Dynamic Compression Intake closing / opening points
#21
Race Director
I wonder if I'll get any opinions on my question about cranking compression here? Or...maybe need to quiz guys at SpeedTalk?
#22
Le Mans Master
Smaller, tighter chamber fits into theory about quenching to smaller size (and improving flame-front). It's been over 7 yrs but I remember reading why larger chamber was better in the AFR Eliminator "architecture". Maybe it had to do with flow rates in/out of the cylinders?
The calculation on DCR makes sense....to an extent. If it uses a percentage of the "swept" volume of the cylinder to pare down the SCR accordingly, it doesn't necessarily account for the varying ability of the valves to supply air during the intake cycle. (especially below .050)
#23
Race Director
Originally Posted by MatthewMiller
It isn't intended to account for that. It's still just a compression ratio, but gives a better idea of the actual amount of compression going on. That remains just one of several factors that determine cylinder pressure. Flow rates of the entire intake tract, including valve size and angles, air density on any given day, fuel qualities, ignition timing, and some other things I've surely forgotten also help determine actual cylinder pressures. Cylinder pressure is still just one of several factors that determine detonation characteristics.
Flow rates...and length of intake tract is something I might revisit in the near future. Hopefully, you won't mind if I piggyback/resurrect an August thread you PM'd a couple of weeks ago?
Drawing clear conclusions about some of these building considerations might make good "stickies" to add to the top of the forum?
(Unless C4's are getting so old/cheap these days that builds will be fading into the sunset!?)
#24
Le Mans Master
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Dynamic compression is just a math calculation that determines the compression after the intake valve closes. It calculates the compression by determining the amount of swept volume left in the cylinder compared to the total non-swept volume left above the piston. You do not need a calculator to figure it out you can use geometry to determine the numbers if the calculator does not work.
Nowadays I see some fairly simple DCR calcs requiring few parameter specifications. My advice is to try as many DCR calcs as you feeling comfortable with its complications. You can compare them also. I guess I have to admit I don't know enough about how the DCR calc works to suggest one for others. But I would recommend using a DCR calc that I can understand what the correct parameters are to use and has a math formula I can use.
I guess what I'm saying is you need to use and compare a DCR calc until you are proficient using it. It may take longer than you expected but it doesn't cost a dime to crunch numbers.
#25
Le Mans Master
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Now I've been reading again from the professionals and some will advance the I/L as much as 12 degrees. They say the motor acts like it has a tighter LSA but not the overlap. Now with a mature motor like yours you might have fun toying with the cam.
Good luck.
#26
Le Mans Master
Yes, but using the intake closing angle accounts for all of that with a single number. The valve closes whenever the valve closes, regardless of the ramp rates that got it closed.
#27
Race Director
That said, I understand the DCR (by nature) is an approximation.
#28
Le Mans Master
..... Intake valves close after bottom dead center on the compression stroke ... how far after BDC determines how far up the cylinder the piston is when the intake valve closes ... how far up the piston is determines the actual compression distance or dynamic stroke of the engine which is where the dynamic compression ratio comes from ..... In other words , if the piston has traveled 1" up into the cylinder when the intake valve closes that is where the actual or dynamic compression begins ... your 3.750" stroke engine now has a 2.750" stroke in which to compress the fuel air charge ... this obviously doesn't take VE into account ... this is just swept volume vs chamber volume .....
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#29
Le Mans Master
I'm thinking a slower/milder ramp has more time between the .050" and fully open/closed event. During that interval, less air gets in -- to be compressed. Less air lowers compression. You can't compress a vacuum.
That said, I understand the DCR (by nature) is an approximation.
That said, I understand the DCR (by nature) is an approximation.
What you're describing is really in the realm of VE, not CR. And it is certainly true that different ramp rates affect cylinder filling (VE). It is one of many things that affect VE, such as overlap, port size and shape, air density, rpm, exhaust scavenging, etc. VE and DCR both affect the power, efficiency, and knock resistance of an engine (as do still other variables).
