Im building an engine (mostly planning stages right now actually...but parts are slowly coming in) Im looking at some SRP pistons.....and im liking their Flat Top pistons...$452.99 for all 8. Used with 5.7" rods and 3.750" stroke (building a 383) Anyways...the CR specs are as follow:

58cc - 11.8:1
64cc - 11.0:1
70cc - 10.3:1
76cc - 9.7:1

I will be using AFR Aluminum 195cc heads.....and MAYBE a 100HP shot of nitrous if I feel like going a bit faster (no more than 100hp though). I emailed AFR...and they told me they could mill my heads to whatever I wanted.....but they recommend I start with 74cc heads and go from there because and I quote:

"I would recommend using our 74 cc version and angle
mill the heads to your desired cc. The 74 cc chamber enshrouds the valves
a little more when you mill it down. The chamber can also be milled."

So my question...how does the process of milling take a 74cc head down to say a 70cc head? Wouldnt you have to ADD material to the head to have less cc rather than take away? Or am I missing something here? (perhaps Im thinking in the opposite way) Second question..with aluminum heads, pretty much a daily driver (though I'll be in college so I wont be driving THAT much) and running pump gas...what CR should I be going for? Thanks in advance.

 

The deck (the flat bottom part of the head that mates with the block) is machined away (by thousanths of an inch). Imagine just setting it on a belt sander until it gets shorter (although what they actually use is a mill that sweeps across the head). This makes the combustion chamber smaller because it's closer to the block deck. As you can imagine, this move the intake side of the head, so that surface has to be worked as well, or the intake won't mate up to it.

 

Here's a quick image of it that I did in paint. It isn't the best picture but I think that it demonstrates the idea...

 

ahhh I see...now that makes sense...thanks for clearing that up for me guys. I thought they actually went in and milled all the intake and exhaust paths.

 

I would buy the smallest cc head possible, and use dished pistons to the desired compression. Milling is nice, but for 2 resons it is better to go the other way. Milling can get expensive, and it can depreciate the the ammount of lift for the cam. And the other thing is that you actually get more hp, because the dish on the pistons forces the compression to the wedge of the cylinder, and away fom the flat part of the chamber. Bottom line: more torgue, less detonation. Ever heard the term quench? Also, try to use the thinnest gasket possible for the same reasons.

Compression is around 2-4% hp a point. This means a 8.5:1, 200 horse 350 is going to make 15-20 more horsepower on compression alone, if it goes to 11:5:1.