Guys

Just want a few opinions from those who have a bit of experience in Chevy pistons. Not technically a "Holden" question but the engine is for a HK GTS327.

I'm just buying some bits for the 427 BBC for one of my HK's before the ATO adds GST to imports under $1000.

I have organised most stuff, just looking at pistons. I was originally going to build this as a replica 1969 L88 engine except using cast LS7 990 heads. Problem is with the replica TRW pistons (50cc dome) the compression was going to be way too high even with the whopping mechanical cam I have for it, around 11.8:1. So I decided to use some smaller dome pistons (around 30cc) to end up around 10:1 and use a smaller hydraulic roller cam which will keep up cylinder pressure but won't lose power or torque (as roller cams do). The two pistons choices for this size dome and floating pin are KB Hypers or SRP forged.

The pistons in question are the following:

https://www.summitracing.../int/parts/srp-212133-8
https://www.summitracing...int/parts/uem-kb227-030

The engine should top out at 475hp give or take 25hp.

My concern with the KB hypers is the high top pistons ring. I have read that you need to be VERY careful of the top ring gap, as it is closer to the top and the cast alloy has lots of heat rejection (compared to forged pistons) and the top rings expand massively. Ring gaps like 30+ thou are necessary. Any detonation will also cause rapid growth in the top ring.

So for the $ saving is it worth using KB pistons?

I get forged is better, but just checking.

With Hypers it isn't heat distortion, it is heat rejection. Forged have close to zero silica in them, and they drag heat out of the combustion chamber, hence expand more, hence the bigger bore clearance required. Hypereutectic cast have a high silica content and reject heat back into the combustion chamber, hence expand less hence allow for tighter bore clearances (this on top of the nature of the material with regards to expansion too). But as a result the tops get hotter, and the top ring gets hotter thus expands more than the lower rings. What bothers me is there is a formula for what ring gap to have for the top ring, and I think this just seems way to finicky for me. However the price differential is huge, and the cast piston is a far better choice for a street engine apart from the above.

450-500hp out of a 427 isn't a big deal either, sounds like a lot but in reality it is equivalent to 324hp-361hp out of a 308. A stock L34 with a hp pack fitted (cam and big Holley) fit into this range, and they ran cast pistons plus raced in Group C with the same pistons with over 400hp. If you scaled the heads and manifold i'm using down to 308 size they'd be bigger than what was run in Group C, so a 427 with rectangle port, open chamber heads has more potential hp than 500. Bump the compression up, wilder cam and higher octane fuel and this combination would probably see closer to 600hp, which is why I think there are good but expensive forged pistons around. I'm only aiming for 450hp, anything more is a bonus!

Forged pistons handle detonation better by whole orders of magnitude too, which in a carby fed, mechanically controlled spark advance engine is probably a good idea.

I think i’m going to go for the 4032 SRP. Are the 1/16 top rings cool? The closed chamber versions run 5/64.