Relationship between Compression, Cylinder pressure and cam ? - Chevelle Tech
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  #1  
Old Apr 21st, 10, 6:17 AM
Rokker Rokker is offline
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Default Relationship between Compression, Cylinder pressure and cam ?

Can someone please enlighten me a bit about the relationship between static compression, dynamic compression, cylinder pressure and cam ?

Hereīs my thinking:
Low static Compression (8:1) builds low cylinder pressure and you can run a small camshaft with little overlap and that will help you retain adequate cylinder pressure, since you don`t have much bleed off.

Go big on the static compression say, 12:1 small on the cam, huge filling of the cylinders, big risk of detonation, lots of torque down low, not much hp at high rpms ?

12:1 static compression, huge camshaft =adequate cylinder pressure right ?
I guess this is oversimplifying it ? How big effect on cylinder pressure will a camshaft have say compared to the static compression? Considerable or marginal.

Thing is I donīt understand why people donīt run 12:1 compression and huge camshaftīs and getting the dynamic compression down with the cam?
Won`t the compression help getting the appropriate vacum for brakes back, and the "loss" of torque down low due to a big camshaft will become smaller since the compression still helps with filling the cylinders. And the result is a strong low rpm motor (big compression) that really shines in the high rpms due to both the cam and compression ?

Won`t the detonation issues come back when you have higher rpms, into where that big cam really shines (5000-6500rpm) or will the cylinder pressure stay constant throughout the rpms ?

Letīs just for now keep the mixture and ignition out of this, thinking that they are both fully optimised. No compromise of jetting a tad richer or retarding the ignition a few degrees.

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Old Apr 21st, 10, 6:37 AM
Calculated Risk Calculated Risk is online now
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

Compression has nothing to do with vaccum. I saw that and just wanted to get that out of the way....

Camshaft has everything to do with dynamic compression ratio (DCR). You have no compression building untill the intake valve closes. Some people don't realize how far up the piston is in the bore before the intake valve closes. The bigger the cam, the longer your delaying the intake valve from closing, the more stroke your wasting (on the compression stroke). This is why big cams and low static compression ratio's (SCR) don't work together. So yes you can use a "big" cam to lower the dcr, but when you start out at 12.5:1 scr, your a bit out of range to fix that with a cam and still have a viable pump gas combo.
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Old Apr 21st, 10, 6:58 AM
Rokker Rokker is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

Doesn`t big compression fill the cylinders more because the "suction" (hmmm-hahah) is also bigger ?
Bigger compression = bigger suction ??
If the suction is bigger I would think that the flow of air is more too, and that the flow of air is more because there`s more suction/vacum.

Am I all confused or ?
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  #4  
Old Apr 21st, 10, 8:39 AM
travis g travis g is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

In my experience, increased compression will increase idle vacuum a bit, which makes me think that "suction" is being increased somewhat.
I do know, that what your talking about is not a 1:1 relationship. With a bigger cam, you will regain *some* of the lost low end torque by increasing the compression, but never all of it. With the intake valve closing later, your compression stroke is shorter...meaning that your taking a smaller volume of air and fuel and squeezing it harder. You could have the same cranking compression readings, and the same dynamic compression ratio, but with all else the same, the smaller cam (or the earlier closing intake valve) will generally trap more air/fuel at lower rpms and create more low end torque. Once things get spinning, dynamic compression becomes less important and airflow and cam timing become more important for making power.
Here is a real world example for you. 2 similar weight pickups, both nearly identical in all aspects except 1 is a 8.5-1 compression 350 with a 260 duration comp cams high energy cam (212@.050, 110 lsa), the other has about 9.5-1 compression and a 350hp 327 GM -151 cam in it (222@.050, 114 lsa). Both crank 150-160psi. In a 1/4 mile, the smaller cammed truck beats the other truck by 1 to 1 1/2 car lengths...but in the first 200 feet the smaller cammed truck is more like 4-5 car lengths ahead. Once the bigger cammed truck gets wound up, it is reeling in the smaller cammed truck the whole way down the track.
Of course, if the bigger cammed truck would have put a 2400 rpm or so stall convertor in there to make up for the reduced off idle output, he would have won.
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Old Apr 21st, 10, 9:33 AM
Calculated Risk Calculated Risk is online now
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

Quote:
Originally Posted by Rokker View Post
Doesn`t big compression fill the cylinders more because the "suction" (hmmm-hahah) is also bigger ?
Bigger compression = bigger suction ??
If the suction is bigger I would think that the flow of air is more too, and that the flow of air is more because there`s more suction/vacum.

