difference b/w SCR & DCR?
12-28-2006, 05:33 PM
#4
That link explains it perfectly. If you can't wrap your head around the math and such, try looking at it like this.
The easiest way to think of it is like a two stroke engine. It cannot produce any more cylinder pressure than allowed by the port timing. As the piston goes back up, it closes off the port and seals the cylinder at that point. Even though the entire stroke length is longer, only the length of stroke reamaining at that point contributes to compressing the mixture in the cylinder.
In a four stroke, the same principal applies. The difference is, we can easily alter that effective length through changes to the camshaft. Retarding a cam closes the intake valve later and reduces the length of effective stroke. Adding duration, while keeping the same Intake Lobe Centerline will also reduce the length of effective stroke (with symetric lobe profiles)
Static compression is the ratio of the combustion volume to the volume of the combustion chamber + the entire cylinder volume. Dynamic compression is the ratio of the combustion volume to the volume of the cylinder that remains when the intake valve has closed.
The easiest way to think of it is like a two stroke engine. It cannot produce any more cylinder pressure than allowed by the port timing. As the piston goes back up, it closes off the port and seals the cylinder at that point. Even though the entire stroke length is longer, only the length of stroke reamaining at that point contributes to compressing the mixture in the cylinder.
In a four stroke, the same principal applies. The difference is, we can easily alter that effective length through changes to the camshaft. Retarding a cam closes the intake valve later and reduces the length of effective stroke. Adding duration, while keeping the same Intake Lobe Centerline will also reduce the length of effective stroke (with symetric lobe profiles)
Static compression is the ratio of the combustion volume to the volume of the combustion chamber + the entire cylinder volume. Dynamic compression is the ratio of the combustion volume to the volume of the cylinder that remains when the intake valve has closed.
12-28-2006, 06:10 PM
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Originally Posted by hammertime
That link explains it perfectly. If you can't wrap your head around the math and such, try looking at it like this.
The easiest way to think of it is like a two stroke engine. It cannot produce any more cylinder pressure than allowed by the port timing. As the piston goes back up, it closes off the port and seals the cylinder at that point. Even though the entire stroke length is longer, only the length of stroke reamaining at that point contributes to compressing the mixture in the cylinder.
In a four stroke, the same principal applies. The difference is, we can easily alter that effective length through changes to the camshaft. Retarding a cam closes the intake valve later and reduces the length of effective stroke. Adding duration, while keeping the same Intake Lobe Centerline will also reduce the length of effective stroke (with symetric lobe profiles)
Static compression is the ratio of the combustion volume to the volume of the combustion chamber + the entire cylinder volume. Dynamic compression is the ratio of the combustion volume to the volume of the cylinder that remains when the intake valve has closed.
The easiest way to think of it is like a two stroke engine. It cannot produce any more cylinder pressure than allowed by the port timing. As the piston goes back up, it closes off the port and seals the cylinder at that point. Even though the entire stroke length is longer, only the length of stroke reamaining at that point contributes to compressing the mixture in the cylinder.
In a four stroke, the same principal applies. The difference is, we can easily alter that effective length through changes to the camshaft. Retarding a cam closes the intake valve later and reduces the length of effective stroke. Adding duration, while keeping the same Intake Lobe Centerline will also reduce the length of effective stroke (with symetric lobe profiles)
Static compression is the ratio of the combustion volume to the volume of the combustion chamber + the entire cylinder volume. Dynamic compression is the ratio of the combustion volume to the volume of the cylinder that remains when the intake valve has closed.
12-28-2006, 08:01 PM
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Originally Posted by Wnts2Go10O
ive always thought of it as theoretical vs real world compression. if that makes any sense.
These numbers are fairly meaningless, but it's a number people like to relate to, sort of like flow bench numbers.
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12-28-2006, 08:16 PM
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Originally Posted by LS1 Swapper
These numbers are fairly meaningless, but it's a number people like to relate to, sort of like flow bench numbers.
12-28-2006, 08:40 PM
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Originally Posted by hammertime
Try running a great big cam without much compression and see how 'meaningless' DCR is. You are not likely to achieve very good results if you are not taking the Dynamic Compression into account when settling on a set Static Compression ratio. 11.5:1 compression sounds rediculously high until you take into consideration that a lot of cams out there require at least that to perform in a 346 ci engine.
12-28-2006, 11:39 PM
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Fyi, n00b here. I think these DCR posts are pretty interesting. And the recent post with the easy to use DCR calculator helped me a lot. If you have low/med/high static compression it's great to know if a cam with low/med/high duration and/or LSA is in the range that will work well for you.
