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working out compression ratios
#4
I just went through all this a month or so ago. To fully calculate it you need to know the head volume accurately. I used a perspex plate over the chamber sealed with a smear of vaseline. The plate has two 1/8th holes in it, one a filler and one to let the air out. Then you can fill the chamber completely and measure the amount of volume. You are supposed to use a burette but I don't have one so a big syringe is what I used. You need to have the spark plug you intend to use in place of course. Different plugs will affect the volume slightly.

You need to know how big the gap there is from the top of the piston at TDC to the top of the block. You also need to know the volume of the gasket. I worked out the area buy placing it on squared paper and counting the squares. Then worked out the volume from that and it's thickness. I am not sure how much thinner a gasket gets when it is tightened down or if it is enough to matter.

You also need to know the volume of the valves if they sit up from the block when fully closed.

So the fixed volume is the head volume + the gasket volume + volume from the deck to the piston top at TDC - the volume of the valve heads.

The swept volume is the maximum volume the piston moves in the bore from TDC to BDC figured out from the stroke and the piston diameter. It's this times the number of cylinder that gives you the engine size.

Compression ratio is: swept volume + fixed volume / fixed volume as you say. You need to do it for every cylinder separately really. And ideally for a smooth running engine you want them all the same.

I have a spreadsheet somewhere for calculating it all. I'll see if I can find it. Mine worked out to be rather low, about 5:1 but I eventually want to add a supercharger so that is about right for when I do that. I think from memory original cars were about 4.8:1 and later ones 6:1 or there abouts?

Simon

I found the spreadsheet I used. I can put it on my web site as a link if people are interested? I don't seem to be able to attach it here. Would be good if someone can check there are no mistakes in it!

Simon
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Messages In This Thread
working out compression ratios - by JonE - 29-09-2018, 02:20 PM
RE: working out compression ratios - by Chris KC - 29-09-2018, 03:35 PM
RE: working out compression ratios - by JonE - 29-09-2018, 06:28 PM
RE: working out compression ratios - by jansens - 29-09-2018, 10:43 PM
RE: working out compression ratios - by jansens - 30-09-2018, 07:12 AM
RE: working out compression ratios - by JonE - 08-10-2018, 08:28 PM
RE: working out compression ratios - by jansens - 30-09-2018, 11:14 AM
RE: working out compression ratios - by JonE - 30-09-2018, 12:34 PM
RE: working out compression ratios - by jansens - 30-09-2018, 08:57 PM
RE: working out compression ratios - by JonE - 01-10-2018, 08:59 PM
RE: working out compression ratios - by GK5268 - 08-10-2018, 10:00 PM
RE: working out compression ratios - by JonE - 09-10-2018, 09:39 AM
RE: working out compression ratios - by JonE - 10-10-2018, 08:53 PM
RE: working out compression ratios - by JonE - 12-10-2018, 11:04 AM
RE: working out compression ratios - by JonE - 11-10-2018, 07:36 AM
RE: working out compression ratios - by jansens - 11-10-2018, 08:14 AM
RE: working out compression ratios - by JonE - 11-10-2018, 10:00 AM
RE: working out compression ratios - by jansens - 11-10-2018, 10:17 AM
RE: working out compression ratios - by Dave Mann - 11-10-2018, 10:23 AM
RE: working out compression ratios - by Dave Mann - 11-10-2018, 08:30 PM

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