390 compression ratio...What? - Ford Truck Enthusiasts Forums



390 compression ratio...What?

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Old 08-02-2005, 11:48 PM
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Talking 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!!!
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Old 08-03-2005, 12:07 AM
xfordman xfordman is offline
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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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Old 08-03-2005, 12:09 AM
Randyb12 Randyb12 is offline
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you need to call the dealership you got it from and ask them. There are so many things that affect CR there is no way to tell you unless you remove the heads and take alot of measurements.
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Old 08-03-2005, 12:16 AM
FalconStng FalconStng is offline
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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
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Old 08-03-2005, 12:30 AM
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Talking

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...
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Old 08-03-2005, 12:42 AM
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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.
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Old 08-03-2005, 12:45 AM
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Bear 45/70 Bear 45/70 is offline
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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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Old 08-03-2005, 01:03 AM
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Well said Bear! I'm too long winded!!! HAHA
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Old 08-03-2005, 01:17 AM
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Talking

Quote:
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.
So, then my 390 is completely "stock" when it comes to the compression ratio...if that is true, then what was the "stock" compression ratio of a 1976 390 truck engine? Is it as low as 8 to 1? Or 9.5 to 1?

Thanks guys! We are close...
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Old 08-03-2005, 01:38 AM
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Most of the light truck 390 engines ran 8.6 to 1, while the car 390's ran about 9.5 to 1 for 2 barrel engines.
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Old 08-03-2005, 12:04 PM
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BINGO!!! Thanks Bear...
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