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Engine Building Fundamentals: Rod To Stroke Ratio

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Rod To Stroke Ratio


00:00 - Another key aspect to engine design, which we're going to discuss in this module is the rod to stroke ratio.
00:07 More often than not, this is an aspect of engine design that's out of our control, and we'll be working within the confines of whatever the engine manufacturer decided on.
00:17 Regardless of this, though, it's a term you will hear, so it's worth understanding its relevance.
00:23 In some more advanced builds, we may also have the opportunity to adjust the rod to stroke ratio, so we need to know what these changes will mean.
00:32 Rod to stroke ratio is calculated by taking the length of the conrod, measured from the centre of the big end journal, to the centre of the wrist pin, and dividing this by the stroke of the engine.
00:44 For example, if we look at the Mitsubishi 4G63 engine, which uses a 150 millimetre long conrod, and an 88 millimetre stroke, the rod to stroke ratio is 150 divided by 88, which equals 1.70.
01:02 If we were to look at the stock rod to stroke ratios for a range of production engines, we would find that the ratio varies quite dramatically, anywhere from as low as, perhaps, 1.4 through to 1.8 or more.
01:15 In fact, in some very specialised race engines, such as those found in Formula One, we could expect to see rod to stroke ratios well in excess of 2.0 being used.
01:27 So now, let's find out what the rod to stroke ratio actually means, and how it will effect the operation of the engine.
01:34 Primarily, the rod to stroke ratio effects the angle of the connecting rod between the crankshaft journal and the piston, for a given angle of crankshaft rotation.
01:46 As we increase the rod to stroke ratio, we find that the angulation of the connecting rod is reduced.
01:53 In particular, this reduces the sideways thrust load applied to the piston skirt and the cylinder wall, which can reduce wear, as well as frictional losses in the engine.
02:04 A lower rod to stroke ratio, on the other hand, increases conrod angulation, and produces increased loading on the thrust face of the piston and the cylinder wall, increasing wear and frictional losses.
02:19 This situation is particularly undesirable in engines that're required to operate at high engine speeds.
02:25 Aside from the thrust loading, the rod to stroke ratio will also effect the piston position and piston acceleration with regard to crankshaft rotation.
02:37 To explain this better, let's first look at a diagram of piston position versus crankshaft rotation.
02:44 In this diagram, we have the piston position plotted relative to crankshaft rotation for two different length conrods, a 100 millimetre conrod, and a 500 millimetre long conrod.
02:57 Obviously, we're unlikely to be able fit a 500 millimetre long conrod into our engine, but it illustrates how the conrod length effects the piston position as the crankshaft rotates.
03:11 We can see that as we increase the length of the conrod, and hence the rod to stroke ratio, the piston spends more time at TDC, or close to it.
03:21 This is referred to as dwell.
03:24 Now, let's see how the rod length effects the piston acceleration.
03:29 We've just seen that a longer rod to stroke ratio results in the piston dwelling longer at TDC, and this also infers that the piston's acceleration around TDC must also reduce.
03:42 On the graph, you can see here we have the acceleration of the piston plotted relative to crankshaft rotation, for a few different conrod lengths.
03:52 We've added a theoretical example with a 5,000 millimetre long conrod to demonstrate what happens as the rod to stroke ratio approaches infinity.
04:03 When we increase the rod to stroke ratio, and achieve more dwell at TDC, this results in more cylinder pressure acting on the piston as the piston begins moving away from TDC, and this can aid torque production.
04:18 Remember that we're aiming to generate peak cylinder pressure around 16 to 18 degrees after TDC, in order to generate peak torque, so the more cylinder pressure we have here, the more torque the engine will produce.
04:33 The slower piston acceleration around TDC can also improve cylinder filling at high RPM, improving engine volumetric efficiency.
04:43 The compromise here is that a longer rod to stroke ratio can hurt low RPM torque, as intake air velocity is reduced.
04:53 With a short rod, for example, the piston accelerates away from TDC on the intake stroke faster.
04:59 This means that for each degree of crankshaft rotation the piston is moved further away from TDC than if we used a longer conrod.
05:09 The result is that the volume inside the cylinder increases faster, and this creates a higher intake vacuum at the start of the intake stroke.
05:19 A higher inlet velocity can also promote improved mixture preparation with a more homogenous mixture of fuel and air.
