Compression Ratio
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Instructor
Joined: Jan 2004
Posts: 139
Likes: 18
From: Surprise Arizona
Compression Ratio
Please educate me on the compression ratio formula. I have been trying to follow the Higher Octane issue initiated by "VETTE LT4 being discussed on the Forum but got lost somewhere down the road when the well schooled Forum mechanics got too technical for this retired auditor. My basic question is how is the compression ratio determined and what does it mean. I have a '67 coupe with a 327, original 300 hp (all stock from the factory). The Technical Data and Specifications page of the '67 Owners Manual says that the compression ratio for this engine is 10.25:1. What are they telling me? :confused: :confused:
[Modified by Bandito, 12:50 AM 2/10/2004]
[Modified by Bandito, 12:52 AM 2/10/2004]
[Modified by Bandito, 12:50 AM 2/10/2004]
[Modified by Bandito, 12:52 AM 2/10/2004]
Drifting
Joined: Feb 1999
Posts: 1,807
Likes: 28
From: CA central coast
Re: Compression Ratio (Bandito)
Maybe this is too simplistic an answer, but...
Compression ratio is the volume of space in a cylinder (bore in the block including head gasket height + combustion cavity in the head) when the piston is all the way down divided by the smaller volume of space when the piston is all the way up. It is not easy to calculate because of the irregular shape of the head's combustion chamber, and any irregularities in the shape of the top of the piston. The ratio is always expressed as x:1. For instance the ratio of a volume of 780 cubic centimeters with the piston down and 78 cubic centimeters with the piston up will be 780/78 and gets expressed as 10:1
The volume of a combustion chamber can be measured by a process called "CC'ing" the head; basically this means taking a known measured volume of a fluid in a calibrated container and pouring it into one combustion chamber of a head... when the chamber is full, you simply check to see how much the volume of fluid in the calibrated container has decreased. I'm frankly not certain how they measure the volume used by a piston-top irregularity (dome, valve recess, etc), but I suspect it is similarly measured.
These measurements are not normally done by the home mechanic, but rather by a professional performance machine shop.
So, if one had his engine "stroked", for instance, that means the crankshaft arm that is connected to the piston is lengthened a bit, so it causes the piston to travel upward and downward a bit further than before, and the volume within the head's combustion chamber remains the same, the compression "ratio" is thus increased. Similarly, if one resurfaces the block's "deck" surface and or the head's surface, the volume in the top of the combustion chamber is slightly reduced while the piston travel remains the same, so the compression ratio increases.
[Modified by waynec, 8:54 AM 2/10/2004]
Compression ratio is the volume of space in a cylinder (bore in the block including head gasket height + combustion cavity in the head) when the piston is all the way down divided by the smaller volume of space when the piston is all the way up. It is not easy to calculate because of the irregular shape of the head's combustion chamber, and any irregularities in the shape of the top of the piston. The ratio is always expressed as x:1. For instance the ratio of a volume of 780 cubic centimeters with the piston down and 78 cubic centimeters with the piston up will be 780/78 and gets expressed as 10:1
The volume of a combustion chamber can be measured by a process called "CC'ing" the head; basically this means taking a known measured volume of a fluid in a calibrated container and pouring it into one combustion chamber of a head... when the chamber is full, you simply check to see how much the volume of fluid in the calibrated container has decreased. I'm frankly not certain how they measure the volume used by a piston-top irregularity (dome, valve recess, etc), but I suspect it is similarly measured.
These measurements are not normally done by the home mechanic, but rather by a professional performance machine shop.
So, if one had his engine "stroked", for instance, that means the crankshaft arm that is connected to the piston is lengthened a bit, so it causes the piston to travel upward and downward a bit further than before, and the volume within the head's combustion chamber remains the same, the compression "ratio" is thus increased. Similarly, if one resurfaces the block's "deck" surface and or the head's surface, the volume in the top of the combustion chamber is slightly reduced while the piston travel remains the same, so the compression ratio increases.
[Modified by waynec, 8:54 AM 2/10/2004]
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,270
Re: Compression Ratio (Bandito)
Compression ratio = (swept volume + clearance volume)/clearance volume. It's the ratio of the total cylinder volume at BDC divided by the cylinder volume at TDC.
The swept volume is the cylinder displacement, which can be computed from bore and stroke.
The clearance volume is the volume at TDC and can be considered to have four components - the cylinder head portion of the combustion chamber, which is usually specified in cc, the volume between the block deck and piston crown, which can be determined by measuring the distance between the block deck and piston crown (known as the deck clearance). The third volume is that added by the head gasket, which can be computed from the head gasket opening diameter and compressed thickness. The fourth volume is that added or subtracted by a dish or dome on the piston, which is usually specified by the piston manufacturer.
The dimension that the rebuilder should measure to compute compression ratio is the deck height, and I always recommend that this be measured prior to removing the pistons from the block on teardown. This will allow you to compute the original and new CR based on the components you plan to use and any planned machining. Chevrolet's specified CRs are usually about a quarter to a half point higher than actual.
New pistons should have a specified compression height (the distance between the pin center and crown), and this should always be compared to the OE piston as any difference will affect deck clearance. Block deck height (distance from crank centerline to block deck) for a SB can be computed from the following formula.
Deck height = deck clearance + pist. comp. ht. + 5.700 + stroke/2
The nominal production deck height is 9.025", so your measured deck clearance on teardown will tell you what your actual block deck height is, and this can be used to determine deck clearance with the new pistons if they have a difference compression height. It will also tell you if your decks are parallel to the crankshaft axis.
You should find more detailed explanantions in books such as "How to Hotrod Your Small Block Chevy.
Duke
The swept volume is the cylinder displacement, which can be computed from bore and stroke.
The clearance volume is the volume at TDC and can be considered to have four components - the cylinder head portion of the combustion chamber, which is usually specified in cc, the volume between the block deck and piston crown, which can be determined by measuring the distance between the block deck and piston crown (known as the deck clearance). The third volume is that added by the head gasket, which can be computed from the head gasket opening diameter and compressed thickness. The fourth volume is that added or subtracted by a dish or dome on the piston, which is usually specified by the piston manufacturer.
The dimension that the rebuilder should measure to compute compression ratio is the deck height, and I always recommend that this be measured prior to removing the pistons from the block on teardown. This will allow you to compute the original and new CR based on the components you plan to use and any planned machining. Chevrolet's specified CRs are usually about a quarter to a half point higher than actual.
New pistons should have a specified compression height (the distance between the pin center and crown), and this should always be compared to the OE piston as any difference will affect deck clearance. Block deck height (distance from crank centerline to block deck) for a SB can be computed from the following formula.
Deck height = deck clearance + pist. comp. ht. + 5.700 + stroke/2
The nominal production deck height is 9.025", so your measured deck clearance on teardown will tell you what your actual block deck height is, and this can be used to determine deck clearance with the new pistons if they have a difference compression height. It will also tell you if your decks are parallel to the crankshaft axis.
You should find more detailed explanantions in books such as "How to Hotrod Your Small Block Chevy.
Duke
