As I'm sure most of you know, some models of the 1.8T come with
Variable
Valve
Timing (VVT). I thought I'd start by posting my understanding of how it works, and some interesting findings. PLEASE- if I have made a mistake, let me know!
Which cars have VVT? I don't know all the engine codes that have VVT, but in general 2001 up Audi/VWs have VVT (North America only, I'm sadly ignorant about the rest of the world). It is quite easy to tell if a car is so equipped; it will have an electric solenoid protruding from the back of the head.
What does it do? The VAG VVT system is designed to improve cold start emmisions by advancing the intake camshaft during coldstarts. It is only operative for a few moments and has no performance purpose.
How does it work? The VVT mechanism is contained completely in the cam chain tensioning mechanism. (This is significant because it means that all heads can be configured with or without VVT depending on which tensioner is used.) The Non-VVT tensioner has spring pressure on the upper (slack side) surface and uses oil pressure to apply downward pressure on the tension side- this force in minimal.
The VVT tensioner has springs on both sides that apply tension to the chain. It also has a larger piston thats direction of thrust is controlled by oil pressure. Default is an upward thrust that places the tensioner in the same basic position as the Non-vvt tensioner. When the electric solenoid is activated, the pressure switches sides, effectively shortening the bottom run of chain and advancing the intake camshaft.
Why do we care?Sorry for the delay. In any case there are two reasons we care.
1. The possible performance advantage- advancing the intake cam to create some overlap(stock cams have 0 overlap). Under some running circumstances this could be of benefit- some expermentation would show waht works. HOWEVER, I think this is a bad idea. The VVT shifts oil pressure tot he drive side of the chain, and greatly increases the load on the phenolic tensioner. I suspect that if it were to run in that position very much, the phenolic would wear away quickly. There is alos the matter of the solenoid itself. It is doubtful in is rated for continuous duty, and if programming required it to be activated often, it might fail. If someone has a bad VVT tensioner, I can construct some sort of duty cycle test. I would be much more interested in a tuner turningt he VVT off altogether.
2. Since the intake changes phase, if one were to put in aftermarket camshafts, piston to valve clearance could be an issue. I am convinced that larger cams allow the 1.8T to make much more power, but certainly wouldn;t want anybody to bend valves trying a set out. With smaller grinds like the CAT 3651, it isn't a problem- but it may be with larger ones. (I'll be checking this out in the future)
Are the VVT and Non-VVT cams different?Good question!!! The answer is maybe, but I dont think so. Here is what I do know:
1. The lobes on VVT and non-VVT cams are identical. I had them checked on a cam doctor and the results are below (top lobe is stock intake)
2. The part numbers for the VVT and Non-VVT cams are different.
Non-VVT 058 109 021B
VVT 058 109 021M
3. The part numbers for the cam gears are different, but one supercedes to the other (sorry, will post numbers later)
4. The cam timing events for listed in the Bentley manual are the same for VVT and non-VVT engines (AEB, AWP) as follows
Timing events at 0 lash and 1mm lift
Inlet opens after TDC 18 degrees
Inlet closes after BDC 28 degrees
Okay, that must mean that the cam gears are pressed onto cams in a different position, so I bolted a head to a shortblock and installed a set of stock cams. My thought was that if I used a Non-VVT tensioner and measured a matching non-VVT intake cam's events, I could then install a VVT intake cam while still using the non-VVT tensioner and see the difference in timing.
Things turned out differently than I had planned. I always do the exhaust cam first since it establishes the relationship between the cams and crankshaft. I had never degeed in a set of stock cams before (just line up the dots, there is no adjustment) and was surprised to find the timing numbers off @7 degrees.
Okay, I must be off a tooth, so I retarded it one tooth. now I was off @7 degrees the other way. After checking my set-up several times, reestablishing TDC twice and even changing the head to one I absolutely, positively knew wasn't cut, I came to the conclusion that : A. Bentley is wrong B. My set-up is incorrect C. the factories method and mine for checking valve events different.
With the exhaust cam lined up with the rear cam cap the exhaust cam timing is advanced @ 7 degrees
Bentley Actual
Exhaust cam opens before BDC 28 35
Exhaust cam closes before TDC 8 15
I don't know how to account for the discrepancy, but figured I'd check out the intake too.
I found it was off by the same 7 degrees advanced, so at least I'd be able to install the VVT cam and see the difference.
To my surprise, the timing specs came out exactly the same!!
My conclusion is the cams are exactly the same, but I'm a bit uneasy with it. I'm also perplexed about the stock timing specs being off by 7 degrees. Your thoughts, comments and criticism is appreciated.
