Ok, time to pick your brains on a kind of advanced topic- dynamic compression ratio. I'm specifically intersted in how cam selection affects this value. What i understand so far is that intake cam timing (advance/retard) and duration will affect the dynamic compression ratio.
Let's take an n/a ecotec L61 for example.
Say we have a 12:1 static compression ratio using stock camshafts. Chances are it will need 93 octane right? The stock camshafts have relativly little duration and the intake valve is completely closed early in the compression stroke (or maybe before it, not sure), such that not much reversion occurs
Now install something like comp stage 3's. The intake valves are open much longer, such that some of the charge is bled out during the compression stroke.
The root of my question is how much does cam selection affect dynamic compression ratio? Is it a very mild effect, or could it mean the difference between pump and race gas? I've playted with some of the calculators our there but the camshaft variable is usually in intake valve closed - degrees ATDC, which i'm not too sure on for any cam.
I'm trying to fully understand this in terms of an n/a engine; i know forced induction will change the game completely
well, i don't have numbers for you, ryan, but i can tell you that camshaft selection has a LOT to do with dynamic compression. and like you said, even if you pick very aggressive cams, that would need, with high compression, race fuel, getting adjustable cam gears could change that(i think pcm tuning will change things a bit more though). when picking out cams and position, i would go with powerband. you can do much more to be able to run pump gas through the pcm than anything else, so to get the most out of the setup, assuming we're still talking about this n/a eco, i would go with cams that would put the peak hp in best operating rpm's based on transmission gearing the car has, and operating condition of the car, and then tune accordingly based on what fuel restrictions i have, unless of course the two greatly disagree.(you won't make a 350 hp n/a eco on pump fuel!lol)
another thing to keep in mind with what you said about cylinder bleed off is engine rpm. the faster the engine is moving, the faster the intake velocity is, which means the cylinder can come up quite a bit and still be filling more. the force of the piston traveling upward on compression stroke isn't going to immediately force air out of the intake valve. some more aggressive cams can run at higher rpm's so that the engine hasn't closed the intake valve even at 60-80 degrees abdc and its still not pushing air out. something to think about...
i love cam talk. its the most interesting part of the engine imo.
Thanks for the insights Dave.
So i guess i should step back and make sure my interpreatations of c/r and fuel requirements are correct.
As i understand it, you take any engine with a certain dynamic c/r, and based on that (and head/chamber characteristics) there will come a certain point where the fuel will pre-ignite. At that point, no tuning will save you, only lowering the c/r (static or dynamic), and/or running a higher octane fuel will stop the pre-ignition. Retarding timing won't do anything because the charge is igniting before the spark plug fires. Some people run richer in this zone, but i think that's more of a half assed way of fixing a problem.
Now if you have a build on the verge of the event i described above, and you change cams such that your dynamic compression ratio is LOWER, did you just buy yourself some room? Will that same engine above that was pre-igniting now be ok with the same grade of fuel? Keep in mind i'm not trying to make this as an agruement to change cams. I'm trying ot understand if it is a welcome byproduct though.
well, yes, you would be able to bleed off and get some wiggle room, but the point at which that would happen is different for every engine setup, and i could never tell you what that point would be for any setup. however, i just want you to think about what changing the cams would do to your powerband...in order to change it enough if you're that close to a problem, it might kill your power. you'd have to adjust the cam timing(i'm talking about just using agjustable gears, cuz i doubt ppl in this position would want to buy a new set of cams after they already purchased a set) so that you can bleed off some pressure, but if you have the same duration as before, just moved, you just changed your overlap, which will change(most likely lower) your peak hp rpm, which in turn, will most likely(but very arguable!) lower your peak power by quite a bit. its opening up a can of worms that could lose a lot of volumetric efficiency when its all said and done. i say a mix of running a bit richer along with really spending time with the spark tuning.(you can make more power when running a bit richer anyways, and this is going to be in more effect at higher rpm's when more air will be entering the cylinder anyway, so you'll want more power).
i've toyed around with the thoughts you have quite a bit in my search for what cr to run and how much boost for what i want. in the end, i'm going to go with a more conservative cr(9:1) with polishing of the combustion chamber and i'm spending the extra dollar on coatings. there are different coatings you can get for the calves, chamber, and piston to aim all of the heat toward the piston to drive it down harder to make your engine put the heat it creates to better use. i would spend more time on this, as i have, and am continuing to do, since i feel there is much more power to be had through this.
sigh, well, i'm off to work now. i look forward to reading progression of this thread when i come home!
Edited 1 time(s). Last edited Monday, November 01, 2010 11:44 AM
does this have anything to do with us and all the LE5 swaps? lmao
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Cams play a critical role in DCR beyond ABDC. Ramp rate, lift, duration and timing are all very important parameters. When you increase cylinder filling (i.e. VE) for a given RPM, you are effectively increasing DCR for that RPM. Without an elaborate piece of software, I doubt you will get a good estimate of Dynamic Compression Ratio, especially considering it is RPM dependent.
