Help with Compression Ratios
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Senior User
Joined: Apr 2012
Posts: 425
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From: Fort Worth, Texas
Help with Compression Ratios
OK so I started just trying to build something inexpensively and close to stock. Well I've kinda blown that out of the water now and have some questions for the Gurus here.
I am using the 35-349-8 comp cam with roller lifters and probably 1.7:1 roller rockers. I just ordered a set of Ford Racing X306 heads with a 64cc combustion chamber. I'd like to land somewhere near 10:1 or a little less to avoid super high octane gas. Did I screw up with the 64cc heads, should I have gotten the 58's?
When I punch everything in the compression calculator I get 11:1 with flat pistons and 8.9:1 with dipped pistons (not sure what stock is here so I used 19.5).
Here are the values I used, open to corrections as some were defauly on the calculator I used.
4" pistons
3.5" stroke
4.03 Head gasket bore
.o21 compressed head gasket thickness
64cc Combustion Chamber
-19.5 piston dip and 0 for flat tops (This I am unsure of and is critical in accurate numbers)
0.016 Piston Deck Clearance
I am using the 35-349-8 comp cam with roller lifters and probably 1.7:1 roller rockers. I just ordered a set of Ford Racing X306 heads with a 64cc combustion chamber. I'd like to land somewhere near 10:1 or a little less to avoid super high octane gas. Did I screw up with the 64cc heads, should I have gotten the 58's?
When I punch everything in the compression calculator I get 11:1 with flat pistons and 8.9:1 with dipped pistons (not sure what stock is here so I used 19.5).
Here are the values I used, open to corrections as some were defauly on the calculator I used.
4" pistons
3.5" stroke
4.03 Head gasket bore
.o21 compressed head gasket thickness
64cc Combustion Chamber
-19.5 piston dip and 0 for flat tops (This I am unsure of and is critical in accurate numbers)
0.016 Piston Deck Clearance
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Well you didn't say what exactly you're starting with here, other than it's a 351. The stock combustion chambers on E7 heads is 64cc (this is a "nominal" figure, it can vary + or - 2-3 ccs) The figure you're giving for the head gasket thickness is about half what most head gaskets are (they're in the .040 range) So if you had E7's on it before, and you go with the stock replacement pistons for your year's motor, the comp ratio will not change, unless you're boring it over size, then it will change slightly, like a quarter point at most. Just as a reference, the 69 351W 4 bbl motor had a 10.5 to 1 ratio, that was with flat tops (if I'm not mistaken) and 59 cc chambers.
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Senior User
Joined: Apr 2012
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From: Fort Worth, Texas
I did mention I ordered the Ford Racing Aluminum heads with 64cc combustion chambers. In fact that was one of the questions if I should have gotten the 58cc heads and used the factory pistons with the dips in them. Another question was what is the volume of the factory dip in these pistons.
Here is the calculator I am using.
Engine Compression Ratio (CR) Calculator
Here is the calculator I am using.
Engine Compression Ratio (CR) Calculator
FTE Legend
Joined: Jun 2006
Posts: 31,930
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From: Ottawa, Ontario
Even "flattop" piston are going to have valve eyebrows.. they better or you can forget about using that cam with 1.7 rockers, so even these will have a certain dish volume and compression ratio won't be quite as high as the calculator suggests.
Also head gasket bore is going to be more like 4.1" and compressed thickness more like 0.035-0.040".
Also head gasket bore is going to be more like 4.1" and compressed thickness more like 0.035-0.040".
Thread Starter
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Senior User
Joined: Apr 2012
Posts: 425
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From: Fort Worth, Texas
Thank you Conanski and do you happen to know the dish volume on the stock pistons? Or where I can find pistons with a dish volume of -11cc?
By the way I am getting close to finishing this project. Ill send you a PM with a link to some pics when I get done.
By the way I am getting close to finishing this project. Ill send you a PM with a link to some pics when I get done.
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Postmaster
Joined: May 2008
Posts: 2,703
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From: La Vergne, TN
Even "flattop" piston are going to have valve eyebrows.. they better or you can forget about using that cam with 1.7 rockers, so even these will have a certain dish volume and compression ratio won't be quite as high as the calculator suggests.
Also head gasket bore is going to be more like 4.1" and compressed thickness more like 0.035-0.040".
Also head gasket bore is going to be more like 4.1" and compressed thickness more like 0.035-0.040".
but still for the OP..
you need to check PTV clearance no matter what.. i did and had a ton of room..
and yeah you made a mistake by going 64cc heads.. personally feel like ya made a mistake by going Ford x306 head....
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Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Those are the heads you ORDERED, not what you have NOW. Pistons with those dishes ? Hard to beat Silvolite, or SpeedPro. There were several dish volumes used over the time period the 351W was produced, Silvolite & SpeedPro offer em all.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
.030 over 351(731,2 cc per cylinder swept volume), 11 cc dished pistons (assuming a zero deck here, anything sort of that will reduce the ratio) 9.5 head gasket volume (this is what a .043 gasket works out to be) and 64 cc heads = 9.66 to 1.
Thread Starter
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Senior User
Joined: Apr 2012
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From: Fort Worth, Texas
mine have valve reliefs and 58 cc heads with that cam and 1.72's and have no clearance issues.. we've had this discussion before conanski..
but still for the OP..
you need to check PTV clearance no matter what.. i did and had a ton of room..
and yeah you made a mistake by going 64cc heads.. personally feel like ya made a mistake by going Ford x306 head....
but still for the OP..
you need to check PTV clearance no matter what.. i did and had a ton of room..
and yeah you made a mistake by going 64cc heads.. personally feel like ya made a mistake by going Ford x306 head....
What ratio should I be aiming for I thought mid 9's was my target? I am looking for 300-325 HP and around 400 ftlbs of torque. Its a daily driver for now but someday ma hot rod it out when my kids are out of college. I'd like to also be able to run most any gas but md grade isnt out of the question.
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Senior User
Joined: Apr 2012
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From: Fort Worth, Texas
Well I feel really stupid at this point as my head just got here and I just realized 529 was for 1 head. They will take it back so if not the Ford Racing heads what do you suggest? That puts 1000 dollars into play for a good set of heads.
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Senior User
Joined: Apr 2012
Posts: 425
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From: Fort Worth, Texas
Everything I read says there are better options out there for the same money. Me personnally I just want the best bang for my buck. At 529 a pair that was hard to believe but at 1060 a pair its quite possible. I am also looking at the Edelbrock 5023's for 939 a pair with a 60cc combustion chamber. The Edelbrocks come with studs over pedastal for rocker mounts.
Honestly I have no clue I am just spit balling and trying to read all I can before I bolt it together. Plans are to finish the machine work next week, then start assembly the next week and drop it in around the end of the month. Take her to the dyno first week of April for a custom tune.