05:29 So as you can see, there's no perfect rod to stroke ratio that we can apply to every engine, but there are some general guidelines we can form.
05:38 Since we've seen that a longer rod to stroke ratio tends to improve cylinder filling at high RPM, at the expense of cylinder filling at lower RPM, the expected RPM ceiling of the engine can help us decide what rod to stroke ratio to target.
05:56 For an example, it's generally accepted that a rod to stroke ratio in the vicinity of 1.75 is close to ideal for an engine that's likely to reach 7,000-8,000 RPM.
06:09 If we're only going to be revving an engine to perhaps 5,500 RPM, we could get away with reducing the rod to stroke ratio into the 1.5 to 1.6 vicinity.
06:21 At high RPM, we can use the sport bike manufacturers as a guideline.
06:26 Here, it's common for engines revving to 12,000-13,000 RPM to use a rod to stroke ratio of 1.9 through to two to one.
06:36 While it's hard to get solid data, it's generally accepted that Formula One engines that were revving to 18,000 RPM were using rod to stroke ratios in the region of 2.8 to one.
06:49 Now that we know what the rod to stroke ratio is, and what might be suitable for our application, let's talk about how we can make changes to the stock rod to stroke ratio.
07:00 Obviously, if we're dealing with any specific engine, aspects such as the stroke and rod length are defined by the manufacturer.
07:08 However, this doesn't necessarily mean that there's nothing we can do to alter the rod to stroke ratio, and in some instances, it can be beneficial to do so.
07:17 One of the most common reasons for adjusting the rod to stroke ratio would be to help negate the effects of fitting a longer stroke crankshaft to the engine.
07:27 In this situation, if we retain the stock rod length, the rod to stroke ratio will be reduced by the virtue of the new, longer stroke crank, and, along with this, come the negatives we've just discussed.
07:40 Often, we can improve the situation by using a slightly longer aftermarket conrod to go along with the longer stroke crankshaft.
07:49 Now, under normal circumstances, this would inevitably end up with the piston coming out of the top of bore at TDC, since the compression height of the piston has remained unchanged.
08:00 In order to rectify this, we can use special pistons with a reduced compression height.
08:07 Or to put it another way, we can lift the wrist-pin higher in the piston.
08:11 This will often result in the wrist-pin actually interfering with, or cutting through the oil control ring, but this can be dealt with by using a lower rail to support the oil control ring, along with moving the ring pack higher on the piston.
08:27 Using these techniques, in some cases, we can often achieve a longer stroke combination with a minimal reduction in rod to stroke ratio.
08:36 In other instances, we may have the opportunity to use a block with a taller deck height, which allows an even longer conrod to be used, either with or without a longer stroke crankshaft.
08:48 This gives the engine builder numerous options to adjust the engine stroke, conrod length, and rod to stroke ratio over what may, in some instances, be quite a wide range, in order to get a combination that suits their requirements.
09:01 Let's look at another example.
09:04 Using the Mitsubishi 4G63.
09:06 As we found out earlier, the stock stroke is 88 millimetres, and the stock rod length is 150 millimetres, giving a rod to stroke ratio of 1.70.
09:18 A common modification with the 4G63 is to swap in the 4G64 crankshaft, which provides 100 millimetres of stroke.
09:26 If we keep the stock 150 millimetre long conrod, though, this will give us a rod to stroke ratio of 150 divided by 100, which equals 1.5 to one, which is getting very low for an engine that we want to run to high RPM, due to the increased rod angularity, and the thrust loading on the piston skirt.
09:48 If, on the other hand, we fitted the same engine components into a 4G64 block, which offers an addition 6 millimetres of deck height, we now have the opportunity to fit a longer, 156 millimetre conrod, which now brings our rod to stroke ratio to 156 divided by 100, which is 1.56.
10:11 Still not perfect, but certainly an improvement.
10:15 Another common option in the Mitsubishi tuning world is to use the stock 88 millimetre crankshaft in the taller 4G64 block.
10:24 This allows an even longer 162 millimetre long conrod to be used, which gives us a rod to stroke ratio of 162 divided by 88, which is 1.84.
10:37 This is much better suited to extremely high RPM operation, so you can see that with a range of factory option parts, combined with some special aftermarket parts, it's possible to have quite a significant impact on the final rod to stroke ratio of the engine.