Am I all confused or ?
Suction bigger? I think your confused. The right cam with the right overlap will increase scavenging and increase volumetric effiecency, but that is still not a function of compression.

I think you need to consider that air is NOT being sucked into an engine. But rather the downward piston movement is creating a low pressure, this causes atmospheric pressure to rush into the engine. Compression has ZERO effect on the size of the low pressure depression. Exhaust scavenging also plays a part in the equasion, the exiting exhaust gas pulls air fuel mixture in and also out the exhaust with it.
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Old Apr 21st, 10, 10:03 AM
93Polo 93Polo is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

Yes valve events/cam design are used to change (bleed or add) dynamic compression(DCR). DCR is what you are ultimately concerned with when selecting the octane needed to run your motor. LS1tech had some major discussions calculating optimal DCR. Playing with all the trade offs is how the respected engine builders make their money and power. The specifics of what each motor likes may change but the concepts are the same. One of the most in depth Camshaft/DCR discussions on LS1tech is linked below.

http://www.ls1tech.com/forums/genera...n-part-ii.html

J-rod and pianoprodigy have both posted up DCR calculators.
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  #7  
Old Apr 21st, 10, 2:39 PM
trmnatr trmnatr is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

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Originally Posted by Calculated Risk View Post
Suction bigger? I think your confused. The right cam with the right overlap will increase scavenging and increase volumetric effiecency, but that is still not a function of compression.

I think you need to consider that air is NOT being sucked into an engine. But rather the downward piston movement is creating a low pressure, this causes atmospheric pressure to rush into the engine. Compression has ZERO effect on the size of the low pressure depression. Exhaust scavenging also plays a part in the equasion, the exiting exhaust gas pulls air fuel mixture in and also out the exhaust with it.
Agree except compression can change the rate of the low pressure, so can rod length

Everything else I agree with and your a turbo guy
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Old Apr 21st, 10, 4:48 PM
MEJ1990TM MEJ1990TM is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

I've got one question. I read that dynamic compression becomes less important as RPM's start coming up. Is that because at higher RPM's you are getting closer to the static compression ratio some how? I don't see how that is possible since no matter what the speed is the point at which the intake valve finally closes doesn't change.
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Old Apr 21st, 10, 4:59 PM
erics327 erics327 is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

With an increase of static CR you can get back some, but not all, of the low end torque you lose with a big cam. You can compensate for the shorter compression stroke with increased static CR, but you will also have a shorter power stroke with a longer duration cam due to an earlier exhaust opening. You effectively have a shorter stroke engine, which is also like a smaller engine, at low speeds before you reach the rpms and air speed which your valve events become efficient.

Eric
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Old Apr 21st, 10, 5:12 PM
MinionII MinionII is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

I theorize that at higher rpm, the intake charge is subjected to high cylinder pressure for a briefer duration, prehaps reducing the tendency to burn un-smoothly or before being ignited by the spark plug.
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Old Apr 21st, 10, 6:16 PM
kirkwoodken kirkwoodken is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

Overlap does not bleed off compression. Compression pressure is determined by the intake valve closing point and the static compression ratio and/or boost. (We don't want to get into rod length.) This is another fallacy put forth by some magazine articles. However, increased overlap can be the result of increased cam timing, which does cause less running compression. In other words, increasing the Lobe Separation Angle makes an engine idle better because there is less exhaust dilution of the incoming mixture, not because the compression is higher or the intake valve closes sooner.

Poor idle vacuum is caused by exhaust dilution and lower compression, which are the result of long duration cams/late intake closing.