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Whalesac wrote:Cams play a critical role in DCR beyond ABDC. Ramp rate, lift, duration and timing are all very important parameters. When you increase cylinder filling (i.e. VE) for a given RPM, you are effectively increasing DCR for that RPM. Without an elaborate piece of software, I doubt you will get a good estimate of Dynamic Compression Ratio, especially considering it is RPM dependent.
very much agreed! i completely forgot to mention all of the other parts of the camshaft profile and what role they play! everything play a role, and its all so finicky that its just really difficult to figure out exactly the best way to get the best dcr for a certain setup...which is why the ppl that are good at it get the big bucks, lol!
IamQwibby (Eco Meatcake) wrote:does this have anything to do with us and all the LE5 swaps? lmao
Yes it does, if you understand basic engine principals and read the description on the linked wallace racing calculator the reason should become clear as to why the LE5 doesnt like L61 cams (at least not in stock cam timing).
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Leafy wrote:IamQwibby (Eco Meatcake) wrote:does this have anything to do with us and all the LE5 swaps? lmao
Yes it does, if you understand basic engine principals and read the description on the linked wallace racing calculator the reason should become clear as to why the LE5 doesnt like L61 cams (at least not in stock cam timing).
What you have when you put a stock l61 head AND cams on an LE5 bottom end is a relatively high static c/r with the wimpiest cams that came in any ecotec. Factor in the lack of VVT and it's not surprise to me that particular combo disappoints. I would not recommend that combo to anyone to be honest.
Whalesac wrote:Cams play a critical role in DCR beyond ABDC. Ramp rate, lift, duration and timing are all very important parameters. When you increase cylinder filling (i.e. VE) for a given RPM, you are effectively increasing DCR for that RPM. Without an elaborate piece of software, I doubt you will get a good estimate of Dynamic Compression Ratio, especially considering it is RPM dependent.
So the calculator posted is pretty much useless? There is not input variable for any of the additional factors you discuss, so i don't see how it could be useful. Thank you for the insight that DCR is RPM dependent though. That very much makes life difficult....uh....interesting lol. It sounds as though a cam that increases VE at a given RPM could increase it's DCR and hence increase its tendency to knock on a given grade of fuel. Not the answer i was looking for, but it's an answer lol.
Leafy wrote:IamQwibby (Eco Meatcake) wrote:does this have anything to do with us and all the LE5 swaps? lmao
Yes it does, if you understand basic engine principals and read the description on the linked wallace racing calculator the reason should become clear as to why the LE5 doesnt like L61 cams (at least not in stock cam timing).
I know anyone who has asked my advice in this swap I always say cams is a must, I went with comp stage 3's
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oldskool (eco meatcake) wrote:
Whalesac wrote:Cams play a critical role in DCR beyond ABDC. Ramp rate, lift, duration and timing are all very important parameters. When you increase cylinder filling (i.e. VE) for a given RPM, you are effectively increasing DCR for that RPM. Without an elaborate piece of software, I doubt you will get a good estimate of Dynamic Compression Ratio, especially considering it is RPM dependent.
So the calculator posted is pretty much useless? There is not input variable for any of the additional factors you discuss, so i don't see how it could be useful. Thank you for the insight that DCR is RPM dependent though. That very much makes life difficult....uh....interesting lol. It sounds as though a cam that increases VE at a given RPM could increase it's DCR and hence increase its tendency to knock on a given grade of fuel. Not the answer i was looking for, but it's an answer lol.
I would hardly call it "useless." I'm sure it does a decent job at showing how the different parameters affect DCR, but I would use it with a grain of salt.
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Whalesac wrote:oldskool (eco meatcake) wrote:
Whalesac wrote:Cams play a critical role in DCR beyond ABDC. Ramp rate, lift, duration and timing are all very important parameters. When you increase cylinder filling (i.e. VE) for a given RPM, you are effectively increasing DCR for that RPM. Without an elaborate piece of software, I doubt you will get a good estimate of Dynamic Compression Ratio, especially considering it is RPM dependent.
So the calculator posted is pretty much useless? There is not input variable for any of the additional factors you discuss, so i don't see how it could be useful. Thank you for the insight that DCR is RPM dependent though. That very much makes life difficult....uh....interesting lol. It sounds as though a cam that increases VE at a given RPM could increase it's DCR and hence increase its tendency to knock on a given grade of fuel. Not the answer i was looking for, but it's an answer lol.
I would hardly call it "useless." I'm sure it does a decent job at showing how the different parameters affect DCR, but I would use it with a grain of salt.
I would call it useless. The factors that it takes into account which affect DCR are intake valve closing and boost pressure. The other factors are related to SCR. Since i was talking n/a talk, it's leads one to think DCR would be based on intake valve closing. You said it's RPM dependent and also depends on other cam properties. After the discussions here, i don't think it's something that on online calculator would calculate with certainty. Hell, i have no clue where you would begin calculating such a widely variable concept (DCR) that is RPM dependent.
^^^^ Rod length has no bearing on Static Compression Ratio in any way shape or form. Bore and stroke only affect SCR in as much as it relates to cylinder volume at TDC...but there is no variable for that in the calculator. Instead, there is a separate variable specifically for SCR. If they only relate to SCR, then there would be no need to make them an editable variable. I will admit, though, I am a bit confused. Rod length and stroke are certainly necessary for the R/S ratio, but I am at a loss to why bore and # of cylinders are variables in that calculator.
