The Ford Barn

46fatford · 05-31-2023, 06:06 PM

I have read conflicting information about the compression ratio given with each of these heads. I currently have a 1950 8CM mercury with standard bore and 4 inch stroke crank. I was wondering if the EAC heads would yield a higher compression ratio than the 8CM heads. I know the EAB Ford heads would definitely yield a higher compression ratio, but I like the look of the mercury script on the heads especially since it's in a 1950 mercury coupe. Are the EAC heads like the EABs when added to a 4 in crank mercury engine, or is the compression change nominal enough to not warrant changing to EACs at all?

tubman · 05-31-2023, 06:25 PM

"Basic Flathead Theory" : Measure the clearance over the piston (with clay or aluminum foil balls). Have the heads milled to give a nominal .045 over the piston. Use a die grinder (or even a Dremel) to remove any irregularities and bring the chamber volume to consistency. Done.

This "willy-nilly" replacing heads or milling a set amount will not yield consistent results. I have done 5 sets of heads (2 aftermarket and 3 stock) and have found variations between heads in the same set. The last set of Edmunds heads I did needed .035" off of one and .025 off the other to bring them to consistancy. (Yes, aftermarket heads need to be custom fitted, too.) Stock heads can have even greater variations.

Custom fit your heads guys; it will yield the best results (both in power and fuel economy). It's also kinda fun and makes you feel like a real "Hot Rodder".

cadillac512 · 05-31-2023, 08:10 PM

Yes, EAC heads will raise compression over 8CM heads. How much? Approx .4 to .5 depending on engine size. The 8CM are about 83 cc vs about 76cc for EAC.
Tubman's advice is sound and will yield very positive results. It's tedious and fiddly but the benefits are real...not huge, but real.

Terry

scicala · 05-31-2023, 09:53 PM

Quote:

Originally Posted by cadillac512

Yes, EAC heads will raise compression over 8CM heads. How much? Approx .4 to .5 depending on engine size. The 8CM are about 83 cc vs about 76cc for EAC.
Tubman's advice is sound and will yield very positive results. It's tedious and fiddly but the benefits are real...not huge, but real.

Terry

Terry,

Would you happen to know what the cc's are on the EAB heads ?
Or do you know if they will raise the compression even more than the EAC's ?

Sal

tubman · 05-31-2023, 10:25 PM

One problem. Even the newest "EAB" and "EAC" heads are over 70 years old. Unless you find a truly untouched engine and remove them yourself, you can never be sure that the heads have not been resurfaced or otherwise modified. Thus, most of the time you can never be sure of what you have. In my opinion, someone putting another set of heads on an engine without knowing where the clearances are may be making a big mistake. In my experience, optimum combustion chamber turbulence (quench) yields more benefit interms of performance than the increased compression.

The first flathead I built over 30 years ago was a teenage magazine-readers dream : a 276" merc with a Holley 390, an MSD ignition, and a set of new Offenhauser heads. I just took the heads out of the box and bolted them on. Between the SBC ignition curve in the MSD and the obvious lack of combustion efficiency with the unmodified heads, I was never really happy with the way that engine performed. I ended up selling it (and the '36 3-window it was in) after 5 years. I have a 255ci Merc with carefully fitted Edmunds heads, a small base 2G, and a Mallory dual point in my current '51 club coupe. I am very happy with the way it performs and believe that the '51 would outrun the '36, even giving away 400 lbs. and 21 cubic inches.

mfirth · 06-01-2023, 06:55 AM

Sponsored Links (Register now to hide all advertisements)

When I first got my 53 with an 8BA it had 8CM heads with domed pistons. It ran better that way than it did after a rebuild with Offy heads and flat top pistons. The .o45 to .050 is far more important than which head you have.

Ol' Ron · 06-01-2023, 08:11 AM

If you read my book I go into this. The 6.8/7.2 is a figment of some ones imagination, star measuring!
Gramps

scicala · 06-01-2023, 10:06 AM

So my question never really got answered. Let me re-word it. Did the EAB heads originally have smaller chambers than the EAC heads ?

Sal

38 coupe · 06-01-2023, 08:17 PM

On paper EAB heads have smaller combustion chambers than EAC heads.
My experience is in line with everyone else's on this thread. Tight piston to head clearance will be felt in the seat-of-the-pants, simple head swapping is a total crap shoot.

cadillac512 · 06-01-2023, 10:33 PM

Quote:

Originally Posted by scicala

So my question never really got answered. Let me re-word it. Did the EAB heads originally have smaller chambers than the EAC heads ?

Sal

Yes. The EAB heads are approx 71cc vs EAC at 76cc. EAC heads kept the compression ratio of the 4" stroke Merc engine the same as the 3.75" stroke Ford with EAB's.

Terry

scicala · 06-02-2023, 04:37 PM

Quote:

Originally Posted by cadillac512

Yes. The EAB heads are approx 71cc vs EAC at 76cc. EAC heads kept the compression ratio of the 4" stroke Merc engine the same as the 3.75" stroke Ford with EAB's.

