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By slick56 - 12 Years Ago
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How are the compression ratios arrived at with these heads? On J Mummert's site, the 272 2bbl has a ratio of 8:1, the 292 4bbl has 8.4:1 and the 312 has 9:1. Are the heads actually different, or does piston/deck height on the different engines determine compression?
Reason I ask is I have recently acquired a C2 truck engine with ECZ-C heads.
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By The Horvaths - 12 Years Ago
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It's (theoretically) the ratio of max chamber volume to min chamber volume.
So, assuming that the max chamber volume changes by a significant percentage and min is very close to the same value, compression ratio will increase. Thus the differences.
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By Hollow Head - 12 Years Ago
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ECZ-C's are good for turbo engines. We went 8.885 on a quartermile this summer with set of those heads. Not bad for a heads that everybody says not to use in performance engines and lot's of compression or boost  .
http://www.youtube.com/watch?v=8_jATbrygbU
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By charliemccraney - 12 Years Ago
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Keeping it simple, compression is (Cylinder Volume + Combustion Chamber Volume) / Combustion Chamber Volume
There are different heads with different sized combustion chambers. With the same displacement, a larger chamber decreases compression. A smaller chamber increases compression. Or with the same heads, a smaller cylinder decreases compression and a larger cylinder increases compression.
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By The Master Cylinder - 12 Years Ago
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The Horvaths (8/26/2013)
It's (theoretically) the ratio of max chamber volume to min chamber volume. So, assuming that the max chamber volume changes by a significant percentage and min is very close to the same value, compression ratio will increase. Thus the differences.
HUH???
charliemccraney (8/26/2013)
Keeping it simple, compression is (Cylinder Volume + Combustion Chamber Volume) / Combustion Chamber Volume
Hence, with the larger Cylinder Volume with the larger engine with the combustion chamber staying the same size, the compression ratio increases.
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By jrw429 - 12 Years Ago
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If we start with
the 272 2bbl has a ratio of 8:1, the 292 4bbl has 8.4:1 and the 312 has 9:1.
and then plug into the formula (displacement + combustion chamber volume) / combustion chamber volume = compression, we get
(272 + X) / X = 8 or a combustion chamber volume of 38.86. The 292 would be 39.46. The 312 would be 38.52.
So math says something must be different. If I didn't mess up. Or it could be that "272" etc. is not accurate enough to use in this calculation...
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By pegleg - 12 Years Ago
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You're getting there, You'd also need the volume of the head gasket. your result is then divided by the amount of the chamber, gasket, deck volume and whatever valve cutouts or dishes are in the piston.
In other words you divide the total amount of volume at top dead center into the total volume at bottom dead center. Clear as mud?
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By The Master Cylinder - 12 Years Ago
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jrw429 (8/26/2013)
we get
(272 + X) / X = 8 or a combustion chamber volume of 38.86. The 292 would be 39.46. The 312 would be 38.52.
So math says something must be different. If I didn't mess up. Or it could be that "272" etc. is not accurate enough to use in this calculation...
Yuummm, Ima thinkin' it's not an exact science? More like a ballpark figure?? 
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By Doug T - 12 Years Ago
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Of course Cyl head volume is usually expressed in cc which stands for Cubic Centimeters. The engine displacement is expressed in cubic inches 1 cubic inch is 16.39 cc's. the usual volume of Y cyl heads is ~ 75 cc's which is 4.57 cubic inches. The units must be consistant for the formulas to work.
Or you could just buy my set of G heads
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By The Master Cylinder - 12 Years Ago
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jrw429 (8/26/2013)
If we start with the 272 2bbl has a ratio of 8:1, the 292 4bbl has 8.4:1 and the 312 has 9:1.
and then plug into the formula (displacement + combustion chamber volume) / combustion chamber volume = compression, we get
(272 + X) / X = 8 or a combustion chamber volume of 38.86. The 292 would be 39.46. The 312 would be 38.52.
