hammertime

iTrader: (1)

I don't think the arguement here is that cylinder filling changes, we're all in agreement there. But since you can't change the static compression ratio, DCR will always remain the same without changing the closing point of the intake valve. You are applying your own definition of dynamic to the term, and its simply incorrect.

If I'm not mistaken, what you are describing is more akin to Brake Mean Effective Pressure.

brad8266

iTrader: (23)

yeah thats what i was thinking he was doing

LS1 Swapper

^ Curious how does the cylinder pressure change but the ratio stay the same? Which is exactly my point. DCR is being over simplified so a number can be associated so that people can get their heads around it. To me it's only another part of the equation, but certainly not the end all.

As for saying boost isn't going to change the ratio? Seriously time to go back to high school, or perhaps finish it.

Searching around I found an interesting calculator for this subject

http://www.rbracing-rsr.com/comprAdvHD.htm

hammertime

iTrader: (1)

Ok, using your example above, enter the following values-
Bore in Inches: 3.89
Stroke in Inches: 3.622
Rod Length in Inches: 6.098
Static Compression Ratio: 10.37
Inlet Valve Closes ABDC: 69.5
Boost Pressure in PSI: 0
Target Altitude: 0
You should get 7.96:1, the same value PianoProdigy's sheets give. Now go back and change the boost pressure to whatever you like. Hell, I used 100psi. Let me know if you get anything different than I did.

Any way you slice it, Dynamic compression Ratio remains the same. That link was the first time in this thread that Boost Dynamic Compression Ratio is mentioned. You have only stated that boost changes DCR, which even your example states it does not.

The compression ratio is the volume of the entire cylinder and combustion volume divided by the volume of just the combustion volume. I'm using the term "combustion volume" to mean the combustion chamber in the head, the clearance volume (positive or negative) of the piston, the volume of the head gasket and the volume of any dish or dome. If the combustion volume is 5ci and the cylinder displaces 45ci, then the total volume is 50ci. When compressed into 5ci, it is now 1/10th its original volume - hence a compression ratio of 10:1.

The only difference for Dynamic Compression Ratio is the volume used for calculating is the volume that remains in the cylinder at the exact point where the intake valve closes. To simplify the math for everyone, lets say the valve closes and leaves 40ci left in the cylinder. When that volume is compressed into 5ci, it is now 1/8th its original volume, or a Dynamic Compression Ratio of 8:1. Whether the pressure when the valve closed was 14.7 psi or 100 psi, the volume is what it is, and that is how DCR is calculated.

And thanks for insulting my intelligence

PREDATOR-Z

X2

LS1 Swapper

We are arguing two different points, I don't agree with DCR being more or less useful then SCR as it's simply using valve timing. In short I don't care about the DCR based on valve timing alone as it doesn't tell me anything I consider overly useful. It's just one small part of the picture.

The link I included does show how boost effects compression ratio, you're just looking at it from your point of view(as am I). Not sure why this needs to be argued as it's very obvious. The DCR is calculated in that link the same as your talking about, which is only taking valve timing into account. Again not too ground breaking.

hammertime

iTrader: (1)

Since my words haven't sunk in yet, I'll use these from the link that was first reply
A ratio is a ratio, you can't change it without tearing down the engine.

hammertime

iTrader: (1)

On another note, I'm not sure how much faith I would place in tech info from that site you've listed.
http://www.rbracing-rsr.com/comprAdvHD.htm
More overlap does not lower your DCR. If duration is held constant, then increasing overlap (lowering LSA) will actually raise DCR. Otherwise, just increasing overlap has no relevant effect on DCR.

Other things that calculator has not taken into account:
Humidity
Ambient Air Temperature
Inlet Air Temperature (intercooler anyone?)
Barometric Pressure

I'm not saying boost does not require any consideration, just that it cannot change any of the constants used to calculate the ratio.

Aside from this, as it fits in this thread, boost is not relevant. I only mentioned boost to further illustrate that the volumes are what matters most here. You won't find a pro engine builder out there who thinks that boost raises either compression ratio.

