390 compression ratio...What?
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Lead Driver
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From: Albuquerque
390 compression ratio...What?
Hey guys,
How do I find the compression ratio to my 390? Tell me what I need to provide...and I will try to get it for you.
1976 F-150 4x4 390 reman. from local FORD dealership. Do have some upgrades...what do you need to know???
Thanks a bunch!!!
How do I find the compression ratio to my 390? Tell me what I need to provide...and I will try to get it for you.
1976 F-150 4x4 390 reman. from local FORD dealership. Do have some upgrades...what do you need to know???
Thanks a bunch!!!
Elder User
Joined: Sep 2004
Posts: 863
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From: Grain Valley Mo.
Biz...I'd like to know the answer to that also. I have the same truck 390, same year even. I hear they are very low compression but when I do a compression test I get 175 psi. I woulda guessed that to be at least 9 to 1.
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Elder User
Joined: Feb 2005
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so true. Pull the head, measure the stroke, measure top of piston to deck height, cc the pistons, cc the heads, know the head gasket thickness....calculate...lot's of fun...LOL. The stock truck 390 is about 8 to 1....I've heard anything from 7.9 to 8.2.
Good Luck,
Tracy
Good Luck,
Tracy
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Lead Driver
Joined: Sep 2003
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From: Albuquerque
Ok...so does the cam effect the comp ratio? Or valve springs? Intake manifold? Pushrods? Timing chain? Water pump? Carb? FelPro gaskets, etc. Truck runs great on 86 octane...in case that matters.
Let's assume my 390 was rebuilt with "stock" pistons...and the "stock" valves. After all, I did order a "stock" 390. The only mods I have done to the engine are what's listed above...
Does that help any??? I really do not want to pull the head to find this out...especially since they will need to mill it (or at least check if it needs to be milled)...always told that was a good rule of thumb to have checked.
I'm just looking for something that might be close...
Let's assume my 390 was rebuilt with "stock" pistons...and the "stock" valves. After all, I did order a "stock" 390. The only mods I have done to the engine are what's listed above...
Does that help any??? I really do not want to pull the head to find this out...especially since they will need to mill it (or at least check if it needs to be milled)...always told that was a good rule of thumb to have checked.
I'm just looking for something that might be close...
Posting Guru
Joined: Jul 2003
Posts: 2,232
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From: Colville, Washington
Static compression ratio is not affected by anything you add to the outside of the engine. It is a calculated number that depends on the bore, stroke, head chamber volume, head gasket compressed volume, piston deck height, piston head style(dome, flattop, dished) and likely a couple other things I forgot
!! The camshaft will affect the effective or running compression which can change the compression test results. ie. a long duration cam will cause lower readings than a short duration cam all other variables being equal.
I guess the short answer is we can't do it without those values I mentioned.
!! The camshaft will affect the effective or running compression which can change the compression test results. ie. a long duration cam will cause lower readings than a short duration cam all other variables being equal.I guess the short answer is we can't do it without those values I mentioned.
Post Fiend
Joined: Mar 2004
Posts: 6,056
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From: Union, Washington
Compression ratio is the difference in the cylinder's volume at Bottom Dead Center and Top Dead Center. 1000CC at Bottom Dead Center and 100cc at Top Dead Center = 1000 divided by 100 = 10 for a 10 to 1 compression ratio. The cam can change cylinder pressures, but not compression ratio.
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Lead Driver
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From: Albuquerque
Originally Posted by Bear 45/70
Compression ratio is the difference in the cylinder's volume at Bottom Dead Center and Top Dead Center. 1000CC at Bottom Dead Center and 100cc at Top Dead Center = 1000 divided by 100 = 10 for a 10 to 1 compression ratio. The cam can change cylinder pressures, but not compression ratio.
Thanks guys! We are close...
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