jd
78 ford 400 Compression Ratio
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Junior User
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From: quakake, pa
78 ford 400 Compression Ratio
Hey guys i have a question that i just cant figure out. im trying to figure out my current compression ration on my 400. bin on the computer searching for a couple hours an i cant really come up wit anything. theres gotta be some good guys that no what there talkin about on here so i can get this sraight an choose another cam. everythings stock except the pistons which i have not a clue what kind they are but there stamped .040 on them. so heres what i got so far.
these are the exact pistions that are in the truck
78 ford 400
Bore:4.040 bored.040 over
stroke:4.0
Deck Height:10.267 from this 351 Cleveland & Ford Block Specs
head gasket:
Bore 4.100''
Compressed Thickness .041''
Compressed Volume 9.0cc
Fel Pro 1011-1 Fel-Pro Wire Ring Cylinder Head Gaskets
Combustion chamber CC :78.4
M-Block 351M/400 Parts Reference
piston volume cc: this is the one that i cant find out for the life of me
If anyone can help i would greatly appropriate it. i had this motor appart once already to do the top end which was only done on a tight budge.
thanks
Ron
these are the exact pistions that are in the truck
78 ford 400
Bore:4.040 bored.040 over
stroke:4.0
Deck Height:10.267 from this 351 Cleveland & Ford Block Specs
head gasket:
Bore 4.100''
Compressed Thickness .041''
Compressed Volume 9.0cc
Fel Pro 1011-1 Fel-Pro Wire Ring Cylinder Head Gaskets
Combustion chamber CC :78.4
M-Block 351M/400 Parts Reference
piston volume cc: this is the one that i cant find out for the life of me
If anyone can help i would greatly appropriate it. i had this motor appart once already to do the top end which was only done on a tight budge.
thanks
Ron
Fleet Owner
Joined: Dec 2009
Posts: 20,678
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From: NorCal
'took up your question as a challenge and also couldn't come up with an answer even after looking up the specs in the Federal-Mogul catalog.
Federal-Mogul Resource Center - Part Details
The specs page however does state a 7.9:1 compression...
Btw, that PN is a .030 over piston.
Federal-Mogul Resource Center - Part Details
The specs page however does state a 7.9:1 compression...
Btw, that PN is a .030 over piston.
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Junior User
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From: quakake, pa
Thanks hio i appreciate it. Yea i no it was a .030 over i was just to give you guys a under standing of wat it looks like. But idk what i should do the cam i would like to get i need a 9:1 compresstion ratio which i doubt that just .040 would do that
Fleet Owner
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From: NorCal
To trick the engine into more compression with the cam, the intake valves would have to close sooner. I'm no cam expert so I think ya ought to contact CompCams and get their input on your build. Their site has a provision to send an email or even discuss it in a forum.
COMP Cams® - Technical Support and Contact Page
Logistics Pro
Joined: Feb 2011
Posts: 3,522
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From: Townsend, MT
False! There will be a greater amount of air drawn in to be compressed. Every .030 over adds about 10% of a point.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
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From: NorCal
You're both wrong. 
Compression ratio is the ratio of cylinder volume with the piston in the bottom of the bore divided by the cylinder volume with the piston at the top of the bore.
When you bore the cylinder it's volume is larger. Your volume increases when the piston is at the bottom of the bore a lot more than it increases with the pistion at the top of the bore. If the piston comes to deck height or higher then the volume with the piston at the top of the bore does not increase, but the volume with the piston at the bottom is larger. That makes the compression ratio higher with a bored cylinder.
Compression ratio is the ratio of cylinder volume with the piston in the bottom of the bore divided by the cylinder volume with the piston at the top of the bore.
When you bore the cylinder it's volume is larger. Your volume increases when the piston is at the bottom of the bore a lot more than it increases with the pistion at the top of the bore. If the piston comes to deck height or higher then the volume with the piston at the top of the bore does not increase, but the volume with the piston at the bottom is larger. That makes the compression ratio higher with a bored cylinder.
Post Fiend
Joined: Mar 2011
Posts: 8,501
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From: Lynchburg, VA
Just as an example, If your engine operates at a volumetric efficiency of 75%, and each cylinder is 500cc, the amount of air drawn in would equate to 375cc at atmospheric pressure. If you increased the cylinder volume by 10%, to 550cc, and the volumetric efficiency stayed at 75%, you'd then be drawing in the equivalent of 412.5cc. So for a cylinder volume increase of 10%, the increase of air volume would be 9.09%.
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Junior User
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From: quakake, pa
Thanks guys for the info regardless i think comp will tell me i gotta find the comp ratio before they give me any info i talked to them on my first build an they gave me the cam i have now an i calc the overlap an its only 1. The thumper cam that needs has a overlap of 35.5 which will be a big step but i have a good knowlege with motors. Bin around them since i can remember. We dont take out cars or toys to the dealer we figure out the prob an fix it.
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Junior User
Joined: Aug 2012
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From: quakake, pa
Hey guys i just wanted to let us no i used summits cr cal just to ballpark it an it cam up 8.5:1 i no this might not be right but lets just say it was. My other question now is what is the best way to bump that 8.4:1 to 9:1?
Fleet Mechanic
Joined: Jan 2012
Posts: 1,668
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From: Saskatchewan
As far as I know TMyer inc has the only flat top pistons for a 400. I believe it put mine around 9.3:1. There are other ways to increase comp ratio, I think there are some Aussie cleavland heads that bump it up too? Check out the 335 engine forum here. It's a great place to learn about 400s. Good luck to you.
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