This is something I've never fully and completly understood. People will often say you need a certain CR to properly "power" the cams. Why is that?

I understand more compression equals a more powerful combustion... but that will be the case with or without performance cams. How does a higher CR get air in and out of the engine any quicker, or in higher quantity? After all that's what you buy performance cams.
Thanks
-Greg P

 

I think what you're referring to is the fact that aggressive cams often favor high-compression setups because their design effectively lowers the engine's dynamic compression ratio.

 

My theory on this topic is that since the engine has a higher CR the space between the valves and the piston becomes smaller. Therefore in order to provide proper timing, duration of lift and valve clearence between the piston there has to be a cam designed for this specific application. And some cams that are designed for this sort of application to work with a higher CR to be more efficient at letting air and fuel in and exhaust gasses out of the engine.

 

What is meant by "dynamic compression ratio"? How would a high performance cam lower this?

 

Dynamic compression ratio is the effective compression ratio observed by the intake charge. For example, a 10:1 compression ratio would provide an effective 10:1 ratio for completely filled cylinders. However, at partial throttle, the cylinders are only partially filled, so a 50% fill would yield only a 5:1 compression ratio on the 10:1 motor. This also occurs at WOT at high RPM where performance camshafts make their power; the cylinders are not totally filled due to fluid flow restrictions. Therefore a higher static compression ratio allows the motor to opperate efficiently with a limited intake charge and make that power.

 

So as the dynamic compression ratio drops do to air flow restrictions, the higher static compression ratio will keep the motor operating more efficently. Perhaps this is why on high rpm race engine's we see super high CR (13-15:1). At high RPM's these High CR's would keep the motor efficent when it's dynamic CR drops. Thanks guys.
-Greg P

 

I don't have time to answer this question again but this was my answer to a guy who wanted to run somewhere between 14 and 15:1 static compression ratio on the street. I snagged it from my notepad. Feel free to post your opinions or rebuttals.

Text from previous post:

What are you planning to do with this engine? Race only I would hope.

How high do you plan to rev this engine? Compression ratio is directly related to the RPM range the engine runs in. The higher the compression ratio you run the more duration you will need to run on the cam. This is because the static compression ratio is not the one the engine sees. There are three types of compression ratios. Static, effective, and dynamic.

Static compression ratio: chamber volume + swept volume / chamber volume. In your case you are looking at 14-15:1

Effective compression ratio (this is the important one): The compression ratio that is measure begining at the point when the intake valve closes. If you think about it you can't begin building compression in the cylinder until you close the intake valve, otherwise it will just blow back into the intake. On a modern 4-valve engine running 91-93 AKI (octane) fuel the effective compression ratio can be somewhere around 8 to 8.5:1. Two valve heads can run about 7.5-8:1.

Dynamic compression ratio: Effective compression ratio x volumetric efficiency. There is 1 point in the rpm band (<U>peak torque</U>) where volumetric efficiency is highest. At lower rpm the incoming air/fuel charge does not have enough inertia to overcome the force of the piston coming back up on the compression stroke and some is forced back out the intake valve, but as rpm builds and the velocity of the A/F charge increases it is able to resist this tendancy and can almost fill the cylinder to 100% capacity (but it rarely does except on extremely well prepared engines, contrary to popular belief).

So for an engine that runs a compression ratio that high you are going to have to spin the engine pretty damn fast (say 10,000 minimum) to make any power. At those speeds you better have a valvetrain that can handle that RPM, not to mention having a stout bottom end.

-Scott Tucker


Modified by AutoEng2002Si at 12:57 PM 1/9/2006

 

Thanks for the very informative post AutoEng2002Si!
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AutoEng2002Si &raquo;</TD></TR><TR><TD CLASS="quote"> Dynamic compression ratio: Effective compression ratio x volumetric efficiency. There is 1 point in the rpm band (peak torque) where volumetric efficiency is highest. At lower rpm the incoming air/fuel charge does not have enough inertia to overcome the force of the piston coming back up on the compression stroke and some is forced back out the intake valve, but as rpm builds and the velocity of the A/F charge increases it is able to resist this tendancy and can almost fill the cylinder to 100% capacity (but it rarely does except on extremely well prepared engines, contrary to popular belief).


