Does anybody know if there is a formula for figuring out dynamic compression ratio?? If so, where can I get it? Thanks!!!

 

I got out my bible (Auto Math Handbook by John Lawlor) and thumbed to the equations for compression ratio and the Dynamic ratio wasen't mentioned in the whole chapter. And he covers everything from blood alchol to bore and stroke! Maybe I'm the only idiot who doesn't know what Dynamic Compression is, but could you explain what exactly it is or what your looking for? Maybe he calls it something else in his book. If I understand what it is I might be able to help.

 

Dynamic Compression Ratio is the compression ratio the cylinders actually see during operation. It is always lower than the static compression ratio on a naturally asperated engine. 8.4:1 was once thought to be the maximum safe dynamic ratio.

Static Compression Ratio is the 'common' compression ratio determined from the cylinder volume, chamber volume, etc. This is actually a theoretical best for a narurally asperated engine, requiring 100% efficiency in head, intake and carburator flow. Therefore, because of flow in-efficiency, even a high performace 12:1 compression ratio race engine probably doesn't see more than 8.4:1 dynamic compression ratio.

For static ratio to equal dynamic ratio there has to be 100% cylinder filling, a 302ci engine would have to ingest 302 cubic inches of air for every intake cycle. In the real world this doesn't happen, generally even a highly efficient engine uses less than 75% of the air it could.

 

You may not have answered his question but you answered mine. I've heard about that not long ago, but I didn't really pay attention, as you can tell. Thanks for the info!

 

I kinda figured it would be difficult to find such info......oh well.

 

I'd think that formula should be out there and it's a good question to ask. Valve overlap is a big factor in Dynamic Compression and engines vary in comp ratio from cam to cam. Hope someone comes forward with it.
Tony

 

An engine dyno gives volumetric efficiently percentages. That could be used to calculate the dynamic ratio. My high school science teacher swore NO naturally aspirated engine could achieve 100% volumetric efficiency, but I've seen it as high as 103%. Most high performance engines run around 85%. There's no way to calculate it on paper without the VE numbers and the static compression ratio.

 

I believe that if you go to the library and research some of the better books written on physics and engine development and design you will find what you're looking for.

I read about this in Smokey Yunicks (Spelling of Name Not Sure as well as Grumpy Jenkins book on smallblock chevy's.

I think the other post is right about VE and static comp ratio.
There are a couple of other variables in there somewhere, but I think those are the biggest of the equation. As far as the 8.4 dynamic ratio. They once said that a man would die if he went over 60 mph too. Much of the old school research is still valid, but with modern fuels and materials/thermo coatings I think we have only begun to see what the future holds.

Think about it. Some guy just last year came out with the engine without poppet valves that he is selling the heads to go on most any kind of engine. A 302 pumping out 400 plus hp with nearly 700 at 10,000 rpm possible! All of this naturally aspirated......10 years ago I bet 1 out of 1000 engineers would have taken him seriously.

good luck, I'm glad to hear that I'm not the only wannabe geek that is interested in this stuff.

 

Alright fellas, I'll answer this for what it's worth. First off, I'm a Junior Civil Engineering student, with mechanical engineering aspirations, and I've taken enough physics, dynamics, and mechanics classes to rule the universe (or at least it seems like it). To actually calculte dynamic compression, run a compression test. Get the numbers, which usually range from 120-190 psi for a street engine. Take this number and divide it by the ambient air pressure. Many factors contribute to this. There is no reasonable way to calculate this, you must do the test, or rely on a computer simulation like Dyno 2000. Simulations like this use the cylindar filling method of claclulating torque, thence power is a function of torque and engine speed. The simulation actually models the filling of each cylindar to come up with dynamic compression, and then integrates this function across the RPM range. you still can't calculate HP without Torque, as HP= (torque *RPM)/5252. This explains why HP and Torque are always equal at 5252 RPM, check it out.
I hope this clears things up a bit
John F. Daly III
The TorqueKing

 

I finally found a program to figure this out. Here's where I got it..
http://cochise.uia.net/pkelley2/
I have calculated the DCR for two of my engines so far. But I'm not sure what to think about the results. My 351w that I'm in the process of building has a DCR of 8.3874 (10.52 Static). And the 351m in my Bronco has a DCR of 7.7252 (8.88 Satic). I just don't know what these numbers translate into....any info on this would be greatly appreciated.