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Whalesac wrote:^^^^ Rod length has no bearing on Static Compression Ratio in any way shape or form. Bore and stroke only affect SCR in as much as it relates to cylinder volume at TDC...but there is no variable for that in the calculator. Instead, there is a separate variable specifically for SCR. If they only relate to SCR, then there would be no need to make them an editable variable. I will admit, though, I am a bit confused. Rod length and stroke are certainly necessary for the R/S ratio, but I am at a loss to why bore and # of cylinders are variables in that calculator.
how does rod length effect dcr? i thought it only effected thrust walls, and that longer rods aid in torque growth, etc... i've never heard this before... but am quite interested!
Dynamic compression.
The definition is loosely defined by where the piston position is in relation to the intake valve during the time it opens BTDC and when it closes ABDC (which is why I'm confused on why the calculator doesn't include when the intake valve opens). The true variable which defines how much HP an engine makes and at what RPM that power is made is more directly related to cylinder pressure (or what they call "cranking pressure".)
Peak torque will normally occur at the point in which the engine has the highest amount of cylinder pressure. This is why camshafts, and their timing in relationship to when the valves open and close in relation to crankshaft position (or piston position), have such a huge effect on what RPM an engine makes most of its power. The real goal when trying to increase HP is to increase the RPM at which the engine makes torque most efficiently. Since HP is calculate from the formula ((Torque x RPM)/ 5252 = HP), it's a given that torque is the number which needs to be focused on.
The reason the rod length is used in calculating the dynamic compression ratio is because it has a direct effect on how much time the piston spends at TDC and BDC, called dwell. The rod's job is to change the rotating energy of the crankshaft into reciprocating energy based on leverage. When the length of the rod is increased, the amount of leverage goes up. Because of the amount of leverage goes up, the longer it takes to get the piston to move. The more time the piston spends at TDC and BDC, the less time it has to move throughout it's stroke. The piston speed will increase at the beginning of its decent and toward the end (near BDC), as well as its time ascending from BDC and toward its end (at TDC). The time throughout the middle if its cycle (half way between TDC and BDC) remains relatively unchanged. But this slight increase in speed during the intake stroke will help create a larger pressure drop (read "vacuum") and help to increase cylinder pressure. The other benefit is the amount of dwell at TDC during the combustion process. Once the spark ignites the fuel, the amount of time the heat has to build before the piston begins its decent (force on the crankshaft) increases also. How much this truly helps is debatable.
I don't agree with using online calculators to determine dynamic compression. As I said, cylinder pressure is what needs to be focused on. Plus there are so many other variables which effect cylinder pressure. Restriction through the air cleaner, air filter, throttle bore, intake runners (in the manifold and the cylinder head), valve size, valve overlap, exhaust runners, primary tube size, reversion in the exhaust and the intake manifold, as well as RPM all greatly effect how much cylinder pressure is built and at what RPM.
The only real way to determine cylinder pressure is after the engine is built and tested with a pressure gauge. Even then the RPM is going to be limited to how fast the starter is able to crank the engine over, hence the term "cranking pressure". If you're going to use online calculators, use them as a guide, not gospel.
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is there a calculator that might calculate some kind of estimate of DCR related to what PSI your engine pressurises during a cranking pressure test? or is that completely irrelevant?
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z yaaaa wrote:is there a calculator that might calculate some kind of estimate of DCR related to what PSI your engine pressurises during a cranking pressure test? or is that completely irrelevant?
That's a good question! My bet is on irrelevence though, cause Whalesac said DCR is rpm dependent. You won't spin your engine fast enough in a compression test to see the effect of rpm. Intersting thought though. I always equated a compression test with static c/r.
DCR isn't dependent on RPM. Cylinder pressure is.
DCR is just a loose calculation based on where the intake valve closes in relation to the piston position ABDC.
E.G. (Keep in mind, there is much more to this than I'm explaining here. This is just to simplify the explanation so more people can better understand how it's calculated.)
If the stroke of the engine is 3" long and the engine has a SCR of 10.0:1, but the intake valve doesn't close until the piston is 1/2" on its way back up, then the DCR is calculated using a 2-1/2" stroke. This will effectively lower the DCR to, let's just say, 9.0:1
The only time RPM would effect the DCR is on engines with variable valve timing on the intake lobes.
Here's a link that has a much better explanation about DCR than I can give.
http://www.empirenet.com/pkelley2/DynamicCR.html
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Ok there's two conflicting schools of thought. I originally thought of it how AJ explained it (roughly), however whalesac introduced RPM dependence. The way AJ explains it, cam timing and duration would have a very significant effect on DCR. So i guess the question is how much does DCR affect what grade of gasoline you can run without pre-ignition?
AJ wrote:Did you read the link I provided?
That's actually the main purpose of the site I linked you to. What DCR can you run on 91 octane.
I intend to read it, it's very long lol. My point was two seemingly knowledgeable yet conflicting ideas were posted.
