Calculating compression?

[Ed]

Hi guys,

I am planning my motor rebuild and need to know how exactly compression is calculated.

from what i (think i know) it's the size of the combustion area in relation to the stroke?

if i am aiming for a CR of say 10:1, what volume is required for a 308?

I can CC the head combustion chambers quite easily at work using basic lab tools.
going on some references I have head gaskets are usually about 8cc.

I was thinking of setting the squish to almost zero then making any adjustments to overall volume using various pistons whether it be dished, flat or even domed top should it need.

anyway.. back to the question, how do I calculate the volume necessary?
some pointers in engine compression fundamentals would be great and very helpful.

also has anybody had their cylinder bores re-sleeved?  is it a pricey process?  I am imagining it would be.

Cheers

Ed

[Glenn 'Stinky' Stankevicius]

If you collect Street Machine mags they had an article in the last year or two about this. I think it was in Bob Kotmel's "Stage Write".

[FB_MAD]

I'm going to have a go at this ED but I'm stretching a 20 yr+ memory here from when I was an apprentice and this is all off the top of my head so to speak so hoping its right.I'm sure someone will correct any mistakes I make.
To calculate cylinder volume use Pie (is this correct spelling?) x R square x height.
So pie = 3.142 x cylinder radius squared x piston stroke.

So a 308 with 4 inch bore and  3.062 inch stroke would be:
Pie (3.142) x 2 (radius) x 2 (radius) x 3.062 (stroke)=38.483216 cubic inches for each cylinder.Then times by 8 cylinders and you have the total cubic inches.

3.142 x 2 x 2 x 3.062= 38.483216 cubic inches x 8 (cylinders) = 307.865728 cubic inches which Holden called the 308.

You already know how to cc your combustion chambers and head gasket so divide total swept volume by total combustion cc and theres your compression ratio.

Now...cylinder sleeving is costly but works and with original bore old engines getting harder to find a lot of blokes I know are going the resleeve way especially for rare ones like Torana XU1 engines for example.

Hope this info helps (and is correct!!),

Terry.

[Ed]

hey Terry,

very helpful, yep it's def Pi Rsqared.

ok cool, that's way too easy.

will convert the measurements to centimeters now for (cc's).
=
3.142 x 5.08 x 5.08 x 7.77748 = 630.6269  cc's per cyl.
x 8 = 5045.015 cc's  ok so far so good, approx 5L right.

so for 10:1 compression  the cc =
approx 63cc? (ie 630:63)

or should it be
57cc? (570:57, where the total cc = 630)

Stinky, sorry dont collect SM mags just buy em when i see something of interest.  unfortunately missed that issue.

Cheers

Ed

[FB_MAD]

Yeah, had a rethink and your right ED, it's Pi (not Pie!!).
So Pi x radius squared x height is it.
Your decimal conversion looks right as Gregorys manual puts 308 cubes in at 5044 cc.
So as per your calculation each cylinders swept volume is 630 cc and your combustion chamber is 63 cc then you have 10 to 1 compression as you said.
If you had a 57 cc combustion chamber you'd have approx 11 to 1 compression, and you'd probably have to run Premium fuel with that compression.

[Dr_Terry]

Hi Guys.

You've oversimplified this a bit.

You have to calculate the volume of the head gasket hole 0.040" X bore dia (convert this to cc).

Also you have to calculate the small volume left because the piston doesn't come all the way to the top of the bore. Say it misses out by 0.020" then this volume is 0.020" X bore dia convert this to cc).

Therefore your combustion chamber volume is the sum of these 3 parts. So your combustion chamber that is 63 cc is actually around say 69 cc.

Then when calculating the compression ratio, you do it this way.

The total volume above the piston at BDC (this is swept volume PLUS combustion chamber) divided by the total volume above the piston at TDC (combustion chamber volume).

If we use your figures of 630 cc swept volume & my estimate of 69 cc) this calculates as.

630 + 69 = 699 total vol

699 / 69 = 10.13.

If you use the 57 cc chamber the estimated actual chamber is 63 cc

630 + 63 = 693 (total volume)

693 / 63 = 11 .0

I know these figures have come out to near yours, but this would differ with other engines. But my point is that the calculation is done including the combustion chamber volume with the swept volume.

Dr Terry.

[Ed]

Also you have to calculate the small volume left because the piston doesn't come all the way to the top of the bore. Say it misses out by 0.020" then this volume is 0.020" X bore dia convert this to cc).

this is squish height right?

is there an accepted way of measuring valve to piston clearance at TDC?

thanks for the info, it's very helpful.

Cheers

Ed

[Dr_Terry]

Hi Ed.

Squish height usually refers to the gap between the top of the piston & the face of the head at TDC, which is the sum of the deck height gap & the head gasket gap.

Piston to valve clearance is usually measured using plasticene on top of the piston & then turn the motor over manually. Also the minimum piston to valve is sometimes not a TDC, it can be just before or just after TDC because the valve has a peak opening point as well.

Dr Terry.