AVCS and Interference

Hi everyone!

I'm trying to wrap my head around avcs in an ej25. From scouring forums I have some conflicting information:

1. At zero intake advance, the valve opens at 5* ATDC

2.The max advance on the intake cam is 30* of crank rotation

3 The ej25 is an interference engine, in which the valves and pistons occupy the same space


If all this is true, wouldn't the intake valves collide with the pistons at TDC, making 5* of intake advance impossible, let alone 30?

I'm sure either there's a gap in my understanding or my information is not correct. And help would be appreciated!

https://youtu.be/QmaVxsyzSac?t=449

I tried to make a good start point for you, but generally speaking yes it is interference, however the stock camshafts do not have a super high ramp rate, so the camshaft is not at maximum lift as soon as the valve starts to open and close.

Lots of factors impact piston to valve. With variable valve timing (AVCS in Subaru), you're advancing/retarding the valve events based on various conditions (stating the obvious here). But because there are so many factors, it really comes down to checking PTV yourself, especially if you've built a custom engine. How far the piston is in the hole, head gasket thickness, valve events of the camshafts etc. I have ZERO experience with Subaru engines, but whenever you swap camshafts with longer duration than stock, higher lift than stock and earlier intake valve opening events etc, then you have to check PTV.

The way to do it in a VVT engine would be to check PTV with fixed adjustable cam gears, and then to "degree them in". Then you'll know how far you can go with cam advance etc. I'd imagine, however, that with higher performance camshafts, that you're typically going to care a bit less about advancing the intake cams as the valve events on performance camshafts are longer in crank degrees. This means that the exhaust cam closes later and the intake cam opens earlier..

Again, lots going on, but I imagine that "interference engine" in this application is a conservative term, meaning that you'd have to do some insane intake advance to come close. The compression on these engines is so low, that the piston is going to be farther in the hole plus a thicker head gasket that you're not going to be nearly as tight as you would if you ran a higher compression/thinner head gasket set up with higher lift camshafts.

You won't make contact with AVCS as it has its limits, assuming OEM or typical mods. As stated above, at the point the valve starts up the ramp, it will not be full extended at the the point that starts.