Cylinder Pressure

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Bad Sport

Bad Sport

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I need a general understanding of duration vs. cylinder pressure. What builds more, less etc. Why/how is it used when building an engine.

These cam dynamics escape me.

How does it work. Keep it simple please. :D
 
It's not necessarily the duration. It has more to do with when the intake valve closes. Imagine just one cylinder. When the piston is at the bottom of the intake stroke just starting it's way up on the compression stroke. The sooner the intake valve closes, the more mixture is trapped in the cylinder and the more pressure will be created at TDC. If a camshaft has a later closing event, pressure starts to be built later and not as much pressure will be produced. Is that simple enough?
 
hi, also, port velocity and flow are important along with valve closing. faster velocity will fill cylinder faster along with valve closing at the right time produces very high cylinder pressures. which makes more torque and power.
 
RustyRatRod said:
It's not necessarily the duration. It has more to do with when the intake valve closes. Imagine just one cylinder. When the piston is at the bottom of the intake stroke just starting it's way up on the compression stroke. The sooner the intake valve closes, the more mixture is trapped in the cylinder and the more pressure will be created at TDC. If a camshaft has a later closing event, pressure starts to be built later and not as much pressure will be produced. Is that simple enough?
Click to expand...

That makes sense, now to find a cam card for a cam I have.
 
Lonewolf3165 said:
Look up Dynamic compression vs. static compression ratio. That may shed some light on it.
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Well, I have read the link posted above a time or two, having found it a while back and bookmarked it.

I'm still "in the dark" I guess.
 
Yeah, that's what it is, what I'm trying to determine is how duration, valve timing etc affects cylinder pressure and or compression.

I have read countless times on here about bleeding off cylinder pressure etc, etc. I'm trying to get a grasp on it.

Some things come easy to me, but this ain't one of em. lmao
 
More advertised duration,usually needs more physical compression (milled heads/flat top pistons,etc). Split pattern cams,bleed off duration to a degree as well. RRR's definition,nails it dead on. If that intake valve hangs open longer,cylinder pressure drops by that valve hanging open .10 degrees is a HUGE difference here. You bump the mechanical compression up,to compensate. It's a fine line of balancing it here.
 
Bad Sport said:
OK, here's the card:
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IMO, that is an excellent grind for a street/strip car. I wouldn't run it with anymore than about 9.5:1 though. Even using the United Engine formula and adding 15* to the IVC it's still only 50*. That's pretty mild.....but that's a good thing because it will keep cylinder pressure pretty high while not having to use a stupid high static compression ratio. IMO 9.5 would be dead perfect. It oughtta run like a ***** in church.
 
RustyRatRod said:
IMO, that is an excellent grind for a street/strip car. I wouldn't run it with anymore than about 9.5:1 though. Even using the United Engine formula and adding 15* to the IVC it's still only 50*. That's pretty mild.....but that's a good thing because it will keep cylinder pressure pretty high while not having to use a stupid high static compression ratio. IMO 9.5 would be dead perfect. It oughtta run like a ***** in church.
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Well, 9.5 is what I'm shootin' fer, lol.


Even using the United Engine formula and adding 15* to the IVC it's still only 50*.
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You lost me.
 
From the link above:

Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.
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I understand how to check the cc volume, but how do you check cylinder volume (sweep volume) as they are discussing it in the article?

Lost in space. :D
 
Bad Sport said:
From the link above:



I understand how to check the cc volume, but how do you check cylinder volume (sweep volume) as they are discussing it in the article?

Lost in space. :D
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Funny that you mention that. I'm thinking about doing a thread on how to find that out. I have most of the pictures taken, just have a few more pictures to take and process, then I can start a "How to" thread. It's not that difficult if you do it the way that I do.

If you sit tight and give me some time, I will take the last few pictures and start the thread. Then I can come back to this one and give a link.
 
Bad Sport said:
Well, 9.5 is what I'm shootin' fer, lol.




You lost me.
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The United Engine dynamic compression ratio calculator requires you add 15* to the IVC IF it is measured at .050", which yours is.

Simply put, dynamic compression can also be called effective or running compression, since it takes into account the camshaft events.
 
And when we talk about 9.5 to 1 compression are we speaking Dynamic or Static? I see there's a difference.
 
Bad Sport said:
And when we talk about 9.5 to 1 compression are we speaking Dynamic or Static? I see there's a difference.
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Static: mechanical compression ratio,(head cc's head gasket thickness,piston deck height). It becomes dynamic,when the cam events are applied.
 
Abodybomber said:
Static: mechanical compression ratio,(head cc's head gasket thickness,piston deck height). It becomes dynamic,when the cam events are applied.
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So when figuring dynamic, say 9.5 to 1, we would need a higher static compression?

If I read that right in the article. :banghead:
 
Bad Sport said:
So when figuring dynamic, say 9.5 to 1, we would need a higher static compression?

If I read that right in the article. :banghead:
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IIRCif I am not mistaken,the dynamic usually drops from the static ,when you apply valve timing events.(I have used a couple of grinds from Crower like this.They pull like crazy,IMO...
 
9.5 is static in this conversation. Dynamic for pump gas they SAY is 8.5 max, but I like to play it safe and stay just under or right at 8. Because of things like idling in traffic on a hot humid day causing heat soak, detonation becomes more of a problem. Better to be safe than sorry.
 
RustyRatRod said:
9.5 is static in this conversation. Dynamic for pump gas they SAY is 8.5 max, but I like to play it safe and stay just under or right at 8. Because of things like idling in traffic on a hot humid day causing heat soak, detonation becomes more of a problem. Better to be safe than sorry.
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Ah, OK, well that makes sense too, because in the article it did say something about the "safe range" to be in, which was actually lower than the static.

From the article:
It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane.
Click to expand...

When compression is discussed in so many threads on here, it leaves one wondering which one we are talkin about.

Cool, now to figure what I have, and where I need to be.
 
Looks like with that cam, you're going to need to stay at 9:1. I got it at 9.2 and with that cam your dynamic compression is almost 8.2. That's skatin for a pump gas street motor with no quench, IMO.
 
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