So DCR is a useful data point, but it's important not to conflate it with VE. And it's important to know that neither DCR or VE are sufficient to predict everything about an engine's output or knock characteristics.
#30
Le Mans Master
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..... Intake valves close after bottom dead center on the compression stroke ... how far after BDC determines how far up the cylinder the piston is when the intake valve closes ... how far up the piston is determines the actual compression distance or dynamic stroke of the engine which is where the dynamic compression ratio comes from ..... In other words , if the piston has traveled 1" up into the cylinder when the intake valve closes that is where the actual or dynamic compression begins ... your 3.750" stroke engine now has a 2.750" stroke in which to compress the fuel air charge ... this obviously doesn't take VE into account ... this is just swept volume vs chamber volume .....
It also requires accurate data for piston crevice vol/top ring location, piston crown height, deck height, piston dish/dome/valve reliefs. Or just an accurate liquid volume instead. Regardless you need an accurate static volume to begin with.
I do recall formulas I saw decades ago that did include a VE parameter and those calcs seemed intimitating to use. I also read of many confused posters trying to use it. Well maybe time has allowed DCR calcs to progress enough for the hobbiest to use. I see now some fairly simple calcs but again I believe you need to use it enough to become proficient.
#31
Melting Slicks
Gave this some thought today and without seeing the complicated models and it falls apart with rpm. Air flow is measured in CFM some time is an important input. In this case time is RPM as the faster the engines turns the less time for the so called leak to reduce cylinder pressure. Given airflow at lets say 0.002 lift and rpm a correction curve might be drawn up. The tip off was very simple, if memory serves me correctly 8.5 DCR and 155 psi cranking compression were mentioned. The DCR predicted under 125 psi and 155 recorded.
#32
Race Director
Originally Posted by ddahlgren
if memory serves me correctly 8.5 DCR and 155 psi cranking compression were mentioned. The DCR predicted under 125 psi and 155 recorded.
I'm left wondering about the accuracy of the gauge, the barometric pressure on the day I tested, and (mostly) if I landed significantly lower in DCR than I planned.
Maybe I need to BOOST it now?!?!
It would be interesting to know where you came up with 125psi?
#34
Race Director
That seems clear and straight forward. Makes sense....except that my cranking pressure comes out correct IF I used static instead!
I'm trying to think of a reason why it would hit full static compression with a compression gauge. Must have to do with multiple strokes (since it takes SEVERAL to hit max PSI).
It'll come to me.... Don't RUSH me! LOL
So...I guess that means the other couple of 383s I found posted -- with pressures around 200psi are running SCR above 13:1? Apparently much bigger cams and more "bleed off".
I'm trying to think of a reason why it would hit full static compression with a compression gauge. Must have to do with multiple strokes (since it takes SEVERAL to hit max PSI).
It'll come to me.... Don't RUSH me! LOL
So...I guess that means the other couple of 383s I found posted -- with pressures around 200psi are running SCR above 13:1? Apparently much bigger cams and more "bleed off".
#35
Melting Slicks
That seems clear and straight forward. Makes sense....except that my cranking pressure comes out correct IF I used static instead!
I'm trying to think of a reason why it would hit full static compression with a compression gauge. Must have to do with multiple strokes (since it takes SEVERAL to hit max PSI).
It'll come to me.... Don't RUSH me! LOL
So...I guess that means the other couple of 383s I found posted -- with pressures around 200psi are running SCR above 13:1? Apparently much bigger cams and more "bleed off".
I'm trying to think of a reason why it would hit full static compression with a compression gauge. Must have to do with multiple strokes (since it takes SEVERAL to hit max PSI).
It'll come to me.... Don't RUSH me! LOL
So...I guess that means the other couple of 383s I found posted -- with pressures around 200psi are running SCR above 13:1? Apparently much bigger cams and more "bleed off".
