DCR not as important as thought...
Thread Starter
Joined: Aug 2005
Posts: 2,649
Likes: 1
From: Mont Belvieu, TX
Thread Starter
Joined: Aug 2005
Posts: 2,649
Likes: 1
From: Mont Belvieu, TX
From what I can understand is that the exact same "DCR" in two different engines will have different results. There is no "magic" number that you have to have in order to make the engine perform correctly.
As has always been my stance, it is a secondary spec. Make the rest of the Valve events correct for the application and let it fall where it may.
As has always been my stance, it is a secondary spec. Make the rest of the Valve events correct for the application and let it fall where it may.
This is no new news but it is an important point to make. Optimizing DCR will gain you power but there are way too many other variables involved such as valve events, head flow, and quench etc. to let it be the end all deciding factor. It does still remain a good rule to go by but for serious engine builders it is just one of many factors that must be considered to truly optimize an engine package.
It will be a while until I get my swap done to see if it all works or not but I think I have my bases covered.
DCR @ 8.5:1
SCR @ 10.8:1
L92 Heads, light exhaust porting
.035 quench
6.0L engine
DCR @ 8.5:1
SCR @ 10.8:1
L92 Heads, light exhaust porting
.035 quench
6.0L engine
Originally Posted by UDHarold
The intake valve closing point mainly tells where the intake valve closes.
Get your favorite cam grinder to grind 2 cams with the identical profiles, but with TWO different LSAs, say 106 and 108, or 110 and 112.
Install one cam in the engine, then dyno it.
Install the other cam on the IDENTICAL intake centerline, so that opening and CLOSING points are the same for both cams.
The cams will have different cranking compressions, different power curves. This is not to say one is better than the other, only different.
What effects the REAL dynamic compression is HOW MUCH air and gas is in the cylinder when the intake valve closes, not at what degree ABDC the valve closes.
The different exhaust opening points affect the amount of back pressure, ie, reversion, that is present when the intake valve opens BTDC. The amount of reversion affects the rate of cylinder filling on the intake stroke, and the amount of air and gas in the cylinder when the intake valve closes. This is what gives you the actual Dynamic Compression. The operative word is DYNAMIC.
Other cam designers may differ in opinion.
Get your favorite cam grinder to grind 2 cams with the identical profiles, but with TWO different LSAs, say 106 and 108, or 110 and 112.
Install one cam in the engine, then dyno it.
Install the other cam on the IDENTICAL intake centerline, so that opening and CLOSING points are the same for both cams.
The cams will have different cranking compressions, different power curves. This is not to say one is better than the other, only different.
What effects the REAL dynamic compression is HOW MUCH air and gas is in the cylinder when the intake valve closes, not at what degree ABDC the valve closes.
The different exhaust opening points affect the amount of back pressure, ie, reversion, that is present when the intake valve opens BTDC. The amount of reversion affects the rate of cylinder filling on the intake stroke, and the amount of air and gas in the cylinder when the intake valve closes. This is what gives you the actual Dynamic Compression. The operative word is DYNAMIC.
Other cam designers may differ in opinion.
Originally Posted by UDHarold
Here are a few examples, all with the exact same closing point:
44-80 304 108 LSA
40-80 300 110 LSA
36-80 296 112 LSA
Wouldn't you expect these 3 different cams to fill the cylinder with different amounts of air and gas, even though they all have the identical intake closing point?
The cranking compression is related to how much air and gas is in the cylinder when the intake valve closes, not where the intake valve closes.
This gets even more confusing when you get into the actual valve lift curve, which may be different for 2 cams with the identical opening and closing points.
It gets even more confusing when you start looking at unsymmetrical cams vs symmetrical cams, which may have the identical opening and closing points, but different LSAs.
The actual valve lift curve, including where the LSA/ICL are, is what governs the cylinder filling, and therefore the cylinder pressure.
The opening and closing points are just part of this valve lift curve. It is generally easier to explain things in relationship to something simple, like where the valve opens and shuts, than to get into a complex discussion about shapes of valve lift curves.
UDHarold
44-80 304 108 LSA
40-80 300 110 LSA
36-80 296 112 LSA
Wouldn't you expect these 3 different cams to fill the cylinder with different amounts of air and gas, even though they all have the identical intake closing point?
The cranking compression is related to how much air and gas is in the cylinder when the intake valve closes, not where the intake valve closes.
This gets even more confusing when you get into the actual valve lift curve, which may be different for 2 cams with the identical opening and closing points.
It gets even more confusing when you start looking at unsymmetrical cams vs symmetrical cams, which may have the identical opening and closing points, but different LSAs.
The actual valve lift curve, including where the LSA/ICL are, is what governs the cylinder filling, and therefore the cylinder pressure.
The opening and closing points are just part of this valve lift curve. It is generally easier to explain things in relationship to something simple, like where the valve opens and shuts, than to get into a complex discussion about shapes of valve lift curves.
UDHarold
From what I can understand is that the exact same "DCR" in two different engines will have different results. There is no "magic" number that you have to have in order to make the engine perform correctly.
As has always been my stance, it is a secondary spec. Make the rest of the Valve events correct for the application and let it fall where it may.
As has always been my stance, it is a secondary spec. Make the rest of the Valve events correct for the application and let it fall where it may.
I believe ths is something that many of us already know. I get the valve events where I like them and look at the DCR after.
I do allow it to play a roll depending on the customer's goals, but in the long run if a combo is 8.2 and everything else looks perfect vs having a combo at 8.5 and everything else looking pretty good I will take the 8.2
I have never designed cam's around DCR. I kinda view DCR as........ if you can have a hot blonde or a hot blonde with bigg ******* might as well take the one with big *******. If I can get a cam setup where I want it and get the DCR on the money too I might as well.
