I have a 402 bored 30 over. I had planed on using a closed chambered head for my motor but money is tight and i found a set of 781's at the right price, free in exchange for time to pull the motor. Heres my combo as it sits now. 402 30 over, comp cam, 274-280 568-578 lift. Pistons were the biggest dome my engine builder could get i think. holley DP 750, pete Jackson gear drive. hei-msd ignition. Edelebrock RPM air gap intake With the closed chambered heads my engine builder guessed id be around 10-1, with these open chambered im guessing it will be around 8-1 or so. Will the cam i have be mismatched now? any other unforeseen problems with this combo? How will my motor perform? bad low end? ect....

If compression was 10 to 1 and U were running 100cc heads going to the 781's at 119cc it would put U right around 8 to 1 or so. Youre engine builder has no idea what pistons were used??? cast/forged etc, doesnt say on reciept???? It doesnt sound like the builder used the largest piston available they make them 12.5 to 1 and also 11 to 1 in forged trw/speed pro's. A 396 forged replacement pist#2240 has
a .200 or so dome and would net about 9.7-9.4 with closed chamber head and a forged 402 replacement has approx. .100 dome and would be about 9.25 as would most of the cheaper cast 396/402 replacement pistons. If the smaller .100 dome pistons were used comp would probably be lower than 8 to 1. U should find out what pistons U have before U swap heads. With your cam size I would think U wouldnt want to go below 9 to 1. Why are U changing heads to begin with???? There is really nothing wrong with the 65-70 closed chamber heads. If your motor is indeed 10 to1 I wouldnt swap the #781's on to it, I would think the comp drop would hurt performance more than the flow of the #781's. Are the 781's equipped with 2.19/1.88 valves???? I bought a 396 awhile back from a guy that had forged 11.5 to 1 pistons (had a .300 dome) but had the ported #781's with 2.19/1.88 on it he said motor ran great plenty of power but the heads dropped the comp ratio down to where he could run pump gas in it (9.5-10 to 1 or so). I would look into before ripping the heads of your motor.

well the motor came with 292 open ports, but i wanted closed, by the advice of my engine builder. But the ones i was gonna buy i cant cuz of money constraints. so now im left with 292s that need work or 781s that i can pretty much just bolt on. was just wondering what the difference would be between the open and closed. My engine builder knows the size of the pistons i dont have the papers with me. sorry for the confusion