I had a friend who rebuilt an early/mid 70's cutlass (rebuilt it back in the 80's). I think it had around 8.5:1 static compression. He put in a cam with big duration and a single plane intake. Given that he wasn't going to change the static CR I think a DCR calculator would have helped him in his choices a lot.
I had a friend who rebuilt an early/mid 70's cutlass (rebuilt it back in the 80's). I think it had around 8.5:1 static compression. He put in a cam with big duration and a single plane intake. Given that he wasn't going to change the static CR I think a DCR calculator would have helped him in his choices a lot.
12-29-2006, 12:49 AM
#12
Originally Posted by LS1 Swapper
Not theoretical but calculated based only on the simple volume between piston up and piston down.
Originally Posted by LS1 Swapper
Interesting, so what is my compression ratio going to be at WOT 5000rpms? How about at idle? same? doubt it. Again DCR is just as useful to me as SCR. Thanks for the pointer on high compression for bigger camshafts
12-29-2006, 01:12 AM
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Quote:
Originally Posted by LS1 Swapper
Not theoretical but calculated based only on the simple volume between piston up and piston down. <- Static, which is just that
Quote:
Originally Posted by LS1 Swapper
Interesting, so what is my compression ratio going to be at WOT 5000rpms? How about at idle? same? doubt it. Again DCR is just as useful to me as SCR. Thanks for the pointer on high compression for bigger camshafts
How so? Dynamic compression ratio is again just that, dynamic. (Definition: Characterized by continuous change, activity, or progress.)
These calculated DCR's are useful in understanding how a camshaft effects compression ratio, but they don't tell near the full story and the use of 'dynamic' really isn't correct. Think about what contributes to the air filling the cylinders, simply stating it has #.## DCR is pretty meaningless.
If you think your compression is the same with 7psi boost then perhaps it's magic that makes the extra power...
Originally Posted by LS1 Swapper
Not theoretical but calculated based only on the simple volume between piston up and piston down. <- Static, which is just that
Quote:
Originally Posted by LS1 Swapper
Interesting, so what is my compression ratio going to be at WOT 5000rpms? How about at idle? same? doubt it. Again DCR is just as useful to me as SCR. Thanks for the pointer on high compression for bigger camshafts
Originally Posted by hammertime
So which is it? If it's calculated from fixed volumes, then compression ratio is always the same. Of course cylinder pressures change between a restricted and wide open throttle. But both compression ratios remain constant, even with boost added to the equation.
These calculated DCR's are useful in understanding how a camshaft effects compression ratio, but they don't tell near the full story and the use of 'dynamic' really isn't correct. Think about what contributes to the air filling the cylinders, simply stating it has #.## DCR is pretty meaningless.
If you think your compression is the same with 7psi boost then perhaps it's magic that makes the extra power...
12-29-2006, 01:32 AM
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Tooth fairy left my old man this , 8.0:1 scr
under full boogie it rises some feirce
If you think your compression is the same with 7psi boost then perhaps it's magic that makes the extra power...
12-29-2006, 11:54 AM
#15
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Dynamic compression is the ratio of the combustion volume to the volume of the cylinder that remains when the intake valve has closed.
12-29-2006, 12:24 PM
#16
Originally Posted by LS1 Swapper
These calculated DCR's are useful in understanding how a camshaft effects compression ratio, but they don't tell near the full story and the use of 'dynamic' really isn't correct. Think about what contributes to the air filling the cylinders, simply stating it has #.## DCR is pretty meaningless.
12-29-2006, 12:52 PM
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Originally Posted by LS1 Swapper
Interesting, so what is my compression ratio going to be at WOT 5000rpms? How about at idle? same? doubt it. Again DCR is just as useful to me as SCR. Thanks for the pointer on high compression for bigger camshafts
We dont have VTec man. 
12-29-2006, 04:41 PM
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Originally Posted by brad8266
umm,, yeah compression is the same no matter what rpm you are at. How the hell could you change the cams profile while the engine is running?? We dont have VTec man.
We dont have VTec man.If it doesn't change then how is it dynamic?(see definition above) I understand the 'DCR' as used here and other places, I just don't agree that it's dynamic, or more useful then SCR.
Running engine that is FI, say it doubles the air in the cylinder. Would the compression ratio still be the same? The calculated cylinder volume of course never changes but the actual ratio does change.