Well, what is dynamic effective compression ratio? I know what static compression ratio is. Thats the one that everyone refers to, correct? I would like to know what DCR is and how it is affected by the motor's parts and timing. Is necessary to take DCR into consideration when building a motor? How is it calculated? How does it affect power, timing, detonation, etc... I am wondering what DCR is because I saw it on this site when calculating my static compression ratio. Thanks for the help!

http://www.kb-silvolite.com/calc.php?action=comp

 

Dynamic ratio is the compression ratio that is generated under the dynamic conditions of a running engine. It is simply the compression ratio when taking into account the point of closing of the intake valve and rod length. I don't know how to calculate i but there are a million calculators out there that will do so for you.

 

To expand a little on the above, detonation is a result of, among other things, cylinder pressure that rises too high for the fuel octane before the mixture completes burning. This pressure is largely dependent on the compression ratio, but at low speed compression can't begin until the intake valve closes. Therefore a greater intake cam duration (specifically the closing point) lowers this 'Dynamic Compression Ratio' and reduces potential power. By adjusting the mechanical ratio higher to maintain DCR (~9:1 is considered a very safe maximum) power, especially at lower RPM where a big cam is usually weak, is improved.

 

DCR is calculated as if the stroke starts exactly when the intake valve closes (IVC) because theoretically that's when compression can start. To calculate it, you use the actual IVC, not the IVC @.050. Not all cam companies give you this number, but "advertised duration" may allow you to calculate it fairly closely. The link you posted added 15 degrees to the IVC @ .050, which is a fair estimate.

DCR is a useful number when designing an engine because it allows the designer to select the Static Compression Ratio (SCR) which will give the desired DCR which relates to detonation potential with the gas you are using. It can also tell you what effect changing the valve events have on what the engine "sees" as effective compression.

You called it "dynamic effective compression ratio" which is a pretty good description.

If you search on DCR in this forum and other "advanced" forums you might frequent, you'll find lots of discussion, interspersed with some useful information. MadBill says some good stuff. Remember that DCR is an engine design tool, not the be-all-and-end-all solution.

OOPS! They beat me to it because I type so slowly. (I was correct about MadBill, huh?)

 

At least we didn't contradict each other SS! Great minds and all that.
They say a man with a watch knows the time. A man with two is never quite sure.
If both read exactly the same however...

 

My favorite quote about watches is an old fighter pilot axiom that says something about the size of one's watch being inversely proportional to the size of one's....umm, well, you know.

Big watch, little ****

Glad to see we're on the same page with DCR. You've made good points about specifiying the SCR last in an engine disign, not first.

 

Ok, I buy the 9 to 1 bit, but what happens when you throw boost on top of it?

What is the safe effective CR qhen using a power adder?

I calculate using temp in the chamber more than DCR, but just looking for other opinions.

 

Is there a way to find the IVC of givin lobes without contacting the cam manufacturer?

 

Start with the total or "advertised duration" of the intake lobe. Divide by 2. Add the Intake Center line and subtract 180 from the answer.

Example: Comp XR277HR hydraulic roller for LS1 installed @112 ICL. It has 277 degrees "advertised" intake duration.

(277/2 +112) -180 = 70.5 IVC. (ABDC)

If you do the numbers for a milder cam like the XR259HR you get:

(259/2 + 112) - 180 = 67.5 IVC (ABDC)

So the milder cam closes the valve earlier (after BDC, or 112.5 BTDC vs. 109.5 BTDC on the "277") so the DCR will be higher.

Note:

These aren't necessarily exact figures, because Comp uses .006 lift to rate advertised duration, but with lifter plunger movement and valve train deflection, they are pretty close.

If it was a solid with say .017 lash (at the valve or .010 at the lifter with 1.7 RAR), you'd need to subtract a little from the rated duration before you did the math.

 

With that being said, what do you think is more accurate, the formula above or just adding 15 to the IVC, assuming you have the .050" IVC. I did the math both ways on a Comp XE-R 244 intake lobe, 109 ICL, and it went like this: 293/2 + 109 - 180 = 75.5* The .050" IVC is 51*. Assuming you add 15 like the website above says, you get 66*. That's a pretty big diffrence in DCR when it's figured in. I just wonder which is more accurate?

 

I've never seen a cam spec (at least not a hydraulic roller) with lobes so aggressive that the 0.050" duration was only 2 x 15 degrees less than advertised/0.006" lift specs, so I'd go with a spec more like 0.050" plus 25 or thereabouts. BTW, most of the companies list 0.050" and advertised durations on their web sites.
Remember too, there is no 'exact' maximum DCR for all applications. Just as with SCR, it depends on head material, chamber shape, fuel octane, engine operating and ambient temperatures, bore size, altitude, humidity..
When you go to forced induction, it gets even more complicated by boost pressure, compressor efficiency, presence/type/size/efficiency of intercooler... There are sites that calculate DCR based also on boost and altitude, but I don't know if the results are valid.

 

I have a custom cam coming with the following IVC spec

@ .050" 42 ABDC
@ .006" 67 ABDC

25 degrees difference right there as per MadBill's estimate.

Assuming pump gas, alu head, stock bore...What should max Boosted CR be?

 

Here's a site that will give you a number: http://www.rbracing-rsr.com/comprAdvHD.htm

 

Thanks... , but my question is not how to figure it out, but what the MAX Boosted CR would be before Kaboom.

Just looking for opinions. Is it 9, 10, 12 15, 20?

I calculate MAX temperature in the chamber, but have seen Boosted CR's that were quite high and the motor still lived.

 

So it looks like SStrokers formula works out to be a bit more accurate. The lobe I figured showed 24.5 diffrence, so that looks just about right. On another note, should one choose a camshaft based off of DCR? Assuming one was using only XE-R lobes in his camshaft choice, You'd have to go rather small on the intake lobe in order to keep the DCR up. Take for instance my 454 ci LS2. I'd need a cam in the high 230's to keep DCR up around 8:9:1. That just dosen't seem right for such a large motor. I know I'm missing a point somewhere, but I can't seem to figure out what it is.

 

That's why MadBill and a few other very knowledgeable sources around here have suggested to choose the static compression of the motor last or atleast towards the end after your camshaft choice and IVC event have been chosen. From my understanding the IVC point which promotes optimal DCR may not infact promote the best BMEP/VE for a motor, well unless you spec the SCR after the IVC point is chosen as indicated before.

 

Keep in mind that DCR is primarily a low speed phenomenon. At high rpm it has no effect. If it did, you could not achieve volumetric efficiency above 100% as is require for high horsepower output. The inertial and sonic "ramming" effect overcomes whatever lowering of compression ratio that late intake closing causes. In the end, it is cylinder pressure that is the issue, not what the compression ratio is.

 

You're right Critter, DCR has it's biggest effect at low RPM, where it can really crutch a big cam's 'soggy bottom'. The increased SCR is still a plus though at high RPM, where although the V.E. may exceed 100%, there is insufficient time for detonation to occur.

 

What's the stock rod length on an LS1?
Edit: nevermind, looks like 6.125

I come up with a SCR of 11:1 and a DCR of 9.49:1 runnig an F14 with 62cc heads and a .045" gasket.

 

Stock rod length of an ls1 is 6.098"