There is another thing to consider. Let's say you have a 327 with a hot street cam. The intake valve closes at 65* ABDC. You have inefficient 461 heads, so you light off the mix at 40* BTDC. Your intake valve has closed when the piston is 2 1/2" in the hole, and you are firing the plug when the piston is 1/2" in the hole. How much compression heat did you develope in 2 inches travel, with the piston still 1/2" from quench? Not much! You can see that some of the fuel vaporization still needs to take place and that will happen after the fire has been lit.

This example also shows the importance of engine revs, WOT, and the ram effect. And the reason some stock engines light the fire just a few degrees before TDC. IC engines are full of compromises. They are a constant speed/constant load engine. They make great airplane engines, but for cars, they need some help. That help comes in the form of mixture curves, spark curves, variable cam timing, and picking the right compression and cam timing for a particular running range, without it being too bad on the extreme ends.
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Old Apr 21st, 10, 6:20 PM
trmnatr trmnatr is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

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Originally Posted by erics327 View Post
With an increase of static CR you can get back some, but not all, of the low end torque you lose with a big cam. You can compensate for the shorter compression stroke with increased static CR, but you will also have a shorter power stroke with a longer duration cam due to an earlier exhaust opening. You effectively have a shorter stroke engine, which is also like a smaller engine, at low speeds before you reach the rpms and air speed which your valve events become efficient.

Eric
With small engines such as 327's I dont really look at DCR. I have had 11:1 327's make more power than 12:1 327's but we did use lobe seperations under 105 with a pretty long duration

I wrote some stuff on what I do for cams on 1320techtalk.com , I will copy/paste it here as this may give the OP a better idea as to what all he needs to look at
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Old Apr 21st, 10, 7:10 PM
Calculated Risk Calculated Risk is online now
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

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Originally Posted by trmnatr View Post
Agree except compression can change the rate of the low pressure, so can rod length

Everything else I agree with and your a turbo guy

I don't see how compression will change the rate of the low pressure depression. But i will listen to your theroy as long as there are no pictures or specs listed....


Rod length would be a different variable, OP only stated a change in compression(scr), i wanted to leave SCR as the only variable in the equasion, or else the conversation goes crazy vecause of so many variables.
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Old Apr 21st, 10, 7:56 PM
erics327 erics327 is offline
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

I don't remember where I read about it, but apparently with higher CR the rate of change in cylinder volume compared with clearance volume/combustion space above the piston at TDC is greater. In other words, as the piston moves down the pressure differential is greater if the volume above the piston [chamber/gasket/quench volumes] is smaller. If I remember right this only makes a difference early in the stroke since the cylinder volume will equal and exceed the combustion volume sooner.

Eric
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Old Apr 21st, 10, 8:28 PM
novadude novadude is online now
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Default Re: Relationship between Compression, Cylinder pressure and cam ?

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Originally Posted by erics327 View Post
I don't remember where I read about it, but apparently with higher CR the rate of change in cylinder volume compared with clearance volume/combustion space above the piston at TDC is greater. In other words, as the piston moves down the pressure differential is greater if the volume above the piston [chamber/gasket/quench volumes] is smaller. If I remember right this only makes a difference early in the stroke since the cylinder volume will equal and exceed the combustion volume sooner.

Eric
The change in volume happens quicker. If you have 50cc above the piston at TDC, the piston will need to move much less to double the volume than if you have 100cc above the piston.

I am thinking that the space above the piston needs filled with air/fuel too, and you are generating the same amount of "suck" based on displaced volume and valve curtain regardless of volume above the piston at TDC. If piston displacement = 100cc, then you will need 50cc + 100cc of a/f to fill the volume in the first scenario vs 100cc+100cc in the second.

For a given amount of displacement and a certain valve curtain, I would think you have more charge to burn in the 50cc chamber volume scenario, vs the 100cc scenario, since the cylinder should flow the same amount through the valve.

I didn't read this anywhere, so I could be way off-base. Just drawing conclusions as I think this through.
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