Not that it really matters, but I was just thinking about calculating compression ratios when I had this thought.

Example engine is stock L48 with 8.5:1 factory compression ratio.


With the low lift and short duration of the stock camshaft, the cylinders are only going to fill up with a certain amount of air/fuel mixture. Let's call that "MIXTURE A".
By the factory specs, this is the mixture that is being compressed from 8.5 to 1.

Now let's swap the factory cam with a mild performance cam. Let's use Lunati's 235°/240° duration at .050, .490/.490 camshaft used in the Holly Systemax II kit.
The theory here is that the valves are hanging open longer due to the increased duration which in turn will allow more air/fuel mixture into the cylinder. We'll call that "MIXTURE B"

Therfore, as a unit of measure:
MIXTURE A < MIXTURE B.

Wouldn't more air/fuel in the cylinders slightly alter the overall compression ratio?



Discuss...

 

You are asking about Dynamic Compression Ratio (DCR) changes as related to valve timing as opposed to fixed Static Compression Ratio (SCR). So yes, it can change depending upon the valve timing events.

 

Your DCR is changed thought items like the overlap in a camshaft as well as the durations and LSA. Your SCR, which is normally the compression most people talk about, is based on the cylinder volume and rod stroke. This is part of the reason why stoker motors often have a higher SCR due to the increase in the cylinder volume.

 

Right. BUT... if your cylinder will hold 8.5 parts water (due to it's volume), a small camshaft may not be efficient enough to fill the cylinder completely due to late intake valve opening - right? If so, the stock CR may be closer to 8.2:1 rather than the advertised 8.5:1.

 

The advertised compression isn't the actual compression, actual measured compression for a L-48 is closer to 8.2:1 based upon the piston 12cc, head 76cc and .026" gasket volumes and average deck height of .025"

 

DCR is only valid at relatively low rpm. At higher rpm, the engine has the capability to fill the cylinder close to 100 %. This certainly happens at the point of max tq or near that point. At that point the DCR could equal SCR. That is why you can run into detonation, even though when using DCR you are safe.

 

So, educate me on the details.
In lower compression engines 8.5 - 9.0 :1 what would you be looking for as in duration and lobe seperation 110 - 112 degrees vs. an engine in the 9.75-10.25 :1 range. Speaking basically stock engines.
and please keep it simple

 

Keep your DCR somewhere around 8-8,5 and you should be ok.

You will find out that the intake valve on a 8,5 SCR engine is not that much open abdc. My guess is the cam has something like 190-200° of duration at .05"

 

True, but that is not taken into account for SCR. SCR only looks at volume of the cylinder and the reduced volume dictated by that rod. We are not talking about water, but air. Air (gases) will expand to fill any volume they are placed in.

Let me provide a simple example. The bore on a SBC 350 is 4.00 inches, and the engine has a stroke of 3.48 inches. This equates to a volume of 350 cubic inches for the 8 cylinder engine with 43.7 cubic inches per cylinder. Now I am going to leave out gaskets and their thickness to simplify this. If you add a 72 cc cylinder head to the block it will add around 4.39 cubic inches to the volume of a single cylinder. Now an engine with 8.5:1 CR with the 72 cc head will go from a volume of 48.39 cubic inches of volume regardless of air content to 5.69 cubic inches over the process of traversing from BDC to TDC. There is no such thing as a 100% filled cylinder since with increased pressure you can hold a higher mass of gas in the same volume. The only thing that stops us from putting more air into the cylinder is the potential failure of a head gasket or bolts due to pressure.

When you start to bring the pressure into the equation that is when you look at DCR. It will tell you whether you have too much pressure in the cylinder and thus increasing the heat in the cylinder and causing knock or detonation if you do not have a high enough octane.

 

Static Compression Ratio is calculated based on the idea that a cylinder closes the valves at bottom dead center, then the piston is pushed up all the way to the top of the stroke, and the ratio of the beginning trapped volume divided by the final trapped volume is the static compression ratio. It doesn't mean much but people use it as a point of reference. Cam timing is not part of this calculation. In actual practice, the valves never close close to bottom dead center so this calculation of rather limited use. People can understand it so it gets thrown around a lot.

