check my maths

[spyke]

Hi chaps,

I know weve had posts touching on this previously  but ive now got my head back from reconditioning/boring out to 70mm(was an A7 head) and ive just measured my volumes with an accurate burette. Ive got flat top 7.25:1 hepolites but as the A7 combustion  chamber is i guess smaller i was hoping for the comp ratio to not go sky high, as im building the motor to a softish tune.
Ive just had a go at calculating and have come up with 8.05:1 which sounds fine but i would like you to check before i bolt the head on incase i need thicker head gasket or some other dodge to keep it low,if im wrong with my calcs.

Here are the clues:

-Standard bore A10
-head gasket 39 thou
-combustion chamber vol 42cc
-volume above rings/piston crown to top of barrel 2.3cc

I had head upside down and level and tried to avoid the fluid going covex by checking with a straight edge , same with above piston measurement.
I know it would be better to bolt head on to check as would be bit more accurate but i think this method should be close enough to check im in the right field.

You have 60 mins starting nooow,(you can use a slide rule and/or log tables)
no copying, and you will have to be accompanied if you need to use the toilet

cheers Spyke

[MG]

Hi Spyke!

I get 7.7:1 with the uncompressed head gasket thickness, so it might be a tad higher once mounted.

[muskrat]

G'day Spyke, MG,
                       I get 7.27:1.
Volume of head gasket 3.1428(3.5 x 3.5) x 0.01 = 0.385cc + 2.3 + 42 = 44.68cc. Swept volume of cylinder = 325cc divided by 44.68 = 7.27. Am I forgetting something.
 I was good at maths at school (35 years ago). I remember being marked wrong in a test where the ? was how many degrees between the hands of a clock at 6:30 ? I said 15 as the hour hand would be between the 6 and 7. He corrected my paper.

[spyke]

Thanks for the answers guys but as you can see we now have three different ones, 8.05,7.7, and 7.07 ! Can anyone else confirm one of these?
The only thing is theyre all lowish which is what i was looking for .
Do you think that these sort of comp ratios with a 356 cam , ali  head with gold flash sized valves  and a 276 carb should run nicely.?
Cheers Spyke

[muskrat]

G'day Spyke,
                 not enough O's in SMOOOTH with that build.
An A7 head with the lip removed should be the same volume as an A10 and can then take the A10 valves.
 Cheers

[Brian]

Dont think your 276 will be up to it with a ali head. If you want a sidebowl you will have to go up to a 289 but a monobloc would be the best.

Compression and cam are good, I set mine up with a 356 and 7.25-1.

[iansoady]

G'day Spyke, MG,
                       I get 7.27:1.
Volume of head gasket 3.1428(3.5 x 3.5) x 0.01 = 0.385cc + 2.3 + 42 = 44.68cc. Swept volume of cylinder = 325cc divided by 44.68 = 7.27. Am I forgetting something.

Yes, the formula is ([swept volume]+[combustion chamber volume])/[combustion chamber volume] -

ie (44.68+325)/44.68 giving compression ratio of 8.3:1.

[MG]

ok, you planted the seed, so there you go:

Yes, the formula is ([swept volume]+[combustion chamber volume])/[combustion chamber volume]

yep, that is the correct one


head gasket volume:
  the gasket thickness is 0.039inch * 25.4 = 1mm = 0.1cm

  v_gasket = 3.5cm*3.5cm*Pi*0.1cm = 3,85cc

so the compressed volume v_comp = 42cc+2.3cc+3.85cc = 48.1cc

the swept volume v_swept is 3.5*3.5*Pi*8.4 = 323.3cc


CR = (323.3cc+48.1cc) / 48.1cc = 7.71   q.e.d. 

[muskrat]

OK, I see where I went wrong, the h/gasket height is 0.1 not 0.01 and didn't add the comb chamber volume to the swept volume.
  MG.
Cheers

[MG]

To err is human (said the hedgehog and jumped off the hairbrush). 

[LJ.]

You guys are hurting my brain! ???

[iansoady]

ok, you planted the seed, so there you go:

Yes, the formula is ([swept volume]+[combustion chamber volume])/[combustion chamber volume]

yep, that is the correct one


head gasket volume:
  the gasket thickness is 0.039inch * 25.4 = 1mm = 0.1cm

  v_gasket = 3.5cm*3.5cm*Pi*0.1cm = 3,85cc

so the compressed volume v_comp = 42cc+2.3cc+3.85cc = 48.1cc

the swept volume v_swept is 3.5*3.5*Pi*8.4 = 323.3cc


CR = (323.3cc+48.1cc) / 48.1cc = 7.71   q.e.d. 

Correct, I didn't check the gasket calculation.....