Compression Ratio (static / dynamic) and more calculator
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MASSIVE TYPE IV
- Posts: 20132
- Joined: Fri Jul 07, 2000 12:01 am
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HAM Inc
- Posts: 591
- Joined: Wed Jan 14, 2004 7:33 am
If the engine could get it's full gulp of air by the time the piston reached BDC, yes it would make more power. At very low speeds this is possible, but at high speeds the piston outruns the fuel mixture. Leaving the valve open after BDC allows the mixture to continue flowing into the cylinder due to the inertia built up by the mixture trying to keep up with the piston.
There are porting tricks to capitalize on this mixture inertia that extend the rev range of a "short" cam, while retaining the low speed characteristics of the "short" cam. (a broader torque curve is the result)
The reason engines with "long" cams idle funny has little to do with overlap. It is the late closing of the intake valve.
Don I would set your cam straight up. You won't see much difference going either side of a properly ground cam. If the cam description is a high revver and you want it to come on a little sooner, advance it 1 or 2 degrees's. If it is ground for bottom end and you want to raise the revs a little retard it a couple of degree's. The truth is you can spend a lot of time fooling with this detail and never notice a difference. If the cam degrees in within a degree of the cam card I let it ride, even on my race engines.
Moving more than 2 degrees will usually hurt overall performance.
There are porting tricks to capitalize on this mixture inertia that extend the rev range of a "short" cam, while retaining the low speed characteristics of the "short" cam. (a broader torque curve is the result)
The reason engines with "long" cams idle funny has little to do with overlap. It is the late closing of the intake valve.
Don I would set your cam straight up. You won't see much difference going either side of a properly ground cam. If the cam description is a high revver and you want it to come on a little sooner, advance it 1 or 2 degrees's. If it is ground for bottom end and you want to raise the revs a little retard it a couple of degree's. The truth is you can spend a lot of time fooling with this detail and never notice a difference. If the cam degrees in within a degree of the cam card I let it ride, even on my race engines.
Moving more than 2 degrees will usually hurt overall performance.
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MASSIVE TYPE IV
- Posts: 20132
- Joined: Fri Jul 07, 2000 12:01 am
I concur with Len 100% on his post. I run most cams straight up and only alter the settings if the cam does not degree in perfect, which is VERY, VERY rare with a Web Cam.
Even the engine that has the crazy cam in it and insanely ported heads by Len still idled smooth as glass at 1K RPM... it sounded like it wasn't even built- just the way we like them!
Even the engine that has the crazy cam in it and insanely ported heads by Len still idled smooth as glass at 1K RPM... it sounded like it wasn't even built- just the way we like them!
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914fan
- Posts: 189
- Joined: Thu Aug 21, 2003 8:53 pm
This is not yet the updated version. I sent a new one to Tuna the other day. it has a few more things added.
Ham or Jake,
Is there a better way to calculate dynamic compression? I think it would be fun to try and upgrade what I have. Can you provide any calculations or formulas for me to try? If so can you also prodive some valid data. If I try and make up the head flow and etc this will never work. I do not know enough about that to make ligit numbers.
I had thought that you needed dynamic cr to help set static and your selected cam. For example my CR is 11:1 but the cam brings it to a dynamic 9.5:1. That was what I thouht I was trying to accomplish. Is that not dynamic CR, or did I do a goof?
Ham I can send you the updated version (sneek peek) if you e-mail me.
Nick
Ham or Jake,
Is there a better way to calculate dynamic compression? I think it would be fun to try and upgrade what I have. Can you provide any calculations or formulas for me to try? If so can you also prodive some valid data. If I try and make up the head flow and etc this will never work. I do not know enough about that to make ligit numbers.
I had thought that you needed dynamic cr to help set static and your selected cam. For example my CR is 11:1 but the cam brings it to a dynamic 9.5:1. That was what I thouht I was trying to accomplish. Is that not dynamic CR, or did I do a goof?
