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Variable Cam Control Tuning: Vernier Cam gears

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Vernier Cam gears


00:00 - We've discussed the limitations and compromise that's involved with choosing the valve timing events in an engine that uses fixed cam timing, however not is all lost here.
00:10 We can still manipulate that cam timing to suit our own preferences by fitting what's referred to as a vernier adjustable cam pulley.
00:17 This is most commonly a 2 piece pulley where the centre section that's attached to the camshaft can be rotated with respect to the outer section that's being driven by the cam belt or chain.
00:28 You will typically have some reference marks on the cam pulley so that you can make accurate adjustments to the cam timing and then the 2 sections of the cam gear can be locked up so that they can't move when the engine is operating.
00:42 A simpler take on the vernier cam pulley is one where there are multiple keyway slots machined for locating the pulley on the camshaft and by selecting a specific keyway location, along with the matching location between the pulley and the cam chain or belt, the cam timing can be accurately adjusted.
01:01 The downside of this type of cam gear is that it does need to be physically removed from the camshaft and refitted in order to effect a cam timing change.
01:10 The advantage however is that once the cam gear is refitted there's no potential for it to move and have the cam timing alter while the engine's operating.
01:19 This can be a problem with the 2 piece cam gear design where the locking bolts can potentially slip if they're not correctly tightened.
01:28 In some applications you may only have the option of one style of adjustable cam gear while in others you may be able to choose between either.
01:36 Even on a cam control engine you may still have the requirement for a vernier cam gear.
01:41 As we've discussed, many cam control engines only provide continuously adjustable cam control on the inlet cam and retain a conventional fixed cam timing on the exhaust cam.
01:53 Mitsubishi's Evo 9 4G63 and many of the Subaru EJ series engines for example fall into this category.
02:02 In this instance, particularly with an aftermarket camshaft, adding a vernier adjustable cam pulley to the exhaust cam can provide the ability to accurately degree the exhaust cam and see what gains if any can be achieved by moving the cam timing.
02:17 It can be tempting to just fit a vernier cam gear and lock it up on the zero mark and assume that all is well.
02:25 The problem with doing this is that we really have no idea what base cam timing or cam centreline this will result in.
02:32 While I know that this is a technique that many engine builders and tuners employ, it's not one that I can advise.
02:39 If you're dealing with an aftermarket camshaft then it's important to degree the cam accurately before optimising the timing on the dyno.
02:46 If you're adding a vernier cam gear to a factory camshaft then you can usually do a pretty good job of adjusting the cam gear to replicate the factory timing before installing it.