Well I went over to see my buddies turbo H22 Honda that had head gasket problem last night. We pulled the head and found that his oversized intake valves had contacted the tops of the piston valve reliefs and actually broke off pieces of piston with no valve damage.....compression was still perfect. The motor had been clayed with no clearance issues but it had not been clayed for when the engine was in VTEC. To make a long story short the pistons had not been ordered with larger reliefs to accomodate the 1mm oversize valves and they hit the psiton in VTEC. I was thinking this morning about aftermarket oversize valves in the VQ motors and if there might be issues with VTC changing timing when the engine is running. I wonder if its possible that there could be valve to piston contact if the pistons were not specified for an oversize valve size application?? Unlike the Honda I don't think you could test for clearance with the VTC system and its parameter swing since it is not a mechanically controlled system but an electronic one which varies on load, octane, etc. Any thoughts??

 

I would think if you checked the clearance with the valve at max lift and the piston at TDC, it would not matter what the timing is. That point is the closest that those two parts would ever physically be able to come to each other.

 

I think this is only an issue with VTEC. The VQ does not change cam lobes like VTEC (only phase shift). Now, it's possible that valve float could be aggravated depending on what way the phase shift went.
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Jeff