If you include actual valve closing points of a particular cam in an effort to more precisely determine what the given cam and engine combination would compress to in actual practice, you are looking at the Dynamic Compression Ratio. This ratio can be applied to determine if your combination will run on pump gas or if you have enough compression for the combination to run well. The interpretation of the results is based on actual experience with typical aftermarket streetable cams and moderate static compression ratios. Higher RPM power bands generaly need more static compression ratio and more cam timing. Lower RPM power bands need less cam timing and less static compression ratio. The Dynamic Compresion Ratio that a combination needs to have if it is to run on pump gas is in the range of 8.0 to 8.5 if it has a typical aftermarket cam and a reasonable static compression ratio, based on actual practice.

Hope this helps,

-Mark.

 

Jeezzzz,,, I now have a head ache... What were you talking about? yes the beer is cold. That is all I know...

 

When in doubt, a big block and beer is the answer.

-Mark.

 

Measured Static compression ratio...what they listed as 8.5 to 1 or whatever is just that. The difference the volume of the cylinder when it is at BDC vs what it is when it's at TDC. Swept volume.

DCR..is that magical thing we all actually shoot for. This is what makes a motor a good one...or a dog. Cylinder pressure is what it's all about and manipulating DCR is the fine juggling match to work out the compromise between breathing, RPM range, detonation and TQ production. The more the better...up to the capability of the fuel being used.

So you are correct, changing cam timing does drastically affect DCR. There are a bunch of calculators where folks have worked out..or *backed* into solutions based on some experience. They are a decent baseline...but as mentioned there are lots of variables.

I can put a big enough cam in a 12.0 motor so that it will bleed enough pressure off that I can ride around all day long on 87 octane without a ping. That's because the throttle is limiting the amount of air getting into the cylinder. The less amount that goes in...just means there is less to compress. The static compression ratio is still 12.0 to 1....but it doesn't build enough pressure to hurt anything because there isn't much in there to compress. But once I stuff my foot in it and the throttles open, the cylinders start to really get full. They REALLY get full at low speed WOT...which is why you have to watch timing curve so you don't rattle it. As RPM climbs, we have to count on good head work to get the cylinders full...and the longer duration and higher lifts of the serious cams because there just isn't enough time to get the mixture moving. But eventually it will...and then on a motor that was doing fine and not pinging...all of a sudden will cross the line and hit violent detonation.

This is all where the tuning comes in also. I can make the 8.5 motor rattle on 93 octane if I crank enough timing in it. I can also make a 12.0 motor do fine on the same 93 octane. Just have to balance it all. Timing changes cylinder pressure under operating conditions at least as much or more than the cam ever will.


I like to look at the DCR calculators...and they will help if you ahve no other data, but whatever you do, don't think they are the end all to the question. They estimate. The better the heads and intake tract...the higher the RPM etc...all skews the results. They would have me believe my motor can handle well over 12.0 with my cam and run 93 octane. Just ain't happening. My motor shows incredible VE#'s at high RPM and I have to watch cylinder pressure to make it race on 93 octane..which is all I ever use.

If you look at the old Busch motors. They typically had 9.5 compression and cams in the 260's@.050. Way too much cam for a 9.5 motor. But when RPM went up and those incredible heads started flowing...you better not have anything but the best race gas you can find in the tank...or you'll have shrapnel in seconds!

Engines don't always read the same books or websites that we do!


One more thing..overlap has nothing to do in any direct fashion with cylinder pressure. Think about it....it occurs when both valves are open...that's the wrong end of the strokes to be looking at cylinder pressure. Compression doesn't really happen 'till they get closed one more stroke down the line.


JIM

 

Well said Jim!

 

http://victorylibrary.com/mopar/cam-tech-c.htm this article I believe will answer your question Hope it helps

 

Since Smokey, Mr. V and others were actually talking volumetric efficiency x (static) compression ratio when referring to dynamic compression, I'm beginning to think that a different term such as Effective Mechanical Compression Ratio (EMCR) might be a better description of what a lot of us including myself have been calling DCR, as one definition of dynamic is "...characterized by continuous change". Of course, the problem is that it would likely to lead to still more confusion concerning these matters.

