dcr vs scr

[grumpyvette]

basically SCR (STATIC COMPRESSION RATIO is the max cylinder volume at BOTTOM DEAD CENTER COMPRESSED INTO THE VOLUME AT TOP DEAD CENTER ON THE ENGINE STROKE, WITH NO VALVE TIMING INVOLVED

USE THIS

http://www.newcovenant.com/speedcrafter/calculators/compressionratio.htm

DCR (dynamic compression ratio) [/color]TAKES INTO ACCOUNT THE FACT THAT THE PISTON CAN NOT COMPRESS ANY CYLINDER VOLUME UNTILL ALL THE VALVES HAVE CLOSED

 

DOWNLOAD AND USE THIS SOFTWARE

 

http://cochise.uia.net/pkelley2/DynamicCR.html

 

KEEP IN MIND YOUR ENGINE never sees static compression, it can only relate to dynamic compression, the only real use for SCR is a BASIS for finding the working engines DYNAMIC COMPRESSION RATIO

 

LOOK heres a piston location rotational degree chart for a 350 with 5.7" rods

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

 

HERES A CAM SPEC CARD

 

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=119661&B1=Display+Card

 

if the static compression is 11:1 the engine will see close to 7.8:1 with that cam

[Guest Bob L]

Hi grumpyvette,

A question for you. It seems that DCR is based on two ideas: First that no compression can take place when the intake valve is open and, second, that if the intake valve is open at all, it is considered open.

 

I'd be interested in drilling down on both of those points. If you think about the ram effect of the intake, it is possible to have greater than atmospheric pressure on th intke charge at some RPMs; i.e., volumetric efficienies of greater than unity. In this case, the dcr calculation appears a little simplistic, though I might be missing something.

 

Let's consider a thought example. Let's say I have two engines with the same scr but that differ in the following ways. Engine 1 has an intake, exhaust, and camshaft tuned for low RPM performance; Engine 2 is tuned for high RPM performance. Engine 2 would have a later intake valve closing and hence a lower dcr (perhaps much). Assume the engines are otherwise built so that they have the same peak torque. The peak volumetric efficiencies and hence peak cylinder pressures should then be about the same. But the peak cylinder pressures strike me as a good predictor of knock susceptability. Wouldn't the two engines have comparable knock issues, though at different RPMs.

 

The second question is related to the amount the valve would need to be open to prevent the intake charge from being compressed. It seems this would be different at different RPMs. As RPMs got very low, even a little bit of lift could spoil compression. But at high RPMs, it's not clear this is the case.

 

Any thoughts?

 

Thanks,

Bob

[Kevin Shasteen]

.., the only real use for SCR is a BASIS for finding the working engines DYNAMIC COMPRESSION RATIO..,

 

To add to this statement, I would say that SCR's purpose is to correct the compromise in airflow velocity wherein that compromise to the pre-modified condition was brought about by your post-modified cam upgrade.

 

As a result this new cam determines what your DCR is after being insctalled, and it is the DCR that determines to what degree you will have to alter your SCR....for the sole purpose of correcting said airflow velocity: at whatever specific peak rpm your level of performance requires.

 

BobL: "A question for you. It seems that DCR is based on two ideas: First that no compression can take place when the intake valve is open and, second, that if the intake valve is open at all, it is considered open."

 

Bob, I would say there is one more issue that immediately comes to my mind-and that is: DCR also involves "The Position" of the piston within the cylinder at the moment in time that corresponds with the IVC...this is where the DCR 3D Cylinder will encroach on the SCR 3D Cylinder. With a cam upgrade utlizing a narrower LSA and longer Duration-the result is a minimizing of your SCR and requires a correction to your pre-mod SCR in order to correct that compromise in said airflow velocity to better maximize efficiency that matches your post-mod DCR.

 

To some-this may sound like circular reasoning, but it is not.

