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.
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.
Probably because your engine is a low compression model in the 8.5 to 1 range.
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Bear 45/70 Gimp Racing
“There are only two things stopping you, common sense and fear! And not much of the latter.”
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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Blue Saphire 70 F250 42? in the making/Rollerized C6/4:10 Locker
With a Secret BBF/TBA sometime??
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.
.120 down the hole. Just like a 360. You measure that stroke?
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.
Not gonna happen. The 390 2V pistons have a lower compression with the same length rods, hence a lower compression. Even the high compression 2V 390 pistons have no higher a wrist pin height than the 390 4V pistons. The 410 pistons will have an even higher pin height, hence a lower compression. The 360 pistons are known to have the same pin height as the 390 4V pistons.
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Bear 45/70 Gimp Racing
“There are only two things stopping you, common sense and fear! And not much of the latter.”
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.
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.
Well, first off the 70's engine were all low compression compared to the 60's engines(9.5 to 10.5 to 1) and used the pistons with the low compression height (piston pin mounted higher in the piston) to achieve the lower compression. Remember in the early 70's lead disappeared from gas an compressions plummeted. If memory serves (it don't always) the trick was preformed by using the 410 pistons in the 390, there by dropping the top of the piston down by .2".
Oh and your chart is for the engines built in the 1960's, not the 70's.
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Bear 45/70 Gimp Racing
“There are only two things stopping you, common sense and fear! And not much of the latter.”
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