I'm trying to get the parts together to build another motor right now and was wondering what would be the best DCR I can get away on pump gas would be. The quench is going to be .040-.044. And the motor will have aluminum heads. Right now, going through Pat Kelly's calculator I am coming up with about 8.58 DCR with one set of pistons and 7.99 with the other. I am thinking 8.58 would probably be right on the ragged edge of pump gas, but kinda wanted to see what you guy's opinion would be. The car will be street\strip, but probably more street than anything else. Thanks!

I'm at 8.51 with my Canfield heads. So far, no ping or detonation with .062" quench. If you're doing mostly street, I'd go for the 7.99 and try to come up with a thinner head gasket (like a Felpro .015" coated shim) if possible.

For me, when I installed the new heads, it was a big moment when I ran the tank dry of race/pump mix (100 octane) and tried straight 92. The whole purpose of me buying new heads was so that I could run only pump gas...thankfully, and I pray to the car gods, that everything is OK. If I remember right, Eric68 has a slightly higher DCR than I.

The lower compression will also allow you to choose a cam with a tighter LDA, which I am a big fan of.

If the car is mostly for street I wouldn't try to push it to the ragged edge. Exactly where that "ragged edge" is depends on a lot of factors in addition to cam timing and static compression ratio. Quench is important -- combustion chamber design, air inlet temp, engine coolant temp, ignition curve, A/F ratio, gearing, and converter stall all effect how much DCR an engine can handle.

I'm pushing it with 8.66:1 with no problems, but I wouldn't recommend going to the extreme I have. 8.3:1 with aluminum heads and pump 92/93 is a pretty safe bet. 7.8-8.0:1 is pretty safe for iron heads.

Most important . . . if you start getting close to the edge you need to measure everything carefully when putting your engine together. Combustion chambers are probably not exactly the volume advertised, deck height will make a big difference, so will head gasket thickness and piston valve reliefs. If you are up to it you can also calculate crevice volume (that's the volume around the piston crown above the top compression ring.)

"If you are up to it you can also calculate crevice volume (that's the volume around the piston crown above the top compression ring.)"

Eric, I've usually found that to be 1.1cc on 4" bore motors.

It depends on the location of the top ring land, the diameter of the piston crown, and the size of the bore . . .

Are you saying that they all wind up pretty much close to the same on a 4" motor?

The static compression for the 8.58 DCR is 10.52. The heads will be the AFR #1034, so I am assuming the chambers should be pretty accurate, but I haven't recieved them yet because of the 8-12 week backorder!!! The deck height will be 9.005 (should be, we'll see how machine work goes). It gets pretty hot around here in the summer, ambient is upper 90's most summer days, and the car doesn't run as cool as I'd like it to (200-205). So I think I'll go for the lower DCR to keep from having to do it twice. But then again, a bigger cam........lol