 

Dynamic compression ratio isn't really useful, what is useful is the dynamic pressure PEAK, measured in Psi. obviously, you've done that already, so search the internet or a reference manual on what fuel you should use. I think that from around 165-195 psi you need 93 octane, any more than that and you'll need to get into specialty fuel. Below that you'll be okay on 89, an maybe 87, I'm not perfectly sure. The dynamic peak compression is used primarily for determining your Motor Octane Number (MON), basically your fuel requirements. There is one more factor though, your total ignition timing may also effect this. In other words, in an engine that makes 190 psi, it's fine to run 93 so long as you're not trying to put 42 degrees of total timing in it. as long as you keep your timing around 36 degrees total, you'll make max power, and shouldn't have detonation problems. Most all street fuels make best power at about 36 degrees. Good Luck

The TorqueKing

 

hey torque king
i read of a research project at north western called green power
the project was to get the most and cleanest HP from test engines GM supplied a row of 502s (theory even works on a chevy) any way charge density was more important than there origonal premis expected,

as to the comment in one of these replys that you could't easly calculate efficency before the fact MBZ reaserch fomd that to be true in order to design the most effecirnt combustion chamber and primery flow they made 1100 diferent castings and ran them out on the test dyno it seems the results favored the "hemi" (da) with a matching reverse dome piston for a clamshell effect.

so i took all this and put to geather a 400 using off set porting to create a vortex so the cold high density air hits the cylinder wall and rapidly vaporizes. i re shaped the compression chamber and smothed all edges had custom made pistons for the clamshell thing. chose a cam that would work with all this theory. and my dynamic pressure is more than my static. if you cc everything then add for gasket space do the math i get one reading when you put a guage on it and set the motor at tdc firing roll it over one complete time take the reading devide by 14.7 it is at least 15% more if you crank it 2 times it is at least 30% more

i dont know if this means anything but it sure runs good

 

uhh, I'm so impressed by that, I don't even know what to say. That is super-custom, and I'd like to see what that did on the dyno. I have done some research on chamber efficiency, but obviously not so much as to take it into my own hands to build a naturally aspirated engine that exceeds 100% VE. Wow, that is awesome. I would like to know the details, what you used in terms of cams, heads, and exhaust. There is one more thing I would like to interrogate you on: if you are close to sea level, then the ambient atmosphereic pressure would be greater. Also, when you read a cylinder pressure gague, you are getting a number called Gauge pressure. This is the pressure that is beyond the ambient pressure. The real way to get peak cylinder pressure is to use a scinetific gauge that measures Absolute pressure. In other words, before you plug it into the hole, it reads the ambient atmosphereic pressure. You can still use the gauge you have, but you must add the atmosperic pressure to it. To formulize this: gauge + ambient atmospheric = absolute pressure. dynamic compression is thence gotten from: (absolute peak)/(ambient). What I think you may have done wrong is to not add the ambient atmospheric to the gauge pressure. I'm not saying that what you got was impossible, but it is very unlikely. I think what you got is probably 10 years ahead of aftermarket research. Check your number, I want to beleive you, because obviously know what you're doing.

Cadet Second Lieutenant John F. Daly III
South Carolina Corps of Cadets, The Citadel
The TorqueKing

 

i do a bunch of dyno testing ,
and even some of the SuperStock engines
will get 112 to 117 % Ve

some of the Comp engines will be 120+ Ve

 

if you post the following :

1-Intake duration at .050"
2-Intake CenterLine cam is degreed on
3-Intake valve lash
4-Actual measured Compression Ratio

i can calculate your dynamic cranking psi and CR
very close