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Here's the typical diagram showing the impact on duration from different rocker ratios that I'm thinking of:
Adam
Yes, the effect is more prevalent in the middle of the lobe. I remember back in the day when everyone was putting 1.72 rockers on stock 5.0 Mustangs, you could actually hear the slight duration increase at idle.
The Lunati cam card lobe timing events are calculated from the duration @.050 not advertised duration.
The actual closing event for the Intake lobe is 60* ABDC...
BTW That's actually NOT the real degree that the valve will hit the seat because advertised duration's are based on .006 of tappet lift and none of this takes into account rocker arm ratio, valve train deflection, ect..... but why don't you actually breaking out a degree wheel and dial indicator on the valve spring retainer some time and see for yourself.
So how do you calculate those fancy cam lobe events? It's very easy grade school math.
We know the advertised duration is 268 degrees and we know the Intake Centerline is 106*.
To find the Opening Event you take the Duration (in this case the Advertised Duration) and divide it 2. 268/2=134. Then you take the remainder of 134 and subtract from it, the lobe centerline which is 106. 134-106=28* BTDC.
To find the Closing Event you take the Duration and subtract 180. 268-180=88. Then you take the sum of 88 and subtract the Opening Event which we just calculated as 28. 88-28=60* BBDC.
That explains a ton. Thanks for the lesson
Does this approach work with non-symmetrical lobes on your cam?
Originally Posted by rklessdriver
Somethings to ponder about Dynamic Compression Ratio (DCR).
First off - It's a very BIG waste of time to calculate it. It's a made up number that means almost nothing and has been perpetuated on the internet for years as something actually useful, when in reality it's nothing of the sort. It doesn't tell you anything useful and I guarantee you will never build a good engine compromising cam events so that you can fit them to some magical DCR number.
You really don't use this at all? If I said someone had a street engine that they designed with a quench of .065 with a DCR of 9.0 there is nothing you would glean from it?
Heck, I'm reading one of Vizard's books and he seems to start off here. He starts with a max cylinder pressure in mind and deducts 5 psi per octane or 8-10F in intake charge temperature, then uses that to back into DCR.
Then there is one of Grumpys charts that I found useful as a decent starting point:
If you don't start with SCR and DCR, where do you start?
Originally Posted by rklessdriver
Even the name "Dynamic Compression Ratio" is wrong because there is nothing "Dynamic" about it.... it attempts to calculate the mass/volume of Air and Fuel in the chamber based on a static number - The Intake Closing Point/Degree/Event (which I have already pointed out is very flawed because it is NOT the actual point which the valve closes)....
Also if there were anything Dynamic about DCR, it'd take into account engine RPM, Ram Effect and Volumetric Efficiency - All of which effect the mass/volume of Air and Fuel that enters the chamber before the valve closes.... It doesn't even take into account overlap which has a HUGE effect on the start of the Intake cycle.
What you are describing is compression pressure, not compression ratio, no?
Does this approach work with non-symmetrical lobes on your cam?
You really don't use this at all? If I said someone had a street engine that they designed with a quench of .065 with a DCR of 9.0 there is nothing you would glean from it?
Heck, I'm reading one of Vizard's books and he seems to start off here. He starts with a max cylinder pressure in mind and deducts 5 psi per octane or 8-10F in intake charge temperature, then uses that to back into DCR.
Then there is one of Grumpys charts that I found useful as a decent starting point:
If you don't start with SCR and DCR, where do you start?
What you are describing is compression pressure, not compression ratio, no?
Yes the math I laid out to find opening and closing points will work with Symetrical or Asymetrical lobes.
I really do not use DCR for anything. I have calculated it with Peter Kellys calculator for customers that wanted to know what the DCR is on their engine. In the example you give I would glean that someone made a math mistake most likely.
So, I look at opening and closing events based on what I am trying to do. I use my experience with what has worked for us before.... I usually start with the Intake closing point and go from there. Once I have what I feel to be the ideal events mapped out, I calculate out what the durations and lobe centerlines correspond to... then I find a couple different lobe series that have whatever opening/closing rate I feel is appropriate for the application and pick the 2 lobes from each series that get me the closest to my ideal events. Then I look at my options and make a descision on what 2 lobes I'm going to use..... All this is based off experience and when I dont feel comfortable about something I ask someone else I trust to check my thinking.
What I'm describing in my examples are "Dynamic Events" vs Static events. DCR is a calculation based upon a static event and measures an obviously static amount of mass and it does not take into account any dynamic events at all. If you want to compare your DCR with others DCR... thats cool but I havent found anything comparing them and I did so for a while trying to figure out how it might be useful.... I even went back and calculated some stuff my father built back in the 1970s.... far as I can tell, you are not going to know anything useful based off the DCR number and those guides are bunk. I've got engines with 13.25 SCR and 8.7 DCR... guarantee it wont live long on 93octane.... also got engines with 11.50 SCR and 8.7 DCR that run just fine on 93octane.
Just want to add the stroker block was purchased from Coast performance and transmission in Toms River NJ...