Terry

Thank you Terry.

Sal

05-31-2023, 06:06 PM	#1
46fatford Senior Member Join Date: Jun 2010 Location: Indiana Posts: 338	Mercury head comp ratio 8CM vs EAC I have read conflicting information about the compression ratio given with each of these heads. I currently have a 1950 8CM mercury with standard bore and 4 inch stroke crank. I was wondering if the EAC heads would yield a higher compression ratio than the 8CM heads. I know the EAB Ford heads would definitely yield a higher compression ratio, but I like the look of the mercury script on the heads especially since it's in a 1950 mercury coupe. Are the EAC heads like the EABs when added to a 4 in crank mercury engine, or is the compression change nominal enough to not warrant changing to EACs at all? __________________ "..Nothin' outrun my V8 Ford" (Chuck Berry) S.M.I.B.

05-31-2023, 06:25 PM	#2
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,319	Re: Mercury head comp ratio 8CM vs EAC "Basic Flathead Theory" : Measure the clearance over the piston (with clay or aluminum foil balls). Have the heads milled to give a nominal .045 over the piston. Use a die grinder (or even a Dremel) to remove any irregularities and bring the chamber volume to consistency. Done. This "willy-nilly" replacing heads or milling a set amount will not yield consistent results. I have done 5 sets of heads (2 aftermarket and 3 stock) and have found variations between heads in the same set. The last set of Edmunds heads I did needed .035" off of one and .025 off the other to bring them to consistancy. (Yes, aftermarket heads need to be custom fitted, too.) Stock heads can have even greater variations. Custom fit your heads guys; it will yield the best results (both in power and fuel economy). It's also kinda fun and makes you feel like a real "Hot Rodder".

05-31-2023, 08:10 PM	#3
cadillac512 Senior Member Join Date: Apr 2019 Location: Kansas Posts: 921	Re: Mercury head comp ratio 8CM vs EAC Yes, EAC heads will raise compression over 8CM heads. How much? Approx .4 to .5 depending on engine size. The 8CM are about 83 cc vs about 76cc for EAC. Tubman's advice is sound and will yield very positive results. It's tedious and fiddly but the benefits are real...not huge, but real. Terry __________________ "It don't take but country smarts to solve the problem" (Smokey Yunick) '41 Merc Town Sedan / 260" 8CM engine '66 Fairlane four door / "warmed up" 302

05-31-2023, 10:25 PM	#5
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,319	Re: Mercury head comp ratio 8CM vs EAC One problem. Even the newest "EAB" and "EAC" heads are over 70 years old. Unless you find a truly untouched engine and remove them yourself, you can never be sure that the heads have not been resurfaced or otherwise modified. Thus, most of the time you can never be sure of what you have. In my opinion, someone putting another set of heads on an engine without knowing where the clearances are may be making a big mistake. In my experience, optimum combustion chamber turbulence (quench) yields more benefit interms of performance than the increased compression. The first flathead I built over 30 years ago was a teenage magazine-readers dream : a 276" merc with a Holley 390, an MSD ignition, and a set of new Offenhauser heads. I just took the heads out of the box and bolted them on. Between the SBC ignition curve in the MSD and the obvious lack of combustion efficiency with the unmodified heads, I was never really happy with the way that engine performed. I ended up selling it (and the '36 3-window it was in) after 5 years. I have a 255ci Merc with carefully fitted Edmunds heads, a small base 2G, and a Mallory dual point in my current '51 club coupe. I am very happy with the way it performs and believe that the '51 would outrun the '36, even giving away 400 lbs. and 21 cubic inches.

06-01-2023, 06:55 AM	#6
mfirth Senior Member Join Date: Aug 2017 Location: ohio Posts: 986	Re: Mercury head comp ratio 8CM vs EAC Sponsored Links (Register now to hide all advertisements) When I first got my 53 with an 8BA it had 8CM heads with domed pistons. It ran better that way than it did after a rebuild with Offy heads and flat top pistons. The .o45 to .050 is far more important than which head you have.

06-01-2023, 08:11 AM	#7
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,860	Re: Mercury head comp ratio 8CM vs EAC If you read my book I go into this. The 6.8/7.2 is a figment of some ones imagination, star measuring! Gramps

06-01-2023, 10:06 AM	#8
scicala Senior Member Join Date: Jun 2010 Location: Detroit suburb, MI Posts: 3,706	Re: Mercury head comp ratio 8CM vs EAC So my question never really got answered. Let me re-word it. Did the EAB heads originally have smaller chambers than the EAC heads ? Sal

06-01-2023, 08:17 PM	#9
38 coupe Senior Member Join Date: May 2010 Location: South Texas Posts: 1,631	Re: Mercury head comp ratio 8CM vs EAC On paper EAB heads have smaller combustion chambers than EAC heads. My experience is in line with everyone else's on this thread. Tight piston to head clearance will be felt in the seat-of-the-pants, simple head swapping is a total crap shoot.