390 Engine Compression?
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Senior User
Joined: Mar 2003
Posts: 232
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From: Tampa Bay, FL
390 Engine Compression?
Can someone help?
Using '68 390 2v heads --- along with forged flat top pistons (.030) / valve reliefs.
Stock crank and rods ... any calculators out there for compression ratio?
Using '68 390 2v heads --- along with forged flat top pistons (.030) / valve reliefs.
Stock crank and rods ... any calculators out there for compression ratio?
Senior User
Joined: Sep 2000
Posts: 435
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From: Central Oregon
390 Engine Compression?
You have to do some homework, but the www.kb-silvolite.com has a good calculator section. You can take a stab at it with data from the Christ' "Building Big Block Fords" book.
Senior User
Joined: Mar 2003
Posts: 231
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From: Utah
390 Engine Compression?
If I were to guess I'd say about 9.5:1
You'll need to know several factors to figure it out:
bore
stroke
compression height
deck clearance
head gasket thickness (smashed)
valve releif cc's
chamber volume
You'll need to know several factors to figure it out:
bore
stroke
compression height
deck clearance
head gasket thickness (smashed)
valve releif cc's
chamber volume
Freshman User
Joined: Jun 2003
Posts: 36
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390 Engine Compression?
I have the same set up as you and have been trying to figure out the same thing. The stock motor would have 8.6:1 or 8.6:1. Like Chissler stated you need to know several factors. A book called High Performance Ford Engine Parts Interchange is good and can help. Like me you have probably realized that you need to cc your head and do several other measurements. It really is hard to know for sure. If this stuff was easy girls would be doing it.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
390 Engine Compression?
If the casting #s on those 68 heads are C8AE-H then your comp ratio should be around 10.5 to 1 ( 67-69cc chambers) the bigger 71-73 chambered heads will lower it down a little to around 9.75 or there about.
Freshman User
Joined: Jun 2003
Posts: 36
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390 Engine Compression?
I believe the compression ratio of a 2V is 9.5:1 and the 4V is 10.5:1. These ratios are for a stock engine.
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Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
390 Engine Compression?
The difference in the comp ratio, was due to the pistons. The 2 bbl could have either comp ratio in the cars , the 390 in my 68 Monterey had 10.5 to 1, while the motor I pulled from a 68 Galaxie was a 9.5 to 1 motor, both were 2 bbl. The difference was one had dished pistons and the other had flattops. Both had C8AE-H heads. If his replacement pistons have the same comp height as the factory flattops, then he'll get a 10.5 to 1 ratio
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