Anybody had any work done at that shop? He always has a lot of vettes and hot rods outside the shop..
As far as the cam that's in there now, I remember him telling me it is a custom grind......Its similar to comp cam 280/280 duration with a 480/480 lift...
How much would that differ from the lunati cam I mentioned earlier?
What I'm describing in my examples are "Dynamic Events" vs Static events. DCR is a calculation based upon a static event and measures an obviously static amount of mass and it does not take into account any dynamic events at all. If you want to compare your DCR with others DCR... thats cool but I havent found anything comparing them and I did so for a while trying to figure out how it might be useful.... I even went back and calculated some stuff my father built back in the 1970s.... far as I can tell, you are not going to know anything useful based off the DCR number and those guides are bunk. I've got engines with 13.25 SCR and 8.7 DCR... guarantee it wont live long on 93octane.... also got engines with 11.50 SCR and 8.7 DCR that run just fine on 93octane.
Will
I respectfully disagree with this or not understanding exactly what you mean. DCR is not measuring or a measurement of any mass. It is a dynamic measurement when cam events change such as newer vvt in use today. It's a measure of pressure related to the intake valve closing point to tdc as the piston moves up the cylinder building pressure. Pressure cannot start to increase until the valve is closed. If too much pressure is built up by not allowing for an 'optimal' valve closing point for a given cam profile and scr, then detonation may occur. The higher the scr, the higher the risk of detonation with too high of a cylinder pressure for the octane of the gas and engine coolant temperature. This is clearly demonstrated in Grumpy's graph. I don't build hipo engines for a living so the DCR calculator worked for me. My scr is 10.7, dcr 8.3 and combined with afr heads, 180* engine temp and 91 octane, I have not detected any detonation and it coincides with the graph Not saying detonation isn't happening but my engine has spun to 6400 many times and the hyper pistons are doing fine so far. For guys that do build engines for a living like yourself, I assume, and Straub et al, your experience doesn't require a dcr calculation because you just know it will perform fine. But that's not to say DCR is useless as a tool. I feel it helped me along with the advice of others in here to build my engine.
Does this approach work with non-symmetrical lobes on your cam?
You really don't use this at all? If I said someone had a street engine that they designed with a quench of .065 with a DCR of 9.0 there is nothing you would glean from it?
Heck, I'm reading one of Vizard's books and he seems to start off here. He starts with a max cylinder pressure in mind and deducts 5 psi per octane or 8-10F in intake charge temperature, then uses that to back into DCR.
Then there is one of Grumpys charts that I found useful as a decent starting point:
If you don't start with SCR and DCR, where do you start?
What you are describing is compression pressure, not compression ratio, no?
A good point has been made that the "DCR" is really "Adjusted CR" (adjusted for the important intake valve close (IVC) event) to determine the actual volume of air that's trapped in the cylinder before compression starts; it's not actually "Dynamic" or changing, unless you have cam phasing (ex:LS3 and some newer LS engines) or other variable valve timing tech ---The cam phasing is advancing the cam in the low RPMs to move the IVC event forward to increase cylinder pressure & torque to get the car moving and then retards the cam as RPMs increase- moving the IVC event back to increase high-RPM HP. If you were measuring the actual compression ratio, adjusting for cam phasing / VVT, you'd have something that could accurately be called "Dynamic Compression Ratio" (but that's obviously NOT what this "dynamic compression ratio" is doing).
Obviously what actually matters, regarding detonation resistance, is the in-cylinder, combustion TEMPERATURES- the temperature gets too high for the octane of the fuel and it auto-ignites from the temp without a spark required (this is more akin to an explosion, or a diesel engine (with a rapid and high-pressure spike) than the much slower, controlled burning like we want).
In-cylinder pressure is a very close corollary to combustion TEMPERATURE and a pretty good indicator (way better than static CR or DCR), but things like air and coolant temp and whether the head is aluminum or iron and whether the chamber / pistons are coated will further impact the actual combustion temperature/detonation risk.
DCR is way better than static CR but accounts for 2 variables out of 10 or 12 that are influencers. Still useful as a "rough order of magnitude" estimate, IMHO.
IF you've got a DCR of high 9s or 10s in an old SBC and are running pump gas, I imagine you're going to be in trouble in a hurry. So no, not useless; it just shouldn't be used alone as the only indicator.
Rklessdriver's comment about "not using DCR" but at the same time saying he instead looks at the "opening and closing events" is a bit misleading as that's essentially doing the same thing; one with a number and the other with a good understanding/feeling for what the events should be.
-It's like how when some people cook they use precise measurements, particularly when they've never made the recipe before, and the chef who's made the recipe a million times and has been cooking every day forever just says "use a pinch" and literally pinches the 1/2 tsp that's supposed to be in the dish. -They're both trying to ensure that they end up with the "right" amount of salt in the dish, but they're getting there a different way; if the chef then decries measuring spoons and measuring cups as "useless" to new aspiring cooks, we're in a very similar situation, IMO.
