Topic: calculating compression ratios

[valvolux]

apologies if this is covered in another thread, i did a search....honest 

Look, i admit it, im not that bright. Can someone help?
I can't calculate the compression ratio of my 78 CB550K engine

apparently you need the following values:

1. Cylinder Bore Size     
2. Piston Stroke Length     
3. Head Gasket Bore Diameter   
4. Compressed Head Gasket Thickness   
5. Combustion Chamber Volume In CCs   
6. Piston Dome Volume In CCs     
7. Piston Deck Clearance 

So my humble cb550 has a bore is 58.5mm and a stroke is 50.6mm
So that no. 1 & 2 answered.
How do i determine no. 3 & 4 ?
With no. 5 is that the capacity of my engine (544cc) divided by four?
How do i determine no. 6 & 7 ?

I already know the answer - the stock CR is 9:1 but i want to check it
 
I've gone and bought a 605cc kit with 60mm pistons, if someone can enlighten me how CR is calculated, perhaps (i doubt it) with my newfound knowledge i might be able to deduce the proposed CR.

Cheers guys 

[mlinder]

Stock compression is 9.0
The kit should have told you what CR the pistons are.

The simple calulation is

• CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)
• CR = Compression Ratio
• D = Displacement
• PV = Piston Volume
• DC = Deck Clearance Volume
• G = Gasket Volume
• CC = Combustion Chamber Volume

But this doesn't take into account dome or valve reliefs. You'd have to measure both to find out the domes total volume.

Best way is to measure volume with a syringe and liquid at TDC and BDC once you have it assembled. Measurement and the math to figure it out is a pain in the ass.

Alternatively, you could build the dome and valve reliefs in something like rhino3d with the proper measurements which can give you the volume of the area, which would remove the hard math part.

[bwaller]

Those kits have 61mm pistons. I have 61's in one engine (with piston/cylinder clearance the bore is actually 61.04mm)

In my case the pistons are 0.038mm proud of the deck, with a 1mm head gasket which has a bore of 62.5mm.

Combustion chamber volume is exactly 13cc's. (take my word for it)

My little brain works by CR= swept volume + combustion volume divided by combustion volume. So by my calculation CR=10.3:1.

If you get anything different please don't tell me.....my brain hurts 
                                             
                                                   

[Artfrombama]

To get the volume of a cylinder (HGV);
pi x radius squared x height=Yci
3.1416 x diameter x thickness of head gasket=Yci

The sum is in cubic inches, you must convert to cubic centimeters;

Yci x 16.387 = Ycc

Same formula to get swept volume (SV)

To get piston dome volume (PDV), you run the piston to TDC (top dead center) then cc the area on top of the piston,
if the piston is domed, it's a little more complicated.
drop the piston down in the bore 1", then cc the area,
then calculate the volume of a empty cylinder 60mm diameter 1" tall (remember to put everything in cc's, see above).
Subtract the PDV from the calculated volume, if you have a negative number subtract it from the TDCV, positive number, add.

My head hurts, gonna have anuther ML

[paulages]

math formulas are cool and all, the best way to do it is to measure the CR in the real world... it's easy if you're already putting an engine back together.

1) grease the top ring with something heavy enough to seal it, but don't leave too much around the shoulder or it'll throw your reading off.
2) assemble the head with gasket, and fill the chamber up with carefully measured isopropyl alcohol (food coloring helps). use something with Ml or cc increments (a 10 ml plastic syringe can usually be had for free at a pharmacy), and be as careful measuring as possible. position the plug hole at the top so that air doesn't get trapped.
3) grease the bottom ring and position at TDC. measure this volume the same.
4)repeat until you are certain your measurements are accurate.

CR= volume at BDC/ volume at TDC.

for example, in my 718cc engine i had 198cc at TDC and 18.8cc.

198/18.88=10.53/1 compression ratio

[mlinder]

I thought I said that already

[paulages]

I thought I said that already

i missed that part of your post, mark.  it is fun using math though.

[Artfrombama]

CC's put into perspective;
=354.882 cc's