390 Piston question
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Elder User
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390 Piston question
The pistons on my mid 70's rebuilt by ATK 390 come up to about 1/8" below the deck of the block at TDC. Is this normal? If so, any guess as to the compression ratio with stock heads? One head has 76 stamped on it.
Post Fiend
Joined: Mar 2004
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From: Union, Washington
Probably because your engine is a low compression model in the 8.5 to 1 range.
FE "Freakin Expensive"
Joined: Dec 2003
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From: Smith Mountain Lake, VA
Thats why Block decking and or measuring your crank throw, rod length and Piston C.D. is very important in a build when wanting to bring the CR up to a decent range.. But then again I dont know what CR you were shooting for ? Maybe your gonna hang a blower on it ? LOL
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.120 down the hole. Just like a 360. You measure that stroke?
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Are you considering fixing this? Car pistons for a 390 2bbl, or truck 360 pistons may be an answer....
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
Just how would the lower compression pistons help anything, except to lower his compression even more?
Super Moderator
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Notice I said "may be the answer".
If he has 8.5 or so CR now, he may get a higher CR with those other pistons, depending on cylinder head volume. 2bbl car 390s were about 9.5, and others on this board report about 9.5 using 360 pistons with the 390 crank and rods.
More research and figuring of the compression height etc would be in order.
As always, a proven solution would be good, but we're dealing with limited info.
If he has 8.5 or so CR now, he may get a higher CR with those other pistons, depending on cylinder head volume. 2bbl car 390s were about 9.5, and others on this board report about 9.5 using 360 pistons with the 390 crank and rods.
More research and figuring of the compression height etc would be in order.
As always, a proven solution would be good, but we're dealing with limited info.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Notice I said "may be the answer".
If he has 8.5 or so CR now, he may get a higher CR with those other pistons, depending on cylinder head volume. 2bbl car 390s were about 9.5, and others on this board report about 9.5 using 360 pistons with the 390 crank and rods.
More research and figuring of the compression height etc would be in order.
As always, a proven solution would be good, but we're dealing with limited info.
If he has 8.5 or so CR now, he may get a higher CR with those other pistons, depending on cylinder head volume. 2bbl car 390s were about 9.5, and others on this board report about 9.5 using 360 pistons with the 390 crank and rods.
More research and figuring of the compression height etc would be in order.
As always, a proven solution would be good, but we're dealing with limited info.
Super Moderator
Joined: Apr 2004
Posts: 34,472
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So what is driving the CR train here? If Ford listed these engines at 9.5 and 10:1 when new, what, other than combustion chamber size, which we do not know, prevents the same result today?
This does not include the 360, but shows no 390 piston down the hole like the OP describes.
http://www.woodyg.com/fairlane/finfo/fespechart2.html
This does not include the 360, but shows no 390 piston down the hole like the OP describes.
http://www.woodyg.com/fairlane/finfo/fespechart2.html
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
So what is driving the CR train here? If Ford listed these engines at 9.5 and 10:1 when new, what, other than combustion chamber size, which we do not know, prevents the same result today?
This does not include the 360, but shows no 390 piston down the hole like the OP describes.
http://www.woodyg.com/fairlane/finfo/fespechart2.html
This does not include the 360, but shows no 390 piston down the hole like the OP describes.
http://www.woodyg.com/fairlane/finfo/fespechart2.html
Oh and your chart is for the engines built in the 1960's, not the 70's.
