# How to determine Compression Ratio

Discussion in 'The Garage' started by 91GMCSuburban, Nov 17, 2004.

1. ### 91GMCSuburban1/2 ton status

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I've got a TBI motor of which I will slaping some vortec heads on. How do I determine the compression ratio for this combo?

2. ### folkenheathWorthless TrashPremium MemberGMOTM Winner

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If you know all of the specs I have an excel spreadsheet I made that you can input the variables and it gives you compression ratio, as well as dynamic compression ratio and other info.

3. ### 91GMCSuburban1/2 ton status

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would you mind getting me a copy of this file?

4. ### heavy4x41/2 ton status

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Essentially, you'll need to know your bore size, stroke length, distance from top of piston to bottom of head at top-dead-center, and head combustion chamber size.

That's sounds difficult to do, but it's pretty simple. A standard Chevy small block 350 has a bore size of 4.00 inches and a stroke of 3.48 inches. This computes to a single-cylinder volume of 716.62 cubic centimeters. Now, for the total combustion chamber size (volume at top dead center), you need to add the head cc's to the volume between the top of the piston and the bottom of the head at top-dead-center. This is a combination of top-of-piston to top-of-cylinder height at TDC and head gasket thickness. For the purpose of carrying out these calculations, let's call that distance 0.030 inches.

If I remember correctly, the combustion chamber size of a vortec head is 68 cubic centimeters (might be 72...I'm drawing a blank right now). Adding in the volume stated above, we get a total combustion chamber size of 74.17 cc's.

So, volume at bottom-dead-center divided by volume at top-dead-center = 716.62/74.17 = 9.66.

These are all general calculations and will vary based on your head size, whether or not you've bored out your cylinders, and how much clearance you have from the top of the piston to the bottom of the head at TDC.

5. ### 91GMCSuburban1/2 ton status

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thanks,

I like how you use inches and centimeters together so easily /forums/images/graemlins/bow.gif

I understand now.

6. ### folkenheathWorthless TrashPremium MemberGMOTM Winner

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If you give me your e-mail address I can send you a copy of the spreadsheet I made. It accounts for all of the variables. The inputs it uses are..

Number of Cylinders
Bore (in)
Stroke (in)
Connecting Rod Length (in)
Anticipated Redline (RPM)
Piston Volume (cc)
Comb Chamber Volume (cc)
Deck Height (in)
Extra Quench Height (in)
Intake Closing(deg)
Boost Pressure (lbs/in2(psig))
Volumetric Efficiency (%)

And it outputs these outputs:

Cylinder Volume (in3)
Swept Volume (in3)
Engine C.I.D.
Engine Liters
Connecting Rod Ratio
Compression Height (in)
CFM Flow
Compression Ratio
Dynamic Compression Ratio

Of course you don't have to fill in all of the inputs just to get compression ratio. It is fairly simple once you look at the spreadsheet format. /forums/images/graemlins/wink.gif

7. ### folkenheathWorthless TrashPremium MemberGMOTM Winner

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I tried to send you the file with all the info to the e-mail address on your file, it didn't work. Is that old or am I just having problems? Please let me know if you would like the file still. Also, the Vortec heads are 64 cc, which if you have standard flat top pistons you're compression will be 9.85:1, assuming everything else is stock. /forums/images/graemlins/thumb.gif

Inputs A
Number of Cylinders 8
Bore (in) 4.000
Stroke (in) 3.480
Connecting Rod Length (in) 5.700
Anticipated Redline (RPM) 5500
Piston Volume (cc)1 -5
Comb Chamber Volume (cc) 64
Deck Height (in) 9.000
Extra Quench Height (in) 0.015
Intake Closing(deg)5 66
Boost Pressure (lbs/in2(psig))2 0
Volumetric Efficiency (%)3 90

Outputs A
Cylinder Volume (in3) 43.73
Swept Volume (in3) 4.94
Engine C.I.D. 349.8
Engine Liters 5.7
Connecting Rod Ratio 1.64
Compression Height (in) 1.560
CFM Flow4 501
Compression Ratio 9.85
Dynamic Compression Ratio 7.80

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