RaceCal 'Tech Insight' - MBT, Ignition Timing and Knock
This week's Tech Insight focuses on a term that you may (or may not!) have heard used before when it comes to calibrating an engine.
MBT is an important aspect of engine calibration as it is directly related to the torque the engine produces and how safely at your chosen ignition advance.
To get a better understanding of MBT, we should first start with a recap on the fundamentals of the spark delivery on a 4-stroke engine.
The term ‘ignition timing’ / ‘ignition advance’ / ‘ignition angle’ refers to the point in the engine cycle at which the spark event occurs.
In order to ascertain this angle, we must first choose a point to reference this against. This chosen point is known as TDC – ‘Top Dead Centre’, which is the point in the engine cycle where the piston has reached the top of the stroke on the compression stroke.
Our ignition angle is referenced against crankshaft degrees - how many degrees of engine rotation relative to TDC. Ignition angle is most commonly referenced in angles BTDC – ‘Before Top Dead Centre’ – this means how many degrees of crankshaft rotation before the piston reaches the top of the stroke when the spark event occurs. The spark event then begins the combustion.
Whilst not massively common in most ignition maps, sometimes you will see ignition angles ATDC – ‘After Top Dead Centre’, this is where the spark event occurs after the piston has passed TDC and is returning down the bore on its power stroke. In most aftermarket ECUs, ignition angles BTDC will be shown as for example ’18.00’, where as ignition angles ATDC will be shown as, for example, ‘-2.00’. The minus sign dictates this angle is ATDC.
Fundamentally, to create maximum energy from the combustion event to act on the top of the piston on the power stroke, if possible we need to start our spark event so the cylinder pressure smoothly peaks at approximately just before 20 degrees ATDC. This will create the most force, which is then transferred through the rod to crank rotation thus creating torque.
This is when our MBT term comes back into play, if we can achieve the above the engine is at MBT.
MBT stands for ‘Maximum Brake Torque Timing’, also referred to as 'Minimum Best Timing' - this is the optimum ignition angle to create maximum engine torque.
Start the spark event too soon and we run into the risk of knock/detonation, start the spark event too late and we wont achieve optimum cylinder pressure to create torque.
This example graph found online shows a spark event started too early (2), one too late (3) and one that is perfect (1).
Knock/Detonation occurs when the cylinder pressures/temperatures become to great, which in turn causes pockets of un-burnt fuel and air around the edge of the combustion chamber to spontaneously combust. This phenomenon causes sharp spikes in cylinder pressure (you can see this in the graph above) which can induce engine failure. This is because the sharp spikes in pressure can remove the boundary layer of gases that protect the crown of the piston and combustion chamber. Stripping away these gasses opens up these components to the full heat and energy of combustion. Engine heads and pistons are most commonly made of aluminium, aluminium’s melting point is below that of the full temperature of combustion which is why it’s hugely important to keep the gas layer intact for component protection.
A quick search online will furnish you with plenty of pictures showing a piston that has been subject to detonation, almost a pitted like finish as we can see below.
In a more extreme knock event, the whole side of the piston can get melted away very quickly.
Serious engine damage can occur as we have explained above, this is why you must always use a quality knock detection system when calibrating an engine.
We use the PLEX system which can be seen on our webstore here.
In the aftermarket world, its very rare to have access to in cylinder pressure monitoring devices, this is mainly due to cost and is often reserved for high end motorsport programmes as well as OEM manufacturers developing road vehicles.
There are systems available such as the PLEX standalone system, however for an ECU with onboard cylinder pressure monitoring expect to pay £20,000 + excluding the sensors themselves.
With this in mind, we need to find an alternate way to optimise the ignition timing to MBT; this is where the use of the dyno is invaluable. Road tuning is not a suitable way to find MBT effectively as we often here ‘road tuning is better’, MBT is the exact reason why tuning on a dyno is better with final tweaks/driveability adjustments made on the road if necessary.
The tuning process is quite simple when on the dyno when tuning steady state. Steady state tuning is when you use the dyno brake to hold you at a set RPM which allows you to access different load sites in your ignition/fuel map.
For example, we can use the dyno to hold us at 2500rpm, then by varying the throttle opening we can access the varying load sites in the table.
You will centre yourself in your chosen cell, for example 2400rpm and 1000mbar as shown below.
You will then start to advance the ignition timing whilst watching the torque output the dyno is showing, you will continue to advance the timing whilst listening for knock and monitoring torque output. (Scroll to the bottom of this article to see the torque optimisation feature on a Mainline Dyno that highlights MBT to the user)
Some engines are ‘Knock Limited’; this means you will encounter knock before achieving optimal ignition timing, in this case it is not possible to reach MBT, so you have to make a safe compromise between timing, torque and safety.
On the other hand, there are engines that are not knock limited and you will be able to reach MBT and go past it.
How this looks is, as you increase ignition timing the engines torque output will increase, as you continue to advance the timing past MBT the torque output will plateau and then start to drop away. This can happen before knock occurs on some engines.
The above is the reason why you should always optimise ignition timing on the dyno, as on the road your max ignition timing reference is usually up to knock, then retarding the timing a couple of degrees for safety. As discussed in this article, if the engine is not knock limited, you don’t always make the max torque on the edge of knock so your car would be running less torque and more ignition advance taking you closer to knock unnecessarily.
Once you have optimised the timing for the current load site at the given RPM, you will continue onto 1100mbar, 1200mbar, 1300mbar and so on. Once you have completed the optimisation for all the load cells you can reach at 2400rpm, you will move up to 2700rpm and repeat the process over the operating range of the engine.
This is one of the subjects we cover in more detail in our 1-2-1 training sessions, if you'd like to learn how to calibrate your car, email email@example.com to discuss.
We hope this article was insightful, let us know what you think in the comments below, on Instagram or Facebook!