Bled over from another thread talking about dynamic compression ratio affects with changing the Lobe seperation angle:

My theory of why cyl pressure is bled off with a wider LSA is relative to cyl scavaging. Tighter LSAs create more overlap. Overlap is the time that the intake valve and the exhaust valve is open at the same time. At some undetermined point above idle, the velocity of the exhaust gases exiting the cyl creates a low pressure area and helps pull in the intake charge when the intake valve first starts to open before the exhaust valve closes, which is know as cyl scavaging.

Now with a wider LSA you have less overlap and less scavaging thus less air into the cyl causing a lower DCR. At idle or just off idle, when scavaging isnt taking place, you will have more cyl pressure with a wider LSA due to less overlap.

Here is a cool DCR calculator to play with:

https://ls1tech.com/forums/showthrea...dcr+calculator

 

So, what does DCR have to do with this? LOL
Lobe seperation can not change the dynamic compression. The centerlines do however.

 

Ok. Well changing the LSA changes the centerlines of course. Explain...

 

Okay...

220/220 110LSA + 0 = 110ICL/110ECL
-Now tighten the LSA 2*, but keep the same ICL.
220/220 108LSA - 2 = 110ICL/106ECL
-The LSA tightened 2*, but the ICL remains the same.

... Therefore, no change in DCR

You can adjust LSA every which a way, but if the ICL remains the same there is no change in DCR.

ICL and intake duration are the two keys in DCR because these move the IVC.

You already know this. I think I'm missing something in your opening post.

 

so what DCR should people shoot for if they are NA??? and FI??? i have lots of discussion about thunder550s DCR being around 7.5, but he is FI... also seen where turbogibbs dcr is close to 9 (i think)

 

Okay, okay, okay, so let me get this straight. Changing the LSA whilst keeping the ICL the same has no effect on DCR. I see that, but then the new issue becomes: Why do people (including engine builders) use this as gospel when spec'ing an engine to run on pump gas? It's really overlap that bleeds off the cylinder pressure and makes things jive on pump gas. I'm not saying DCR should be discredited by any means, but it should be just one consideration.

 

Arrrrrgh. I'm at home sick with a sinus infection. I don't really have the want, drive or just give a **** to have to think right now. I'll have to pick back up on this later.

 

I'm unsure of what is optimal for pump gas and FI but NA 8.8 is what i've read is the highest you want and that is what I went with. I can tell you from a tuning stand point that having it any higher would not be a good thing. You'd have to pull so much timing that you would have been better off with less compression. Honestly, I wish I would have gone with 11:1 compression so I could have run a tighter LSA. My low to midrange torque isn't where I want it to be.

 

Simply put... Over on tech, there is too large of a focus on DCR instead of valve events.

 

Ok, I started drinking again so I could think more clearly.

Sportside is correct. It is all just a play on numbers. It really is affected by the moving of the intake centerline. If you widen the lsa and keep the same amount of advance on a cam then you get more bleed off....because the intake center line moved.