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vvt for you bofins
- Thread starter Scribbler
- Start date
ok here you go its about toyota but its the same
A Comparison of the Toyota VVT system Vs the Honda V-TEC
On this page I talk about the two different methods used to increase the power output, and whats good and bad about them.
What the two systems are, and why they are used
By using a conventional valve system, to keep a modern multi-valve engine usable for the road, you are limited to about 85hp to 90hp per litre. You can use a bigger camshaft quite easily get a lot more power, but only at higher revs, and at the expense of power at lower revs. So, with a bit of lateral thinking, it is now becoming more common to be able to change that very cam timing that limited the engine power while the engine is running. The Toyota VVT system isnt new, however, as similar systems have been in use for many decades before. But not for a mass production engine and certainly not with the highly accurate control of the modern engine management systems. The Honda V-Tec system is a relative new comer, and by using a system of far greater complexity than that used by Toyota, Honda is now making an engine that produces as much power as many of the better racing engines!
So lets have a look at each system, and how they work ...
Toyota Variable Valve Timing system, or VVT & VVT-i
The VVT-type system has been around and in use by various companies for at least 40 years that I know of.
(I can remember seeing a 1960s catalogue from the US that showed a special cam wheel that bolted onto a small block Ford engines cam, and it had a mechanism that worked like a mechanical advance system in a distributor, so that as the revs picked up it advanced the cam timing. I also believe that Alfa Romeo or Fiat used a similar system back around then, or maybe before)
VVT is simple and fairly effective. It consists of only two main parts; an oil control solenoid and the VVT mechanism itself.
http://www.billzilla.org/vvt1.gif
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 its often called.)
The early VVT system was relatively simple, ie, at a specific rpm (~4400rpm on the 20 valve 4AGEs) 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. Theres 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 pistons fore & aft movement
http://www.billzilla.org/vvt2.gif
So, when the computer signals for the VVT to operate, the OCV opens and thus causes the VVT pulley to advance the inlet cam timing by 30°, reference the crankshaft. (15° on the pulley itself)
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 timings 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 cant feel anything when it happens. You can, however, hear a change in engine note, just before theres 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 20vs 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 (cant get the exact reading because everything happen so fast. Its 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.
Now, back to the above schematic of the VVT. It shows the second evolution of the VVT system - called VVT-i - where instead of the simple on or off positions of the earlier VVT system, this version can make the inlet cam r****d/advance to any angle between the maximum limits, and to do this the camshaft has a position sensor on the back of the head. This means that the engine is even more flexible in its power output than before. The latest version, VVTL-i is described on http://www.billzilla.org/vvtvtec3.htmU face=Comic Sans MS color=#0000ffthis page/U. Its completely different to the original VVT system, and is more like the V-TEC in operation.
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 3SGEs have a single inlet VVT-i and the later blacktop generation four 3SGEs 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, its 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 cant do this, so Toyota has gone to a system much like the ....
HONDA V-TEC
Right. Lets 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 - Thats 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. Its 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. Heres a picture of the valve gear
http://www.billzilla.org/vtecdetail.jpg
http://www.billzilla.org/vtecdetailbig.jpg
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.
You can see from the above pictures, and the one below that theres been a huge amount of effort to make it all work. The cam followers all have small rollers, to reduce friction and allow for a larger cam lobe.
http://www.billzilla.org/vteccut.jpg
The follower system of valve operation, believe it or not, is quite similar to the latest developments in Formula One engine technology. (Though the F1s dont use V-TEC, have pneumatic valve springs, a smaller included valve angle, and so on ...)
Heres a picture of a head thats 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.
http://www.billzilla.org/vteccolour.jpg
Honda have also made a single cam version of the V-TEC, (V-Tir system??) though it only operates on the inlet cam valve timing/duration/lift. As with the twin cam system, it is quite elegant but has many small parts operating under high loads and speeds. http://www.billzilla.org/vtecsingle.jpg
The point at which the V-TEC system operates is a purely rpm derived point, as was the VVT system, and is done for exactly the same reasons. Because of this, you will not gain anything on a standard engine (either type) by using one of the aftermarket controllers that let you alter the rpm at which the systems operate. All youll do is create an unpleasant dead spot in the torque curve.