Ham I can send you the updated version (sneek peek) if you e-mail me.
Nick
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HAM Inc
- Posts: 591
- Joined: Wed Jan 14, 2004 7:33 am
Nick I would love to get a sneak peak at your handi work. I started a chart and never got to finish it. I'm not sure how to find your e-mail address so just e-mail me and I'll respond. I'm at [email protected]
As for dynamic compression ratios I have never seen a formula. I would imagine that some of the multi-million $ race teams have a program to estimate it. The problem is that "dynamic" indicates that the ratio is going to take into account the actual amount of mixture the cylinder gulps down in a cycle. There are SOOOO many factors that influence this (including ex. effeciency) ignition effeciency, burn quality and all of the factors that contribute to it. Plus there is the fact that at high speeds with the right combo an engine can operate at over 100% volumetric effeciency, meaning the amount of fuel mixture that pushes into the cylinder is greater than the swept volume. (This goes back to inertia filling and ex. scavenge)
None of the experienced engine builders I communicate with worry about dynamic ratios. The subject comes up from time to time. But it's not something we chase. It is far more exacting a science to calculate static ratios then make performance observations as other variables are changed one at a time.
It is interesting to calculate the static ratio based on the volume starting at the pistons location when the valve closes. But here again variables enter into play. At what point do you consider the valve closed? I know that sounds crazy, but consider that the higher the speed the less significant the flow at low lifts becomes. I could not care less about the duration at .050" on a 8500RPM race engine. The flow at this lift value is meaningless. On one of these engines the intake valve may be open as much as .200" (or more) at TDC at the start of the intake stroke and may still be open as much as .350" (or more) at BDC. From this you can see that any flow occuring below .200, and on the back side of the lobe below .350, is the result of ex. tuning or inertia or even a combo of the two.
I guess what I'm trying to say is I'm not even close to being smart enough to calculate dynamic compression.
As for dynamic compression ratios I have never seen a formula. I would imagine that some of the multi-million $ race teams have a program to estimate it. The problem is that "dynamic" indicates that the ratio is going to take into account the actual amount of mixture the cylinder gulps down in a cycle. There are SOOOO many factors that influence this (including ex. effeciency) ignition effeciency, burn quality and all of the factors that contribute to it. Plus there is the fact that at high speeds with the right combo an engine can operate at over 100% volumetric effeciency, meaning the amount of fuel mixture that pushes into the cylinder is greater than the swept volume. (This goes back to inertia filling and ex. scavenge)
None of the experienced engine builders I communicate with worry about dynamic ratios. The subject comes up from time to time. But it's not something we chase. It is far more exacting a science to calculate static ratios then make performance observations as other variables are changed one at a time.
It is interesting to calculate the static ratio based on the volume starting at the pistons location when the valve closes. But here again variables enter into play. At what point do you consider the valve closed? I know that sounds crazy, but consider that the higher the speed the less significant the flow at low lifts becomes. I could not care less about the duration at .050" on a 8500RPM race engine. The flow at this lift value is meaningless. On one of these engines the intake valve may be open as much as .200" (or more) at TDC at the start of the intake stroke and may still be open as much as .350" (or more) at BDC. From this you can see that any flow occuring below .200, and on the back side of the lobe below .350, is the result of ex. tuning or inertia or even a combo of the two.
I guess what I'm trying to say is I'm not even close to being smart enough to calculate dynamic compression.
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dstar
- Posts: 3733
- Joined: Tue Feb 25, 2003 12:01 am
Heh, Heh, Heh!MASSIVE TYPE IV wrote:......Even the engine that has the crazy cam in it and insanely
ported heads by Len still idled smooth as glass at 1K RPM...
it sounded like it wasn't even built-
just the way we like them!
Just the way *I* like it TOO!
I'm about getting tired of putzing with this thang.
I think I'm gonna run it straight up and see what she does!
Don
Don