DCR and head flow
DCR and head flow Ok, I've been pondering on this for a while, as a mechanical engineering student with a background in automotive design I understand quite a bit about ic engines. How much does a head swap to something with less pressure drop (higher flow) across the valve seat effect the dynamic compression ratio of a motor. I see all these DCR calculators on the internet that don't take into account head flow at all and I can't see how this is an accurate representation of DCR. Are they all assuming you have 0 pressure drop across the valve seat? If you have a low flowing head that's replaced by something that flows more CFM doesn't the DCR go up? I'm trying to get to the magical 8.5-8.6:1 DCR in my car with a somewhat large cam that most people seem to think has a DCR closer to 7.XX:1, but shouldn't higher flowing heads allow a higher DCR? I know IVC has everything to do with the DCR of a car but I don't see how 470whp and good torque is made in a 346 with a 7.XX DCR. Please excuse this if its a n00b question, I still have yet to read a copy of haywood's book, everything I've learned has been on a much smaller engine when I lead an engine development program at university.
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The problem is that DCR is another basic mechanical linkage type equation, it does not factor in any fluid dynamics. DCR is effectively the remaining compression you have left after the intake valve closes.
Digging into the port flow and its relationship to performance, i think its time you get a copy of heywood. There are so many things you need to factor in and its best learned by reading... and the fact that i dont feel like typing much either
Digging into the port flow and its relationship to performance, i think its time you get a copy of heywood. There are so many things you need to factor in and its best learned by reading... and the fact that i dont feel like typing much either
I realize DCR is how much air you basically have left in the cylinder before the intake valve closes but if you have a better flowing head that more effectively fills the cylinder how does that not effect the DCR? Probably going to pick up a copy of heywood's book next friday.
*edit* I guess if you have a better flowing head reversion of the intake happens at a greater rate? I mean, if it flows better it should flow better in both directions
*edit* I guess if you have a better flowing head reversion of the intake happens at a greater rate? I mean, if it flows better it should flow better in both directions
Last edited by Drew04GTO; Dec 2, 2007 at 10:18 PM. Reason: moment of idiocy
The big missing part of the puzzle with DCR is the pressure differential at the valve at intake valve closing. Slightly before IVC manifold pressure will still be higher than cylinder pressure even though the piston is past BDC and on its way back up. This is due to wave dynamics inside the intake manifold.
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DCR does not change.. because it a a fixed mechanical value
However what was said about wave dynamics is correct.
Air has mass and therefore it has inertia... it takes time to speed up and slow down. And to put it simply: at your target rpm value, you want to time the IVC to match the point at which air stops flowing through the intake port...
The piston is already on its way back up before air stops flowing into the chamber.. and the higher the speed..the later the IVC point becomes to achieve this. High RPM = late IVC, Low RPM = early IVC.. now you can see why varaible valve timing is helpful (and this is only a small part)
For example: you have your IVC/DCR closing point setup so that its ideal for 5000rpms.. below that you will have reversion and above that you will have restriction (or close before the air is finished flowing)
However what was said about wave dynamics is correct.
Air has mass and therefore it has inertia... it takes time to speed up and slow down. And to put it simply: at your target rpm value, you want to time the IVC to match the point at which air stops flowing through the intake port...
The piston is already on its way back up before air stops flowing into the chamber.. and the higher the speed..the later the IVC point becomes to achieve this. High RPM = late IVC, Low RPM = early IVC.. now you can see why varaible valve timing is helpful (and this is only a small part)
For example: you have your IVC/DCR closing point setup so that its ideal for 5000rpms.. below that you will have reversion and above that you will have restriction (or close before the air is finished flowing)
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Dynamic compression ratio.
Characterized by continuous change, activity, or progress
It does change.. and thats why someone assuming that dynamic compression ratio varies only with IVC isn't fully understanding what is going on.
Characterized by continuous change, activity, or progress
It does change.. and thats why someone assuming that dynamic compression ratio varies only with IVC isn't fully understanding what is going on.
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Dynamic Compression ratio is also referred to as "Corrected" compression ratio.
the Word 'Dynamic' may be misleading. Its effectively the cylinder volume at IVC minus cylinder volume at TDC
SCR = Volume at BDC - Volume at TDC
DCR = Volume at IVC - Volume at TDC
In effect, people care far too much about DCR.. its not that important!!! The relationship between SCR and DCR is more important that a single target value for each
Quiz:
2 engines.. Same DCR.. which engine is designed for higher rpms?
Engine 1: SCR 13:1, DCR: 7.5:1
Engine 2: SCR 8.5, DCR 7.5:1
(note, i arbitrarily picked values..)
Also.. bonus, what would you call an engine with a significantly early IVC or significantly late IVC (i.e. what cycle)
Last edited by DanO; Dec 3, 2007 at 08:17 PM.
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well Atkinson cycle is technically correct.. but miller just adds forced induction so yeah that works too!!
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https://ls1tech.com/forums/generation-iv-internal-engine/811089-dcr-not-important-thought.html
more of the same........
more of the same........
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I've been against DCR values even being looked at since the begining.. IVC and SCR are the important factors.. DCR means nothing
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For example, take an LSI stock engine and throw a big cam in it. The powerband has now shifted to the higher RPM range without touching the intake/exhaust, its purely from cam events.
Matching all of the components for the best power band is exactly what a good engine builder/designer/engineer does...
