Porting Exhaust Smog Heads - Double Take 360

In our last installment of the Double Take 360 series, we discussed the virtues of the 340 "X" head. However, it would not be the "X" head that would be installed on our 360, but rather the lowly, late '70s smog heads-#4071051 castings-that came with our E58 salvage yard refugee engine.

To get the most from these stock small-block smog heads, we performed some basic intake porting in our January 2000 issue to see how much flow we could unlock on the intake side. Since our aim with the stock heads is to put together a budget combo, we weren't going for the full-on Pro Stock porting job, but realistic do-it-yourself street porting. Using the equipment at Specialized Motor Service in Riverside, California, I performed all of the cylinder head machining using a Serdi 100 seat and guide machine, while the subsequent port modifications were done at home using an air die grinder and a selection of carbide bits and grinding stones-a true home porting job. All of the flow tests were complements of airflow expert David Vizard's Quadrant Scientific flowbench, which we used last year in our series on big block heads ("Go With The Flow," Mopar Muscle, January, February, March 1999).

This issue, we'll tackle the exhaust side and, in our effort to outperform the fabled "X" head, provide you with the information so you can be armed with your die grinder in hand.

Exhaust: Where To Begin
On the exhaust side, the 360 heads already have the amply-sized, 1.60-inch diameter valve-the same diameter used on the high performance 340s. Nothing special was done here, except to use the sweeping cutter out to the gasket line, as was done on the intake, to reduce valve shrouding. The valve's seat location was dropped slightly with a 5-degree topcut in order to even the exhaust valves to the height of the intakes. (If anything, the exhaust should be lower than the intake.) The seats were a conventional 45 degrees, with a 30-degree top cut and a bottom cut of 75 degrees. We toyed with the idea of keeping things cheap and reusing the stock exhaust valves, but made a change to new Milodon Street Stainless valves. These offer a slight improvement in flow and greater durability than our used stockers. By reusing the stock exhaust valves, the cost of the heads can be reduced by about $100.

Rounding out the other machining possibilities, the guides were judged to be acceptable, if slightly on the loose side, and nothing was done there. Credit the flash-chromed stems on the E-58 heavy-duty police 360's valves for the low guide wear on this high-mileage salvage yard motor. Since the cam we will be using is conservative-with only .480-inch lift-the tops of the guides did not need to be machined down. Also due to the cam choice, the regular replacement umbrella-type valve stem seals will work with the matched single springs (Competition Cams #901-16), precluding the need to cut the guide bosses for PC-type fixed valve seals. Milling would wait until we finished with the porting, and then we'd figure out how much we'd need to skim to dial in the chamber volume, and thus, the compression ratio.

Exhaust, Easy Flow
We had our intake numbers and were looking to achieve 70-75 percent of peak intake flow on the exhaust side, or about 175 cfm. As on the intake, the 340 "X" head would serve as our benchmark, and the stock ports were flowed to get an idea of where we were. These numbers are in Table 2, Columns 1 and 2 on page 68, respectively. In the street lift ranges, we were surprised to see that the 340 "X" head and the 360 smog head ports flowed essentially the same. In fact, the 360 head pulled better numbers down low, indicating that the seat on the smog heads was better in the particular heads we tested.

At high lifts, the 340 head had a marginal advantage over the later heads. Examining the ports, there was little difference in port design between the two castings other than the air injection passage protruding in the low-speed area on the floor of the 360 port.