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Thread Starter
Joined: Aug 2005
Posts: 2,649
Likes: 1
From: Mont Belvieu, TX
I believe ths is something that many of us already know. I get the valve events where I like them and look at the DCR after.
I do allow it to play a roll depending on the customer's goals, but in the long run if a combo is 8.2 and everything else looks perfect vs having a combo at 8.5 and everything else looking pretty good I will take the 8.2
I have never designed cam's around DCR. I kinda view DCR as........ if you can have a hot blonde or a hot blonde with bigg ******* might as well take the one with big *******. If I can get a cam setup where I want it and get the DCR on the money too I might as well.
I do allow it to play a roll depending on the customer's goals, but in the long run if a combo is 8.2 and everything else looks perfect vs having a combo at 8.5 and everything else looking pretty good I will take the 8.2
I have never designed cam's around DCR. I kinda view DCR as........ if you can have a hot blonde or a hot blonde with bigg ******* might as well take the one with big *******. If I can get a cam setup where I want it and get the DCR on the money too I might as well.
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Originally Posted by MPHmotorsports
I have never designed cam's around DCR. I kinda view DCR as........ if you can have a hot blonde or a hot blonde with big ******* might as well take the one with big *******. If I can get a cam setup where I want it and get the DCR on the money too I might as well.
I know that you probably weren't trying to make a sig quote, but I'm 100% sure that someone will pick it up pretty soon anyway... 
TECH Junkie
Joined: Dec 2005
Posts: 3,092
Likes: 11
That is one of the problems with Speedtalk. Most of the guys on that board build high rpm racing motors. It is true DCR is not that big of deal to them when they are talking 7000-9000rpm motors. You have to understand where they are comming from.
However on a street application DCR is one of the tools to be used. I would reread that thread and pay close attention to the comments by MadBill. His comments are more applicable to what we are doing on the street where our motors spend a lot of time between 2000 and 3000rpm.
However on a street application DCR is one of the tools to be used. I would reread that thread and pay close attention to the comments by MadBill. His comments are more applicable to what we are doing on the street where our motors spend a lot of time between 2000 and 3000rpm.
Teching In
Joined: Sep 2007
Posts: 38
Likes: 0
This effective compression ratio is dependent on several factors, but it comes down to volumetric efficiency. As scavenging becomes more effective, volumetric efficiency improves. Yes, this can be affected by LSA, but also at what RPM that LSA and exhaust is tuned to as they are set factors, and throughout the RPM range many things are changing. Others are intake and head flow, these have a direct relationship with V.E. and, again, are tuned to only one specific RPM. Has anyone ever heard of Variable Valve Timing??
The point here is not to believe or unbelieve everything that you hear about on the internet, but to understand that this is a highly complex system, but not a totally mysterious one.
The first time I brought this up alot of eyes rolled............
The point here is not to believe or unbelieve everything that you hear about on the internet, but to understand that this is a highly complex system, but not a totally mysterious one.
The first time I brought this up alot of eyes rolled............
Thread Starter
Joined: Aug 2005
Posts: 2,649
Likes: 1
From: Mont Belvieu, TX
This effective compression ratio is dependent on several factors, but it comes down to volumetric efficiency. As scavenging becomes more effective, volumetric efficiency improves. Yes, this can be affected by LSA, but also at what RPM that LSA and exhaust is tuned to as they are set factors, and throughout the RPM range many things are changing. Others are intake and head flow, these have a direct relationship with V.E. and, again, are tuned to only one specific RPM. Has anyone ever heard of Variable Valve Timing??
The point here is not to believe or unbelieve everything that you hear about on the internet, but to understand that this is a highly complex system, but not a totally mysterious one.
The first time I brought this up alot of eyes rolled............
The point here is not to believe or unbelieve everything that you hear about on the internet, but to understand that this is a highly complex system, but not a totally mysterious one.
The first time I brought this up alot of eyes rolled............
My point was that DCR was brought up a couple of times by people in the "know" around here and then everybody ran with it. I can't tell you how many cams have been designed around here with that as the #1 priority.
I never said it was not a valid thing to look at, it is just something I don't overly concern myself with as I think it is a byproduct of the proper valve events for the application.
LS1Tech Sponsor
Joined: Feb 2006
Posts: 1,170
Likes: 1
From: Rockfield Kentucky
Never ever have I heard the term "DCR" brought up by ANY engine builder or racer that I have talked. And trust me I have talked to many.
Stick with talking about cams too much
To each his own I say!
Robin
Stick with talking about cams too much
To each his own I say!
Robin
This is a pretty good article about pump gas and compression:
http://members.uia.net/pkelley2/DynamicCR.html
http://members.uia.net/pkelley2/DynamicCR.html
Teching In
Joined: Sep 2007
Posts: 38
Likes: 0
The dynamic root equation is Force = Mass * Acceleration. There is simply no account for the dynamic flow responses such as inertia, vacuum, and the tuned lengths, volumes, and timings (think scavenging again) in the calculations that attempt to account for the change in V.E. with respect to RPM due to these characteristics. That being said, let's pretend that the term "dynamic" is a poor choice of words, less than scientific.
However, I see alot of people jumping ship on this concept, to that I would offer this: There are many on this board that can give accurate results/predictions on what this value offers and where the limits are. This practical experimentation is valuable. These limits are absolutely something to heed.
However, I see alot of people jumping ship on this concept, to that I would offer this: There are many on this board that can give accurate results/predictions on what this value offers and where the limits are. This practical experimentation is valuable. These limits are absolutely something to heed.
