compresion ratio
04-16-2002 | 05:03 PM
#1
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compresion ratio
What is the correct formula to calculate compression ratio. Reason for asking is that I have a 502 that is being bored .030. Won't that increase compression if I don't purchase a lower domed piston? The block is being decked for straightness as well.
04-16-2002 | 09:57 PM
#3
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Joined: Jan 2002
Posts: 4,612
Likes: 7
From: ALTO, MI
EXCALABUR when you bore an engine you are increasing the cylinder volume and then compressing that larger volume into the same head volume -- thus you efectively increased the compression ratio. I don't know how much of an increase you will get since it depends on the bore diameter.
04-16-2002 | 11:19 PM
#4
Charter Member #601
Joined: Dec 2000
Posts: 3,276
Likes: 5
From: Holland, Mi USA.
voodoo,
I dont think you will see much of a difference. My formulas are at home. There are web sites that will do the calculation for you. Do a search on the board for past links or search on the net.. I know someone mentioned this in the past.
Dont worry about changing the dome on the piston, it will be a small change and wont impact your operation. If you want a close guesstimate. Take the bore and stroke and figure your volume then add the overbore dimension, and refigure your volume. If it is 1 or 2cc, I believe it will be somewhere around .1 on your ratio..
If you have trouble and want more info, email me and I will look for the formula at home this weekend..
Dick
I dont think you will see much of a difference. My formulas are at home. There are web sites that will do the calculation for you. Do a search on the board for past links or search on the net.. I know someone mentioned this in the past.
Dont worry about changing the dome on the piston, it will be a small change and wont impact your operation. If you want a close guesstimate. Take the bore and stroke and figure your volume then add the overbore dimension, and refigure your volume. If it is 1 or 2cc, I believe it will be somewhere around .1 on your ratio..
If you have trouble and want more info, email me and I will look for the formula at home this weekend..
Dick
04-17-2002 | 12:02 AM
#5
Platinum Member
Joined: Sep 2001
Posts: 5,769
Likes: 150
From: tn
502 bore = 4.470" Stroke = 4"
swept volume= (pi)*(2.235squared)*4.00=62.76cu.inches
.030 over (4.500")
swept volume= (pi)*(2.250squared)*4.00=63.605cu.inches
Now, we must assume a particular combustion chamber size for reference (the actual size does have an effect on the final values, but since we are going to come up with a relative index, the deviation is insignificant).
Let's assume a flattop piston and a flush deck height and a 120cc combustion chamber (7.3 cu.inch). You'll see in a minute why it is okay to make these assumptions...
CR = (swept + chamber)/chamber
stock: (62.76+7.3)/7.3 = 9.60:1
+.030: (63.605+7.3)/7.3 = 9.71:1
Difference: 9.71/9.60 = 101.14% of the previous CR.
If you are currently 7.0:1, you will be 7.08:1
If you are currently 12.5:1, you will be 12.64:1
There's the math. Easy to see that it doesn't make a big deal..
mike
swept volume= (pi)*(2.235squared)*4.00=62.76cu.inches
.030 over (4.500")
swept volume= (pi)*(2.250squared)*4.00=63.605cu.inches
Now, we must assume a particular combustion chamber size for reference (the actual size does have an effect on the final values, but since we are going to come up with a relative index, the deviation is insignificant).
Let's assume a flattop piston and a flush deck height and a 120cc combustion chamber (7.3 cu.inch). You'll see in a minute why it is okay to make these assumptions...
CR = (swept + chamber)/chamber
stock: (62.76+7.3)/7.3 = 9.60:1
+.030: (63.605+7.3)/7.3 = 9.71:1
Difference: 9.71/9.60 = 101.14% of the previous CR.
If you are currently 7.0:1, you will be 7.08:1
If you are currently 12.5:1, you will be 12.64:1
There's the math. Easy to see that it doesn't make a big deal..
mike
04-17-2002 | 01:06 AM
#6
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Posts: n/a
The formula (IIRC) is on the oreder of:
First get the "displaced" volume [lets call this 'Vscrape']
Next you need the 'static' volume (same as above except replace stroke with deck height, and then add head gasket volume (compressed gasket height * 'hole size'[which aint the same as your bore])
[lets call this 'Vblow', ]
Lastly you need to know the combustion chamber volume [Vdetonation]
The compression ratio is then
Now this is oversimplified to assume a 'flat top' piston. So if you are running funky(tm) pistons adjust by putting that change in your 'Vblow' will work.
So for a stock 502 its basically 1020 : 120 which is 8.50:1, that .030 overbore adds ~14.75ccs, and brings you to 8.625 or so... So the answer is: in a lower compression motor you don't have to worry about it.
You can thank your 118CC combustion chambers for that... If you had a small chambered head you could end up in more trouble (you could see a full .2 increase...)
Now decking the block can cause much more complex issues. When you reduce the 'Vblow/Vdetonation' number you make LARGE changes to compression ratio, and thats the easiest complication. If the block is decked much you will have to at least give some consideration to:
- Compression (.060 shaved =15cc= about 10.1:1!)
- Valve Clearance (watch your lift!)
- Intake manifold alignment**
- Valvetrain geometry**
The first 2 are rather obvious, the last two will cause ulcers until you diagnose them... Well the intake thing could be obvious in a boat, if it causes a gas leak and its only a small explosion
Now don't even think about using my equations, just do what was suggested and use one of the myriad of online calculators
--Adam
First get the "displaced" volume [lets call this 'Vscrape']
Code:
(pi really)
2 |
|BORE| \ /
Stroke * |----| * 3.141592
| 2 |
[lets call this 'Vblow', ]
Lastly you need to know the combustion chamber volume [Vdetonation]
The compression ratio is then
Code:
Vscrape + Vblow + Vdetonation
-----------------------------
Vblow + Vdetonation
So for a stock 502 its basically 1020 : 120 which is 8.50:1, that .030 overbore adds ~14.75ccs, and brings you to 8.625 or so... So the answer is: in a lower compression motor you don't have to worry about it.
You can thank your 118CC combustion chambers for that... If you had a small chambered head you could end up in more trouble (you could see a full .2 increase...)
Now decking the block can cause much more complex issues. When you reduce the 'Vblow/Vdetonation' number you make LARGE changes to compression ratio, and thats the easiest complication. If the block is decked much you will have to at least give some consideration to:
- Compression (.060 shaved =15cc= about 10.1:1!)
- Valve Clearance (watch your lift!)
- Intake manifold alignment**
- Valvetrain geometry**
The first 2 are rather obvious, the last two will cause ulcers until you diagnose them... Well the intake thing could be obvious in a boat, if it causes a gas leak and its only a small explosion
Now don't even think about using my equations, just do what was suggested and use one of the myriad of online calculators
--Adam
Last edited by Mother; 04-17-2002 at 01:11 AM.
04-17-2002 | 10:56 AM
#10
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Joined: Feb 2001
Posts: 591
Likes: 7
From: Saugerties, NY
What is the "displaced" volume or the "swept" volume that both of you are talking about? Is that the formula for figuring out the volume of a cylinder? This is the volume that will be compressed into the compression chamber? I don't understand why the "static" volume needs to be incorporated into the formula? Couldn't you just add the "head gasket" volume without the "static" volume in the second equation?
