scdyne
February 4th, 2008, 05:53 AM
While I may be a little ahead of things on the tuning of VVT that's no reason to ignore it all together. Over the last few months I have been compiling SAE docs on VVT control, reading through all the FSM's of GM vehicles that use VVT and logging the heck out of it on my E67 equipped vehicle. At this point I can say that I have a very good understanding of how it works mechanically, a very good understanding of how it works electronically and a respectable understanding of how it works mathematically.
However there are still a few areas that are gray for me in relation to the details of each aspect of the VVT system.
For anyone not familiar with GM's version of VVT here is a quick rundown:
GM uses a cam phaser to advance or retard the timing of the cam in relation of the crank position. Depending on the application either intake or both intake/exhaust can be controlled with an almost infinite advance or retard within the mechanical limits of the phaser. It doesn't increase lift it just changes the position of the cam. The main advantages to this is internal EGR and the control of the dynamic compression ratio. A quick Google search on 'VVT cam phasing' will explain further.
Where things become gray:
What is the default cam position?
At idle my cams are 0 deg 0% duty cycle.
How many degrees of advance/retard are there?
This has got to be specific to each engine. Mine (LE5) shows 25* total intake/exhaust. I have seen GM text that says as much as 50* for some engines. Cam or crank degrees? I don't know.
Can changing tables in EFILive cause valve/vale or valve/piston contact?
A question we all need to be 100% sure of for tuning. I doubt it since if there was a problem with the oil control valve it could change the timing and have the piston and valve try to occupy the same space.
How does one calculate dynamic compression ratio based on cam phasing.
This isn't magic, but the point is that we want to know it for boosted applications. My engine static C/R is 10.4:1 and some of us are boosting. knowing it and controlling it can really starve off knock at high load and rpm conditions.
However there are still a few areas that are gray for me in relation to the details of each aspect of the VVT system.
For anyone not familiar with GM's version of VVT here is a quick rundown:
GM uses a cam phaser to advance or retard the timing of the cam in relation of the crank position. Depending on the application either intake or both intake/exhaust can be controlled with an almost infinite advance or retard within the mechanical limits of the phaser. It doesn't increase lift it just changes the position of the cam. The main advantages to this is internal EGR and the control of the dynamic compression ratio. A quick Google search on 'VVT cam phasing' will explain further.
Where things become gray:
What is the default cam position?
At idle my cams are 0 deg 0% duty cycle.
How many degrees of advance/retard are there?
This has got to be specific to each engine. Mine (LE5) shows 25* total intake/exhaust. I have seen GM text that says as much as 50* for some engines. Cam or crank degrees? I don't know.
Can changing tables in EFILive cause valve/vale or valve/piston contact?
A question we all need to be 100% sure of for tuning. I doubt it since if there was a problem with the oil control valve it could change the timing and have the piston and valve try to occupy the same space.
How does one calculate dynamic compression ratio based on cam phasing.
This isn't magic, but the point is that we want to know it for boosted applications. My engine static C/R is 10.4:1 and some of us are boosting. knowing it and controlling it can really starve off knock at high load and rpm conditions.