So math says something must be different. If I didn't mess up. Or it could be that "272" etc. is not accurate enough to use in this calculation...
Jim, I just reread your post. You have to use the displacement of one cylinder in the formula, not all eight.
And yes Doug. Good point. You have to be consistent with the units of measure.
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By jrw429 - 12 Years Ago
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Actually, I used the whole engine displacement. Divide by 8 for one cylinder. So for an engine displacing 272 cubic inches, whole engine combustion chamber volume (at 8.0:1 compression) works out to 38.86 cubic inches. Thats 4.86 cubic inches per cylinder. Or 79.6 cubic centimeters.
If you know the displacement (difference between total volume at BDC minus total volume at TDC), and if you know the compression ratio, then you can calculate the combustion chamber volume (aka volume at TDC). You don't know how much of that is attributable to cylinder head volume or gasket thickness or how far above or below the top of the block the piston travels.
In practice, you are calculating compression ratio and all the other bits are known or need to be determined. So this calculation may seem weird. But I think it is correct.
And the original question was - is there a difference between heads on the different engines. The calculations show different combustion chamber volumes, so it seems that yes, there was a difference. Beyond the calculations, I too am interested in if there really was a difference in heads or not. And why the numbers work out as they do.
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By PF Arcand - 12 Years Ago
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A side note on all these cylinder volume calculations. According to John Mummert, most of Fords advertised compression ratios on Y-Blks were optimistic by about 9 percent. As for ECZ-C heads, Mummert says the main performance limitation with them is that the lower port will not flow as good as the upper, even with a good porting job. However, they are not prone to combustion chamber failures like sometimes occurs with early "G" heads, particularly if they are milled very much.
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By The Horvaths - 12 Years Ago
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My apologies if I over-simplified it. It's that whole day-job-gets-in-the-way-of-taking-time-to-formulate-an-intelligent-answer thing.
One thing to take into account when trying to nail the ratio down, as mentioned before, is head gasket volume. Were they doing the original calculations with a specific head gasket in mind? If so, did they match head gasket opening to cylinder diameter or to a universal-fit gasket? In other words, a head gasket that just fits a 292 or 312 would cause a greater minimum chamber volume on a 272 due to its diameter being much greater than the cylinder diameter. It never fails, I need to go to my day job so I don't have time to poke some figures into a spreadsheet.
OK, I took the time to poke numbers into a spreadsheet and, using 73cc for chamber volume on the heads and a 0.04" head gasket thickness, I came up with the following (assuming head gaskets meet bore diameter, which is an unwise assumption):
272 = 7.97/1
292 = 8.44/1
312 = 8.94/1
That's close enough for government work.
BTW: another pair of unsafe assumptions were used to get those numbers. Those are a deck height of zero and a piston top volume of zero.
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By Y block Billy - 12 Years Ago
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Dont forget the 312 has a longer stroke than the 292-272 so you are compressing more air not only in diameter of the cylinder but the stroke also!
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By pegleg - 12 Years Ago
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Jim, The heads are all the same. Check out Mummert's site, I think he has specs for most Ford y heads.
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By Ted - 12 Years Ago
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Simply increasing the engines cubic inch displacement will increase the compression ratio with everything else remaining equal. Something as simple as boring the engine 0.030” over and insuring the piston has the same deck height will increase the compression ratio.
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By DANIEL TINDER - 12 Years Ago
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pegleg (8/27/2013)
Jim, The heads are all the same.
I believe some U.S. 272s came with heads that had been milled at the factory, so C/R matched the 292 (?).
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By Ted - 12 Years Ago
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Daniel is correct in that some 272’s did come with factory milled heads which creates some confusion in regards to combustion chamber volumes on those particular heads. One of those head casting numbers were the ECL-B heads where they were factory milled ~0.035” for use on the 1955 182HP Special 272’s versus no milling for the 1955 292 Thunderbird engines. Both engines were rated at 8.5:1 CR.
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