 

As far as your Two Engine Example: I would suggest not basing it on two engines w/the same SCR and different DCR's, rather I would say it was just the opposite. In that two engines could have the same DCR but their SCR's would differ simply because one was tuned for high rpm while the other was tuned for lower rpm. If their SCR's are identical then one of those two engines have components that are mismatched: simply because an engine built for peak high rpms will have ports that are larger than an engine built for peak low rpms. I think because all of us grew up reading perf.mag's-which will never utter the "Dynamic CR" words and always concentrate on SCR-then it is hard to grasp the issue that DCR should actually be considered first yet it is your DCR that determines your optimum SCR.

 

Your "Total Chamber Volume" is the mathematical variable that when plugged into the CR formula will determine what level of performance is possible and which engine components should be used.

 

The DCR & SCR are not means to end all: they are merely tools. The final performance of an engine is a total package and not based merely on DCR or SCR (hence your mentioning VE%): they are pretty important tools but not the only tools. Remember that DCR & SCR are both "Compression Ratios"...they are not linear numbers.

 

Kevin,

(Yea,Still an Inliner)

[Guest Bob L]

Thank you for the response kevin. I still have a couple of questions if I may.

 

DCR also involves "The Position" of the piston within the cylinder at the moment in time that corresponds with the IVC...this is where the DCR 3D Cylinder will encroach on the SCR 3D Cylinder.

I understand this to mean that the bottom or base of the DCR cylinder is the position of the piston at the IVC. Is this correct?

 

With a cam upgrade utlizing a narrower LSA and longer Duration-the result is a minimizing of your SCR and requires a correction to your pre-mod SCR in order to correct that compromise in said airflow velocity to better maximize efficiency that matches your post-mod DCR.

 

I was assuming that different heads, cams, and intakes were chosen so that the engines have the same peak torque. My assumption, perhaps unreasonable, is that the peak volumetric efficiency is the same in both cases, call it 0.9. In this case, the engine would produce something on the order of 1.2 lb-ft of torque for each cubic inch.

 

Thanks,

Bob

[Kevin Shasteen]

..' date=' DCR also involves "The Position" of the piston within the cylinder at the moment in time that corresponds with the IVC...this is where the DCR 3D Cylinder will encroach on the SCR 3D Cylinder.

I understand this to mean that the bottom or base of the DCR cylinder is the position of the piston at the IVC. Is this correct? Bob[/quote']

 

Yes, to make it even more clear: the bottom of both the DCR & SCR 3D Cylinders will be the Top of the Piston. Where SCR involves B x S the DCR involves (B x Remaining Stroke after IVC).

 

[qote=Bob L].., I was assuming that different heads, cams, and intakes were chosen so that the engines have the same peak torque. My assumption, perhaps unreasonable, is that the peak volumetric efficiency is the same in both cases, call it 0.9. In this case, the engine would produce something on the order of 1.2 lb-ft of torque for each cubic inch..,

 

Theoretically you could but to what end? Your example was about two engines-one tuned for HIGHER RPM PEAK POWR while the second was tuned for LOWER RPM PEAK POWER with the same SCR but different DCR's. If you took this approach your power output for the engine tuned for higher rpm peak power would be inefficient (mismatched components to the cam chosen-AKA: wasted money and time). You could manipulate the airflow velocity for both engines in order to obtain peak power at the same time...but the SCR's would have to differ otherwise one engine would be prone to inappropriate combustion problems at the higher rpm's.

 

If you build an engine for higher rpms and you want that engine to reach max rpms quicker you dont do so by suffocating it w/a minimized SCR, rather you do so by gearing of the trans, differential & wheel/tire combo: otherwise why bother? HP is a function of how much torque occurs over time and distance and you wont get the maximum HP unless your engine is operating at maximum efficiencies: those efficiencies are: 1) VE%, 2) Mechanical Efficiencies 3) Thermal Efficiencies. As Efficiencies increase-so too does your tolerance for a higher SCR w/out fear of inappropriate combustion.

 

You cant apply a simple linear number to DCR or SCR-it doesnt work that way: simply because they are a Ratio...they wont stand alone by themselves in a mathematical equation.

 

DCR and SCR each has a working mathematical relationship to one another where Displacement has the final say-and therefore DCR nor SCR can not be mathematically separated into a simple linear/incremental/exponential number.

 

.., I'm not sure that answered your question(?): did any of that make sence?

 

Kevin,

(Yea,Still an Inliner)