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| Compression ratio question https://www.slantsix.org/forum/viewtopic.php?t=12617 |
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| Author: | 70valiant [ Wed Apr 13, 2005 5:25 pm ] |
| Post subject: | Compression ratio question |
Could anyone tell me my CR from these numbers Head CC'd at 44cc's deck looks to be .140" down bored 30 over. I tried with my virtual engine calculator butr I dont know what "Cbr cc's" means Thanks Phil |
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| Author: | SlantSixDan [ Wed Apr 13, 2005 8:33 pm ] |
| Post subject: | |
9.58 by my math, assuming your 44cc and your 0.14" are accurate. Sounds like you've milled the head. Don't forget to factor in whatever head gasket you're using. |
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| Author: | 70valiant [ Thu Apr 14, 2005 4:02 am ] |
| Post subject: | |
Thanks Dan, I forgot to add the thick Fel-Pro at .39 compressed. and yes I asked for 0.100" to be taken off the head at the machine shop. |
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| Author: | Super6 [ Thu Apr 14, 2005 3:15 pm ] |
| Post subject: | |
I would expect "Cbr cc's" is short for "Chamber cc's", I.E. combustion chamber volume; in your case 44 cc's. With the .039" head gasket I come up with 9.75:1. -S/6 |
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| Author: | SlantSixDan [ Thu Apr 14, 2005 3:30 pm ] |
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One of us is making an error in our calculations. I came up with 9.58 without the gasket's 0.039" thickness; you came up with 9.75 with the gasket. Rerunning my math to include the gasket, I come up with... 9.78, which is within rounding technique differences of your 9.75. Error looks to have been in my original calculation! Next time I won't try to calculate CR and watch Star Trek at the same time. 
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| Author: | Super6 [ Thu Apr 14, 2005 4:12 pm ] |
| Post subject: | |
That's why I 'programmed' Excel to do the math for me...I am the king of fat-finger error on every calculator ever made! 
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| Author: | NewLancerMan [ Thu Apr 14, 2005 4:18 pm ] |
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Quote: Next time I won't try to calculate CR and watch Star Trek at the same time.
What you really needed was one of these:  You and ST, scary! MJ |
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| Author: | 70valiant [ Thu Apr 14, 2005 6:27 pm ] |
| Post subject: | |
The guys over at moparts gave me a couple of links that have CR calculators http://www.straightline-perf.com/ . Would my bore be 3.43 with the 0.30" over bore? if so they came up with 9.797:1. |
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| Author: | SlantSixDan [ Thu Apr 14, 2005 7:27 pm ] |
| Post subject: | |
Yes, your bore is 3.43. The math isn't very complex. Works like this: (bore ÷ 2)² × π × (stroke) = piston swept volume (bore ÷ 2)² × π × (negative deck height + head gasket compressed thickness) + (combustion chamber volume) = TDC volume (piston swept volume) + (TDC volume) = BDC volume Therefore: (BDC volume) ÷ (TDC volume) = compression ratio. Don't forget to keep your units consistent; work in inches (linear and cubic) or in centimetres (linear and cubic). |
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| Author: | 70valiant [ Thu Apr 14, 2005 7:42 pm ] |
| Post subject: | |
the nice thing about the calculator on the link I posted is that you do not have to do any standard to metric conversions. you input the bore, stroke, gasket thickness, deck height, and gasket bore in inches and chamber volume in cc's. then it tells you the displacement of the engine in cubic inches(228.69), litres(3.75), and static compression ratio(9.797:1). |
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