First off, I'm a mechanic and have been for 25+ years. Very knowledgable in all areas or automotive repair except auto trans repairs/rebuild. Just saying this beceuse on these forums you never know the knowledge level of those you are talking to.

In discussion of cams I've heard a lot of mention of dynamic compression. I know what static compression is but have neverheard the term dynamic compression.

Teach me.

Re'

 

This type of discussion usually opens up a can of worms, but here is the precis:

Static Compression Ratio (SCR) is calculated by dividing the total volume in the cylinder with the piston at BDC by the total volume in the cylinder with the piston at TDC.

Total volume is the Swept Volume (SV) which is the area of the bore times the stroke (bore x bore x stroke x .7854) plus the Clearance Volume (CV) which is the combustion chamber volume, any piston notching, dish or dome (+ or -), volume of the hole in the head gasket, and volume from the piston to the top of the block, which may also be + or - depending on whether the piston is in or out of the hole.

So the SCR = (SV + CV)/ CV. On a 3.48 stroke 4.03 bore engine, the total SV is (4.03 x 4.03 x 3.48) or 44.389 cu-in. or 355.11 cubes for a V8. If this was a 10.0:1 SCR engine, the CV would be 4.9321 cu in (about 80.85 cc).

The so-called Dynamic Compression Ratio (DCR) is an attempt to calculate what the engine sees because the intake valve closes AFTER, not at BDC (ABDC). Theoretically, compression doesn't start until the cylinder is sealed (intake closed).

To calculate DCR, use the SV of the cylinder at the point of intake valve closing. Determining where the piston is in the stroke at a certain number of degrees ABDC isn't easy, so generally a software program is used.

Let's say, for example, the intake closes when the piston is .50 inches up from BDC. If you calculate the SV with the piston .50 up the bore, the (effective) stroke is (3.48-.50) or 2.98 inches. Now the "effective" SV is 38.012 cubic inches. The CV doesn't change, so the engine thinks it has a smaller displacement with the same CV so the calculated CR is lower. (38.012+4.9321)/4.9321= 8.71, the calculated DCR.

DCR is a tool which may be used along with many other factors to evaluate an engine. The secret is not to get hung up on it, or become a slave to it as many folks have done. That's why bringing up DCR opens a can of worms.

Confused yet?

 

Where people fail to understand DCR is how wave propogation inside the intake manifold continues to fill the cylinder after BDC. We do not pick cams with late IVC to pump air back out of the cylinder.. the idea is just crazy.


People see how a high compression car seems to get along better with a big cam rather than a lower comression car with the same cam and think... "oh the DCR is good". The reason for this has absoulutely nothing to do with DCR. It is instead because volumetric effeciency is closely tied to compression ratio and exhaust pressure at EVC and intake pressure at IVC (see above). At EVC you have a mass of exhaust at the pressure of your exhaust (at EVC) and the volume of your total clearance volume. This left over inert mass (residual gas) displaces volume you could otherwise be filling with a fresh air/fuel charge. The larger your cam the more residual gas (which is your reversion) you will have. This hurts you but you can make up for it with higher compression. Higher compression, less of that volume @ EVC pressure you have and less residual gas.

M=PV/nRT. P and V have a huge say in what your residual gas mass will be!

 

In various post I've heard DCR being refered to as being too high or too low the a perticular cam. How do I know what my DCR should be for the cam I have?

Re'

 

 

LMAO@Alvin

There are DCR calculators on the net...but they will only get you close. Alot of engine builders see this as BS because No one REALLY knows the VE of an engine until you run it. And SO many factors affect VE...that it really is pointless. The calculators you see onlline do not take into account your intake tract in front of the throttlebody OR your exhaust past the exhaust port. Both have a huge affect on how effecient the engine is.

Even DA has an affect on VE. If your engine has high VE DCR goes up, but there is no way to find out.To REALLY get it dead nuts you would have to run it on an engine dyno with your full ex system and itake tract in place and change cams until you made the most power, even then it may not et as good as it would with a different cam that made LESS horsepower.

And we could go on and on...makes my head hurt.

David

 

Dynamic compression = ACTUAL COMPRESSION RATIO Like mentioned above, impossible to accuratly calculate and is affected by many things including but not limited to valve overlap and duration, exhaust scavenging, intake and throttle body design, ect.

 

The can of worms overflows...

 

The domino has been kicked over

The internal combustion engine, even as much as we(I say we, not as in ME, but as in OldSStroker and the like) know now is still a bit weird.


David

 

So... Scr=(( Bore X Bore X Stroke X .7854)+cv)\cv ...?

 

In easy terms

Static compression ratio is what most understand

Dynamic compression ratio is the amount of compression left after the intake valve closes

air has inertia.. depending on speed it will keep flowing after the piston has started its way back up.

combine all that, and you get the optimum Intake valve closing point and SCR.. (and as mentioned.. the calculation of that is a can of worms.. but can be calculated.. via extensive mathematics/fluid mechanics or programs such as GT-Power)

 

This is extremely complicated, especially when typed out on a forum. The easiest way to comprehend it is visually, graphed out. There is a book by Greg Bandish. It's a tuning book that shows, not necessarly dynamic compression ratio, but how timing affects cylinder pressure. Either way, check it out it's very informative.

 

A simple way of understanding dynamic compression is it is what you would see on a compression tester when doing a compression test. Take the same enging and throw in a cam with different valve events and what you see on the compression tester would probably show a different pressure even though the static compression of the engine hasn't changed.

 

Dynamic compression closely follows volumetric eff. or torque output + mechanical ineff.. (ie indicated torque)

 

We have created a new simplified dynamic compression ratio calculator at https://oztrack.com.au/dynamic-compr...io-calculator/ use as much as you like and bookmark its page.

 

Hi NSW, your Dienumbic compression pressure is controlled by MANY more factors.
I would ADD, piston pin offset, Intake runner/exhaust runner details, and Valve Overlap@ .050"
I created IR intakes that worked well on an engine with an ICL of 96*
THUS, ICL should be added.
Lance

 

WHO is NSW??

 

Sometimes these resurrected threads make me literally laugh out loud.

 

THAT "term" was created when England sent prisoners from Wales England to Australia.
New SOUTH Wales (NSW) is where the poster resides.
Lance

 

OK, but that would be like saying "Hi, California" to either one of us. But thanks for the geography lesson.