[gtofreek]

I don't think your understanding what I'm saying.

Think about it like this. If you have 2 cams that close the valve at the same time, but 1 cam lets in twice as much air and fuel than the other, do you really think both cams will build the same cylinder pressure during compression stroke?

[johnta1]

So, if you retarded the same cam it wouldn't change the pressure?
(same lift still)



Interesting reading:

Cam Timing

[65 GTO Convert]

Originally Posted by gtofreek

I don't think your understanding what I'm saying.

Think about it like this. If you have 2 cams that close the valve at the same time, but 1 cam lets in twice as much air and fuel than the other, do you really think both cams will build the same cylinder pressure during compression stroke?

It is dynamic compression RATIO. Cylinder pressure or cranking compression can be related but are different. If I have an engine and it has 8.0:1 DCR and 10.5:1 static CR and at 70 F and 29.92 BP it cranks 200 psi. If I go Denver and at 70 F and 24.895 BP it cranks 166 psi I still have 8.0:1 DCR and 10.5:1 static CR.

[johnta1]

^^^

[gtofreek]

Originally Posted by johnta1

So, if you retarded the same cam it wouldn't change the pressure?
(same lift still)



Interesting reading:

Cam Timing

Of course it would. But that's not what I was talking about.

[screamingchief]

The biggest problem with DCR based theory is that it only truly relates to what is happening when the engine isn't actually running.

Once the engine is actually running, all bets are off, as numerous other dynamics come into play that the DCR calculations cannot even begin to account for.

That's the point that Harold B. is often trying to get across when he dismisses the whole DCR thing as a waste of time.

And I believe that's what Paul (gtofreek) was talking about too.

Cylinder filling dynamics in a running engine are based on far more than just the things that DCR calculations account for, and that includes the IC event too.

So if you need to know what the DCR is for an engine while it's cranking over, then I guess DCR theory tells you everything you need to know.

Though it doesn't tell you much of anything useful beyond that.

DCR wont tell you how much HP a given engine may (or may not) make.

And it wont tell you with any reliable accuracy if you can run on a given octane fuel either...

FWIW

Bret P.

[65 GTO Convert]

Is DCR useful? Some people think so, others do not. Will DCR change in a running engine due to things like push rod defection, YES. What DCR is not, is a way to tell what the pressure in the cylinder will be @ IVC. What it is, is a way to tell how much the charge @ IVC weather it is .8 bars or 1.6 bars will be compressed. Things like inertia ramming of the intake charge will increase the pressure @ IVC but that does not change the DCR.

[p4msi1]

Well, with 130 psi cranking compression, I will remove the 30cc dish pistons and replace them with flat tops. At this point I don't care if I have to run a little race gas. I'm looking for the HP. My suspition is that I can get it to run on pump gas on the street and race gas at the track. I will also read that cam article posted earlier.

Steve