And if you do, I wouldn't use them

helicoil

iTrader: (104)

A ratio is a ratio, you can't change it without tearing down the engine.[/QUOTE]

+1

Definitely cylinder pressure changes between idle and 5000 WOT obviously due to inreased cylinder filling/volumetric efficiency thru valve events (overlap), etc, but the DCR does not. The swept volume in the hole measured when the valve closes is a 'fixed' relationship that will occur at the same point everytime, at any engine RPM or cylinder pressure - boosted or atmospheric.

Pressures CAN change dramatically and do under operating conditions. That is where the increased power comes from.

However, I also agree that the DCR calculations hold similar weight to the head flow number reference you mentioned. It is just 'one' variable. Explain how an engine with a cylinder head with higher flow numbers can be outperformed by an engine with heads with smaller flow numbers..... happens all the time. Same with DCR comparisons. Performance is based on the overall package, not just any one piece. It all has to work together.

As far as DCR is concerned there cannot be a 'blanket' number that should be set as a goal that is the same for every engine. Too many variables......combustion chamber design, plug location, quench, cooling system efficiency, operating temperature, bore size, etc. The DCR of 8.5:1 in a 3.900" bore engine may work well on 91 octane, such as in a LSx engine, but may detonate like crazy in a 4.625" bore BBC engine w/ 124CC iron heads on 91 octane if the engine has a really kick *** cylinder heads/manifold combination simply to due the dynamic (running) pressures generated from this engine. DCR is a good tool for indicating cam profile influences on an engine. In my mind it is similar to taking cranking compression measurements

The true 'dynamics' affecting cylinder filling cannot be represented through DCR numbers solely. VE (volumetric efficiency) is another dynamic measurement of the whole package relative to the atmospheric conditions at hand.

How many of you have used what GM gives us through the scan tool PID called Load, which is really a measurement of VE in percentage. A great tool for diagnosing breathing problems on everyday cars and trucks, but also an indicator of how efficient the engine is. Has anyone exceeded 100% VE on their normally aspirated LSx engines with FAST 90's or LS6 intakes? Many production cars I have tested yield results in the 80%-85% @ 5000RPM WOT range and several normally aspirated race engines I have ran on engine dynos can yield 120%+ VE numbers How can a 'normally' aspirated engine make over 100% VE, meaning how can it intake and exhaust more than what is in the available atmosphere? Could this be from DCR, overlap, the fifth cycle...... great topic to debate, I mean discuss. How volumetric efficient at idle is an engine with 8.5:1 DCR.

helicoil

iTrader: (104)

Read this link, it is not as simple as many believe it to be on this site. If it was everybody would go fast.
http://victorylibrary.com/mopar/cam-tech-c.htm

helicoil

iTrader: (104)

[QUOTE=LS1 Swapper]We are arguing two different points, I don't agree with DCR being more or less useful then SCR as it's simply using valve timing. In short I don't care about the DCR based on valve timing alone as it doesn't tell me anything I consider overly useful. It's just one small part of the picture.


+1

massls1guy

iTrader: (9)

Just wondering what you DCR debate guys have for automotive backgrounds. Do you guys work in the automotive industry, have techincal training, automotive engineering degrees or something like that. Good thread btw.

PREDATOR-Z

DCR is not the MOST important factor and as some of you mentionned, it is part of the overall combo efficiency.
Now this is LSx tech so let us keep it relative to that parameter (LS1)

From R&D and experience (LSx going into its 10th year), we have found that an 8.5>8.7 range contributes to increased performance (more HPs) and still allows us to run 91 octane.

I'm sure you folks have noticed that we all jump at the news of X cam combo has made such and such #s. After many applauses some try to duplicate and are simply unnable to do so. That is when applauses turn into BS flags.

But slapping in that cam X and making BIG power is simply not enough. A combo has a plathera of details to be matched in order to optimize a certain setup. Matching supporting mods, quench, SCR, DCR, lifts, flow, swirl, valve angle ...and the list goes on.

Point is that if you slap X cam in a 10.1:1 combo and it yields x.xx DCR, it will simply make less power than same X cam with 8.7 DCR.
Now you may say it is a meaningless increase, but that increase can be as much as 4>5%