Just a thought...

 

A website with some really good info:

http://www.tmossporting.com/tabid/3714/Default.aspx

Crane cams back in the 80s had a system to increase fuel economy that used 12.5:1 compression pistons but the cam had a special grind that held the intake open longer to bleed off pressure back into the intake to allow 87 octane fuel. It worked off the fact high compression ratio increased the signal to the intake creating a better mixture and port velocities. The downside was reversion.

Rhoads lifters come to mind also.

 

Jim is correct!

At low engine speed or cranking rpm the L-48 with a wimpy cam is has plenty of time to fill the cylinder

The bigger roller cam at low rpm and cranking speed has enough over lap where both valve are open that it bleeds off the trapped gasses. So you get lower cranking compression and lower dynamic compression ratio.

As rpm builds fluid dynamics come into play. Once you get some object moving it keeps moving. So you open the valve the A/F rushes into the cylinder. It is hard to reverse the flow direction once it gets moving.

Highly tuned intakes add to the ram effect over a small rpm and can actually over fill a cylinder. that is where you get max power

 

Now George....put down the coffee man.....look at what you just wrote.

Overlap doesn't bleed off trapped gases affecting compression. Again...there isn't any compression happening at that point.

The piston is actually moving upwards. The exhaust is open and it's flowing out the pipe. The intake is open and the exhaust movement will help get the column of air in the intake moving....but think about it...if exhaust is flowing out the exhaust...the pressure in cylinder must be higher than what atmospheric is right? So exhaust also will try to flow back into intake...reversion. Until pressure drops below atmospheric....ain't much moving into the cylinder.

This is where the magic of cam and head design come into play. This is also where a lot of fallacy concerning the value of very low lift head flow gets popped around. When the valve opens the piston is rising and no real movement is going to happen until piston starts to descend and gets out of the way. Guess what....many cams are open .075-.110" at TDC. By the time real flow gets moving, the valves are up in the .300+ lift range. At .200 lift the piston isn't very far down the cylinder. Don't get me wrong....you need valves open as fast as you can. But there are neat things you can do with this stuff. Ultradyne...and the Voodoo cams designer actually moves valves very slowly initially as piston approaches TDC...then rapidly hauls butt as piston descends. This cuts *effective* overlap by having valve not as far open as you might think it would from looking at the .050 opening numbers. At .050 it's moving...but below that it is slower by design.

Also think about the low lift flow on the other strokes. There are actually folks who flow heads in reverse to design port and valvejob to reduce the lower lift flow to cut reversion. The point is that when the valves are at low lift...the piston is usually going the wrong direction to matter much.

Another side note to cam/heads. I disagree with folks who say there is no need to open a valve more if the head is stopping flow at say .500 lift. If you open it further..you will have more duration at the highest flowing point of the head. Lift is always good!

I know this is side info...but it's related. What affects compression is the closing point of the intake. That's at a completely different part of the 4 strokes than overlap. Overlap does nothing for compression other than in a related fashion it might help cylinder filling. Lower dynamic comes from longer duration or retarded cams. With the longer duration for the higher RPM breathing, you need more compression to make up for the later valve closing. I can design a cam with lots of overlap...and still have incredible cranking compression. I would put it on a 100* LSA with short duration. I would have a bunch of overlap and a very advanced intake lobe...and then I could advance it some more and make even more cylinder pressure.


Skunkworks-I like the term Dynamic exactly for the definition you gave. "Continuous change". That's the whole deal...with every movement of the throttle plate or change in RPM..the dynamic compression is changing based on cylinder fill. The problem to me is that the calculators everyone plays with measure a static deal by calculating the cylinder pressure based on piston position in relation to the valve. They have no way to really account for the way an engine behaves when it *all comes together* and all that cool flow starts happening. DCR calculations are out the window in most cases at that point.


JIM

 

that's why i say most guys here can get a better cam pick from experienced guys like Jim or gkull then by spending 100 hours playing cam designer on a DCR program.