The cylinder heads you have were designed when the best thing for pump gas was a low static compression ratio. However, this design and the lower ratio negatively affected the power output and efficiency. At the time, this was all they knew. That was back in the late 50s and early 60s. The heads remained basically unchanged until the mid to late 80s, when they started applying more developments in cylinder head flow and chamber design. That was still 30 years ago. Can they be made to work well and make power? Most certainly. Are they "the best" idea when you are starting from a clean sheet of paper? Hell no.
No, We had higher octane/leaded gas back then.
In no way were the old heads designed around low comp/octane, if they were...they'd be closed chamber already.. they were actually designed around better gas then we have today. See 10.1-10.5 was not really that high back then, 11.5 + max wedges and hemi's were considered high compression , like how we have closed chambered max wedge heads, now that adds up.
I don't know about you...but when a compression raise in needed, we change pistons...just like chrysler did with the 340, 440,383,426 hemi,max wedge etc....
while the squish is more efficient=cleaner faster burn=hp
you also get some worser valve shrouding, slight flow loss, nothing major.
If the OP has $1300 more to put into this build, go for it.
If it was me, I wouldn't worry about it and I'd mill them heads to get 10.2 with those pistons, then select my cam and not have any problems with detonation.
The problem is you have a 4" stroke which by its very nature imparts a lot of ability to create compression, and you have a large open chamber which by it's nature is prone to detonation and ping. The way to minimize that trend with the parts you have is to lower the static compression ratio even more. That in turn lowers the overall output, response, and efficiency of the engine package. So to me, if you are already thinking of building basically from scratch, it makes sense to use technology that is readilly available, doesn't cost much more, and get the best overall result.
Polish the chambers, open them to bore size and mill to desired cc.
most people never consider that since the SHARP chamber edge hangs into bore on an LA open chambered head, that is where the detonation usually comes from-glowing hot sharp edges.
I only know 2 people that open the chamber up to the bore, and I'm one.
to the op, compression is an air/fuel mixer in itself, let alone if the carburetor and port volume=velocity is keeping or trying to keep it all suspended/mixed , oddly enough the thing thats involved in detonation is also trying to help by atomizing it all.
In terms of part details...
-The smaller you can make the combustion chamber, the better it will run on pump gas. You are looking at pistons with a 23cc dish, and a chamber volume of arouns 72ccs at the smallest. The combustion chamber volume, the space the air anf fuel mix and burn, is 95ccs and at no time are the piston and head close enough to help the mixture stay mixed. This means there is a greater chance of areas of poorly mixed air/fuel, and areas that simply will not burn, thus losing power and efficiency and making it easy for ping or detonation.
-With the RHS heads at 63cc, and the piston dish at 16.5, the overall chamber size is reduced by almost 20%. And you also have the piston and head coming close together which really reduces the poorly mixed areas. That means you don't have the ping or detonation even tho you have a higher static (and higher dynamic) compression ratio.
I think the overall benefits are worth the initial investment if you are at the point of building a non factory engine size. Hope that makes some sense.
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I have milled la heads down to 59cc no problem, it took about .050 to get there, currently mine are at 60cc with the kb356 giving me 10.1 comp-175 dynamic and runs 89 octane.
What I'm seeing in the pics of those icon 847 is a step head, is it possible to get that up out of the hole to zero deck the dish ledge?
What Im saying is... maybe you can mill the block some [way cheaper than heads] and get that step close to the chamber for sum real quench
hell if you could get it .020 out of the hole, use a thin .019 steel head gasket and mill the heads down to 60, you'd be stylin.
those icon pistons 847-846 make it so you either have to run race gas, buy closed chambered heads, or have to mill a lot off the heads.
