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  #1  
Old 06-14-2002, 03:20 AM
bens7d2 bens7d2 is offline
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Default whats my compression ratio?

Guys, just wondering what the true comp ratio is on a stock 72 318? What about with 308 heads milled .020 with the thinner MP head gasket? stock 318 heads milled .020? Is there a formula or resource where i can find this out at? Thanks Ben
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  #2  
Old 06-14-2002, 03:58 AM
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ehostler ehostler is offline
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Unless you measure everything, you're only going to be able to get a best guestimate. The best thing to do is measure cylinder volume at BDC, then measure at TDC. Find out what the compressed volume of the head gasket is. Measure the volume of the combustion chamber.

Compresion ratio = volume at BDC/(volume at TDC + head gasket volume + combustion chamber volume)
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Old 06-14-2002, 10:22 AM
Jims451 Jims451 is offline
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Like ehostler said, its only guessing unless you can measure everything.
To actually calculate compression ratio from parts here is the important measurements you need:
#1. Cylinder head volume (measured.)
#2. Head gasket volume, or thickness and inside diameter which can be used to calculate volume.
#3. Piston to deck height (how far down the cylinder is the piston at TDC?)
#4. Volume of piston dish or dome.
#5. engine bore and stroke.

You can calculate number 3, if you know the engine block height (usually taller than spec), the connecting rod length, and the piston to pin height (compresion height.)

The formula for converting Diameter and distance (or Bore / stroke) is:
PI *[(Diameter/2)^2]* distance, or PI * [(Bore/2)^2]*stroke

To convert head or gasket cc's to CID, just multiply the cc by 0.061023744

So now you can calculate:
Piston swept volume in CID (for one cylinder) by using the: PI * [(Bore/2)^2]*stroke

Then you need to find the volume above the piston at TDC. This is the cylinder head volume + head gasket volume + piston dish volume (will be negative if the piston is a dome) + Piston to deck height (will be negative if piston is above deck).

Compression ratio is just:
(Piston swept volume + volume above the piston) / Volume above the piston
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Old 06-14-2002, 03:01 PM
440BB 440BB is offline
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Idea your compression ratio

the compression ratio for a 72 318 is (8:6:1) with a whopping 150hp,260ft/lbs torque . but thats stock its much better with the heads it might get to 9:0:1.
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Old 06-14-2002, 06:36 PM
jimbo7074 jimbo7074 is offline
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Default compression

Using the thinner gaskets compared to the felpros was worth 3/10 of a point compression on my 360,and shaving your heads .020 should reduce your head volume by 4cc's.Even though the 318 was rated at 8.6-1 compression,don't count on that,more like8.1-1.
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Old 06-14-2002, 09:56 PM
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MoPaul MoPaul is offline
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http://users.erols.com/srweiss/calccr.htm
I like this calculator, but there are others. This will do no good without the measurements, but may become useful should you get them. Just thought it might be good for future reference.
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Old 06-14-2002, 11:37 PM
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dave571 dave571 is offline
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There's a calculator at www.kb-silvolite.com

As mentioned earlier, be wary of rated compression vs. real compression.

My 400 was rated at 8.2(or 8.3) :1 in the factory service manual. It actually measured out at a pathetic 7.5:1

Now it measures out at 9.9:1. What would that be advertised at? Maybe 10.7:1???

Also raises the question of what do the original 10:1 rated motors really have for compression?
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Old 06-14-2002, 11:48 PM
jimbo7074 jimbo7074 is offline
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Default compression

Years ago I saw an article on a blue printed 70 440 that was rated at I believe 10-1.When everything was measured it came in at a whopping 8-1 compression delivered from the factory!
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Old 06-24-2002, 04:28 PM
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AAR4fun AAR4fun is offline
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I tried the site, it's pretty close. The posted formula is more accurate.
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