Compression Ratio Calculations

[Guest Anonymous]

COMPRESSION RATIO? WHAT IS THAT?

 

NOTE: IF YOU HAVEN'T THE EXPERIENCE OF COMBINING CYLINDER VOLUME, QUENCH AREA, GASKET THICKNESS, +/- DECK CLEARANCE, SWEPT VOLUME AND STATIC COMPRESSION RATIO INTO ONE PACKAGE AND USING THIS TO EXTRACT THE TRUE "DYNAMIC COMPRESSION RATIO", THEN I DON'T THINK YOU CAN HELP HERE.

 

I admit that I am terrible with math, but to make matters even worse I find that the available formulas online are neither compatible with my brain or each other for that matter.

 

I am using the Dyno 2000 Program to simulate an engine build. I am working the figures for a 4.625 bore x 4.750 stroke BBC. Everywhere I read, these blocks are always referred to a 632cui. block. I spent a lot of time doing all the cc and cui conversions back and forth and finally came up with a list of values and was ready to go. And I did go, using a certain Dynamic Compression Calculator online. As I was concluding, I happened to notice that the calculator was giving a result concerning 638cui instead of the standard 632cui. I mulled this over a little, did the calculation manually and concluded they were allowing for the Piston to wall x top ring depth areas. It appears that this figure is around 0.8cui/cyl. with this particular build. However I am not sure I am correct about this situation at all.....just guessing.

 

Another situation exists when I try to change the C/R by reducing the piston "Compression Height". I expect this to lower the C/R but it has no effect whatsoever on the Dyno 2000. Apparrently this sort of action is not factored in.

 

Then I tried to dish the piston (it's a ROSS flat top) and nothing there either. I am familiar with adding new item to Dyno 2000 but I am stuck here. Does everyone finally have to rely on the piston vendor to figure out what they need?

 

Also, I have E-mailed Dyno 2000 tech help to get an explanation of just what their shown values of compression mean overall. I mean are these numbers for the dynamic ratio or what? And what do they consider detonation safe compression-wise on preium pump gas for example? Come! Let us reason together!.....LOL

[Kevin Shasteen]

Spiirit,

 

The Bore x Stroke you have chosen, 4.625 x 4.750, doesnt equate to 632...it equates to 638.4; so the displacement calculators are correct.

 

Example:

 

Displacement Formula is,

Bore x Bore x Stroke x .7854 x #of Cylinders

 

4.625 x 4.625 = 21.390625

 

21.390625 x 4.750 = 101.6054688

 

101.6054688 x .7854 = 79.80093516

 

79.80093519 x 8 = 638.4074813

 

or, 638.41 cubic inches.

 

So the Dyno 2000 program isnt taking into consideration the Piston's Top Ring cc's, rather it is simply calculating the bore & stroke alteration you have made. The piston's top ring cc's will be included in the "Total Chamber Volume cc's" once you have chosen your "Compression Ratio". This will be the Static Compression Ratio & not the Dynamic Compression Ratio. Even tho there is no "data input" for your Dynamic Compression Ratio their program will calculate the DCR once you have chosen your camshaft's profile: without the DCR designed into the program you could not get a theoretical dyno run w/out it.

 

As far as the Displacement input-this is basically for the program itself so that the program, when ran, recognizes distinctions between engine "family series": such as certain distinctions between a small block -vs- a big block...as in its breathing capabilities and or limitations.

 

This way, once the computer recognizes a certain engine's family series it can then calculate the other formulas that go into the internal combustion engine; as in cylinder heads, intake systems & exhaust systems, ect., ect.

 

BTW: dont look at Dyno 2000 as an absolute, rather-look at it as a means for recognizing patterns. If you learn the math yourself-then you can apply that knowledge to the computer programs for a deeper insight if/when you actually do use a computer dyno type program. As far as I am concerned, the missing link to understanding IC spark engines is in understanding the Dynamic Compression Ratio, as it is the bridge that bridges the gap in choosing secondary engine components relative to engine performance expectations.

 

You may also want to read the book Desktop Dyno that came w/the earlier outdated Dyno program. The book has some interesting comments about camshaft timing events, airflow....and the book also addresses the hp/torque issues you asked about in another thread.

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

ITS NOT THAT HARD AFTER YOUVE DONE IT A FEW HUNDRED TIMES,READ this THEN

(1) down load this software

(bottom of the page)

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

 

heres one just for static compression

 

http://www.turbofast.com.au/TFcomp.html

 

heres one for supercharged

 

http://www.turbofast.com.au/TFcompB.html

 

only dynamic compression counts, the engine compresses NOTHING untill both valves are closed

 

this will also help

 

