working out compression ratios - Printable Version +- Austinsevenfriends ( http://www.austinsevenfriends.co.uk/forum)+-- Forum: Austin Seven Friends Forum ( http://www.austinsevenfriends.co.uk/forum/forumdisplay.php?fid=1)+--- Forum: Forum chat... ( http://www.austinsevenfriends.co.uk/forum/forumdisplay.php?fid=14)+--- Thread: working out compression ratios ( /showthread.php?tid=1810) |

working out compression ratios - JonE - 29-09-2018
I'm posting this as found it difficult to find things in one place on the web. I wanted to know the exact CR on a Ricardo head machined for high lift cam [i.e. its got maximum volume possible] as I don't know the condition of the bottom end of this engine and whilst I want to use it as my LC head is quite badly corroded. I don't want to inflict big stresses on the engine. It's the first of the 1 5/16" crank and presently running with a Ruby manifold (ha!) which I will keep, with a 1" SU eventually. I found that my pistons are +040, and the table in the back of the 750MC Companion gives 774cc. That is thus 193.5cc per cylinder. I have a little weedkiller millilitre dispenser and I filled 35, then 30ml, in order to pour into the compression chamber with a spark plug fitted. 1 cubic centimetre = 1ml (seems impossible, but google says..) I could get 30ml in, but unsure about the meniscus bit... so it may be a bit less. 193.5 divided by 30 gives 6.45 Compression Ration (29 would give 6.67) which seems just about ok if I use a standard gasket rather than a thin one. I'd like to know where I can improve my maths and what else I haven't allowed for (or have done wrong) - will it be lower because of the gasket thickness for instance? RE: working out compression ratios - Chris KC - 29-09-2018
30 ml is about right for a sports head. You could kill the meniscus with a spot of fairy but it may give you a froth problem instead! If being fussy you could subtract about 1.3cc for valve head intrusion and add 3cc for head gasket thickness (assuming 25 thou compressed). Do your pistons stroke to top of bore or stop 20 thou short? Are both valves fully closed at bdc on inlet? 6:1 is a nice figure for a reliable road going car. I ran 10:1 in my youth but figured it couldn't last... In my experience quoted figures often ignore the head gasket. RE: working out compression ratios - JonE - 29-09-2018
I'll let you know when I get the naffing head off! I'm also advised that if at TDC the pistons are level with the top of the block (ie deck height is zero) then compression ratio is: (Cylinder swept volume + combustion chamber volume + gasket volume) divided by (combustion chamber volume + gasket volume) So with the 3 from Chris's post... 193.5 + 30 + 3 30+3 is a tad higher at 6.86 : 1 (standard LC head is 5:1) So if my pistons are shy of the top, that is going to bring it down a touch, right? I'll be hoping they are... RE: working out compression ratios - jansens - 29-09-2018
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 RE: working out compression ratios - Tony Press - 30-09-2018
(29-09-2018, 10:43 PM)jansens Wrote: 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. The standard Compression Ratios seem to be 1923-1933 4.8 (or 4.9) to 1 1933-1936 5.2 ? to 1 1936- 1939 6.0 (or 6.2?) to 1 I understand that Compression Pressures are between 17 and 20 times the Compression Ratio in a good engine giving 4.8 to 1 = 80 to 96 psi 5.2 to 1 = 88 to 104 6.0 to 1 = 102 to 120 I get around 100 psi from the standard early head and 110 psi from the 1936 head. Cheers, Tony. RE: working out compression ratios - jansens - 30-09-2018
(30-09-2018, 02:39 AM)Tony Press Wrote: ) to download. It's in .xlsx format. I don't have a modern enough version of Excel to test it but you can import it fine into Google sheets. |