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Here is my NA build. I want to be on the safe side of DCR and SCR on Pump 91, conservative side of caution. Trying to get my SCR to be 10.7-10.9, DCR under 8.4. Living in AZ 100+ weather and 91 pump is not so great here. I hear a lot talk about SCR, but NOTHING about DCR.
I would do the second cam. I have a 402 ls2 tuned by NicD (highly, highly recommended) and my quench is about .55 which is a little high and my scr is 11.4 and dcr is like 8.5. Going from memory of course. This setup runs fine in AZ weather. 91 pump. Anyways. Runs great and took timing just fine.
To find clearance find the pistons compression height usually listed on the website and if block is zero decked then deck height is zero obviously. If it's stock I would research it and find stock deck height. I forgot what it was.
The simple math calls for dividing the stroke in half (3.48 / 2 = 1.74 inches) and then adding the connecting rod length and compression height. This is then subtracted from the engine's deck height to find the deck clearance, which can be positive (above deck) or negative (below deck).
Added my compression height on the piston. 1.330" to OP. Based that, I came up with 0.001 (3.622/2 + 6.098 + 1.330 = 9.239) LS2 Deck Height is 9.24. So I guess it would be close to zero.
Oh if you already have the first cam in the car I wouldn't spend the money changing it. Just imo.
I have the BTR stg 3 cam, but nothing is in the block yet. I would like a bigger cam since my dished pistons will have valve reliefs, and could make more power while obtaining a lower DCR. I used this cam for few thousand miles, so essentially could sell the BTR and get another cam pretty easily..
Yeah, it's going to be a pretty big difference. The advertised intake duration of my cam is +54° more than duration at 0.050" lift. Lobe opening and closing ramps aren't same so can't necessarily just split the difference.
Yeah, it's going to be a pretty big difference. The advertised intake duration of my cam is +54° more than duration at 0.050" lift. Lobe opening and closing ramps aren't same so can't necessarily just split the difference.
So the calculator I’m using, and others I found, compute DCR based on IVC @ .05. I believe the Wallace racing uses .006.
http://www.wallaceracing.com/dynamic-cr.php
Found the card for XER (found online), one of the cams above, so IVC is 74??
So if 74 is correct, based on my SCR of 10.78:1, my DCR is 7.91:1
Yes. Most LS hyd roller cams have a 48-51 degree ramp to .006 from .050. Your static is around 10.7 with a 59cc head and -10 dishes. This is why I believe in fact the original cam is just fine for your project.
Your dynamic is around 8.1-8.3 with the 231 cam
Last edited by Bspeck82; Aug 30, 2021 at 08:59 AM.
Thanks guys. This definitely changes things, and makes a lot more sense. I figured I was missing something. I could change the pistons I’ll be going with. But I do like the fact of having the option of a -10cc dishes piston if I decide to swap heads out down the road and change my compression for FI. But for NA purposes, I may go with a flat top.
Will update this thread with the combo I select. I’ll be running the final build on an engine dyno, so I’ll share my results from that
Thanks guys. This definitely changes things, and makes a lot more sense. I figured I was missing something. I could change the pistons I’ll be going with. But I do like the fact of having the option of a -10cc dishes piston if I decide to swap heads out down the road and change my compression for FI. But for NA purposes, I may go with a flat top.
Will update this thread with the combo I select. I’ll be running the final build on an engine dyno, so I’ll share my results from that
Lots of great info here. My only comment is DCR is largely unknown unless you have flow test equipment and pressure monitors. With cams the more appropriate reference is Effective Compression Ratio (ECR), as mathematically, we only know what is compressed after the valve closes.
We can’t assume dynamic as this is movement of air, and isn’t something you can assume without data of some type. None of that data is calculated by any ECM, other then straight AFR/EQ from VE. At best, this number only helps us guess the actual dynamic compression ratio. There is simply no way of knowing how much air a particular cam actually compresses under flow, aside from our VE with PERFECT AFR numbers. Even then, it’s an assumption considering scavenging and reversion are not factored in.
With all of the above, if you are playing with the compression limits on your fuel, there may be places in the RPM band where it’s completely fine, and other places where it will simply detonate. No confirmed number on 93, but all I’ve seemed to find on it hints at a 8.63:1 ECR. Above that and there will be issues. Since DCR technically cannot be calculated, I’d keep the ECR well under that number to be safe. My 427 Camaro is at 8.51:1 and I have had to reduce timing significantly to eliminate knock on that particular motor. Even N/A, timing is as low as 14 on ramp in, and up to 18 WOT.
Lots of great info here. My only comment is DCR is largely unknown unless you have flow test equipment and pressure monitors. With cams the more appropriate reference is Effective Compression Ratio (ECR), as mathematically, we only know what is compressed after the valve closes.
