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Variable Cam Control Tuning: Cam Angle Targets

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Cam Angle Targets

04.26

00:00 - So far we've discussed the basics of continuously variable cam control however we haven't talked about what the numbers in a cam target table actually mean.
00:08 And this can be initially confusing, particularly if you've come from an engine building background and you're familiar with camshaft specification cards.
00:18 If we look at the camshaft spec card for a Kelford TX272R cam for a Mitsubishi Evo 4-7 with fixed cam timing, we can see there are 3 pieces of information that can be used to degree the camshafts correctly.
00:31 These are the valve opening and closing points labelled IVO and IVC for the intake and the suggested camshaft centreline.
00:40 If we're using the valve opening and closing points for degreeing the camshaft, then we also need to factor in the amount of valve lift when these timing events occur.
00:49 Due to the gentle way the valve is lifted off the valve seat and then closed back down it's very difficult to accurately time the camshaft at zero lift so instead the industry standard is to use either 50 thou lift or 1 mm lift depending on the cam manufacturer.
01:07 It's worth noting here that 50 thou is also not the same as 1 mm and hence you can't readily compare valve duration between 2 cams where the valve timing events are specified at different lift values.
01:19 The other parameter that's more often discussed when talking about cam timing is the camshaft centreline.
01:25 This simply defines the point in the engine cycle where the valve reaches peak lift.
01:29 For an exhaust cam, this is defined in degrees before top dead centre while for the intake it's degrees after top dead centre.
01:37 This is the bit that gets tricky because those familiar with camshaft degreeing expect the cam target tables to represent camshaft centreline values and they seldom do.
01:48 In most instances, if we're considering a variable intake camshaft, the mechanical installation is designed so that when the actuator isn't being energised, the cam will sit in its most retarded position.
02:00 This could be likened to the camshaft resting position.
02:03 Of course for every rule there are exceptions and there are a few engines such as the Subaru FA20 where the cam can be both advanced and retarded from the natural resting position but this isn't a common situation.
02:17 When we look at the numbers in the intake cam target map, we'll usually see positive values and these represent degrees of cam advance from that natural resting position or in other words the cam's most retarded position.
02:30 It's also worth mentioning that the values represent degrees at the crankshaft rather than degrees at the cam.
02:37 This is a subtle but important difference since the cam rotates at half crankshaft speed so every degree of movement at the cam corresponds to 2° at the crankshaft.
02:48 On the other hand, when we consider a variable exhaust cam, the resting position when the actuator isn't being energised, will usually be the most advanced position.
02:57 This combination in a dual varibale cam engine results in stable operation and a smooth idle due to the fact that advancing the exhaust cam and retarding the inlet cam has the effect of reducing valve overlap.
03:10 Consequently we'll also typically find that the peak power values produced with a variable cam engine, with the cam actuators disabled will be reasonably close to the peak values we'll achieve when the cam control system is active and the timing is optimised.
03:25 It's the mid range where we see the predominant gains, not high RPM performance.
03:30 Since the exhaust cam will be resting in the most advanced position, we'll be retarding this with our cam control tuning and hence often when we look at the numbers in the cam target map, these will be represented as negative values.
03:42 In both instances though we tend to see similar trends to properly tune cam target maps where the cam position will sit at the resting position at idle in low RPM then as the RPM increases towards peak torque, the intake cam will advance while the exhaust cam is retarded.
04:00 As we move past peak torque and move towards the rev limit, we tend to move both cams back closer to their resting positions.
04:07 Of course this is a very generalised picture but it gives you an idea of what you can expect to see.