I was running the numbers and have 2 different answers for the DCR of this combo.
SB 385cid 350 40 over 3.75 stroke
5.7 rods
Edelbrock RPM heads 70cc
flat top 2vr -5cc
CC XE274 cam
.039 gasket
0 decked

Pat Kelly's calculator says 10.4 SCR and 8.5 DCR
while Performance Trends engine analyzer V 3.0 says 10.4 SCR and 7.5 DCR.
Either one of them is wrong or I missed some #'s.
Anyone care to run the #'s and see what you come up with?
The cam is I-230 E-236 @.050 Adv is I-274 E-286

IO 31BTDC IC 63ABDC
EO 77BBDC EC 29ATDC

Anything I missed?

Todd.

I've had the same problem, but I think Pat Kelly's calculator is more accurate. The Engine Analyzer will be somewhat close if you be sure to choose Seat-To-Seat timing in the cam specs, not .050, but you probably knew that.

I saw that, too. I know how mine is calculated but I have no idea how PT calculates theirs.

I know the piston location is correct on my calculator. I comparied the piston location to a chart on Isky's site, sadly not there anymore. I did Dload it and you can look at it here (http://cochise.uia.net/pkelley2/ncrank1.pdf). For instance, take a look at 75 ABDC for 5.7" rods the piston location is 2.444 on the chart. My calculator yields 2.4437 rounding to 2.444. Once the piston location is deterimed, this value is used for the stroke length in a standard CR calculator. Most all of the info for this type of DCR was gleaned from David Vizard's book on camshafts and valvetrains. My numbers match with his. He used a different method, working with percentages. I found that difficult and, seemingly, requiring some mystical knowledge smile.gif so I did it differently.

I feel confident about my calculations. I wish I knew how PT does it.

It all depends on what the real seat timing is. Most advertised durations are specified at about .006" lift, which could be 20° short of full seat-to-seat duration. If you look at the specifications of the GM performance cams like the L79 3863151, it has a .050" duration of 221°/221°, a .008" duration of 284°/284° and a seat-to-seat duration of 306°/306°.

In EA3.0 you have a choice of mild or aggressive grinds, so I think it interpolates the DCR based on this selection and the geometry of the installation. I put in the full seat timing of the mild L79 cam in a 10:1 385 and got 7.08:1 DCR. If I put in the .050" numbers it comes out to 7.37.

There is another 050 calculator here.

http://www.rbracing-rsr.com/comprAdvHD.htm

This one doesn't give you a DCR ratio, but it DOES calculate cranking compression. The numbers from this calculator were with 5 psi on my engine. Not sure if it was a fluke or what . . .

There seems to be a little controversy over what measurement one should use when calc DCR --- 050 or advertised. How much air can push back past a valve into the intake when it is only open .006 - .020" anyway?

JMO

Originally posted by Eric68:
There seems to be a little controversy over what measurement one should use when calc DCR --- 050 or advertised.JMO No controversy at all. Seat timing all the way. If you measure the timing events with a wheel, use that, it is more accurate. For a more approximate answer, use the published advertised duration. .050" timings will not work for this.

Matt, Right on, I went back and had to change the cam to seat to seat and the DCR is the same as Pat's program.

What I'm after is a combination that will run on 91 octane pump gas. The heads and cam are being used in my 350 so I plan on keeping them and choosing a piston to give me the ratio I need for the 383.

Looks like 8.5 is too much, if I install the cam back to 110 CL the DCR drops to 8.3 but might still be too high. Looks like a longer duration is in order or maybe dished pistons.

Todd.

OK Pat, but will a cam with lower intensity ramps (like my 294s for example) have the same DCR as a cam with the same adv duration and much more radical ramps (more duration @ .050, and .200)?

It seems to me that there would be a difference between a cam that is only 294* adv and 250* @ 050 and a cam that is 294* adv with 260* @ 050.

According to the calculator I posted in the link above, there would be a significant difference in cranking compression between the two cams. and when talking about RPM bands, duration @ 050 seems to be the big factor (along with LSA).

I'm not doubting the theory, Pat, in fact I think your calculator is great and use it often. Just trying to figure the whole lobe intensity vs. DCR thing out.

DCR, as both David Vizard and I use it, can only be calculated with the seat time not the .050" time. Cams with the same intake duration, SCR, and ICL will have the exact same Dynamic Compression Ratio regardless of the steepness of the ramps. The Dynamic Cylinder Pressure can, and probably will, be vastly different. You have to remember that DCR, as the DCR Calculator calculates it, is a simple calculation that substitutes the calculated position of the piston when the intake closes for the actual stroke length of the crank. Nothing else is involved. Using the .050" timing will position the piston much lower in the cylinder and give an artifically high DCR. Low intensity cams, like the 278/288-204/214 cam in my driver, are problematic. Using the adv dur seems to give too low of a DCR, for the way the engine performs, but using the .050" dur gives way too high of a DCR and the engine should be pinging itself to death (it runs fine on regular 87 octane). I don't really know the answer. Remember, none of this is carved in stone. The DCR Calculator, mine or others, are really guides to help match the cam and compression ratio. A very aggressive cam could have detonation problems even though the DCR calculates to be OK because the higher cylinder pressures of an aggressive cam could push the engine into detonation.

Eric, I ran the calculator you posted. Using the intake closing time of my cam (73º ABDC, with boost pressure and altitude at zero) it gave 7.81:1 for the DCR. My calculator gave 7.82. Pretty close, I'd say.