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post #1 of 8 (permalink) Old 03-21-2010, 03:32 PM Thread Starter
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FE compression ratio question

i recently built a 390 fe and have a question about the compression ratio. my mechanic, who has never done an fe, said that the piston at top dead center is about an 8th of an inch down in the cylinder and he thinks the motor is going to have bad compression. he told me this after he put my heads on so i wasn't able to take any measurements and see. the motor was a 68 mercury 4 barrel and i did some research and found out they came with 10.5:1 compression. the motor was rebuilt a year ago by the 2nd last owner so i dont know if he put in low compression pistons or something like that. i know for sure that it was bored out 30 thousandths and a mild cam was put in as far as i know the pistons and crank are just like stock other than the larger bore. so do all the fe's have the pistons set down that far or will mine have lower compression? any info would be great thanks
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post #2 of 8 (permalink) Old 03-21-2010, 03:58 PM
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Re: FE compression ratio question

you let someone that never did a FE build your engine -- you are a brave -brave soul-

theres some serious oiling issues to take care of when building a FE engine--

Democracy is two wolves and a lamb discussing what's for dinner. Liberty is a well armed lamb willing to contest the majority decision".
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post #3 of 8 (permalink) Old 03-21-2010, 04:03 PM
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Re: FE compression ratio question

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Originally Posted by rpadget View Post
so do all the fe's have the pistons set down that far or will mine have lower compression? any info would be great thanks
No they don't all have the piston down that far. Sounds like you have the later truck type pistons. Probably close to 7.5 comp (depending on a lot of other things).
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post #4 of 8 (permalink) Old 03-21-2010, 05:34 PM Thread Starter
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Re: FE compression ratio question

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Originally Posted by Rich27028 View Post
you let someone that never did a FE build your engine -- you are a brave -brave soul-

theres some serious oiling issues to take care of when building a FE engine--
o really what kind of oiling issues?
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post #5 of 8 (permalink) Old 03-21-2010, 05:44 PM Thread Starter
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Re: FE compression ratio question

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Originally Posted by Rich27028 View Post
you let someone that never did a FE build your engine -- you are a brave -brave soul-

theres some serious oiling issues to take care of when building a FE engine--
i didnt do a real crazy build just heads cam and intake. it is supposed to make around 425 horse. i thought with a mild build like that i wouldnt have to worry about stuff like that?
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post #6 of 8 (permalink) Old 03-22-2010, 12:12 PM
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Re: FE compression ratio question

425 horse- no way in hell-maybe 350 -maybe
with the pistons down in the bore that far - something is very wrong--

Democracy is two wolves and a lamb discussing what's for dinner. Liberty is a well armed lamb willing to contest the majority decision".
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post #7 of 8 (permalink) Old 03-22-2010, 12:47 PM
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Re: FE compression ratio question

I can only say that if I was building an engine for someone and their choice of piston/rod put the piston top 1/8" down in the hole I'd sure as h$$$ call before bolting any heads on the motor. FEs are a special animal and there are several issues that require careful attention. I can't tell you exactly what because I don't do FEs. Oiling is a top end issue and usually requires a restrictor of some sort in the oil feed to the top, also the rocker shaft bolts have to be in certain locations. For learning, you might want to visit fordfe.com, join and ask specific questions there too.

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post #8 of 8 (permalink) Old 03-22-2010, 01:04 PM
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Re: FE compression ratio question

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Originally Posted by Rich27028 View Post
425 horse- no way in hell-maybe 350 -maybe
with the pistons down in the bore that far - something is very wrong--
X2. If the pistons are that far down, they are most likely the mid '70s truck piston.
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