check my math please

[bullpeters]

If my bore is 25mm and my stroke is 30mm then if the piston is 4.3mm from the head at TDC then my compression is 7:1 yes?
I figure: volume of cylinder is 12.5 x 30 = 14726
Divide by 7 = 2103
Which is a cylinder diameter 25 x 4.3 high.
Allowing for a flat head etc
Before I go and cut something, does that sound right?

7:1 is about right for hit and miss model I am hoping

 

[Swifty]

I'm going to throw my ideas in, piston dia and stroke will give you swept volume. You have to take into account the total volume of the cylinder with the piston at BDC, allow for the volume left at TDC and then work out compression ratio.

Using your figures for dia and stroke, and 4.3mm left at top of stroke,

Total volume at BDC is 16836
Total volume left at TDC is 2110

So 16836 divide by 2110 equals 7.98 which is the compression ratio.

Paul.

 

[petertha]

In the case of flat top piston & either flat head or even a recessed head whose chamber is same piston diameter, the math is easier yet, you don't have to introduce area's or volumes into the calculation.

CR = (30 + 4.3) / 4.3 = 7.98 1)

If for example the head had a circular recess combustion chamber of same piston diameter, say 2.5mm length, the calculation becomes:

CR = (30 + 4.3 + 2.5) / (4.3 + 2.5) = 5.41 1)

 

[dave-in-england]

Total volume = 14726.22 cu. mm.
left over volume = 2110.76 cu. mm.

ratio = 6.97:1

 

[Cogsy]

Total volume = 14726.22 cu. mm.

total volume = (approx) 16837cu. mm. Cylinder is 25mm x 34.3mm, stroke is 30mm.

Had to post the correction - your sig made me do it

 

[GailInNM]

In the "for what it's worth" department, I like lower compression ratios and use from 5:1 to 5.5:1 for hit and miss engines. With higher compression ratios you can get more power but at the expense of having a higher low end RPM. With higher compression ratios the engine has to run faster to have enough flywheel energy to overcome the higher power necessary for the compression stroke. I have seen models that use as low as 4:1.
Gail in NM

 

[dave-in-england]

total volume = (approx) 16837cu. mm. Cylinder is 25mm x 34.3mm, stroke is 30mm.

Had to post the correction - your sig made me do it

~~~~~~~~~~~~~~~



DAMN !!!!!

This is just like one of those sneaky gotcha college exam questions.

OK i'll start again !

Total volume = 16836.97 cu. mm.
left over volume = 2110.76 cu. mm.

ratio = 7.967:1

:wall:

.

 

[Swifty]

~~~~~~~~~~~~~~~



DAMN !!!!!

This is just like one of those sneaky gotcha college exam questions.

OK i'll start again !

Total volume = 16836.97 cu. mm.
left over volume = 2110.76 cu. mm.

ratio = 7.967:1

:wall:

.

Figure that I arrived at in post #2, allowing for rounding.;D

Paul.

 

[bullpeters]

I will go with the 4.3mm space. I can easily lower the compresion but harder to raise it. I only have cam and rocker arm and I can see if it will work. If its finished by christmas I will be happy

 

[canadianhorsepower]

I will go with the 4.3mm space. I can easily lower the compresion but harder to raise it. I only have cam and rocker arm and I can see if it will work. If its finished by christmas I will be happy

if you want 7/1 you need 5mm over your piston
swept volume 25*25*.7854*30=14726
combustion chamber 25*25*.7854*5=2454.
total volume = 17180
divided by combustion chamber(2454)=7.000/1

 

[Charles Lamont]

As petertha said, if the combustion chamber is cylindrical, and the same diameter as the bore, then the bore is not relevant to the calculation, only the stroke and combustion chamber height are needed.

To my mind nearly 8:1 is far too high a compression for a hit-and-miss engine. Something more like 4:1 would make for an easier starting, slower idling, and more-like-full-size model, surely?