What's the piston dome volume/cc ? For example: my 420 c.i. stroker has 19cc DISH pistons, 4.040 bore, 4.1 inch stroke on the crank, and a 6.2 inch rod. With .005 deck clearance, and .040 head gasket compressed thickness, my combination yields 10.3 to 1 c.r.
my gt40's came in at 62cc so I had 30 thou shaved off of them to get them at 59-60cc. My engine has 10.2 to 1 static compression but by the time you figure my cam timing it puts me at 9.4 to 1. with aluminum heads I would shoot for 11 to 1 static and 10 to 1 with cast iron heads. Use this to help get a better idea of compression with cam timing. A bigger cam will let you get away with higher compression by letting the cylinders bleed off some pressure at low rpm. http://www.kb-silvolite.com/calc.php?action=comp2
Without knowing the cam, I'd start out with 9.5 cast iron heads, or 10.5 aluminum heads. Cam does make a difference, so you would tweak up or down from there. That's just my .02, and worth what you paid for it!
As of yet I have not selected a cam. What I read was the GT40 heads on the Cobra were shaved to get 62.5cc by Ford, but the stock ones on the Explorer like mine are sopposed to be 65.5cc. These KB pistones are dome 2.60cc. One thing I'm not sure of is where this piston's 1.608in compression height will put me on deck clearance. I calculated it will leave me at 0.008in below , but I 'm not sure I'm running any of these numbers right.
Speaking of deck clearance... Some 302 blocks were 8.209 tall, and some were 8.229 tall. You can see what year your block is and then see if that was one of the "tall" years. However, after 40 years, your block may have been decked sometime in the past. Best thing to do is physically check it. They say it isn't a good thing to have your piston too far down in the hole in performance applications.
I take the known deck height and subtract all of my lengths to get clearance. Ex: 8.229" - half of the stroke (1.5" for stock 302) - rod length - compression height of piston - any amount of machine work on the block deck to shorten or true up deck surface... The remaining measurement is your deck clearance. (Add the thickness of the gasket to your volume, but it doesn't count as deck clearance.) When deciding on compressed volume, they say to keep as much as possible in the head. The volume that you leave in the block is far less effiecient in transferring power to the piston. That is what Ford did in the 70s... They needed to drop compression, but did it the 'lazy' way and added .020" to the deck height, rather than make larger combustion chambers. After a few years of this, they realized their error and went back to the original deck height and put huge chambers in the heads.
This is what I came up with using the c.r calculator on Summits website. Of course you'll have to verify what the deck clearance is. But, this is the numbers that you provided. bore= 4.030 stroke= 3.00 combustion chamber= 65.5 effective dome volume= -2.60 deck clearance= .008 compressed gasket thickness= .040 number of cylinders= 8 Compression Calculator compression ratio= 9.59 to 1 Bore: (diameter) in. mm Stroke: in. mm Cylinder Head Volume: cc in. Effective Dome Volume: Use (-) for Dome and (+) for Dish. cc in. Deck Clearance: in. mm Compressed Gasket Thickenss: in. mm Number of Cylinders: 4 6 8 10 12 Compression Ratio : 9.59 : 1Total Displacement (in.3) : 306.13Total Displacement cc's : 5018.52
And That was only part of the equation, they also added huge grand canyon dished pistons to the mix from 72-76.
When my speed shop built my 393 Cleveland, the original comfiguration of the stroker parts would have had my compression way up even with the open chamber CJ heads. We went with a set of Keith Black forged reverse dome pistions and with the open heads the compression settled in at 9 to 1. Before the engine was completed, a nice set of Boss 351 closed chamber quench heads became available for sale. Assembled it should come in at 10 to 1 - 10.5 to 1. Reading responses in this thread, it seems that I should still be OK with premium pump gas.
You'll be fine, as you now have quench areas with the closed chamber heads. With the open chambered cleveland heads, there is no quench at all.
Agreed. Open chamber heads supposedly detonate at 9.5 with 93 octane. Closed chambers have the advantage of 'quench' and are said to be good for at least another point of compression.
Ya, I thought it was the right thing to do too I was reading up on quench heads about the same time my set came up for sale. When I talked to my engine builder he completely agreed with me and that was the deciding factor on making the purchase. What else is nice is the reverse dome of the KB pistons mirrors the quench chamber of the head. Then when I read the reinforcing comments on this forum, I feel even "more better"
So I finally got around to CCing the cylinder heads. Scrounged together some bits and pieces to accomplish the task. The 0.006" they took off the heads didn't appear to change the volume that much. They measured from 64.5 - 65.5cc. With that number in mind I'm looking at these pistons. (Keith Black - KB116-030) which I found in a kit on Summit for $264. Here's a link to the gallery of pictures from the CCing process. PicasaWeb Gallery
