Understanding compression ratio's-tons of ref's

[1Gary]

While I am considering which pistons for the Ole Yellar 383 I have to consider static vs dynamic compression ratios.The vacuum the engine produces because of it being a TBI and which cam choice is also a consideration.I found this post on Chevy talk and thought I would pass it along to you all.It's long and you have to be willing to study at length.But is is surely a complete and good info to know if your interested.

its obvious some of the guys on this site need to understand the differance between static and dynamic compression ratios, and thats understandable as its a difficult concept to grasp at first
but you need to understand it before selecting a combos components

the differance between STATIC COMPRESSION RATIO AND DYNAMIC COMPRESSION RATIO is where the piston is in the cylinder when the valves close and the piston can accually start compressing the REMAINING VOLUUM IN THE CYLINDER VS the STATIC COMPRESSION THAT ASSUMES THE PISTON STARTS COMPRESSING THE INSTANT IT LEAVES BOTTOM DEAD CENTER AND STARTS UPWARD ON THE COMPRESSION STROKE!

let me try and explain, the short version is that the PISTON COMPRESSES NOTHING untill BOTH VALVES ARE CLOSED, .......thats the only compression ratio that matters,.... since its the only compression ratio the engine ever sees.

static compression is simply the differance between the cylinder volume at BOTTOM DEAD CENTER(BDC) and its compressed volume at TOP DEAD CENTER (TDC), into the combustion chambers,... dynamic compression takes into account that on the pistons upward compression stroke the valves have not yet closed and nothing gets compressed by the piston untill they do, that of course depends on the cam and rockers, pistons and connecting rods, the cylinder voluum, the rod/stroke ratio, ETC.,used, in the combo, and the rpm levels to some extent BTW, ALUMINUM HEADS can usually operate at a higher dynamic compression simply because ALUMINUM releases heat to the coolant much faster than iron, its the lower heat levels that remain in the cylinder that help prevent detonation..when you increase the dynamic compression the heat levels in the heads combustion chamber rise , the differance in the RATE heat leaves the cylinder allows a slightly higher dynamic compression level from aluminum before the same HEAT levels are REACHED & MAINTAINED in the combustion chambers

heres a calculator for static cpr, which you need to figure first

http://www.rbracing-rsr.com/compstaticcalc.html

or
http://users.erols.com/srweiss/calccr.htm

or
http://not2fast.wryday.com/turbo/compre ... sion.shtml
let me point out a few things
first look at this chart

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

then lets assume your 350 sbc engine has a static compression ratio of 11:1 but youve installed this cam

http://www.cranecams.com/?show=browsePar...lvl=2&prt=5

looking at the cam specs we see that the effective stroke is not the 3.48" that the static compression ratio is measured from ,at BDC, BUT from about 2.6 inches from tdc where the valves close as the piston moves upward, so your true working compression is closer to 8.1:1 NOT 11:1

heres a longer more detailed explanation and access to the software to figure dynamic cpr with the cam your useing in your engine

http://cochise.uia.net/pkelley2/DynamicCR.html

http://www.diamondracing.net/cocalc.htm

heres some differant calculators

http://www.kb-silvolite.com/calc.php?action=comp2

http://www.wallaceracing.com/dynamic-cr.php

http://www.smokemup.com/auto_math/compression_ratio.php

http://not2fast.wryday.com/turbo/compre ... sure.shtml
average the results

keep in mind that you can easilly run a stattic compressio of 11:1 with aluminum heads if you keep the cam timing in a range so that the DYNAMIC COMPRESSION is CLOSE TO 8:1
take the time to understand the concept,it VERY IMPORTANT

read this

http://www.diamondracing.net/cocalc.htm



http://victorylibrary.com/mopar/cam-tech.htm

http://victorylibrary.com/mopar/otto-c.htm

http://victorylibrary.com/mopar/cam-tech-c.htm

http://kb-silvolite.com/article.php?act ... 42f4eb4c49

http://victorylibrary.com/mopar/rod-tech-c.htm

http://victorylibrary.com/mopar/piston_position-c.htm

http://www.iskycams.com/techtips.html#2003

http://victorylibrary.com/mopar/rod-tech.htm

http://www.zhome.com/ZCMnL/PICS/detonat ... ation.html

http://www.chevytalk.org/threads/showfl ... Post397334

if cams are a mystery please take the time to read these, it will get you a good start

http://www.newcovenant.com/speedcrafter ... haft/1.htm
(read LESSONs 1-8)


http://www.wighat.com/fcr3/confusion.htm

http://www.chevyhiperformance.com/techarticles/95298/

http://www.idavette.net/hib/camcon.htm

http://www.centuryperformance.com/valveadjustment.htm

http://www.totalengineairflow.com/tech/valvelashing.htm

http://www.chevytalk.com/tech/engine/Cam_Selection.html

http://www.chevytalk.com/tech/101/Cam_Theory.html

http://www.babcox.com/editorial/ar/ar119736.htm



http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html

http://home.wxs.nl/~meine119/tech/camqa.html

http://www.chevytalk.org/threads/showfl ... Post200511

http://www.crower.com/misc/valve_timing_chart.html

http://www.speedomotive.com/Building%20Tips.htm

]

http://chevyhiperformance.com/techarticles/94138/

http://www.aera.org/Members/EngineTech/engine.htm

http://www.zhome.com/ZCMnL/PICS/detonat ... ation.html



I just want to justify this by saying really good combo's are never easy and require tons of planing in advance.Right now I am looking at Mahle dished pistons part Number sbc425030I26 that would produce 9.1 compression ratio with 64cc heads.I worry that might be too low of a compression ratio and I would be disappointed.I am researching flat top four eye brow pistons with 72cc heads and still be able to run 87 gas.I'll post about that after I have looked hard at that.