Poor Man's - knocking issue

[gnaveh]

I've just built a poor man's 4.6 stroker,
Everything is stock 4.0L except the crank & short rods of 4.2L (258 engine).
Injectors - Bosch/Ford 24Lib/H, Fuel regulator 40-50 PSI.

During the 100+ miles since then, the engine works really perfect - except for the very annoying phenomena of knocking under an even slight load.

I was wondering if it's an early-ignition issue (i.e. spark is created too early) or a pre-ignition issue (i.e. fuel auto-ignites due to the little bit higher compression ratio).
I tried fulling with 98 Octane (instead of 95) and the problem totally disappeared. However it was only a try, I'm not into keep paying for the higher octane gazoline...

What do you think - which of the two is my problem and how you recommend resolving it?

Thanks a lot!
Gitai

[Cheromaniac]

Which pistons and which head gasket did you use? Sounds like your engine has the unwanted combination of highish CR and quench height that makes it more prone to detonate. Looks like premium fuel is your only option.
Which spark plugs did you use? Check their color just in case the engine's running lean.

[Russ Pottenger]

Unfortunately one of the drawbacks of the poor mans stroker combination is if you're not careful on how much you surface the cylinder head you'll have a relatively high static compression ratio matched with a excessive quench distance.

Unfortunately this won't help you, but my solution is to zero deck the block and lower existing dish in the piston through a CNC program I have bumping it up to a 22 to 26cc dish while maintaining a tight quench.