Compression ratio

[rallyrob]

I'm going to start by saying I feel rather daft to start with here.
Ive got all the workings of the compression ratio of my engine, done by an old friend that I know longer have contact with these days.

But for the bloody life of me I can't get my head round the workings or how he got the final answer.
So this could turn out to be rather long winded.
But can someone try an explain it all to me please?

Ive googled it too and all I really seem to manage to do is confuse myself even more so.
I know there's a few conversions in there an things like that too, and Math's never was my strong point.
And apologies for the photo being the wrong way round, I ponced about with that for ages trying to get it to come out the right way up, I just gave up in the end.

[Graham]

your measurement for head gasket thick ness and pistons down bore are they inches, so 0.025 and 0.012"

maths is simple add up all the volumes above the piston at tdc, then add that to the swept volume of a cylinder ie 528.41cc then divide by the volume above the piston

[rallyrob]

Yea they're in thou Graham. And Im pretty sure the CC he's got there is already including the pistons down the bore and the headgasket thickness too. I think thats the bit of his workings nearer the bottom of the page 37" = 93.98 =6.5cc

[MK1_Oz]

Chamber + piston Down Bore Vol + Head Gasket Vol + Stroke Vol

Divided by

Chamber + piston Down Bore Vol + Head Gasket Vol

Chamber 47.5cc
Gasket 4.36cc (pi x r2 x 0.025" = 3.14 x 4.675^2 x 93.5/10/2)
Piston clearance 2.09cc (3.14 x 4.675^2 x 0.025")
Bore 528.35cc (3.14 x 4.675^2 x 76.95/10)

So it then becomes;

(47.5 + 4.36 + 2.09 + 528.35) / (47.5 + 4.36 + 2.09)

[rallyrob]

Ok thanks, I still haven't got it.. Think I'll carry on reading some more...
Should have paid more attention at schoool.

Give me a spanner and Im pretty good at most things but ask me something technical and Im easily baffled.

[MK1_Oz]

Which bit is baffling you? Oh yeah, the bit I totally stuffed up..... This is what I should have written;

Chamber 47.5cc
Gasket 4.36cc (pi x r2 x 0.025" = 3.14 x [93.5/2/10] x [0.025 x 25.4 / 10])
Piston clearance 2.09cc (3.14 x [93.5/2/10] x [0.012 x 25.4 / 10])
Bore 528.35cc (3.14 x [93.5/2/10] x [76.95 / 10])

Further explanation for Gasket 4.36cc (pi x r2 x 0.025" = 3.14 x [93.5/2/10] x [0.025 x 25.4 / 10]);

The volume of a cylinder is pi x radius squared x height. For your example this means 3.14 (pi) x bore/2/10 (to give the radius then convert from mm to cm) squared x 0.025 (inches of gasket thickness) x 25.4 (to convert inches to mm) / 10 (to convert mm to cm).

[rallyrob]

Ok guys, thanks for the help... Ive finally got the same answer...
Im quite pleased with myself now..
Im still not completely sure but Ive got it once so Ive just keep going over it again and again until I have
a better grasp on on it...
I also found a good site that had another full explaination about swept volume clearance volume
and so on and so forth...
Thanks Graham and Mk1 Oz

[stevieturbo]

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

[rallyrob]

A quick question surrounding this subject again..

So if the pistons had valve pockets how do you account for those in the calculations?
Would you just cc the piston crowns/valve pockets and add that into the calculations like you do
with the combustion chambers? Please...

[katana]

Yep - easiest way is do a dry build, seal rings with a bit of grease, bolt head on and cant whole over till plug hole is horizontal and burette in fluid to bottom of spark plug hole = total chamber volume. Also helps when using domed pistons.

[MK1_Oz]

Are the pistons installed? If not just cc the piston cutouts as you would the combustion chamber. If you follow Katana's suggestion would you not have to do his method at BOTH TDC and BDC?

[rallyrob]

Well at the moment the pistons are still flat tops anyway.. But if I were to change the cam then it might need cut outs.
So this was just an enquiry anyway really...
Thanks
Guys...

[katana]

Are the pistons installed? If not just cc the piston cutouts as you would the combustion chamber. If you follow Katana's suggestion would you not have to do his method at BOTH TDC and BDC?

No - because stroke is constant Pi R sq'd x H (stroke). So measure chamber capacity @ TDC add stroked volume then divide sum by chamber capacity = CR.

[Forest_rallying]

The easiest way to find your swept cylinder volume is to used Vizards method. Bore x Bore x stroke x0.785 = cylinder cc. x by four gives you the engine capacity.

To find the piston cutouts volume or the piston intruder if they are high dome pistons we need to lower the piston down the bore 10 mm and work the volume out using the above formula. This equates to 64.72cc with a 90.8 mm std Pinto bore. Lightly grease the ring land and carefully fill and measure the cylinder with fluid, not water. It's best to cover the top of the bore with a sheet of Perspex with 2 holes in it to get an accurate reading.

If the figure is more than the 64.7 cc then that's the cc of the valve cut outs so that figure needs to be added to the cylinder head and gasket volume when working out your compression. If the cc is less than the 64.7cc then that's the volume the piston intruder is. That figure then needs to be deducted from the gasket and head volume.

The above method was taught to me by an experienced engine builder of many years. It requires the head off but the block can be left in situ.

[MK1_Oz]

No - because stroke is constant Pi R sq'd x H (stroke). So measure chamber capacity @ TDC add stroked volume then divide sum by chamber capacity = CR.

I only had to think about it for another minute and I would have realised that