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Variable Cam Control Tuning: Inlet vs Exhaust

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Inlet vs Exhaust


00:00 - In the perfect world, we'd like to be able to vary the cam timing independently on both the inlet and the exhaust cams but that's not always feasible.
00:08 A single cam engine for example will allow the inlet and exhaust valve timing to be altered but not independently since the inlet and exhaust cam lobes are both ground on a common camshaft.
00:20 That's not to say there still aren't advantages to being able to move the inlet and exhaust valve timing together but we have no way of influencing the valve overlap.
00:30 With cam control engines we also need to understand that adding this technology to the engine adds to the cost and complexity of the design and the manufacturers need to factor this in and weigh up the advantages with the cost.
00:44 For this reason, we'll often find that cam control is only applied to the inlet camshaft and the exhaust will be a conventional fixed cam design.
00:53 The reason this is done is that we find that the engine performance is much more sensitive to the valve timing events on the inlet compared to the exhaust.
01:02 That's not to say there's no advantage to adjusting the exhaust cam timing, just that the results will be more significant on the inlet side so if we need to choose one or the other, it's going to be the inlet that we'd want.
01:16 To highlight just how much more sensitive the engine is to the inlet cam timing vs the exhaust, let's look at the cam timing maps from our Toyota FA20 engine.
01:27 These maps have been optimised on the dyno and if we start with the inlet cam targets, we can see that these vary under load anywhere from a peak of about 35° down to a low of about 3° at high RPM so we have a total swing of about 32° of cam movement.
01:45 On the exhaust cam map, under the same wide open throttle conditions, we can see that the cam target is only varying by about 10°.
01:52 To put this in perspective, let's look at the dyno results from the same engine with the inlet cam initially fixed at 0° and then with the optimised timing.
02:01 We can see that this results in a significant difference in performance, particularly at low RPM.
02:08 Repeating this test with the exhaust cam fixed at 0° and then with optimised timing, we can see that there is still a difference in power but it's much less than what we saw with the same movement range on the inlet.
02:21 As we've already discussed, in engines that use cam control on the inlet and a fixed cam timing on the exhaust, we can still potentially see some benefits from adding a vernier adjustable cam gear to the exhaust side.
02:32 Another unique engine design which influences what we can do with the cam timing is an engine which uses a scissor drive cam system.
02:41 This is a popular system with Toyota and they've used it on their 1UZ-FE and their 1ZZ-FE engines just to name a couple.
02:49 This system drives the inlet cam in a conventional way via a cam pulley at the front of the cam which is in turn operated by a cam belt in the case of the 1UZ-FE or a chain in the case of the 1ZZ-FE.
03:03 The exhaust cam however is driven off the inlet cam rather than via a separate pulley.
03:07 This is achieved via a helical gear on each cam.