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Old 03-04-2009, 10:37 PM
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figuring out compression ratio

bbc 427 ci. 105cc head. 40cc dome. .040 head gasket. how do i figure out my compression ratio? am i missing info to figure out my compression ratio?

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Old 03-04-2009, 11:53 PM
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Yes, you also need the bore size(not piston size, but actual size the cylinder is bored to) and crank stroke. You also need to cc the piston dome, the advertised volume is rarely accurate. Plus, if you have an early 427 block that has notches at the top of the bore for the exhaust valve , you need to take that into account also! Since many compression formulas assume the bore in the head gasket to be round(a BBC is not), an error of some amount will occur, unless you actually measure the volume of it. Some gasket companies put the compressed thickness and head gasket volume on the packaging now, so maybe that will save a phone call to the manufacturer. Depending on how accurate you want your results to be. And finally, you need to know the piston deck clearance....
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Old 03-05-2009, 07:42 AM
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online java cr calc
http://www.wheelspin.net/calc/calc2.html
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Old 03-05-2009, 08:22 AM
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875cc each cylinder
105cc head gasket
-40cc piston dome
10cc head gasket
5cc piston below deck (typical)

so

(875 + 105 -40 +10 +5) / (105 -40 + 10 + 5) = (875 + 80) / 80 = 11.9

11.9:1 cr

assuming the deck height is 0.020". Could be 12.7:1 if the block has zero decked.
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Old 03-05-2009, 06:32 PM
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ok thats interesting. ive been running it on premium pump gas. i didnt know i could run the comp ratio on pump gas
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Old 03-05-2009, 06:47 PM
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go to google and type in "engine cr" it is a compression ratio calculator and it is the most accurate one i have came across so far and it is FREE! to download and use. you just type your info in and it does the rest.
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Old 03-05-2009, 10:40 PM
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everywhere i look or check i seem to get a different answer on my compression ratio. from one of my old books i some how got 9:1. everything else i get is either low 6's to high 11's and 12's. im stumped 4.25 bore 3.76 stroke.

Last edited by dman16; 03-05-2009 at 10:47 PM.
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Old 03-06-2009, 02:03 AM
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I cam up with 12.1:1 on the compression. you will also need to know your deck clearance and head gasket bore.
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Old 03-06-2009, 10:08 AM
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most of those calculators you have to list your piston dome volume as a negative number if you don't you will wind up with a real low number because it thinks you have a hole that big on the top of your piston instead of a dome
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Old 03-06-2009, 04:00 PM
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When you have a domed piston, you have to deduct the volume of the dome from the chamber volume when figuring the entire space on the piston downstroke. The piston dome takes away 40cc of space that would normally be occupied by fuel/air mixture on the intake stroke, so deducting it from the chamber on the cylinder fill stroke and on the squeeze stroke is the correct way to do it.

You would need 5 volumes to figure the static c.r., cylinder volume (.7854 x bore x bore x stroke x 16.387), chamber volume, head gasket volume, piston volume and piston deck height volume.

The cylinder volume is 875cc, figuring a stock bore and stroke of 4.25 x 3.76.

The chamber is 105. Deduct the 40cc piston dome and figure the chamber at 65 for both the cylinder suck and squeeze computations.

Guestimate the head gasket at 10.

Figuring a 0.005" piston deck height, compute .7854 x 4.25 x 4.25 x 0.005 x 16.387 and find 1.16cc's in the piston deck height.

You've deducted the 40cc dome from the chamber, so figure the piston as a flat-top with a zero value for this computation. Just think of it as the same thing as cutting the dome off the piston flush with the piston crown and attaching the dome into the chamber, thus reducing the chamber volume.

875 cylinder
65 chamber
10 gasket
1 piston deck height
zero piston
Added up, we find the cylinder draws in 951cc's on the suck stroke.
Now, add up all values except the cylinder...
65 chamber
10 gasket
1 piston deck height
zero piston
Added up, we find the the mixture is squeezed into 76cc's of space on the compression stroke.
Divide 951 by 76 and find a 12.51:1 static compression ratio.
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Old 03-06-2009, 09:51 PM
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bore of head gasket is 4.370 thickness is .039 of an inch.
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Old 03-07-2009, 12:48 AM
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Quote:
Originally Posted by dman16
bore of head gasket is 4.370 thickness is .039 of an inch.
.7854 x 4.370 x 4.370 x 0.039 x 16.387 = 9.585cc's.
What's the piston deck height?
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Old 03-09-2009, 11:12 PM
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the piston deck height is .020
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Old 03-10-2009, 07:53 AM
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http://www.rbracing-rsr.com/compstaticcalc.html
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Old 03-10-2009, 12:37 PM
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OK, so 4.649cc's in the piston deck height and 9.585cc's in the gasket.

875 cylinder
65 chamber
9.585 gasket
4.649 piston deck height
zero piston
Added up, we find the cylinder draws in 954.234cc's on the suck stroke.
Now, add up all values except the cylinder...
65 chamber
9.585 gasket
4.649 piston deck height
zero piston
Added up, we find the the mixture is squeezed into 79.234cc's of space on the compression stroke.
Divide 954.234 by 79.234 and find a 12.04:1 static compression ratio.
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