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.

The more cylinder volume at TDC the less compression
you have but at .030 over it would not be significant

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.

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

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

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

FWIW I started that post long before mcollin replied, but got busy, doh!

--Adam

Most piston companies adjust the dome configuration to the overbore so the c/r doesn't change significantly.

Thanks for all the replies guys!

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?

Fbronco -

The volume "swept" by the piston is one part of the initial volume. The rest of the initial volume is the "static" or "unswept volume consisting of the space that remains when the piston is at Top Dead Center. Imagine the piston TDC. The amount of liquid that would occupy the remaing area is the static volume.

Since the ratio of compression is determined by:

(Initial Volume) divided by (Final Volume)

then it becomes easy to see that these are represented by:

(swept + static) divided by (static).

Omitting the static volume from the total Initial Volume will not give a correct representation of the CR.

I have no idea whether I am helping you visualize this. Hope I'm helping.

M

mcollinstn, that helps. My problem is that I don't see how replacing the stroke with the deck height in the formula for the swept volume will get you the space between the piston at TDC and the top of the deck. I do understand that you need this along with the gasket volume and combustion chamber in head to get the true compression ratio. Maybe I'm not sure what the "stroke" actually means. If that is the length the top of the pistion moves from bottom to top, then you would still need to figure out how far the piston is away from the deck height when it is at TDC? Right? And use that measurement to replace the stroke in the formula you used for "swept" volume?

Thanks

Ahh, Bronco, thats not how it works, so it shouldnt make sense :)

The deck / piston clearance could be calculated with the following info:

Deck Height

Crank Throw (half of the stroke)

Rod length

Distance from the center of the piston pin to the top of the piston (Piston 'height')

It would be

Deck - (Throw + Rod + Piston)

I've got too much time on my hands so I drew this out to hopefully help everyone visualize this discussion...


--Adam

Jeez, you guys draw some really neat pictures.:D Now, as Yosemite Sam would say, "Alright rabbit, draw a gun!"

This is what happends when you are an unemployed sales engineer with a background in network architecture, hehe..

You get programmed to turn dry stuff into a pretty picture... Then get deprived of the need. hehe

:)

--Adam