Below is the Nissan version of V-TEC, the VVL system. Its basically exactly the same as V-TEC in design and operation and so I assume is used under licence. This engine is the SR-16-VVL bluetop and they make about 175hp from the factory. Theres a similar N1 version that has a red coloured cam cover and theyre reported to make 197hp. Theres only suposed to be about 400 bluetops and 80 redtops made, and they were fitted to the faster versions of the Nissan Pulsars in Japan. This engine is my own, and its going into my http://www.billzilla.org/newrcar.htmU face=Comic Sans MS color=#0000ffMallock/U racing car. On the right is how it looked when I picked it up in Malaysia.
The VVT and V-TEC in operation in the real world
The Toyota engines seem to run slightly more aggressive cams than the Hondas, and so at lower revs they seem to (anecdotal evidence here ...) be a bit more pleasant to drive and make a little more power. Theres also less of a transition when the cam shifting systems operate, but this is obvious due to the Honda system swapping over to a much more racy cam profile. I think that the Hondas may seem to be a little flat at lower revs because of this relatively large contrast, but Id have to drive one and see a dyno chart to make verify this.
Pros - Both systems allow you to have an engine thats quite a lot more powerful and yet still driveable than a conventional engine would otherwise possibly be. The V-TEC is the obvious choice for outright power, and the Hondas certainly seem to rev a heck of a lot more than the Toyotas do. (The S2000 red lines at a stratospheric 9,000rpm - stock!)
Cons - You are pretty much stuck with limited modifications to the engine, eg, air filters, extractors, etc, to get more power. The reason for this is the very system that give the engine all that extra power - The cams & VVT/V-TEC. You can of course use larger cams to get more power, but this defeats the purpose of having the VVT/V-TEC in the first place. Youll most likely lose power at low revs, and not gain a great deal at high revs. (The VVT will gain proportionally more than the V-TEC, however, as the V-TEC head is optimised - well, compromised - for the big cam & small cam and so using a larger cam may not help much at all)
So, if you want an engine with power like a racing engine, then youre better off building a straight race engine right from the start. Or maybe a turbo engine ...
The other concern I have is the longevity of these sorts of engines. I believe that the VVT system would be largely trouble free for the life of the engine provided that you keep the oil clean and change it regularly. Even more so with the V-TEC, as with all its little bits & pieces in close formation in the head Id hate to think what would happen if some of those little locking pins didnt engage properly at 6000rpm+. All that being said though, I have it on reliable advice that Honda have never had a warrantee claim for any V-TEC engine in the area of the head and/or valve gear. Quite impressive.
I think that perhaps the best long term solution to getting large amounts of power from a relatively small engine is still by using a turbo, but if you like to hear the engine scream at high revs then one of these two systems is the way to go.
ripped from http://www.billzilla.org/vvtvtec.htmhttp://www.billzilla.org/vvtvtec.htmhttp://www.billzilla.org/vvt2.gif
I havnt read all that yet but the Clio one only advances the intake cams by 16 degrees. But there are uqite a few conditions that need to be met for it to adjust the timing. As far as I remember they are above 1200rpm and below 4500rpm. Oil temp above 80 degress (not cooling temp ie the one on the dash) and full throttle.
the vvt operates in the following conditions(conditions shown are for the 172 f4r other models are different)
1,coolant temp above 80c
2,inlet manifold pressure greater than 850mbars(note-full throttle is atmospheric pressure which is 1000mbars)
3,between 1800rpm and 6500rpm
4,it only has two positions
the vvt is operated using an oil pressure operated dephasor on the inlet cam only. it advances the cam timing on the inlet cam by 16degrees.One reason for this is to create valve overlap.The dephasor has oil channels in it. A solenoid(operated by an earth signal from the ecu) in the cylinder head directs oil down oil ways in the cylinder head and the cam shaft centre.The oil then goes into the dephasor into the channels that are reqd(one set of channels for vvt on and one set for off). the oil acts on vanes(inside the dephasor channels) which forces the centre part of the dephasor(attached to the cam) to change position depending on which channel the oil has been directed down. Also when the oil is directed down the channels it pushes off an unlocking deplunger to allow the dephasor centre to be moved.The plunger locks back in place when the dephasor has moved to its reqd position. The renault system is the same idea as the first toyota version explained in dogmauls previous post. Not half as clever as the honda v-tec system.
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