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Old 11-13-2008, 05:08 PM
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Compression Ratio

I'm getting ready to order a Federal Mogul CSMHP773-311 engine rebuild kit from Summit Racing for my 454, but I don't know what the compression ratio will be with my heads. The kit lists that the approximate compression ratio is 8.69:1 with 116.9cc heads. After looking up info on my heads ( casting # 346236 ) I found that they are 120cc heads. Does anybody know what my compression ratio would be with this engine kit? I'm looking to build it around 8:1, it's gonna be used in my rock crawler so I'm thinking the lower compression ratio should help keep the engine temp down at low speed / higher RPMs.

I'm also thinking about using the COMP Cams Xtreme Energy 4x4 cam kit # COMP Cams K11-239-3. Would this be to much cam for a 8:1 compression ratio engine? Should I go with a Xtreme Energy 4x4 cam kit with less duration? I'll be running an automatic transmission with a stock stall speed. I will mostly be rock crawling, but will make a trip or two a year out to the dunes too & will need a little more RPMs out of it.

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Old 11-13-2008, 08:45 PM
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Quote:
Originally Posted by Tribal
I'm getting ready to order a Federal Mogul CSMHP773-311 engine rebuild kit from Summit Racing for my 454, but I don't know what the compression ratio will be with my heads. The kit lists that the approximate compression ratio is 8.69:1 with 116.9cc heads. After looking up info on my heads ( casting # 346236 ) I found that they are 120cc heads. Does anybody know what my compression ratio would be with this engine kit? I'm looking to build it around 8:1, it's gonna be used in my rock crawler so I'm thinking the lower compression ratio should help keep the engine temp down at low speed / higher RPMs.

I'm also thinking about using the COMP Cams Xtreme Energy 4x4 cam kit # COMP Cams K11-239-3. Would this be to much cam for a 8:1 compression ratio engine? Should I go with a Xtreme Energy 4x4 cam kit with less duration? I'll be running an automatic transmission with a stock stall speed. I will mostly be rock crawling, but will make a trip or two a year out to the dunes too & will need a little more RPMs out of it.
I did this the other day, while there's lot of compression ratio calculators out there, just google it. It's mighty helpful to know what goes into the computation, so here what I wrote a few days ago:

"Static Compression is Total Volume enclosed by the cylinder and head divided by the Compression Volume above the piston.

SCR=TV/CV

Total Volume consists of the sum of these elements:

- Cylinder Swept Volume (CSV), which is bore area times stoke. bore area is our old friend of 1/2 bore diameter, times itself (which is squared), times pi (3.1416 being close enough); that answer times the stroke gives what's called the cylinder's "swept volume". since you're probably working in U.S. measures you need to convert this to cubic centimeters (CCs) which is cubic inches times 16.4. All the following volumes for piston crown to deck clearance and head gasket thickness is computed the same way circular area times thickness times 16.4 gets to volume in CCs

- Piston Clearance Volume (PCV), which is bore area times the distance from the piston crown to the cylinder block deck.

- Head Gasket Volume (HGV), which is gasket bore area times the gasket thickness. The gasket bore is usually larger than the cylinder bore.

- Piston Head Volume (PHV), which is the positive or negative volume contained on the top of the piston. A dome makes a negative volume resulting a smaller compression volume and a higher ratio. A dish or valve pockets make for a positive volume resulting in a larger compression volume and a lower compression ratio. These volumes should be available in the technical specs from the piston manufacturer.

- The combustion chamber volume contained in the head is either from the manufacturer's technical information or direct measurement if you have the head a set of valves and a graduated beaker measured in CCs. A 100 milli-liter graduated cylinder is sufficient, easily available and inexpensive as opposed to a burette which is really expensive and provides an accuracy the average person doesn't need to the requisite data for computation. Keep in mind that manufacturer's almost always list chamber volumes a little tighter than they actually are, this leaves some space for head milling without fouling out in the tech inspection line. "

You can get to most any combination of the effect of piston volumes, deck to piston clearance, gasket thickness, or needed chamber volume by running the equation backwards and subtracting the volumes of the parts not in question at the moment. I the original response the other day the fellow was looking for combustion chamber volume and I did not include the immediately preceding paragraph on combustion chamber since that's what he need to derive from the equation. The next paragraph was intended to show how to run the original equation and its components backward to get to the combustion chamber volume needed to hit a desired compression ratio when you already know the piston crown, crown to deck clearance and gasket volumes.

"To get at the answer you're looking for take the original formula and run it backwards to where Total Volume (TV) = Compression Volume (CV) times the Static Compression Ratio (SCR). TV=CV X SCR

Now take the Total Volume and subtract the volumes for the Piston Clearance Volume (PCV) plus the Head Gasket Volume (HGV) plus the Piston Head Volume (PHV). The result is the combustion chamber volume needed to get the SCR you're looking for.

Some handy conversion factors you'll need are 25.4 milli-meters (mm) to an inch and 16.4 cubic centi-meters (CC) in a cubic inch. "



The cam you sight (K11-239-3) already calls for a 2000 plus RPM high stall converter. It's a cam that will be a lot happier with compression getting up around 9.5 to 10.5. Not having your exact dimensions but assuming a .025 crown to deck clearance an .050 gasket and 120 chamber over a flat top piston I guesstimate your compression around 7.7 to 1, awfully weak for that cam. I'm not so sure about the selection of hyperutectic pistons for what you're doing either. A forged piston is a lot less likely to become the point of failure in hard working motor.


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Old 11-14-2008, 03:50 PM
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Some one need to maek a rubber stamp out of compression calculations. I mean is it that hard to find on Google?
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Old 11-15-2008, 08:00 PM
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I was thinking maybe this cam kit ( COMP Cams K11-231-3 ) would work better since the compression ratio is so low. I don't need a ton of power, I building a fairly light weight buggy that this will be going into, I just want something with a fair amount of torque that won't be overheating on me. I don't want to use a higher stall converter because I want more low RPM control for rock crawling.
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