My question is why ar the 75 and 76 heads not used. Is it because they are 78cc instead of 66? Do they not flow as good? Can they still be used to build a 450 horse engine? Just curious thanks
They are of an open chamber design. The combustion chamber is large, and about the size of the bore. Closed chamber heads(67-74) have a much smaller combustion chamber.
They make bombproof, into the next millennium small animal feeding and watering bowls, though. Sorry to sound sorta cynical, but crap, couldn't they have just severely dished the pistons instead of killing off two good production years? You can't turn right onto a dirt road and not eventually come across a small block Chevy head. You'd have to navigate the world twice to run across one of ours. Then it'd be a pair of '75's. With a crack under the spring pad. Original spark plugs permanently fused in place. Rust flooded water jackets. The Chevy's would be in the back of your truck from the machine shop by the end of the week. But I still have stacks of 'em out in the tool shed. Probably '76's. o No:
I can do that right now. For a stock bore, using 78cc for the combustion chamber size, .040 head gasket, 0 deck, and flat top pistons, I get 11.6:1.
I think it is just the shape and size of the combustion chamber. They should flow just fine, but who needs flow with forced induction.
Larry, Assuming zero deck, flat tops w/reliefs, .020 gaskets, do you think quench will still be ok? Or would a .040 gasket be better? I do not have a set of late model heads to compare chamber design to...??? Thanks for any input you may have!:TU:
With flat top pistons and 0 deck, the quench distance would be equal to the head gasket thickness. I honestly don't know how that open chamber would work. Any piston reliefs (notches) will lower the compression some.
I made the mistake of buying a set of new ported Bulldog heads off the board here. They were the 78 cc variety and the seller didn't tell me and I didn't know at the time they were made in two different chamber sizes (68&78 C.C.) . When I went to put them on the car I noticed how large the chambers were and just about threwup because I knew what it meant. To make a long story short I put them on with a .027 head gasket to get as much compression as I could after talking to Greg Gessler. Needless to say the engine isn't even close to what it was with ported steel Stage 1's that were 68 C.C.'s. So not a happy camper. Part my fault and I only wish the seller had told me they were 78 C.C.'s I would not have bought them. Moral of the story is they might be OK for another application but they won't work on the average piston about .040 below the deck build. Don't do it! Peace, Mike D.
Larry improved flow with forced induction is a plus.I know of a test done with ported to non ported heads and the power did increase with the same psi of boost.Some of the power increase may have come from the increase in port volume from the porting. Jamie
The open chambers of those heads will be slower burning than a closed chamber.I've never used a set but this would leave me to believe that more timing lead would be needed. Jamie
The quench distance would be the head gasket thickness plus the depth of the open chamber. In Mopar land there's so many of these late model open chamber heads available cheap that they make reverse dome pistons to bring the quench distance down to .040. This allows you run a full point more compression without detonation problems and use $40 core heads. Perhaps a Mopar reverse dome piston piston could be used in this application...?
The reason i asked is because about 6 years ago i built a 455 for my 65 skylark. I used a 75 block and heads ( NO KNOWING THEY WERE BAD WERE THE WORST YEARS FOR HEADS) We converted them to a stage 1 valves, and did some minor bowl work and port matching. The bottom end on the other hand i am happy with. I am running the JE 11:1 flat top pistons that only weigh 794 grams! I run a crower cam (234/245 @ .05) .517/.535 lift. Eldabrock intake, headers, holley 850 dp. This thing runs like a beast. Turbo 350 trans with 10" coan 3500 stall. 3:55 gears in the back. It doesn't have a problem going down the road at 30 mph and just blowing the tires off:TU: My question is how much compression is this motor running, and how much would i pick up with a set of ta alumumin heads? thanks Bryan
Oh no doubt, but forced induction has the potential to make more power than porting alone. The two together are definitely a plus.
I was using the Pat Kelley calculator. It doesn't take that into account, and with flat top pistons and 0 deck, the quench distance is listed as the head gasket thickness.
I had occasion to dyno a motor that had ported 75 heads on it, converted to STG 1 valves, some years ago. Was a project that was brought to me assembled, so I checked it over, and then we dynoed it. Torque was off quite a bit, although it was 12-1 compression. The heads had been milled a bunch. We ended up putting a set of similarly ported 73 heads on it, with the closed chamber, and picked up a bunch of torque, without really changing anything. I can only assume it was due to the better quench of the closed chamber. JW
So what I am gathering is that i really screwed up with these heads. :spank: I was hoping to make 450 hp and 500 ft/lb of torque with this combo. Do you think its possible or not? Also what kind of compression does this thing have. Zero deck, with JE 11:1 pistons and .040 quench. thanks bryan
With a bore of 4.343" (0.030" over), zero deck, flat top pistons with no valve relief, 0.040" thick gaskets, 78cc chambers, I calculate 11.7:1. Valve reliefs will lower that number. If you want to be precise, give us all the actual measurements. I have a formula that even takes into account the piston diameter and distance from the crown down to the top ring which is additional volume nobody bothers to consider. Devon
I run a JE flat top piston with 20cc valve reliefs, stage 1 valves, shaved the heads just for cleanup. With the bigger valves and clean-up cut, I'm sure that reduces the cc of the head by 1 or 2cc's. head gasket of .040. I want to say the pistons are .006 in the hole at TDC. thanks
