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How to Determine Compression Ratio?

Rob79er

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Apr 13, 2005
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60
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Dallas, Tx.
Without tearing down the engine to measure, is there a way to determine the compression ratio that a car has?
I would like to know what I am running but don't have an idea on how to check it.
 

waterboy1976

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Aug 18, 2003
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510
Location
Cleveland,Ohio
Corvette
1973 4-speed Coupe FOR SALE/ SOLD!!!!
Ive always used the head clearance, piston height and chamber to calc compression...
 

Hib Halverson

Technical Writer for Internet & Print Media
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Jan 10, 2001
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71 04 12 19
If you don't know the chamber volume when the piston is at top dead center and you don't want to tear the engine down to measure all that, it is impossible to determine compression ratio without some expensive equipment.
 

TJay

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Mar 20, 2006
Messages
157
Location
SouthWestern Ontario
Corvette
1972 Yellow Coupe
The link in the above post if all the information is know will give you the compression ratio. But there are also other factors, such as rod length, cam which change the valve stroke compression on closing.
The simple way to understand compression ratio is if you ...

Send a piston to the bottom (valves open) and you could pour 10 gallons of water through the sparkplug hole.
Than sent the piston TDC (valve closed) and you could now only pour in 1 gallon, thats a 10 to 1 compression. Compression is based on volume... piston size + valve CC, bore, stroke etc.
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
Without tearing down the engine to measure, is there a way to determine the compression ratio that a car has?
I would like to know what I am running but don't have an idea on how to check it.

STATIC compression is strictly a function of measurements, nearly all of which require teardown to determine.

DYNAMIC compression, which is a more accurate determinant of sensitivity to detonation, adds valve timing to the above measurements for a completely different calculation.

No way to determine either one without a teardown, and pushrod length has nothing to do with compression ratio.

:beer
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
One cannot just use the advertised or claimed cc's on the few components involved (e.g. piston displacement, chamber in the head).

Accurate measuring can only be done by disassembly of the engine. Anything less, one would just be fooling oneself and then wonder with "X" CR why your Vette is not giving you "Y" 1/4 mile times.

Gerry:cool
 

glen242

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Joined
Aug 30, 2004
Messages
135
Location
Moon Twp. PA
Corvette
1976 Mahogany
The link in the above post if all the information is know will give you the compression ratio. But there are also other factors, such as rod length, cam which change the valve stroke compression on closing.
The simple way to understand compression ratio is if you ...

Send a piston to the bottom (valves open) and you could pour 10 gallons of water through the sparkplug hole.
Than sent the piston TDC (valve closed) and you could now only pour in 1 gallon, thats a 10 to 1 compression. Compression is based on volume... piston size + valve CC, bore, stroke etc.

Rod length and cam , I believe, will not change Static CR, it will change Dynamic CR.

Most on line calculators are accurate IF the input is accurate.
 

goingballistic

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Joined
Apr 22, 2004
Messages
197
Location
Brockport, NY
Corvette
1981 Beige Coupe
This is something I found on the net a few years ago. I built a CR calculator from this in Excel, ( yea I'm an engineering geek). I was and just finished building my motor, 425HP at the crank. My objective was to get close to 10.5 to 1 so I could run pump gas, actual came out to be ( static) 10.2 to 1, so I was pretty happy. Any way the stuff below shows you the components of how you calculate the CR. Hope it helps.

Compression ratio calculations

Compression Ratio = (Swept Volume + Top Dead Centre (TDC) Volume) divided by TDC Volume

Swept Volume = (pi x Bore Diameter x Bore Diameter x Stroke) divided by 4

TDC Volume = Head Volume + Gasket Volume + Deck Volume + Dish/Dome Volume

Gasket Volume = (pi x Gasket Bore x Gasket Bore x Compressed Gasket Thickness) divided by 4

Deck Volume = (pi x Bore Diameter x Bore Diameter x Deck Clearance) divided by 4

Dish/Dome Volume is published or measured in cc.

Note: Any volume below the piston compression height is a positive value. Any volume above the piston compression height is a minus value.

Dimensions: When calculating, use cubic centimeters (cc) or cubic inches, but not both. Refer to the following conversions:
1 inch = 2.54 centimeters (cm)
0.061 cubic inch = 1 cc
1 cubic inch = 16.387 cc

Example: Find CR for a Chevy 350 using 0.03 inch oversize pistons with a Dish/Dome Volume equal to 5 cc:
Head Volume = 76 cc = 76 x 0.061 = 4.636 cubic inches
Dish/Dome Volume = 5 cc = 5 x 0.061 = 0.305 cubic inches
Gasket Thickness = 0.035"
Gasket Bore Diameter = 4.166"
Deck Clearance = 0.005"
Deck Volume = (pi x 4.03" x 4.03" x 0.005") divided by 4 = 0.064 cubic inches
Gasket Volume = (pi x 4.166" x 4.166" x .035") divided by 4 = 0.477 cubic inches
Swept Volume = (pi x 4.030" x 4.030" x 3.480") divided by 4 = 44.389 cubic inches
TDC Volume = 4.636 + 0.305 + 0.064 + 0.477 = 5.482 cubic inches
COMPRESSION RATIO = (44.389 + 5.482) divided by 5.482 = 9.097
Now, those of you who only sleep well at night having measured Head Volume and Dish/Dome Volume yourselves are undoubtedly saying, "published values can introduce error into compression ratio calculations". Well, you're right, and if cc-ing equipment is at your disposal, using it can help achieve an accurate compression ratio, which in turn can be used to calculate the optimum camshaft profile for the application. [end]
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
I bought the Performance Trends' CR calculator. (http://www.performancetrends.com/) It is a pretty good one.

But again, the advertised cc displacement for most heads, piton dishes and what not are not to be trusted. The advertised chamber size for the L81 head I got from Sensei is about 75.85 cc's but it measured nearly 80 cc's. Some of the chambers got dished valves, so perhaps about 1.5 cc's account for that (when compared to flat valves).

Also, the deck height will affect this, and my L81 block had an average 0.033" above the deck.

GerryLP:cool

P.S. What I meant with "the deck height will affect this" is that a lot of piston manufactures claim advertised piston displacement with a deck set at Zero height. The qunch also is affected by the deck height.
 

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