Yeah hoffie GO!!!

 

No, I'm pretty sure I'm not.

That's exactly what I said already three or four posts ago.

I already said that too. (also several posts ago.)

The valve releifs have almost no overall effect on the compression ratio nor does the process of manufacture. (forged vs. billet vs. hyper) You're talking miniscule amounts of metal that's removed from the piston for valve notching.

It's the shape of the face of the piston, e.g.: is it domed or dished, concave or convex that helps determine CR. If it's concave, or "dished," it lowers the CR ratio. If it's convex, or "domed," it raises it.

I'm already well aware but thanks for the explanation.

"Lift" on a cam shaft is how far the valve is open. Since the valve is typically open during the combustion stroke (All the way up top and making the turn to move down) there is absolutely NO reason the piston would be at the bottom of the cylinder while the valve is open. If that were the case you wouldn't have any positive pressure inside the cylinder since it'd all blow out the open valve as it travels upward in the bore.

Overall lift absolutely does have something to do with piston to valve clearance, as well as duration. If the valve opens too far or too long metal contacts metal.

This isn't opening up the "what about overlap or dual pattern cams" can of worms.

It's not that I don't get it. The problem is you appear to be trying to throw around a lot of key phrases that don't make any sense.

As I said in my other post: The compression ratio is a byproduct of other mechanical "goings on." You don't need the "optimum compression ratio" to run a big cam.

What you need arethe right parts. In having the right parts you'll have a given compression ratio.

The parts make the compression ratio. The compression ratio doesn't make anything.

 

Are you referring to the X1 from MTI or G5X1 from LG.

 

X1 from MTI. I ran 11.30.s with it (cam only). It lopes hard and sounds nasty, it's not overly large so you can keep the idle down around 800, making it sound twice its size.

 

I'll try to clarify since you missed my point.

YOU said:"The compresion ration is a byproduct of chamber volume, piston selection, etc...". The dynamic compression is based on intake valve closing point and that changes cam to cam.

the compression ratio can't be changed on a stock piston bottom end unless you mill heads or use thinner gaskets.....so your first sentence:
The compression ratio has NOTHING to do with cam fitment. is not correct since if you try to run 11.6:1 with a 232/234 cam the thinner gaskets and milled heads will cause the cam to not fit. There is no other way to raise compression on a stock bottom end. So how can you say compression (raising it) doesnt affect piston/valve clearance?

On a final note: changing the cam (and only the cam) to a larger one LOWERS the dynamic compression. Was that what you objected to me saying? That is of course due to the fact that a larger cam closes the intake valve later causing the DCR to drop. Same effect as leaving in the cam and lowering the static compression.

I stated originally that a 232/234 113 or 114 cam with optimum static compression of 11.6:1 -11.7:1 will not fit without notching the stock pistons. There is no way to increase a 6 liter to that static compression level while maintaining a safe P/V distance....i still stick to that.

When you figure out how to run 11.6:1 on a stock bottom end, without notching the piston, with a 232 xer lobe, you let me know.

Are you kidding??? 2cc pison reliefs change compression the same as milling the stock heads .016" That is negligable you? That is about 1/4 of a point in compression.

 

You're talking in circles here and you're giving me a headache.

When people ask you "What's the compression ratio of an LS2" you don't go spouting off about dynamic CR. They're asking "what is the static CR."

Example: The compression ratio in my Cobra motor is roughly 8.5:1.

When the turbos are up and running I'm sure the compression under boost is going to be MUCH higher but that's not what we're talking about.

I don't know how to say this any more clearly: The static compression ratio never changes with a cam swap unless you change a piston, mill a head, or change a head gasket.

I don't know anyone who asks themselves "Hmmm... when I put this cam in, what is my compression ratio going to be".

They say, "I'm running a motor that is X compression (Static), I have Y pistons, and a valve will or will not make contact with a piston. If that's the case I need to notch them so I can run this big *** cam."

Compression ratio isn't discussed until you're trying to figure out what kind of power to expect. It's got little to do with the physical fitment of the cam.

1/4 point of compression isn't something you'd even notice. What do they say, every point is worth 10%? We're talking less than 10 hp give or take on a car making 400 hp in a perfect world.

My Cobra went from 250 hp/ 260 lbft of torque at 9.85:1 compression to 240 RWHP/ 250 RWTQ at 8.5:1 compression. That's 10 hp/tq on more than a full point of compression loss.

What I object to is the positioning. The first person who starts calling around for a cam swap and says "I'm looking for a big cam but want to make sure I have the 'optimum' compression ratio" is going to look like a fool.

 

That's my point right there: it's not the compression ratio that makes the cam not work, but the parts your using. The milled heads, the thinner gaskets, etc...

As I keep saying, the compression ratio isn't a part you install. It's what you end up with when you install parts.

 

Yeah you're right, compression isnt a part. I dont recall saying that. What was I thinking. Stupid me. Take care.