62Ranchero200
Famous Member
Greetings Ford Six Fans,
Background: Have a '74 250 with "early" 300 rods and custom RaceTek forged pistons, .005" deck height, Clay Smith 274/274/108 hydraulic cam, 1.6:1 adjustable stamped rockers, Smith Brothers custom pushrods, CI Al head (base valve size, no porting), highly modified Holley 2300/4412 "500 CFM" 2 BBL carb, CI headers with true dual exhaust, "X" crossover and FlowMaster Series 40 mufflers, custom curved DS II with MSD-6AL. Very solidly built, with ARP main and head studs, ARP rod bolts, statically and dynamically balanced and blueprinted.
Now that I've replaced my C-4 auto trans with a T-5z and an 8" 3.80 Traction Loc is being built for me, I'd like to warm the engine up a bit. I'm not in a hurry to do this, but I want to start planning for this now. My intended approach is a longer-duration, higher-lift cam (I'm thinking the Clay Smith 280/280/108 hydraulic, but that's a subject for a different thread) and more compression (would like to add no more than a point of static compression). I will probably try a 4 BBL carb as well, after the cam and compression are sorted out.
With the custom curved DS II (I run 20 degrees of initial advance but limited to 16 degrees of added mechanical advance; probably don't develop enough vacuum to get much vacuum advance) and running 93 octane gas, the Ranchero has never detonated, so I'm thinking I could go with 1/2 point higher compression with the cam I have, 1 point higher compression with a longer duration, higher lift cam (since that will reduce dynamic compression).
Having just installed the T-5, I'm not eager to pull it back out immediately to work on the short block ... so I'm hoping that it would be possible and advisable to gain some compression by milling the head. The head will need to come off to change the cam, so that would be a great time to mill the head. My CI Al head has 55 cc combustion chambers. Is there a rule of thumb for the shape of the small six head that indicates how much must be milled off of the head to reduce combustion chamber size by 1 cc? Is there a maximum limit to how much can safely be milled off of the small six head before overheating or the probability of cracking become too likely? Are these considerations any different on the AL head than they would be on an iron head? I want to be able to continue to drive the Ranchero on the street, even on long drives such as to regionals in other states, and also in Houston traffic on hot (100 degree) Houston days. More than anything, that AL head is not only expensive but precious (in that only ~100 or so of them exist at the moment), so I don't want to mill too much and damage the head in any way.
What issues am I likely to encounter if milling the head for compression? Hopefully my adjustable rockers will be able to compensate for the altered valve train geometry, if not I may have to order shorter push rods. What else?
I searched for these topics on the forum and found many posts about milling the head to increase CR, but I couldn't find a rule of thumb that correlates thousands of an inch milled from the head with cc reduction of combustion chamber, or a maximum limit. They are probably buried somewhere in the 100's of posts about milling heads for compression.
Thanks,
Bob the Builder
P.S. I put this post here, rather than in the Al head forum, since I don't know if the guidelines for milling a head for compression are any different for the Al head than for an iron head; and also, more people will see it here.
Background: Have a '74 250 with "early" 300 rods and custom RaceTek forged pistons, .005" deck height, Clay Smith 274/274/108 hydraulic cam, 1.6:1 adjustable stamped rockers, Smith Brothers custom pushrods, CI Al head (base valve size, no porting), highly modified Holley 2300/4412 "500 CFM" 2 BBL carb, CI headers with true dual exhaust, "X" crossover and FlowMaster Series 40 mufflers, custom curved DS II with MSD-6AL. Very solidly built, with ARP main and head studs, ARP rod bolts, statically and dynamically balanced and blueprinted.
Now that I've replaced my C-4 auto trans with a T-5z and an 8" 3.80 Traction Loc is being built for me, I'd like to warm the engine up a bit. I'm not in a hurry to do this, but I want to start planning for this now. My intended approach is a longer-duration, higher-lift cam (I'm thinking the Clay Smith 280/280/108 hydraulic, but that's a subject for a different thread) and more compression (would like to add no more than a point of static compression). I will probably try a 4 BBL carb as well, after the cam and compression are sorted out.
With the custom curved DS II (I run 20 degrees of initial advance but limited to 16 degrees of added mechanical advance; probably don't develop enough vacuum to get much vacuum advance) and running 93 octane gas, the Ranchero has never detonated, so I'm thinking I could go with 1/2 point higher compression with the cam I have, 1 point higher compression with a longer duration, higher lift cam (since that will reduce dynamic compression).
Having just installed the T-5, I'm not eager to pull it back out immediately to work on the short block ... so I'm hoping that it would be possible and advisable to gain some compression by milling the head. The head will need to come off to change the cam, so that would be a great time to mill the head. My CI Al head has 55 cc combustion chambers. Is there a rule of thumb for the shape of the small six head that indicates how much must be milled off of the head to reduce combustion chamber size by 1 cc? Is there a maximum limit to how much can safely be milled off of the small six head before overheating or the probability of cracking become too likely? Are these considerations any different on the AL head than they would be on an iron head? I want to be able to continue to drive the Ranchero on the street, even on long drives such as to regionals in other states, and also in Houston traffic on hot (100 degree) Houston days. More than anything, that AL head is not only expensive but precious (in that only ~100 or so of them exist at the moment), so I don't want to mill too much and damage the head in any way.
What issues am I likely to encounter if milling the head for compression? Hopefully my adjustable rockers will be able to compensate for the altered valve train geometry, if not I may have to order shorter push rods. What else?
I searched for these topics on the forum and found many posts about milling the head to increase CR, but I couldn't find a rule of thumb that correlates thousands of an inch milled from the head with cc reduction of combustion chamber, or a maximum limit. They are probably buried somewhere in the 100's of posts about milling heads for compression.
Thanks,
Bob the Builder
P.S. I put this post here, rather than in the Al head forum, since I don't know if the guidelines for milling a head for compression are any different for the Al head than for an iron head; and also, more people will see it here.