VVT - Variable Valve Timing

VVT – Variable Valve Timing The VVT-type system has been around and in use by various companies for at least 40 years that I know of. INCLUDEPICTURE "http://www.billzilla.org/vvt1.gif" \* MERGEFORMATINET This diagram shows a few more bits & pieces, but you can clearly see the main two - the VVT pulley and the OCV. (Oil Control Valve, or oil solenoid as it's often called.) The early VVT system was relatively simple, ie, at a specific rpm (~4400rpm on the 20 valve 4AGE's) the computer signals the OCV to open, this lets oil pressure go through a special gallery in the #1 inlet cam bearing, through the centre of the inlet cam to the VVT pulley. There's a small piston in the VVT pulley, and once it gets enough pressure behind it, it starts to move outwards, causing the outer part of the pulley to turn in relation to the inner part, due to the helical spline that guides the piston's fore & aft movement. The rpm at which this happens is worked out by running the engine on a dynamometer with the inlet cam in both the fully advanced and fully retarded positions. Since the two different cam timing's will make different power throughout the rev range, (advanced inlet give more top end power at the expense of low end power, and vice-versa) there is a point where the power will be identical for both cam settings, and this is where the VVT is programmed to operate. Because the power output is the same with the VVT in either position, you can't feel anything when it happens. You can, however, hear a change in engine note, just before there's a big increase in power! More detail on the the VVT logic - The VVT comes in three types for the 20 valve. To the best of my knowledge, silvertop 20v's pre May 1993 have the VVT actuate at about 4400rpm. Post May 1993 they seem to work on throttle position and ignore revs. The blacktops seem to work like this, as described on Club4AG – 1. Starting. When you crank the starter there will be VVT operation until the engine fires up, obviously to allow more air into the engine to allow an easier fire up. 2. Coolant temp. There is absolutely NO VVT operation when the coolant tempt is below 50°C except for that brief moment when you operate the starter. Reason obvious, who want to stress a cold engine. 3. Engine rpm. VVT will operate in any rpm between the range of 1500 and 7200 when the inlet manifold pressure is right. The min and max range can be a little out because I was reading from the car tacho. Trust me they are very close. 4. Engine load/inlet manifold pressure. This seems to be the single most important parameter controling the system. The VVT will NOT operate if the inlet manifold has more than about 5 inches of vacuum (can't get the exact reading because everything happen so fast. It's very close.). This is very close to zero vacuum which is atmospheric and that is about the maximum load the map sensor will read to tell the engine in an NA car. As you can figure out the throttle will usually be in the more than 3/4 position for this to happen. 5. VVT will work without the speed sensor. There are two engines that commonly use the VVTL-i system, the 1ZZ-FE/2ZZ-GE series and the latest (in 1999 & onwards) 3SGE, as used in the sporty Altezza. The early generation 'redtop' four 3SGE's have a single inlet VVT-i and the later 'blacktop' generation four 3SGE's have dual VVT-i controllers, one on the inlet and the other on the exhaust cam, and makes 200hp from 2 litres. So, using VVT technology, it's pretty easy to get around 100hp per litre. Toyota has now gone to the third evolution of the VVT, and it not only alters the cam timing, but it also alters the valve lift as well. The 'old' VVT system simply can't do this, so Toyota has gone to a system. HONDA V-TEC Right. Let's not muck around. For straight power output, the V-TEC system craps all over the VVT system. The latest Honda V-TEC engine, as used in the S2000 sports car, makes 240hp odd out of only 2 litres - That's a sparkling 120hp per litre. The V-TEC system is far more complex than the VVT, but it allows you to not only alter the cam timing, but to alter the valve duration and lift at well. It's really like having two engines in one - A 'sedate' one for grocery-getting, and the other a red-blooded high revving screamer. How it does this, however, is with a multitude of 'fiddly bits'. Here's a picture of the valve gear. HYPERLINK "http://www.billzilla.org/vtecdetail.jpg" \t "_NEW" INCLUDEPICTURE "http://www.billzilla.org/vtecdetailsm.jpg" \* MERGEFORMATINET HYPERLINK "http://www.billzilla.org/vtecdetailbig.jpg" \t "_NEW" INCLUDEPICTURE "http://www.billzilla.org/vtecdetailbigsm.jpg" \* MERGEFORMATINET Ok, pay attention - This is where it starts to get tricky! What happens when the engine computer decides to make the V-TEC shift to 'grunt' mode is this - Up until that point, the valves are operated by the pair of cam followers that run directly on top of each valve. A hydraulic valve opens in the head somewhere, allowing oil pressure to fill the pivot shaft that the cam followers swing off. The oil is then directed to a tiny set of pins that live in the inner follower. These pins push outwards when the valves are shut, locking the inner cam follower to the two outer followers. The inner follower runs on a cam lobe that sits between the outer two, and is much bigger. This is the lobe that has the larger duration and lift, and so suddenly allows the engine to breath a lot better. The follower system of valve operation, believe it or not, is quite similar to the latest developments in Formula One engine technology. (Though the F1's don't use V-TEC, have pneumatic valve springs, a smaller included valve angle, and so on ...) Here's a picture of a head that's been cross-sectioned. If you look very carefully at the right hand cam, you can just see the larger of the two sets of cam lobes hiding behind the smaller ones.