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

 

some of the best basic cam info youll find so read this first http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm (lessons 1-8

http://www.mercurycapri.com/technical/engine/cam/lca.html

 

http://ctfba.tripod.com/main/technical/cams/cambasics/cambasics.htm http://ctfba.tripod.com/main/technical/cams/cambasics/GraphAttack.htm http://ctfba.tripod.com/main/technical/cams/cambasics/GraphAttack.htm http://moparjimsgarage.virtualave.net/camvalve.html

http://www.federal-mogul.com/speedpro/camshafts_speed_pro.html look closely at the duration used for each MATCHING rpm range. ALSO KEEP IN MIND THE DCR AND OVERLAP MUST MATCH

look here

these are the valve timeing overlap ranges that are most likely to work correctly

trucks/good mileage towing 10-35 degs overlap

daily driven low rpm performance 30-55degs overlap

hot street performance 50-75 degs overlap

oval track racing 70-95degs overlap

dragster/comp eliminator engines 90-115 degs overlap

but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range

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

I was asked about a previous post,my example of how do headers work? what I think your refering to was when I was trying to explain how headers help pull the exhaust out of the cylinders by useing the energy from the previously fired cylinder mass of exhaust rapidly moveing away from the exhaust valve causeing a negitive pressure at the exhaust valve that helps scavage the cylinder. what I said was you can demonstrate that negative presure caused by a rapidly moveing mass by getting a 2.5"x5' long section of pvc pipe hold it horizontally and insert a full coke can with some tape wrapped around it so that it just slides easily into one end of the 5' long pvc pipe, now put your hand over the end of the pipe you just installed the full coke can into and rapidly drop the other end strait down, as the coke can slides too the floor you will feel the negitive pressure on you hand caused by the falling mass. now a cylinders volume of exhaust moveing through a primary header tube works in much the same way, while it weights much less than the coke can its moveing far faster and carries enought energy that the same negitive pressure is formed at the exhaust valve by the slug of exhaust gas traveling in the header primary tube, now ideally if the header primary tube is long enought and of the correct dia. to still contain the mass of exhaust gas at the rpm that matches the cam timeing and cylinder volume,a peak of that negative pressure will be forming behind the exhaust valve as it opens during the rpm range where the engine spends the majority of its time, now temp. cylinder volume, compression ratios,cam timeing and interior pipe dia. all effect the rpm range that this effect peaks at effiency wise but in most v-8 engines a dia equal too the exhaust port and about 32"-39" will put that negative pressure wave caused by the previous fired cylinder at the exhaust port in the 4000-6000rpm range, look at this chart,

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

now figure that exhaust is 1300f or 977k so thats about 25" at 6000rpm and about 39" at 4000rpm to get that pressure wave to work for you scavageing the cylinder(helping to pull exhaust out and the new intake charge into the cylinder.)hers stuff to read,

 

http://www.burnsstainless.com/TechArticles/Theory/theory.html

 

http://www.engr.colostate.edu/~allan/fluids/page5/page5f.html

 

http://www.engr.colostate.edu/~allan/fluids/page5/page5f.html

 

http://www.feuling.com/randd/articles/art11.htm

 

BTW it should be obvious why shorty headers don,t make as much power,....the primary tubes being much shorter are also much less effective at controling that negative pressure wave timeing and strength over anywhere near the same rpm range.

 

 

read this

http://www.mercurycapri.com/technical/engine/cam/lca.html

keep in mind the same thing is happening on the intake side of the engine in that properly designed intake ports use the inertia of the coluum of air moveing into the intake/port/cylinder to INERTIALLY RAM the the coluum of air into the cylinders durring some rpm ranges, if the two effects can be matched to the correct cam timeing the cylinders can be packed with more that 100% of the air normally filling the cylinders voluum/space at outside air pressure there bye allowing the engine to build exceptional torque by burning greater amounts of fuel/air mix for its size at that rpm range

[Guest Anonymous]

Well, Thank you Kevin!

 

Yes, I picked up that 632 cui. number after seeing it repeated three or four times while looking at custom blocks. I then had no reaso to suspect it was erroneous and never checked it. Later I started running into 638 online and it also started changing my dyno entry to that effect. Finally it sunk in and I checked and you know what..... ha ha! Another one for you is that some of the sites call out 630 instead of 632 or 638. Very interesting!

 

As an added note of interest, I E-maled those Dyno guys with the question about their compression ratio box data (whether static or dynamic) and they stiffed me....LOL! By Using other programs and reading up online, it soon became apparrent that they were showing static values, ya.

 

Also I was confused about "chamber Volume" in cc's with one critical (to me) program calculator because they were NOT specifying the fact that they were including the "Compressed Gasket" volume as inclusive with the "Head chamber" volume, or so I thought anyway. I stay up very late some nights doing numbers and stuff, I see it is giving me problems, a few.....LOL

 

Thank's again!

[Guest Anonymous]

A 632 is really a 631.5. It is a 4.600 bore with a 4.75 stroke. A 638 is 4.625 bore. On the dyno 2000 compression calculator run it as a flat top and subtract the cc of the piston dome from the CC of the head combustion chamber and it works fine.

[Guest Anonymous]

Ya Tex. I agree with all that ok. Thanks!