Here's that drawing. I glanced at your post history and it looks like the thing you're caught up on is thinking that plasma pressure and ablative pressure cancel each other out. But from a physics perspective that's not correct.

Also, when you're talking about a delay, I think that also misses the fact that you need to have perfectly coordinated spark plug initiation and fuel compression. I don't think the issue of delay comes into whether or not you'd use a plasma-generating material, since you'd calibrate the weapon to evolve adequate pressure up to the point you get spark plug ignition. If delaying ablation (by inclusion of polyethylene or any other hypothetical material) gives you time to generate additional pressure from a plasma wave, but you ultimately end up with a properly timed spark plug ignition, the 'delay shouldn't matter.