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Cr Calculated Estimation - Could Someone Check My Maths?

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#1 BaronVonchesto

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Posted 30 March 2019 - 07:44 AM

For a 99H block, standard bore diameter is 2.542". According to haynes, a the dished pistons my car came with would give a CR of 8.3:1

 

So a 998cc engine is 249.5cc per cylinder.

When compressed 8.3 times, thats 249.5/8.3 = 30.06cc per cylinder

 

A 20 thou overbore is (0.02/2.542*100%) = 0.78678% larger, so the new volume per cylinder should be:

(1.0078678*249.5) = 251.46cc

Assuming the dish profile is the same as standard, the compressed volume should remain the same, hence the new CR will be:

251.46/30.06 = 8.365 or approx 8.4:1

 

Now if i were to switch to flat top pistons, minispares says the CR goes up to 10.4:1 meaning the compressed volume on a std piston is 249.5/10.4 = 23.99cc

so if i were to fit a 20 thou overbore flat top piston, the new CR would be 251.46/23.99 = 10.48:1

 

similarly if i were to go to 40 thou overbore the new volume per cylinder is 253.426cc making the CR 8.43:1 for dished and 10.56:1 for flat top

 

Of course theres some error here but this should be a pretty close approximation right?



#2 unburntfuelinthemorning

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Posted 30 March 2019 - 09:12 AM

You've got the right idea and won't be too far out.

 

When the factory started using flat tops the quoted CR went from 8.3:1 to 10.3:1.

 

+0.020" will give 1014cc so 1014 = 253.5cc

                                                4

 

+0.040" will give 1030cc so 1030 = 257.5cc

                                                4

 

Now  __V__  =  C

          CR -1

 

Where V = swept volume of cylinder, C = TOTAL combustion chamber volume and CR = compression ratio

 

So with an 8.3:1 ratio on a 998cc engine  _249.5_   =  34.18cc

                                                                    8.3 -1

 

34.18cc non swept volume minus 6cc piston dish = 28.18cc non swept volume.

 

Transposing the above formula  CR  =  V + C

                                                                  C

 

So  249.5_+_28.18  =  9.85  

              28.18

 

So compression ratio with flat top pistons = 9.85:1

 

There's a discrepancy between the quoted 10.3:1 and my calculation of 9.85:1 so either I've done something wrong or there are more variables at play.


Edited by unburntfuelinthemorning, 30 March 2019 - 09:14 AM.


#3 GraemeC

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Posted 30 March 2019 - 09:33 AM

There are other parts of the non-swept volume that may change with different pistons - mainly the area above the top ring and compression height



#4 Retroman

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Posted 30 March 2019 - 12:25 PM

Working it out using 0.00001% figures and assuming the dish will be the same is not the way to do it

 

To be accurate you need to measure how far the pistons are down the bore, they can vary quite a bit from the factory

 

the cc's in the piston dish and combustion chamber....surfaces may have been skimmed, valves recessed etc

 

https://jscalc.io/ca...RNEglVkKLQir11T

 

The link is useful and saves pressing bottons, but you need actual measurements

 

Another big consideration is the fuel octane, if its down at 95 you want some lower than 10:1 fugures

 

Have a gander at Vizards wisdom....

 

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