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By Neil - 9 Years Ago
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I had my old 272 Ford F-100 engine overhauled by a professional machine shop which constituted having to bore the cyclinders out .080 in order to clean them all up. Of course everything necessary was replaced as required. Lately I have decided to install a mild street cam in it judt for fun. When calling about what grind, duration and all that stuff I ran into needing to know the compression ratio. Can anyone tell me about what it should be. Other than having to bore the cyclinders to clean them up as far as I know everything else is OEM. I've calculated the C.I. to be "about" 336, does this sould about right?
Thank You
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By Barry L - 9 Years Ago
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The standard bore of the 272 is 3.62 , .080 overbore would give you a 3.7 bore which is under the standard 292 bore of 3.75, so for cubic inches, you would
be a bit under a 292, as long as the stroke is the same. Barry L
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By charliemccraney - 9 Years Ago
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You'll need to know which heads you have, too.
Your compression is going to be pretty low, best case with flat top pistons, about 8.35:1 and it could be significantly lower depending on the heads you have.
Since it is a mild cam you want, just take one of the entry level Isky or Mummert cams and it will be good enough. Lots of Ys are running around just like that.
Increasing compression will have a pretty significant cost because it will require different pistons, and possibly a different set of heads.
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By Joe-JDC - 9 Years Ago
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3.620 + .080 = 3.700" if you compute the cubic inches, it will be 3.700 x 3.700 x 3.300 x .7854 x 8 = 283.8561264, or 284 cubic inches. Joe-JDC
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By PF Arcand - 9 Years Ago
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Joe is correct on the displacement. You need to check the head casting letters, then link to Mummert's site to see which heads you have & the advertised compression ratios. Note the correction factors listed at the bottom. Cause your engine is bored out 0.080" the given ratio is likely fairly close. Then you could check the heads to see if they have been planed before? The heads have corner tabs that should be approx. 1" thick if they are stock. If stock you could have them milled to increase compression. As noted in the chart, some heads are not "posted" so some as noted will only accept moderate milling.. Hope this helps..
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By Ted - 9 Years Ago
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As has been brought up, there are some variables in calculating the compression ratio. Charlie’s numbers are a best case ‘high’ scenario so depending upon the cylinder heads being used and where the pistons are sitting in the bore at TDC, the compression ratio could be much lower.
Here’s some numbers that can give you an idea where your compression ratio could potentially be but this assumes a composition head gasket, the pistons being 0.025” in the hole, and 68 cc cylinder heads.
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By DryLakesRacer - 9 Years Ago
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The KB piston site has a calculation section. The one to determine comp ratio is excellent and I have used it many times for my Bonneville engine builds. The Dynamic ratio is always a little disheartening but true none the less. You do need to know all the specs Ted used in his explanation. Good luck.
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