Adam
Last edited by NewbVetteGuy; 10-27-2017 at 01:48 PM.
I respectfully disagree with this or not understanding exactly what you mean. DCR is not measuring or a measurement of any mass. It is a dynamic measurement when cam events change such as newer vvt in use today. It's a measure of pressure related to the intake valve closing point to tdc as the piston moves up the cylinder building pressure. Pressure cannot start to increase until the valve is closed. If too much pressure is built up by not allowing for an 'optimal' valve closing point for a given cam profile and scr, then detonation may occur. The higher the scr, the higher the risk of detonation with too high of a cylinder pressure for the octane of the gas and engine coolant temperature. This is clearly demonstrated in Grumpy's graph. I don't build hipo engines for a living so the DCR calculator worked for me. My scr is 10.7, dcr 8.3 and combined with afr heads, 180* engine temp and 91 octane, I have not detected any detonation and it coincides with the graph Not saying detonation isn't happening but my engine has spun to 6400 many times and the hyper pistons are doing fine so far. For guys that do build engines for a living like yourself, I assume, and Straub et al, your experience doesn't require a dcr calculation because you just know it will perform fine. But that's not to say DCR is useless as a tool. I feel it helped me along with the advice of others in here to build my engine.
DCR is a measure of the volume of the cyl when the intake valve closes and therfore the possible mass of air and fuel in the cyl at that time.
By the definition of the word "Dynamic" there is nothing dynamic about what DCR measures, because it does not measure anything that changes..... it is a stationary measurement. A snap shot of the therotical mass of Air and Fuel in the cyl at the time the valve closes.... however it does not account for the things I stated which do effect the mass of the mixture in the cyl during the intake stroke. And those things are varaible or dynamic in the way they act on the mixture. IME those things will have as much effect on an engines behaviour than wherever the piston is when the intake valve closes.
You are totally incorrect about pressure not increasing until the intake valve closes.... pressure increases in the cyl as soon as overlap stops ie when the exhaust valve closes while the intake valve is open.
Will
Last edited by rklessdriver; 10-27-2017 at 02:52 PM.
pressure increases in the cyl as soon as overlap stops ie when the exhaust valve closes while the intake valve is open.
Let's start discussing this from here. How much would that be in psig when the ev closes? You can pick a scr and cam to use as an example. Ball park number is fine.
pressure increases in the cyl as soon as overlap stops ie when the exhaust valve closes while the intake valve is open.
Let's start discussing this from here. How much would that be in psig when the ev closes? You can pick a scr and cam to use as an example. Ball park number is fine.
I cannot give specifics because it will vary with RPM and far more things than I can discuss here, but when you break down what happens during each cycle, it should be evident to everyone that there will be a positive pressure spike in the cyl as soon s the exhaust valve closes.
When the exhaust valve opens BBDC during the power stroke there is an extremely high amount of still expanding gas.... even thou the piston is traveling downward, this hot gas iniaties "blow down" where by the exhaust gas escapes out the open exhaust valve well before the piston begins its upwards motion.... now somewhere around 20* BTDC on the exhaust stroke the intake valve opens and the intake system sees this as a huge negative pressure (even thou the piston is traveling upwards) drawing in the cool AF mixture, but really it's just the pressure differential because the exhaust valve is open and you have other valves open scavenging the rapidly equalizing cyl (pressure wise).... Now the intake charge is being forced in the cyl at 1 bar (in a NA engine) and you suddenly close off the exit (exhaust valve).... whats going to happen when you have all this super hot gas escaping, drawing in the AF mixture at super high velocity and suddenly slam the escape door closed with the piston only 10* or so after TDC.....
Depending on where you close the exhaust valve and where you opened the intake valve (how much did you use scavenging to jump start the intake stroke) can have a huge amount of effect on how much the AF mixture tries to over run the space available in the cyl before the piston can move further away and create more volume for the incomming mixture. This pressure spike is what causes reversion in the intake runner at low RPM.
Will
Last edited by rklessdriver; 10-27-2017 at 07:52 PM.
Just want to add the stroker block was purchased from Coast performance and transmission in Toms River NJ...
Anybody had any work done at that shop? He always has a lot of vettes and hot rods outside the shop..
As far as the cam that's in there now, I remember him telling me it is a custom grind......Its similar to comp cam 280/280 duration with a 480/480 lift...
How much would that differ from the lunati cam I mentioned earlier?
I'm still learning to use this tool so this is a wet finger in the air approximation using dynosim5, for 383, 4.03x3.75, 5cc relief, 6" rods, .05 quench, Pro Comp heads, Lunati cam, 1.6 rockers (at the crank):
10-26-2017, 04:13 PM
Not saying detonation isn't happening but my engine has spun to 6400 many times and the hyper pistons are doing fine so far. For guys that do build engines for a living like yourself, I assume, and Straub et al, your experience doesn't require a dcr calculation because you just know it will perform fine. But that's not to say DCR is useless as a tool. I feel it helped me along with the advice of others in here to build my engine.