We can’t assume dynamic as this is movement of air, and isn’t something you can assume without data of some type. None of that data is calculated by any ECM, other then straight AFR/EQ from VE. At best, this number only helps us guess the actual dynamic compression ratio. There is simply no way of knowing how much air a particular cam actually compresses under flow, aside from our VE with PERFECT AFR numbers. Even then, it’s an assumption considering scavenging and reversion are not factored in.
With all of the above, if you are playing with the compression limits on your fuel, there may be places in the RPM band where it’s completely fine, and other places where it will simply detonate. No confirmed number on 93, but all I’ve seemed to find on it hints at a 8.63:1 ECR. Above that and there will be issues. Since DCR technically cannot be calculated, I’d keep the ECR well under that number to be safe. My 427 Camaro is at 8.51:1 and I have had to reduce timing significantly to eliminate knock on that particular motor. Even N/A, timing is as low as 14 on ramp in, and up to 18 WOT.
Curious how you are calculating ECR? I've heard of it in terms of boosted applications (SCR @ x psi boost = y ECR). I'm assuming from the 8:63:1 it must be some formula for non-boosted applications.
Last edited by 68Formula; Oct 15, 2021 at 10:44 AM.
Curious how you are calculating ECR? I've heard of it in terms of boosted applications (SCR @ x psi boost = y ECR). I'm assuming from the 8:63:1 it must be some formula for non-boosted applications.
8.63 is the ECR limit for 93 octane. It’s simply the ratio between the IVC event and TDC. Nothing else. It’s a trig function since you have to know the crank angle, rod length, and obviously everything else for your typical static compression. To visualize it I made a video about this stuff a long time ago. Here’s a screenshot from that just showing what ECR is.
Now for that second number you are referring to, that is called Boost Compression Ratio or Final Compression ratio. That number uses SCR to be on the safe side. 12:1 is a typical safe range for pump gas with no other mods.
And yes, the 8.63 ECR is for high compression N/A motors typically, since your static has to be pretty high in order to hit that number. It’s not the end of the world if it’s exceeded, as all of these depend on a lot of outside factors, like IAT and ECT, materials, AFR, timing, etc.
I’ll go on to further explain why DCR isn’t a calculation we can assume. With the ECR pictured above, we have to then factor in actual intake pressure, cylinder temp, humidity of the incoming air, barometric pressure, velocity, and finally VE.
Those factors would have to be added in at a minimum, and plugged into the calculated ECR, which is actually known. In fact, like I said before, mathematically it’s all we really know. If we don’t know the true VE number at “x” RPM, then we don’t know the DCR. Most engines cannot actually move the volume of air that they displace. It’s usually a percentage of the swept volume essentially.
Dynamic obviously refers to “movement” and since there are so many additional factors with that, I simply won’t use it as a calculated reference. That’s just me though. There are other folks out there I’m sure that also think enough into it to feel the same about it. If you look at the progression of generational advances in ECM programming, the engineers seem to feel the same. The difference in detail when calculating VE on a Gen III vs a Gen V is actually quite staggering. Even so, there are still a lot of missing parameters for the real DCR number.
I’ll go on to further explain why DCR isn’t a calculation we can assume. With the ECR pictured above, we have to then factor in actual intake pressure, cylinder temp, humidity of the incoming air, barometric pressure, velocity, and finally VE.
Those factors would have to be added in at a minimum, and plugged into the calculated ECR, which is actually known. In fact, like I said before, mathematically it’s all we really know. If we don’t know the true VE number at “x” RPM, then we don’t know the DCR. Most engines cannot actually move the volume of air that they displace. It’s usually a percentage of the swept volume essentially.
Dynamic obviously refers to “movement” and since there are so many additional factors with that, I simply won’t use it as a calculated reference. That’s just me though. There are other folks out there I’m sure that also think enough into it to feel the same about it. If you look at the progression of generational advances in ECM programming, the engineers seem to feel the same. The difference in detail when calculating VE on a Gen III vs a Gen V is actually quite staggering. Even so, there are still a lot of missing parameters for the real DCR number.
From what you're showing for the ECR calculations, I believe it's is the same formulas the "DCR" calculators common online (or spreadsheets) are using. This one for example: http://www.wallaceracing.com/dynamic-cr.php. They are simply using incorrect terminology (albeit a pretty common misnomer) that's proliferated in the automotive hobby world. In other words as you said, they are not stating true DCR, but rather ECR; and then referring to it as "dynamic" if though it's not taking the true dynamic responses of the system.
I applaud your idea of correctly the terminology. Alas, I think it's gone on unchecked for so many years, that just like when people started referring to every displacement GENIII as an "LS," you'll never get enough traction to make it go away.
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Need Help with my DCR/SCR 
