I am rebuilding the 302 in my 1973 Maverick. Now I am to the point of choosing pistons. My heads have 60 cc combustion chambers. I know that in '73 the blocks went to a taller deck (8.229 vs. 8.206). My block is a D2 casting. Does anyone know if that means that I have the shorter deck, or did they keep the same casting size and mill less off to achieve the taller deck (and lower compression)? The reason I ask is I have narrowed down the pistons to 2 choices. One is somewhat dished and will give me a CR of 8.92:1 for the shorter deck but 8.46:1 for the taller deck. The second is a flat top with relief that will give a CR of 9.85 (which I have been told may be too high) with the shorter deck or 9.29:1 with the taller deck. I have been recommended to shoot for 9-9.5:1 to avoid detonation, but the shop machining my block told me that if I use a good 4 bbl carb and an adjustable distributor that CR's closer to 10:1 are still fine. Any ideas, recommendations, input? Thanks, 73Mav
I agree, you can get away with the 9.85:1 as long as you are careful. Remember, the single greatest way to produce power is to increase your CR, unless of course you are using a super charger or turbo!
Your machine shop should be able to measure the deck height for you. Check the attached link for an informative article on the subject of compression ratios and related matters. I know it says Pontiac at the top of the page but the principals apply to all internal combustion engines. I found this by doing a search on Google for Comp Cams 268 http://www.boyleworks.com/ta400/psp/compression.html
Flat top pistons and deck the block to near zero deck height or get an earlier block to use the flat top pistons in.
mill Mill the block.020" down to the earlier specs. Comp of 9.5-10.0 is possible with the right cam and ignition. Good quality gas makes a difference to. You must also maintain the correct piston to head clearence. That includes the piston at TDC, below the deck and gasket thickness. Not a problem with milling the block.
the easiest way to tell the exact year of the block is to really read the codes. There are 2 main ones on the block, one is the part number (like D2OE-6025-aa), and the other is a casting code, the exact date that it was cast, usually like a "4C15" or something like that. The first number is the last digit of the year it was cast, the letter is the month, and the last 2 are for the actual day. For instance, 4C15 means it was cast 4=1974, c=3rd month, march, and the 15th day. March 15, 1974 in this example. The "taller" blocks were made from 1973-1976. They can be used, but you'll most likely have to do some milling.
You won't really know actual compression ratio until take into account the piston depth at TDC, head gasket thickness, cylinder head cc's, as well as the dome/dish on the piston. The pistons advertise a certain CR, but there are so many variables be sure to check it out yourself to be sure. I measured my clearances on my 1987 factory piston'd 5.0L. The heads and block have both been decked, and I've run a CR of 10.03:1 for a few years now. No detonation problems, but always 93 octane.
You can run 10:1 without problems, if you have a cam with some overlap or aluminum heads. Usually you gain about 4% horsepower with every point of compression, so that extra half point may not be worth it. Ignition curve, cam, haeds, a number of things determine whether or not you will ping. Also keep in mind that if you mill heads and decks, you will need adjustable rockers.
