http://www.type2.com/rvanness/deck.htm Thats for VWs (7:1
) There is another calculator for Minis kicking around someone may beat me to it before I get home tonight - if not I'll put it up
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| To all you mathemeticians out there.... https://ausmini.com/forums/viewtopic.php?f=5&t=32318 |
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| Author: | 1018cc [ Thu May 31, 2007 1:26 pm ] |
| Post subject: | To all you mathemeticians out there.... |
I was just wondering what I'm doing wrong - I'm trying to work out APPROXIMATE compression ratio's of an engine (for when I eventually get around to putting my new head on) and can't get the right answer. So this is what I'm doing:- ((pi*(bore dia(mm)^2))/4)*stroke(mm) = swept volume [divide swept volume by 1000 to get to cc's] Say the combustion chamber vol is 24.5cc's So: (Swept volume + combustion chamber volume) / combustion chamber volume = Compression Ratio. Now - that's the theory. When I plug in the dimensions for a standard engine (I chose a stock 998) bore = 64.57mm; stroke = 76.2mm; combustion chamber vol = 24.5cc's I get a displacement of 998cc's [so that's ok] but a CR of 11.2:1 (I need an answer of about 8.3:1). I know that I should be taking into account dish / dome of the pistons and the head gasket thickness (0.050"????) etc but I'm only after an approximate value. I've been looking at this for a few days and can't see where I'm going wrong. Can anyone here see what i'm doing wrong? Cheers, Nick |
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| Author: | Wombat [ Thu May 31, 2007 1:36 pm ] |
| Post subject: | |
http://www.type2.com/rvanness/deck.htm Thats for VWs (7:1 ) There is another calculator for Minis kicking around someone may beat me to it before I get home tonight - if not I'll put it up
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| Author: | Mick [ Thu May 31, 2007 1:48 pm ] |
| Post subject: | Re: To all you mathemeticians out there.... |
1018cc wrote: I was just wondering what I'm doing wrong - I'm trying to work out APPROXIMATE compression ratio's of an engine (for when I eventually get around to putting my new head on) and can't get the right answer. So this is what I'm doing:-
((pi*(bore dia(mm)^2))/4)*stroke(mm) = swept volume [divide swept volume by 1000 to get to cc's] area of a circle is pi x r^2, not pi x d^2. Area of a cylinder therefore is pi x r^2 x stroke. So try dividing the diameter by two Say the combustion chamber vol is 24.5cc's So: (Swept volume + combustion chamber volume) / combustion chamber volume = Compression Ratio. Now - that's the theory. When I plug in the dimensions for a standard engine (I chose a stock 998) bore = 64.57mm; stroke = 76.2mm; combustion chamber vol = 24.5cc's I get a displacement of 998cc's [so that's ok] but a CR of 11.2:1 (I need an answer of about 8.3:1). Still the original eqn. works as i get 11.18:1 for both eqn's (squares the top and bottom), so I'll get back in my box now  Cheers, Nick |
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| Author: | smac [ Thu May 31, 2007 1:52 pm ] |
| Post subject: | |
What he said ^ Actually, easier still, use this: http://www.csgnetwork.com/compcalc.html |
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| Author: | 1018cc [ Thu May 31, 2007 1:58 pm ] |
| Post subject: | Re: To all you mathemeticians out there.... |
Mick wrote: 1018cc wrote: area of a circle is pi x r^2, not pi x d^2. Area of a cylinder therefore is pi x r^2 x stroke. So try dividing the diameter by two I have typed (pi*D^2)/4 which is the same as pi*r^2 - just depends on how you want to do it. |
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| Author: | Mick [ Thu May 31, 2007 1:59 pm ] |
| Post subject: | |
Yeah you're right, I just checked. oops. |
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| Author: | Mick [ Thu May 31, 2007 2:05 pm ] |
| Post subject: | |
There's nothing lost in the maths then , it's just the extra cc's in the dish and gasket etcetera that throw it out. It never forces that amount into the area of the chamber, so it effectively must be added to the chamber volume (denominatator) |
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| Author: | 1018cc [ Thu May 31, 2007 2:10 pm ] |
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Mick wrote: There's nothing lost in the maths then , it's just the extra cc's in the dish and gasket etcetera that throw it out.
Thanks, that's what I was beginning to think as I couldn't see where I went wrong - I thought I'd get an answer close to around 9.0:1 to be honest. I knew it would be higher CR as I wasn't taking into account those extra volumes - I didn't think it would make that much difference. So that mean's I need (in my equations) about an extra 10cc's of dish + gasket volume etc
Wombat's spread sheet seems to be pretty easy to use and works well. Thanks guys |
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| Author: | Wombat [ Thu May 31, 2007 2:14 pm ] |
| Post subject: | |
This looks interesting - http://www.virtualengine2000.com/Calculator.htm V8's and compact sports - can't look at it at work due to restrictions on downloads - will look at it tonight |
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| Author: | Sprite_Man [ Thu May 31, 2007 2:29 pm ] |
| Post subject: | |
Quote: pi*(bore dia(mm)^2))/4)* is the area of a circle. 
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| Author: | 1018cc [ Thu May 31, 2007 2:52 pm ] |
| Post subject: | |
Mick wrote: Yeah you're right, I just checked. oops.
Sorry Mick - we must've been typing at the same time cause I can't remember seeing that last bit on the end of your post.
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| Author: | KLAS [ Thu May 31, 2007 2:58 pm ] |
| Post subject: | |
i'm using this one http://kb-silvolite.com/calc.php very nice is the dynamic calculator |
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| Author: | smac [ Thu May 31, 2007 7:31 pm ] |
| Post subject: | |
Well bugger me, so it does  Never seen it expressed that way.....maybe cause it's longer.
Don't under estimate the difference the allowance for the gasket makes. Very small differences in thickness have quite an effect. Throw some numbers at the calculator I posted and you'll see. |
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| Author: | Moke'n'Andy [ Thu May 31, 2007 8:03 pm ] |
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http://www.csgnetwork.com/compcalc.html this looked good didnt notice it above - sorry |
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| Author: | matt van'74 [ Thu May 31, 2007 8:15 pm ] |
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if you send me your email address i will send you a spread sheet i have made to calculate CR's. Matt |
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