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working out compression ratios
#1
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?
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#2
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.
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#3
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...
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#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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#5
(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.

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

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.
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#6
(30-09-2018, 02:39 AM)Tony Press Wrote: [Only registered and activated users can see the links Click here to register]) 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.

I made a slight mistake when doing my engine in that I equalised all the chamber volumes first (to 34cc) but then found the valve heads were different thicknesses so my ratios are a little different between cylinders. But we're talking a small amount (5.16, 5.13, 5.16, 5.15) I am not sure it matters? And given those volumes presumably change when the crank whips at high revs meaning the ratios will be changing anyway. It's one reason I did the spreadsheet, so I could plug in different values to see what difference it makes. I suspect the differences I have won't matter too much on a road car (it just upsets my OCD tendencies!).

I am sure the racing guys will have a much better idea how accurate and precise one needs to be with all this. I am not worried about power as much as smoothness. It's satisfying to build a smooth running engine for some reason.

If anyone wants the spreadsheet and can't download it PM me and I can email you a copy. Someone really should check it is sensible!

Simon
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#7
Simon, I tried to validate (and simplify) your spreadsheet but I cant get what you are doing in "Clearance Volume"? have you missed a PI out of this or am I missing something?

Edit:
Attached is my attempt, with values from your spreadsheet - final value is quite different to yours I think you made an error in the way you calculate the ratio at the last step. Hope this helps


Attached File(s)
.xlsx   Compression ratio calculator_MD.xlsx (Size: 15.03 KB / Downloads: 17)
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#8
I wouldnt fret over slight variations, esp on a car where detonation is not a problem. As per recent on oxy sensors, books reckon that for carbs and nondescript distributions (as siamesed) mixture varies by a ratio or so between cyls and degree of fill similarly (possibly improved with a supercharger)
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#9
Hi Mark, am on the wrong computer now to check but I definitely could have got something wrong. I appreciate someone else checking. It's 11pm here now so will check tomorrow when I am on the right computer!

Hi Bob, the question, as always, is what is meant by slight? It's like when the books say check for 'excessive wear'. If I've only ever seen one I have no idea what that means! Absolute measurements make more sense. If something says X should be 2 thou but up to 10 thou is ok that means something. I can measure it. The people who have seen hundreds of these things and have a gut feel for what's ok and what isn't are sadly getting fewer and fewer. When they are gone how is someone like me, who is coming along for the first time, supposed to know what's acceptable or not. I do like that in the Woodrow book, it give acceptable ranges.

I know and appreciate you have a ton of knowledge about this stuff. It's great when you can share it in a measurable way.

Simon
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#10
Simon - what head is yours with, out of interest? It would be good if we built on Tony's notes and put up simplified case histories (head, what treatment,) so that other people don't need to go through it all! I'm presuming also that the state of piston oversizing/bore creates a measurable diffierence as that part of the equation moves depending on the engine, whereas the bit for the head does not?
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