Technical Enlarging SBC Combustion Chambers?

In my quest to find a short block to my liking, what I have come across is the ZZ4/6 from GM. It’s just a rotating assembly, no cam, etc, but a new block, forged crank, hyperuetic flat top pistons with 4 valve reliefs.
The heads I have (based on what they say) are 64ccs but will put me in the 9.6-1 range. I’d like to get that down to 9.1/9.2-1.
So, being the engine I have heads for were a dish with 4 valve reliefs, the only real “quench area” I had was the “ring” around the outer edge of the piston.
Searching Google got me limited info during my down time at work, one thing I found makes sense, (heads on a block, mark where the heads may be in the cylinders and blend in). Okay. Check.
But that’s about all I found.
So my thought (feel free to give a virtual slap on the back of my head) is to remove some of the quench pad area as a start. Why? Well if I take 10-15% of it away say, the actual quench area will still be vastly more than it would have been with the original pistons.
Now here’s what I have the most concern doing that, even though I only had originally a small amount of a proper quench area, how would making the pad on the head smaller affect things?
Or maybe just tell me a better way to remove combustion chamber CCs

Thanks.

Edit: so the question doesn’t get asked “why not buy the proper heads for the CR you want”?
My reply would be along the lines of “please pay for them, I’ll cover shipping”

 

Two head gaskets per side maybe? "Old school"

 

I'd contact any local machine shop that does head milling and ask them to "reverse mill" your heads. I think it is a relatively simple matter of operating the milling machine in the opposite direction. If that isn't possible, ask them to add to your heads whatever they take off the next set they mill. Quite simple really ... aren't you glad you asked

 

That would kill any quench though. The pistons in the short block are .025 down as I recall. The head gaskets I have are .025 or .026 compressed, not in the “optimum “ quench, but I don’t want to be .075 or so at quench either.
@tommyd

 

@borntoloze may I be so kind to offer a virtual head slap to you?
And yes, I’m still laughing!

 

PS. I knew I should have just went to bed after I posted. Lol

 

Dynamic compression is the important number. A slight increase in cam duration if you can tolerate it would erase any difference between 9.2 and 9.6 static.

 

There might be a .042" head gasket set (when compressed) out there.... I'd look into it....

 

I appreciate that, but my need for using what can I chose, etc, isn’t HAMB relevant, just the need for a larger combustion chamber is my quest for what I have.
I ported these heads around 20 years ago. Put them away. Like I do with many things, I lost track of valves, keepers, etc.
So took them to the shop, new guides, valves, springs, keepers, retainers, exhaust seats, surfaced, so I kinda want to use them before I get to old to see how they work

 

I’m sure there is, but for years I’ve read “.040-.045” quench. All things being equal right now I’d be at .050-.051. Would like to not go larger/more than that.

 

Im building a sbc. Using old 461 heads that had 64cc chambers, i cc'd them and found that they actually measured between 58-60 cc. Considering that my 2nd hand balanced rotating assembly had pistons with 1/10" dome, i wanted to get my chambers back down to 64cc.
My local engine shop guy said not to worry to much about it , to focus mainly on slightly radiusing the sharp edges around the combstion chamber to deck area transition. Places that get hotter than surrounding areas. Used a head gasket as a template.
Me, being me, i couldn't leave well enough alone and ground /cc'd the inside of the chambers till i got back to 64 ccs on all the chambers. A very tedious job.
Im still assembling the top end. My searches didn't result in much info either on comb. chamber enlargement. Most of the material i removed was on the plug side of the heads,inside the chamber and
Then softend the sharp edges. Hopefully i didn't weaken the old heads too much.
I haven't run the engine, and cant offer any practical knowledge other than what the machinest told me.

 

Considering all the more compression your dealing with, i tend to agree with Moriarty. Use the gaskets you have, put it together and run it.

