8CM vs. 8RT vs. 8BA heads - differences?
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From: Lewisville, NC
8CM vs. 8RT vs. 8BA heads - differences?
My '53 F100 239 flathead had Mercury 8CM heads on it and a C.R. of only about 5.4 I get differing stories about which heads to put back on it. One source says the 8CM and 8RT are the same, another says the 8RT and 8BA are the same. Does anyone have the straight scoop on what the volumes or CR's are on the different heads? Looking for a daily driver...
-Scott
-Scott
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Your stock bore and stroke (239 c.i.) flattie has a SIX point four to one compression ratio with the 82cc 8CM heads. With 76cc 8BA, 8RT, or EAC heads you would have a 6.8:1 CR, with 71cc EAB heads it would be 7.2:1.
Source: http://www.flatheadv8.org/compratio.htm
I don't think that boosting your compression ratio up to even 7.2 is going to give you much of a power boost. Perhaps there are other things wrong???
Mike
Source: http://www.flatheadv8.org/compratio.htm
I don't think that boosting your compression ratio up to even 7.2 is going to give you much of a power boost. Perhaps there are other things wrong???
Mike
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From: Lewisville, NC
Originally Posted by 45ford
Your stock bore and stroke (239 c.i.) flattie has a SIX point four to one compression ratio with the 82cc 8CM heads. With 76cc 8BA, 8RT, or EAC heads you would have a 6.8:1 CR, with 71cc EAB heads it would be 7.2:1.
Source: http://www.flatheadv8.org/compratio.htm
I don't think that boosting your compression ratio up to even 7.2 is going to give you much of a power boost. Perhaps there are other things wrong???
Mike
Source: http://www.flatheadv8.org/compratio.htm
I don't think that boosting your compression ratio up to even 7.2 is going to give you much of a power boost. Perhaps there are other things wrong???
Mike
-Scott
Fleet Owner
Joined: Jul 2004
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From: NM
The values in the chart are historically correct, but people on other sites who have measured a large sample of the Merc and 8RT heads claim their volumes are typically larger than shown in the chart. That's where the 5.4 value came from.
As soon as Narwhal raised his question about why his flatty might be down on compression and noted it had Merc heads, I suspected this situation. I've heard of it several times on different boards, and it has in all cases dropped compression very evenly down to about where his is. In one case, someone's Ford engine had somewhere along the way had one head replaced with a Merc head and the one bank was down 20 pounds from the other.
How much difference "seat of the pants" it will make is hard to say, the Merc head may breathe better with more area over the relief, who knows?!
As soon as Narwhal raised his question about why his flatty might be down on compression and noted it had Merc heads, I suspected this situation. I've heard of it several times on different boards, and it has in all cases dropped compression very evenly down to about where his is. In one case, someone's Ford engine had somewhere along the way had one head replaced with a Merc head and the one bank was down 20 pounds from the other.
How much difference "seat of the pants" it will make is hard to say, the Merc head may breathe better with more area over the relief, who knows?!
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