Modified by AutoEng2002Si at 12:57 PM 1/9/2006</TD></TR></TABLE>

Are you saying that before peak torque is reached reversion will always occur? If so, what does this have to do w/effective compression ratio? Is it because the higher your effective CR, the higher the torque peak will be moved in the RPM band. If that's the case you would need to rev higher to start making power(I.E. stop reversion and increase VE). That makes sense. However, in honda engines the torque peaks way before hp does. In most cases I've seen the torque drops off pretty sharply after it's peak. So does that mean reversion will start to occur again?
-Greg P

 

So reversion is when air/fuel is pushed backwards and leaked through the valve it just came through, but after the valve closed? what are valve seals for then?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by precisionelite &raquo;</TD></TR><TR><TD CLASS="quote">Thanks for the very informative post AutoEng2002Si!


Are you saying that before peak torque is reached reversion will always occur? If so, what does this have to do w/effective compression ratio? Is it because the higher your effective CR, the higher the torque peak will be moved in the RPM band. If that's the case you would need to rev higher to start making power(I.E. stop reversion and increase VE). That makes sense. However, in honda engines the torque peaks way before hp does. In most cases I've seen the torque drops off pretty sharply after it's peak. So does that mean reversion will start to occur again?
-Greg P

</TD></TR></TABLE>

There is some reversion both below and above the RPM where peak torque is produced (unless you have variable cam phasing).

If the effective compression ratio is higher than about 8.5 for a 4-valve aluminum head engine you will need to run higher octane fuel because, due to the increased compression, the temperature of the air/fuel mixture will be raised. This has nothing to do with engine speed.

If the static compression ratio is raised you will need to change the point the intake valve closes, which will make duration longer. This will move the powerband up higher. I am saying this should be done in an idealcase. Not everyone understands that and a lot of people pick cams with either too much or too little duration.

HP is just a mathematical derivation of torque. You've probably seen the formula:

HP = Torque x RPM / 5250

Horsepower and torque will always be equal at 5250 RPM. Look at some dyno graphs.

Modified by AutoEng2002Si at 2:03 PM 1/10/2006


Modified by AutoEng2002Si at 2:04 PM 1/10/2006


Modified by AutoEng2002Si at 2:05 PM 1/10/2006


Modified by AutoEng2002Si at 2:07 PM 1/10/2006

 

The primary purpose of a valve seal is to keep oil from getting sucked through the clearance between the valve stem and guide and into the combustion chamber.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AutoEng2002Si &raquo;</TD></TR><TR><TD CLASS="quote">
If the static compression ratio is raised you will need to change the point the intake valve closes, which will make duration longer.Modified by AutoEng2002Si at 2:07 PM 1/10/2006</TD></TR></TABLE>
Why do you need to make the intake valve close later if your running more static compression?

 

To keep the effective compression ratio in that 8 - 8.5:1 window. If you raise your static compression but keep the same cam (and therefore the intake valve closes at the same point as it did before - unless you have adjustable cam gears) your effective compression ratio may be too high and you run the risk of detonation.

If I am not making this clear try going to http://www.elgincams.com and go to the '&gt;&gt; Camshafts' option from the menu on the left. This is an in-depth paper on camshafts. He is the master. His camshafts have won the Daytona 500. He grinds Honda cams too, even if your engine is not listed. He treats each project kind of as a custom cam in itself anyway since he has hundreds of masters to choose from that he has collected over the years.

 

Thanks AutoEng2002Si, now I get it :-) So the goal is to keep your effective CR in that 8-8.5 range for the most power.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AutoEng2002Si &raquo;</TD></TR><TR><TD CLASS="quote">There is some reversion both below and above the RPM where peak torque is produced (unless you have variable cam phasing). </TD></TR></TABLE>
Perhaps that is why K series engines have a flat torque band across the board where traditional(B and H series) VTEC engines have a torque peak, then it starts falling. Thank you for clearing this up for me. :-)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Padawan &raquo;</TD></TR><TR><TD CLASS="quote">I think what you're referring to is the fact that aggressive cams often favor high-compression setups because their design effectively lowers the engine's dynamic compression ratio. </TD></TR></TABLE>

Wouldn't aggressive cams increase the engine's dynamic compression ratio? After all, more air should be getting sucked into the engine....