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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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

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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David L


My greatest fear in life is that when I die my wife will sell my Mini and tools for the price I told her I paid for them!

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

What he said ^

Actually, easier still, use this: http://www.csgnetwork.com/compcalc.html

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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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

Yeah you're right, I just checked. oops.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

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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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

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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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

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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David L


My greatest fear in life is that when I die my wife will sell my Mini and tools for the price I told her I paid for them!

is the area of a circle.

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Spritey!
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Lower your standards. Increase your success rate.

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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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

i'm using this one
http://kb-silvolite.com/calc.php
very nice is the dynamic calculator

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Rover 214 atm ....

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.

http://www.csgnetwork.com/compcalc.html

this looked good

didnt notice it above - sorry

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It's not a mini, but still a bloody lot of fun! (and keeps me poor)

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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