Fungus
Part of things
Posts: 960
|
|
Aug 27, 2010 15:59:31 GMT
|
Hi, i just wanted know if there is a formula for calculating compression ratio, and in turn figuring out how much to skim from the head/barrel to achieve a specific ratio? Thought maybe if there are some good replys , a mod could move this ''useful threads'', as i think it could be quite useful for others too Cheers
|
|
|
|
|
|
Aug 27, 2010 16:52:32 GMT
|
Right, quite simply it is: (Combustion chamber volume + cylinder volume + head gasket volume) / combustion chamber volume To calculate the various bits: - Combustion chamber volume: Remove the cylinder head and place on a bench combustion chamber-side upwards. Have a piece of perspex cut big enough to cover the combustion chamber, and drill a couple of holes in it. Smear a thin layer of grease or vaseline around the head surface, just around the edge of the chamber, to crate a seal when the perspex is placed over it. Then fill it with paraffin from a burette or syringe through one of the holes in the perspex, measuring the volume used as you go.
- Cylinder volume: Turn your engine till the piston reaches bottom dead centre. Measure the diameter of the bore, D, using a pair of vernier calipers. Then measure the height from the block face to the piston CROWN, H. The volume is equal to:
pi x (D2/4) x H.
- Head gasket volume: Measure the bore diameter, D, and thickness of the gasket, H, with vernier calipers and calculate as before.
This all assumes you have flat top pistons that stop at the top of the block and don't protrude above the deck height. If you have dished or domed piston CROWNs, or they protrude above deck height then this needs to be taken into account: - Protruding above deck height is the easiest: with the piston at top dead centre, measure from the top of the CROWN of the piston to the top of the deck using the probe of the vernier calipers. Calculate the volume of piston protruding into the combustion chamber from this height and the diameter, as before. Subtract this from the combustion chamber volume in the bottom of the first formula.
- Dished pistons: Turn the engine to top dead centre (or alternatively remove the piston and do this on the bench). Fit your piece of perspex over the top of the piston CROWN and fill and measure with paraffin as described before. Add this volume to both top and bottom of the first equation.
- Domed pistons: This is a bit trickier. Best way I can think to do it is to remove the piston and press the CROWN into a block of plasticine. Then measure the volume of this indentation using the paraffin-and-perspex method. Take this volume away from both top and bottom of the first equation.
|
|
1989 Peugeot 205. You know, the one that was parked in a ditch on the campsite at RRG'17... the glass is always full. but the ratio of air to water may vary.
|
|
Fungus
Part of things
Posts: 960
|
|
Aug 27, 2010 17:08:34 GMT
|
Thats great, thanks
|
|
|
|
Fungus
Part of things
Posts: 960
|
|
Aug 27, 2010 17:29:16 GMT
|
Oh, and why parrafin? Would water do harm to the internals?
|
|
|
|
|
|
Aug 27, 2010 22:47:31 GMT
|
Oh, and why parrafin? Would water do harm to the internals? Erm, no idea actually Everyone says use paraffin, we used paraffin when we did it at uni, so I said paraffin ;D. I guess it might be to prevent rust...
|
|
1989 Peugeot 205. You know, the one that was parked in a ditch on the campsite at RRG'17... the glass is always full. but the ratio of air to water may vary.
|
|
|
|
Aug 27, 2010 22:57:22 GMT
|
hehe, you don't even have to use liquid. What I do is mash plasticine into the head and face it off with a steel rule, then remove and reshape into a box shape. Measure the sides of the box, calculate volume. It's a good rough and ready for when you don't have a burette full of paraffin handy
|
|
To get a standard A40 this low, you'd have to dig a hole to put it in
|
|
|
|
Aug 27, 2010 23:04:10 GMT
|
It's a good rough and ready for when you don't have a burette full of paraffin handy I keep one on the hallway table, next to my keys and wallet ;D
|
|
1989 Peugeot 205. You know, the one that was parked in a ditch on the campsite at RRG'17... the glass is always full. but the ratio of air to water may vary.
|
|
RobinJI
Posted a lot
"Driven by the irony that only being shackled to the road could ever I be free"
Posts: 2,995
|
|
Aug 27, 2010 23:38:42 GMT
|
To add to what Rich has said, in it's truest sense, the compression ratio is simply 'cylinder volume at bottom dead centre/cylinder volume at top dead centre.'
So strictly speaking if we are assuming that the piston CROWN is sitting flush with the block deck at TDC then you would have to include the head gasket in the later half of the equation rich gave, i.e:
(Combustion chamber volume + cylinder volume + head gasket volume) / (combustion chamber volume + Head gasket volume)
Personally I think the easiest way to get your head around it is to calculate the volume at TDC, (so your combustion chamber volume, plus your head gasket volume, plus or minus any piston dish/dome volume, plus or minus the volume of any offset the piston CROWN has from the block deck at TDC.)
Then to find the volume at BDC, simply add the swept area to the volume at TDC. Swept area is simply pi x (bore squared/4) x stroke.
Now add those figures to the 'cylinder volume at bottom dead centre/cylinder volume at top dead centre.' equation and you're done.
While doing all this measuring, it's easy to be caught out by measuring combustion chamber volume in CC, then all other measurements in mm. Remember that if you're working in CC, you need to work the rest using cm, not mm.
|
|
|
|