Thread: 355 street build. DCR, SCR, octane, quench, confusion

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Posted similar question to another board and got the below feedback. Does this jive with the gospel of Techinspector and Denny? i've been reading and searching and reading some more. Looking for closure on this. Help!!


""OK. With an IVC of 66 and the above specs:

SCR-9.62
DCR-7.62
Dynamic cranking pressure: 149 psi

Should be fine running 87 octane. Just be sure the IVC is in fact 66.

I used a quench of .04 (.01 deck, .030 gasket).

SCR by itself does not mean anything. DCR is calculated by using the intake valve closing point. The earlier the intake valve closes on the compression stroke of the engine, the more air gets compressed so you have a higher DCR.

Smaller cams will close the intake valve earlier and larger cams will close the intake valve later. Had you used a bigger cam with a larger IVC your DCR would be below 7.5 and you would need to increase your SCR to get it above 7.5. That's why I said SCR by itself does not mean anything.

Your DCR should be between 7.5 and 8.5. Yours is at 7.62 which is at the bottom of the recommended range. The higher you go with your DCR the higher the octane must be.""

Cranking pressure should not exceed 165 psi for operation on pump gas according to Crane Cams.

You may have shot yourself in the foot with your piston choice. If there is no large flat area on the crown to mate up with the underside of the head, then proper squishing of air/fuel across the chamber at TDC could be a problem. Here's a 12cc D-Cup from Keith Black that has the proper configuration. See the large, flat area on the right side of the photo. If your pistons have only a thin ring of crown material around the perimeter of the piston, there may not be enough area to form proper squish.....
http://kb-silvolite.com/performance....tails&P_id=155

A tight 0.035" to 0.045" squish is one of the best ways to prevent detonation, but again, there must be enough material to form a good squish.

At zero deck with a 0.040" gasket (8.65cc's with 4.100" bore), 12cc pistons and 64cc heads, the SCR will be 9.59:1

With 5.7 rods, 3.48" stroke, 9.59:1 SCR and the intake valve closing at 42 degrees ABDC @0.050" tappet lift, the DCR will be 8.103:1 and should support the use of cheapo pump gas, assuming the squish is right and the ignition system is optimized for the combination.

thanks tech!

i looked at those pistons. ran the KB calculator. ended up with numbers a little different than yours. if i add .005 to deck/gasket height i come down .1 in DCR and SCR. see below. the KB calc 7.6 DCR makes me feel alot better. How did you arrive at 8.1 ? piston face design doesn't seem to affect SCR or DCR, they are all about volumes, but i bet it effects spark and flame propagation. Am i getting too wrapped up in this? is a few tenths SCR or DCR really going to matter in my app? my main concern is detonation and the impact DCR and SCR have on it as well as trying to get a good quench. these three things aren't playing nice with each other. LOL.

ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) Cylinder Head Vol
(cubic in.) 03.904
Piston Head Volume (cc) Piston Head Vol
(cubic in.) 00.732
Gasket Thickness (in.) Swept Volume
(cubic in.) 44.389
Gasket Bore (in.) T.D.C. Volume
(cubic in.) 05.226
Cylinder Bore Diameter (in.) Gasket Volume
(cubic in.) 00.462
Deck Clearance (in.)
Note: Neg. nubmer above deck, Pos. number below deck Deck Volume
(cubic in.) 00.128
Stroke (in.) STATIC COMPRESSION RATIO 09.494

OPTIONAL DATA
Rod Length (in.) Adjusted Stroke (in.) 02.674
Intake Closing Point (degrees)
ABDC @ 0.050 lift plus 15 degrees DYNAMIC EFFECTIVE COMPRESSION RATIO 07.527

Posted by thebigblue:
"How did you arrive at 8.1 ?"

727.35cc's plus 64cc's plus 8.65cc's plus 12cc's = 812
64cc's plus 8.65cc's plus 12cc's = 84.65
812 divided by 84.65 = 9.59:1 SCR

In the KB calculator, enter 9.650 compression ratio, enter 3.480 stroke, enter 5.700 rod length, enter 57.000 intake closing point (intake valve closes at 42 degrees ABDC @0.050" tappet lift. Add 15 to that and enter 57.000 into the calculator).
Result: 8.103:1 DCR.

"my main concern is detonation and the impact DCR and SCR have on it as well as trying to get a good quench. these three things aren't playing nice with each other."

The numbers work out fine. Cut the block decks to square them up and achieve zero deck height. Use a 0.040" gasket. The only thing that concerns me at this point is the configuration of the piston crown. If you don't have enough squish pad on the piston, you've shot yourself in the foot.