Need Help with my DCR/SCR
#1
Need Help with my DCR/SCR
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.
LS2 Block Bore: 4.010
Stroke: 3.622
Heads: PRC 227 Heads CCs: 56
Pistons: -10cc Dish Top, Compression Height: 1.330"
Heads Gasket: .051 (GM 6.0 Gasket)
Deck Clearance: 0.001 (3.622/2 + 6.098 + 1.330 = 9.239) LS2 Deck Height is 9.24
Connecting Rod Length: 6.098 (LS2/LS3 rods)
IVC @ .050: 45.5 [112-2+(231/2)-180]
Advertised PRC 227 Heads Flow:
Lift 0.200" 0.300" 0.400" 0.500" 0.600" 0.650"
Intake: 145, 214, 261, 300, 322, 330
Exhaust: 115, 167, 202, 219, 238, 236
Camshaft: BTR Stage 3 LS1/LS2 NA (33142122) : Specs @ .050: 231 / 242 .671 / .593 112+2
Using this calculator: Static and Dynamic compression ratio
SCR 10.78:1 DCR 8.86:1 - Not sure if I got the DCR right, and if 8.86 is safe.
Maybe I need to change out my cam?
Would this set up make good power?
Going into a C5 Corvette w/ a 3600 stall and 3.90 gears
LS2 Block Bore: 4.010
Stroke: 3.622
Heads: PRC 227 Heads CCs: 56
Pistons: -10cc Dish Top, Compression Height: 1.330"
Heads Gasket: .051 (GM 6.0 Gasket)
Deck Clearance: 0.001 (3.622/2 + 6.098 + 1.330 = 9.239) LS2 Deck Height is 9.24
Connecting Rod Length: 6.098 (LS2/LS3 rods)
IVC @ .050: 45.5 [112-2+(231/2)-180]
Advertised PRC 227 Heads Flow:
Lift 0.200" 0.300" 0.400" 0.500" 0.600" 0.650"
Intake: 145, 214, 261, 300, 322, 330
Exhaust: 115, 167, 202, 219, 238, 236
Camshaft: BTR Stage 3 LS1/LS2 NA (33142122) : Specs @ .050: 231 / 242 .671 / .593 112+2
Using this calculator: Static and Dynamic compression ratio
SCR 10.78:1 DCR 8.86:1 - Not sure if I got the DCR right, and if 8.86 is safe.
Maybe I need to change out my cam?
Would this set up make good power?
Going into a C5 Corvette w/ a 3600 stall and 3.90 gears
Last edited by 1ta ls1; 03-05-2022 at 10:27 AM.
#2
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.
#3
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).
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).
#6
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..
#12
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
Last edited by 1ta ls1; 08-30-2021 at 01:05 AM.
#13
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
Your dynamic is around 8.1-8.3 with the 231 cam
Last edited by Bspeck82; 08-30-2021 at 08:59 AM.
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1ta ls1 (08-30-2021)
#14
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
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
#15
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
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
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1ta ls1 (08-31-2021)
#16
TECH Fanatic
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.
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.
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NSFW (10-16-2021)
#17
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.
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.
Last edited by 68Formula; 10-15-2021 at 10:44 AM.
#18
TECH Fanatic
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.
#19
TECH Fanatic
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.
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.
#20
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.
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 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.
Thanks for the feedback.
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ChopperDoc (10-15-2021)