#36
Race Director
I have to think that gauge was wrong. This calculator shows my SCR would have to be near 8:1 with a DCR below 7:1 to have a cranking compression near 150psi.
Compared to a stock 350, it's too improved/strong for my combo to be that far off. Compared to a 350, it pulls like a 400+ci motor....even at the lowest rpms.
Maybe it could only hold 150psi (due to the seal or something)? I'm not sure I can even say I [at least] proved the cylinders are the same pressure with that gauge.
FWIW, I was checking to make sure all my valves were seating/sealing correctly....in the presence of "tappet" noise....that's been there since day 1 of the build. Finally decided WTF...I should check, just to make sure tapping is "normal". Considering it's a short-duration/high-lift/steep-ramped cam, I've always assumed that was part of the reason for tick-tick-tick.
#37
Melting Slicks
The length of the hose adds to the chamber volume and it takes several pumps to charge the whole system and then the little check valve in the system only flows so much per stroke.
#38
Race Director
Though I can't argue with the logic of this formula (and admit it sounds like something I would also post), something doesn't add up.
I found several stroker builds posting over 200psi as their cranking pressure. Specifically in the 210-220psi range.
210psi/14.5 = 14.5:1 DCR
With a DYNAMIC compression in the 14.5 range, static would be through the roof. What are we talking about here? 17:1 or more?
I even found posts saying cranking compression for a STOCK motor was in the 170psi range. That's 11.7:1 DCR. No way....
I can't believe "dynamic" pressures are THAT high. Even builds much over 9:1 DCR would be toying with the possibility of detonation.
Something seems off with this approach. Right?
Edit: BTW...Looking at record high/low barometric pressures in Kansas City, 1bar can ONLY vary from 14.2psi - 15.2psi
I found several stroker builds posting over 200psi as their cranking pressure. Specifically in the 210-220psi range.
210psi/14.5 = 14.5:1 DCR
With a DYNAMIC compression in the 14.5 range, static would be through the roof. What are we talking about here? 17:1 or more?
I even found posts saying cranking compression for a STOCK motor was in the 170psi range. That's 11.7:1 DCR. No way....
I can't believe "dynamic" pressures are THAT high. Even builds much over 9:1 DCR would be toying with the possibility of detonation.
Something seems off with this approach. Right?
Edit: BTW...Looking at record high/low barometric pressures in Kansas City, 1bar can ONLY vary from 14.2psi - 15.2psi
Last edited by GREGGPENN; 12-12-2017 at 04:23 PM.
#39
Race Director
I posted the question at SpeedTalk if you want to pop over there and read their feedback. They (of course) repudiated the accuracy of the cranking compression calculator I found.
They also agreed SCR and DCR were obviously too high if you divide measured compression by 14.5.
That said, I liked the line of thinking...which is why I posted it at SpeedTalk. I was actually hoping to learn a revised version of the calculation since it would seem fairly helpful.
#40
Melting Slicks
Plus, there's probably some volume loss as the compressed air rises in temperature. In short, the method of applying 14.5psi as a direct multiplier of "compression" doesn't seem to work.
I posted the question at SpeedTalk if you want to pop over there and read their feedback. They (of course) repudiated the accuracy of the cranking compression calculator I found.
They also agreed SCR and DCR were obviously too high if you divide measured compression by 14.5.
That said, I liked the line of thinking...which is why I posted it at SpeedTalk. I was actually hoping to learn a revised version of the calculation since it would seem fairly helpful.
I posted the question at SpeedTalk if you want to pop over there and read their feedback. They (of course) repudiated the accuracy of the cranking compression calculator I found.
They also agreed SCR and DCR were obviously too high if you divide measured compression by 14.5.
That said, I liked the line of thinking...which is why I posted it at SpeedTalk. I was actually hoping to learn a revised version of the calculation since it would seem fairly helpful.