any head modifier went that route in the first place, and whether it has been substantiated by representative, consistently monitored and directly comparative tests.
Looking back at cylinder heads produced by 'recognised' leaders in modified A-series cylinder heads, apart from various original design base/core unit generated similarities they all have something in common - the combustion chamber shape. And for very good reason - the original Weslake design is VERY effective. What's more, it is very effective in numerable applications to include bore sizes and piston crown configurations.