What is the recommended amount of quench? I have the short block all done. Now I need to order parts for the top end. I was thinking about using the Fel-Pro 26191PT (.048") head gasket, but I'm not sure if it leaves enough room for expansion at higher rpms. The flat-top pistons stick out of the bore about .010". That only leaves about .038" quench. Is that enough?

I'm trying to build a good low-end torque, truck engine that is also ready for boost (if wanted). I need to consider both N/A and boost applications for this engine that I plan to sell.

 

That's plenty. Normal range is .035-.045. I'm surprised your pistons are so far out of the hole. Normal, from what I've heard, is about .005"

 

Yeah, I was surprised too. I got my dad to hold a metal ruler across the piston, and I used a feeler gauge to measure the gap between the block and ruler. I easily got a .009" feeler gauge under the ruler. The .010" gauge was a bit tight.

With the 706 heads, I knew the compression was going to be way above stock. I still want to be able to run it on regular 87 octane, so I'm hoping the low quench will help prevent knock. I'm still working on the heads, but I figure the chambers should be around 61 cc by the time I'm done. That should put me in the 10.7:1 compression area. I know that's a bit high for 87 octane, but it might be possible with less timing advance. This engine will have flex fuel injectors, so it should be a hot little 5.3L with E85.

Here's how I measured the piston/block gap:

 

I'm not sure a tight quench is desirable for boosted? You can get to the point where you need to do things to slow down the burn. I'm not sure where that threshold is, maybe somebody with more experience will chime in

 

Is that cross shape raised or recessed with the rest of the piston top? Or is it flush just a grove outlining it?

If it's above, it's probably aligns with the chamber area and not the head deck surface, therefore wouldn't count. Of course if it is in the zone that aligns with the head surface, you'd want to count it to account for minimum distance so it doesn't kiss the surface at high rpm.

 

Only the outline is raised. The interior and exterior are the same height. I didn't measure the height of the outline, but I can't imagine it being more than a couple thousands.

I understand the idea that more quench or less compression is "easier" to deal with when boosted. I also believe that more N/A power = more boost power. You can't please everyone, so I'm trying to build it for an N/A application, but with boost in mind. Yes, it will have more compression than what most boost applications have, but I gave it extra ring gap and less quench. It's not an ideal boost build, but I believe it will handle a good amount of boost.