 

Yes, any gain would come from a tighter squish....Squish is important to a wedge head.....Faster more complte combution and a better resistence to pinging...Even near stock engine can benifit..
On a SBC, I use a steel shim gasket, the stock type piston usually .022 in the hole.. gives a decent .042 squish just like original design....

 

With dished pistons you don't have much quench anyway. I don't dont think a .040 head gasket will change anything

 

What’s the end purpose of the engine? Street? Race? Pull? If you’re trying to get every last hp out of it, maybe try changing it. If it’s just a street motor that might get a few full throttle runs every once in a while, put it together and don’t worry about it. That little bit of difference shouldn’t make it have to have premium fuel or be hard to run on the street. The cam you choose will make more difference in how it performs than a few small points of compression. If you were talking about the difference in a 12-1 and a 9-1, yeah, you might need to do something.
Heck, a lot of old motors ran something like a 5-1 compression and ran fine.

 

1 - 9.5 to 1 should live fine with CA premium gasoline. Yeah, I live here too !

2 - As was said, a camshaft with a couple more degrees of duration will erase some static compression problem. You won't need much !
This will drop some dynamic (vs. static, or measured buy cc's) compression, which is where the REAL detonation problem comes from.

3 - A degree, maybe two (at the most) of valve overlap will erase the static compression problem. That is going from...say 114° to 113°, or 110° to 109° of overlap. This will also help build a little more power output !
Again...this will drop some dynamic (vs. static) compression, which is where the REAL detonation problem comes from.

Mike

 

Cometic sells at least 20 different thicknesses of SBC head gaskets. I lowered a alcohol modified engine from 14.1 to 13.6 for our gasoline super stock that uses SONOCO 110.
Calculated what I needed using the formula on the KB site. Easy-peasy.

 

Instead of using a head gasket as a guide, just bolt the heads on the bare block and flip it over on the stand and look down the holes with a drop light and mark the heads. Lippy

 

Except he's buying am assembled short block, a pretty nice ZZ4 that already has a cam in it designed for that engine.

 

Same as above, he's got an assembled short block, not a bare block.

 

Using intake valves with the depression in the face vs. a flat faced valve can lower compression by .4 . . Milling the valve reliefs on the appropriate set of reliefs if possible on the piston crown might be acceptable. Your getting in to mixture effects & flame fronts milling the chamber .

 

The short block you're buying advertises C/R against a certain CC head, and a certain compressed thickness head gasket. Use a thicker compressed gasket to reduce compression if you need to. That's the easily changed variable.

 

I would start by CCing the chambers to see where you are at. It's all speculation up until that point.

 

I agree , up to this point you are relying on opinions and charts. You need to do the actual math and measure combustion volumes as well as actual piston dome/dish volumes and head gasket thickness.

 

Yep. I would guess about 90% of the 10:1 motors are really more like 9:1.

 

If it really is only 0.4 of a point difference I wouldn’t even give it a second thought

 

C'mon Mike! Can I get one of those virtual head slaps too! I gotta give you my body man input on engine building. You know how that one guy always says he has 500 horsepower, and when it comes to the dyno, he actually has 230hp? I've found its the same with static compression ratio numbers. Until you get out the beaker and syringe and get actual numbers to do your math with, those other numbers are just a guess, and all of the engines and heads I've CC'd that I knew were gonna be 13-1 badass asphalt destroying beasts, turned out to be 9-1 or some shit. So I'd just put those heads on there baby! It's OK to break shit, as long as you're giving it hell!

 

I think you are overthinking this. That being said, if I were to need more chamber volume, I would take material away from the area that is shrouding the valves. You get better flow and achieve your static compression goal.

 

I came here to ask this, too.

I am running 11.25:1 static on an iron-head inline-6, on pump gas. Sure I have to use premium, but so what?

A ZZ4 long block would have aluminum heads, and would be more tolerant of a little more compression.

*and yes, that was with cc-ing the heads, and the block, with the piston all the way up.