calculating comp ratio from cranking comp

[waltcentral]

  I thought I would share something I found interesting. I had been searching to make some sense of cranking compression readings. The formula allows you to plug various compression ratio's and see how the match up.
https://www.apriliaforum.com/forums/showthread.php?221286-Cranking-Compression-Formula

I haven't reached the end of the interweb's yet but getting there.

[motorhogman]

I get a "site can't be reached message.  Tried from here and copy and pasted..

[Burnout]

Look for a dynamic compression ratio calculator....

[cheech]

Look for a dynamic compression ratio calculator....

One here on HTT. An Excel file that opens when you click on it. Reply 3 In Here
Just FYI I was "corrected" as here it's called Corrected compression. LOL
But I and a lot have called it dynamic.

And the formula in that link in the first post to me is highly suspect. It doesn't account at all for intake valve closing event. And I'm sure hopefully some of the physicists on here can explain his VE constant.

[Hybredhog]

     Try this one

www.rbracing-rsr.com

[motorhogman]

     Try this one

www.rbracing-rsr.com

Nice site ! Lots of info.  They sure do have some good looking exhaust systems..

[Hybredhog]

     Try this one

www.rbracing-rsr.com

Nice site ! Lots of info.  They sure do have some good looking exhaust systems..

   Just be ready to wait a couple decades to get one 

[ghostrider]

I use this calculator,  very handy when trying to select a cam and set your compression ratio.

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

[motorhogman]

     Try this one

www.rbracing-rsr.com

Nice site ! Lots of info.  They sure do have some good looking exhaust systems..

   Just be ready to wait a couple decades to get one 

Nice looking.. And I'm sure pricey $$$$$$  Nothing i would venture putting on my stock box 20 year old FLH .. I do like the site though...

[waltcentral]

the forum I linked to was a very general guide and did not include bore and stroke and combustion chamber volume. It sort of assumes we are working with a running engine and makes some assumptions. Is it scientific, no. Is it useful, yes.  In my case I can assume my could not produce the cranking compression of 155-160# if the bike still had the stock compression of 7.5-1.
I have been struggling with  rebuilding  an engine that did not leak, that did not smoke at all, and gets 42-45 MPG. A couple of close buds said why attempt to fix what is not broken. Engine runs and is quiet.  They also said it was near impossible a 1983 Shovel would run as well if it had not been gone through.  Even my spark plugs look closer to what you find on a fuel injected bike.

[hbkeith]

rebuilding  an engine that did not leak, that did not smoke at all, and gets 42-45 MPG. A couple of close buds said why attempt to fix what is not broken. Engine runs and is quiet. 

have to agree with the Buds

[Hossamania]

rebuilding  an engine that did not leak, that did not smoke at all, and gets 42-45 MPG. A couple of close buds said why attempt to fix what is not broken. Engine runs and is quiet. 

have to agree with the Buds

Me too.

[One4Tone]

...a simple way to look at compression readings as a MINIMAL figure is 15 to 1 ratio on a COLD motor with 10 to 1 compression piston should give you 150 pounds on the compression reading...9 to 1 =135  and so on. As I said these are minimal figures. Combustion chambers play into this big time. I am talking standard production engines. Some have quoted much higher figures...that is all good...I would be happy to have readings of 180, 170...I am building a small block 350 chevy block with a 400 crank to get 383 c.i....Now I am using Edelbrock RPM heads #60899, 2.02, 1.6 , 64 c.c. heads. After breakin anything above 150-to 160 pounds reading would make me very happy and the little chevy 2 should be pretty quick.

[rigidthumper]

...a simple way to look at compression readings as a MINIMAL figure is 15 to 1 ratio on a COLD motor with 10 to 1 compression piston should give you 150 pounds on the compression reading...9 to 1 =135  and so on. As I said these are minimal figures. Combustion chambers play into this big time. I am talking standard production engines. Some have quoted much higher figures...that is all good...I would be happy to have readings of 180, 170...I am building a small block 350 chevy block with a 400 crank to get 383 c.i....Now I am using Edelbrock RPM heads #60899, 2.02, 1.6 , 64 c.c. heads. After breakin anything above 150-to 160 pounds reading would make me very happy and the little chevy 2 should be pretty quick.

I "think" you are referring to the 14.7 PSI air has at sea level, multiplied by the static compression? So 14.7 * 10 ( your 10:1 motor) should have a minimum of 147 CCP?

Seems to me that ignores the intake cam closing point, ( which affects the CCP, as does altitude, valve/ring seal, etc)  when the chamber is becoming sealed, and compression of the cylinder's contents actually starts to occur.

[One4Tone]

...well,...I think both valves have to be fully closed to register a reading on the compression stroke... 

[Racepres]

...well,...I think both valves have to be fully closed to register a reading on the compression stroke... 

and I agree... but it ain't very Far into the compression stroke that the Valves are both closed...or there would not be Compression!!!! Yes... the exhaust valve closes sometime into the stroke... but Not that Far!!!

[Burnout]

Intake valve closing timing is what sets the dynamic compression.

[cheech]

Yes... the exhaust valve closes sometime into the stroke... but Not that Far!!!

The compression stroke?? 

[Racepres]

Yes... the exhaust valve closes sometime into the stroke... but Not that Far!!!

The compression stroke?? 

Yea kinda misstated.. but the Exhaust valve is Open going into the Intake stroke...and as Burnout states, the Point at which the Intake Closes, going into Compression stroke,  does determine dynamic compression... My Bad for confusing this Issue...I'll slink away now....

[Hillside Motorcycle]

We've used Accelerator programs here for almost 25 years.

[waltcentral]

  I should have started this post with ESTIMATING comp ratio from cranking comp.

[thumper 823]

Intake valve closing timing is what sets the dynamic compression.

ding ding

we have a winner.

One can change their cold crank a BUNCH with different camshafts.